Hl)6

HARVARD UNIVERSITY

LIBRARY

OF THE

Museum of Comparative Zoology

BULLETIN

OF THE

MUSEUM OF COMPARATIVE ZOOLOGY

AT

HARVARD COLLEGE, IN CAMBRIDGE.

VOL. XXI.

CAMBRIDGE, MASS., U.S.A.
1891.

Reprinted with the permission of the original publisher

KRAUS REPRINT CORPORATION

New York

1967

Printed in U.S.A.

CONTENTS.

Page
No. 1. — Contributions from the Zoological Laboratory. XXIV. Contribu-
tions to the Morphology of the Turbellaria. — I. On the Structure of
Phagocata gracilis, Leidy. By W. M. Woodworth. (4 Plates.) April,
1891 1

No. 2. — Contributions from the Zoological Laboratory. XXV. The Cora-
pound Eyes in Crustaceans. By G. H. Parker. (10 Plates.) May, 1891 . 45

No. 3. — Contributions from the Zoological Laboratory. XXVI. On Some
Points in the Anatomy and Histology of Sipunculus nudus, L. By H. B.
Ward. (3 Plates.) May, 1891 143

No. 4. — Three Letters from Alexander Agassiz on the Dredging Opera-
tions off the West Coast of Central America to the Galapagos, to the West
Coast of Mexico, and in tire Gulf of California, carried on by the U. S. Fish
Commission Steamer " Albatross." June, 1891 185

No. 5. — Contributions from the Zoological Laboratory. XXVII. The De-
velopment of the Pronephros and Segmental Duct in Amphibia. By
H. H. Field. (8 Plates.) June [August], 1891 201

No. 1. — Contributions to the Morphology of the Turbellaria. —
I. On the Structure of Phagocata gracilis, Leicly. By W. M.

WOODWORTH. 1

In the fall of 1887, Mr. H E. Valentine of West Somerville, Mass.,
brought to the Embryological Laboratory of Harvard College some
planarians, with the suggestion that they might be infested with para-
sites. The planarian proved to be the interesting Phagocata gracilis of
Leidy, and the supposed parasites were the pharynges of the complicated
digestive apparatus. At the suggestion of my instructor, Dr. E. L.
Murk, I undertook, the study of this curious Triclad.

The animal, which was afterwards named by Leidy Phagocata gracilis,
was first described by S. S. Haldeman ('40, p. 3) in 1840, under the
name of Planaria gracilis : " Body oblong, suddenly tapering to a point
posteriorly : sides nearly parallel ; head square in front, with a project-
ing appendage on each side : neck narrowed ; eyes (two) sitrated on
each side of the narrower part ; these are oblong and white, with a
black dot at their internal side : ventral opening less than one third
the entire length from the posterior extremity, and from this open-
ing an intestine is sometimes protruded. General color fuliginous,
veined with black. Length, | hi., breadth, ^. Hab. springs in Eastern
Pennsylvania."

In 1848, Leidy published a further description of the species, giving to
it the name of Phagocata ('48, p. 248), because, as he says, " I detected
such a remarkable peculiarity in the digestive apparatus as led me to
investigate its anatomy in detail, and to form for it a separate sub-genus,
characterized as follows : —

"Phagocata, oblonga, plano-convexa, nuda, contractors, mucosa, an-
tica auricularia. Aperturse dure, ventrales, ad os et ad generationem
pertinens. Proboscides multaj.

1 Contributions from the Zoological Laboratory of the Museum of Comparative
Zoology, under tlie direction of E. L. Mark, No. XXIV.

No XXIII. of these Contributions appeared in the Proceedings of the American
Academy of Arts and Sciences, Vol. XXV., under the title, "Preliminary Notice
on Budding in Bryozoa." By C. B. Davenport.

vol. xxi. — no. 1. 1

2 BULLETIN OF THE

"P. gracilis, nigricans, lateribus parallelis, postero acuto abrupte,
plerumque antico recto ; oculis duobus. Long. 9 lin., lat. 1 lin. Habitat
in foDtis Pennsylvanise.

" Description. Oblong, limaceform, naked, convex superiorly, flat
inferiorly, very contractile ; sides ordinarily parallel, convex when the
animal is in a contracted state, convergent anteriorly when elongated ;
anterior extremity with a lateral triangular auricular appendage, straight
in front, by contraction becoming convex or concave ; posterior extremity
abruptly pointed ; ocelli two, anterior, composed of an oblong, semi-
transparent (nervous ?) mass with an intensely black dot of pigmentum
at the internal posterior part ; ventral apertures two ; oral aperture a
little less than one third the length of the body from the posterior
extremity. Color black or iron gray, and in some younger specimens
latericeous."

I have quoted Leidy's description in full, because it seems to me that
the first description of so striking and aberrant a species is of uncommon
interest.

It is noteworthy, that, notwithstanding the faithfulness of the descrip-
tion, and the remarkable peculiarities of the worm, no mention of the
species has been made for over forty years. It is also strange that
Girard should have been ignorant of the existence of Leidy's paper, for
in his list of North American fresh-water Planariaj ('51, p. 2G4) he
uses the name proposed by Haldeman, " Planaria gracilis," and says
that it is " common about Cambridge in pools and rivulets." Ho
adds, in a note, " Planaria gracilis and very likely Planaria tigrina
will not remain in the genus Planaria as soon as we shall know their
internal structure." In a subsequent paper ('51 a , p. 2), "Die Plana-
rien und Nemertinen Xord-Amerikas," the species is described under
the name given to it by Leidy, but no mention is made of the most
Btriking characteristic discovered by that observer, — the multiplicity
of the pharynges.

The structural peculiarities of Phagocata were not simply ignored,
they- were even denied by no less an authority than von Siebold, who
explained the " proboscides " of Leidy as so many processes from the
lip of one normal pharynx. After quoting the description, he says
('50, p. 389) : " T)as erwachsene Thier soil 23 Riissel haben, die es beim
Fressen alle hervorstreckt ; Ref. vermuthet, dass der Riissel eine trich-
terformige ausgezackte Miindung besitzt, und dass die beweglichen Fort-
satze des Riisselrandes fur ebenso viele einzelne Riissel gehalten worden
sind."

MUSEUM OF COMPARATIVE ZOOLOGY. 6

Diesing, like von Siebold, was incredulous ; in his " System," he says
('50, p. 207), "(Esophagus protractilis multi partitus (proboscides
mult* Leidy)." Twelve years later, in the "Revision der Turbellarien,''
he writes ('62, p. 506), " oesophago multipartito."

Stimpson CoS, p. 23) in his Prodroraua apparently followed Diesin
for he says "oesophago protractili multipartito."

Recently, Professor Leidy ('85, p. 49) has figured Ph«gocata gracilis
in a popular account of " Planarians." These are the only descriptions
of Pha&rocata I have been able to find.

Phagocata gracilis, Leidy.

When viewed from above, the general form of the animal is elongated ;
its lateral margins are nearly parallel, being slightly convex posteriorly ;
the widest part of the body is in the pharyngeal region. The largest
specimens measure 30 mm. in length by 4| mm. in breadth. Anteriorly
the sides converge slightly up to about the region of the eyes, where the
diameter increases, thus forming the so called head. This bears the
lateral auriculate appendages. The lateral appendages are rounded,
rather than triangular as described by Leidy; they are continuous with,
and in fact form part of, the anterior extremity ; posteriorly, the sides
converge to a point (Fig. 20 a). The eyes appear as two elongated oval
white spots, with black pigment on the internal edge. They are situated
on the narrow part or "neck."

Haldeman and Leidy have described the head as being " straight in
front." This appearance is seen only when the animal is at rest. It is
then much contracted in the direction of its antero-posterior axis, and is
usually much distorted ; at such times it often appears as a shapeless
black lump, this condition probably being a means of protection (Figs.
20 b and 20 c). When in motion the anterior extremity is usually convex,
but not always, for it may be straight, sinuous, or concave ; these shapes
are only temporary, following each other in quick succession. The head
changes its form especially when the animal approaches some object ;
for this part of the body is functional as an organ of touch ; that it
is suited structurally to be a kind of feeler will be evident from th
description of the nervous system which follows.

Phagocata gracilis has a shiny black appearance when viewed by
reflected light, but by transmitted light it is of a greenish gray color.
The color may vary from black to a reddish brown on the one hand, or
to a light gray on the other. I have seen small specimens which were

4 BULLETIN OF THE

almost milky white. The ventral surface is of lighter color than the
dorsal, and there are light areas about the ventral apertures. The
pigment is densest in the dorsal median line, where it forms a dark
band ; it diminishes toward the sides of the animal, the edges of which
are quite destitute of it. The distribution of the pigment in the head
region presents many variations. In most cases the posterior borders
of the auric ulate appendages show two light spots, and there is a
third one, somewhat triangular in shape, at the anterior end in the
middle line. The marginal area of the head, like that of the body, is
free from pigment. Sometimes the whole head region is light with the
exception of the middle line between the eye spots, where there is an
extension of the dark median band previously referred to. Light non-
pigmented areas occur wherever there has been a reparation of the
tissues resulting from injury.

By an examination of the animal in the natural condition, only a few
of the internal structures can be identified, because of the large amount
of pigment present. When viewed from above, the most striking feature
is a large oblong light region, the pharyngeal cavity with its contained
pharynges. Immediately behind this a similar but smaller spot marks
the position of the penis. From the ventral side the nervous system
may be dimly seen as two long whitish bands united by transverse
commissures and coming together in the head region in a bilobed en-
largement, the brain. Leidy apparently confused these structures with
the excretory organs, no trace of which can be seen on the living animal.
He says ('48, p. 250) : " There appears to be nothing peculiar about
the arrangement of the blood-vessels, if such they be : the term being
applied to two semi-transparent lines passing along each side of the
ventral surface, and a third along the middle of the dorsal surface, the
three freely communicating with each other by transverse lines and
numerous smaller branches, the whole forming an extensive reticulation
upon the surface of the body. At the anterior part of each ventral line,
I distinctly observed a dilatation to exist." And again : " I could detect
no traces of a nervous system." The two <; semi-transparent lines" are
without doubt the longitudinal nerve trunks, and the " dilatations at
the anterior part of each," the lateral enlargements of the brain. What
he means by the " third " line " along the dorsal surface," I cannot say.
When sexually mature individuals are subjected to pressure, parts of
the vasa deferentia and oviducts can also be made out.

Phagocata differs from all other Triclads in possessing many pharyn-
geal tubes instead of one. All the pharynges lie in a common chamber,

MUSEUM OF COMPARATIVE ZOOLOGY 5

and when protruded reach the exterior through a single orifice, but they
open into the intestinal cavity separately. One of these pharynges,
like the single pharynx of other Triclads, joins the intestine at the
junction of its main trunks ; the others are connected with the inferior
median surface of the lateral trunks (Plate II. Fig. 20). The odd
median pharynx is largest, and therefore most prominent of all. The
others, which arise from the intestine farther back, are successively
shorter, as well as narrower, the more remote they are from the median
proboscis. The attachment of the smallest ones is about as far from
the posterior end of the animal as the attachment of the chief one is
from its anterior end, so that the chamber which they all occupy em-
braces the middle half of the body. Although there are about as many
pharynges attached to one of the lateral trunks of the intestine as to
the other, they are not arranged in pairs, nor have their positions any
definite relation to the side branches of the intestine which open into the
lateral trunks. The pharynges are rather less numerous than the side
branches ; they sometimes arise opposite to a branch, sometimes oppo-
site to a space midway between two branches, or at other intermediate
points. The foremost of the lateral pharynges is often considerably
in advance of the corresponding proboscis of the opposite side of the
body (Fig. 20). Leidy ('48, p. 249) has well described the appearance
and action of the pharynges in the living animal. He says : " They
are considerably longer, but narrower, than in P. lactea, and when not
in use are packed together within the animal, so that, when the latter
is placed beneath the microscope and slightly compressed, they will
be seen pressing upon one another in such a manner that, if one
changes its position, it will be instantly occupied by another. Those
which are formed last are smallest, but they soon gain their full size.
If one of these animals be punctured or cut, one or more of the pro-
boscides will be instantly protruded as if they existed under pressure,
and will move about in all directions, appearing as if entirely without
the control of the animal ; or if one of the animals be crushed between
two slips of glass so that the proboscides will be torn from their attach-
ment, they move about involuntarily, always in a line forwards or towards
the mouth. ... In this progressive course they constantly contract
and dilate ; the mouth opens, and any matter in its vicinity rushes in,
when it is closed and the matter passes onwards, and by the alternate
contraction and dilatation of different parts of the same tube it is
thrown backwards and forwards several times, and finally violently
expelled at the torn extremity. When they have escaped from the

b BULLETIN OF THE

ruptures of the tegument produced by crushing, or when snipped off
with a pair of scissors whilst the animal is feeding, they will present
the same curious phenomena. In fact, these curious independent move-
ments caused me at first to mistake the organs for viviparous young,
and it was not until I had frequently observed the animal feeding, and
examined its structure beneath the microscope, after having fed them
upon colored food, that I was convinced of their true nature."

It was these automatic movements of the detached pharynges that at
fir.st led me also to believe that they were parasites. They appear as
long, white worm-like bodies, one end being truncated, the other ragged
and uneven, where it was torn from its attachment. They move about
quite rapidly by means of the cilia with which they are covered, and
waves of contraction continually pass along the length of the tube
from the truncated to the ragged end. The mouth end may be greatly
expanded so as to form a funnel-like structure, or it may be so- con-
tracted as to obliterate the lumen. I did not succeed in satisfying
myself of the real nature of these structures until I examined one of the
animals while it was feeding. I placed one of the Lumbriculidae in a
watch-glass with a Phagocata, which soon attached itself to the annelid
by throwing out its many pharynges, some of which were wrapped about
the victim, while others were thrust into its body (Plate II. Fig. 13).
The soft parts of the prey were rapidly sucked up and swallowed by means
of the peristaltic motions of the pharynges, so that in a short time there
was left nothing but the empty and shrivelled integument.

By far the best reagent for killing is hot corrosive sublimate. An ex-
cess of the salt is added to the saturated aqueous solution and the whole
is heated to the boiling point. A very strong solution can be prepared
in this way, as the salt is more soluble in hot water than in cold. Kennel
('88, p. 455) has recommended the use of 50^, nitric acid. I have used
with entire success a modification of his method, viz. a cold saturated solu-
tion of corrosive sublimate in 50cf o nitric acid. The worm is placed on a
plate in as little water as possible, and when properly extended the fluid
is quickly poured over it. After a few minutes' immersion the fluid is
replaced by a saturated aqueous solution of corrosive sublimate, in which
the worms remain for half an hour and are then washed. I know of
nothing else that will kill so quickly, and at the same time leave the tis-
sues uninjured. For the study of the intestinal tract, unstained speci-
mens were cleared in clove oil. The amount of pigment so obscures the
organs lying beneath, that the ramifications of the intestine could be

MUSEUM OF COMPARATIVE ZOOLOGY. 7

traced only on cleared specimens in which the intestine contained dark-
colored food matter. For staining, Grenadier's alcoholic borax carmine
followed by differentiation with acid alcohol proved to be the most use-
ful and reliable method. I have stained both in toto and on the slide.
Good sections for topographical study were obtained by staining in
alcoholic borax carmine for 24 hours and cutting in the horizontal plane
sections 30 fx. in thickness. By thus lightly staining, the nerve tissue
takes none of the color, and in such comparatively thick sections the
finer branches show as white lines against a red background. Orth's
picrocarminate of lithium is a valuable reagent on account of the select-
ive action of the picric acid for all glandular tissue, which it brings
out in sharp contrast to the red color of the other tissues. I have used
this reagent also with excellent results for macerating. The affinity of
hematoxylin stains for formed substances renders them of little use ;
their intense reaction with the great number of glandular structures tends
to obscure results. For isolation preparations, the best results were
obtained by macerating directly in the stain. I also used successfully
the osmic-acetic method of maceration on fresh material. The isolated
living pharynges were killed in hot \oJ silver nitrate for the purpose of
demonstrating the epithelium. Depigmenting was accomplished by the
use of a 1 of solution of potassic hydrate which was allowed to act for a
few minutes on sections fixed to the slide with Schallibaum's clove-oil
collodion fixative.

Cilia are present over the whole surface of the animal. In material
that had been prepared in hot corrosive sublimate, the middle region of
the ventral surface, where the hypodermis is thinnest, was often desti-
tute of cilia. Likewise at the lateral edges they may be wanting.
These conditions are, however, due to the action of the reagent, since in
the living animal cilia are always present in these places. At the ante-
rior end of the body on either side of the head, the cilia are somewhat
longer than elsewhere. They attain their greatest length at that por-
tion of the margin of the head which forms the auriculate projections.
From the middle of each projection they gradually diminish in length
until, at the anterior tip of the body and at an equally distant point
behind the auricles, they are reduced to the normal length. These two
areas covered by the longer cilia probably correspond to the " Tastor-
gane" of Iijima ('84, p. 366), and are directly related to local modifi-
cations of the hypodermis.

1 cannot find either the short immovable hairs or the long " Geissel-

8 BULLETIN OF THE

haare" seen by Iijima in other Triclads at the anterior margin midway
between the areas of the " Tastorgane " ; nor have I found in Phagocata
that the cilia in fhe head region move in different directions, as Minot
('77, p. 407) has observed in the case of other fresh-water plananans.

There has been a difference of opinion among writers as to the possi-
bility of certain regions of the body being normally destitute of cilia.
Metschnikow ('66, p. 436) and Kennel ('79, p. 125) found cilia covering
the whole surface in Rhynchodesmus and Geodesmus, but Zacharias
('88, p. 542) states that the dorsal surface of a variety of Geodesmus is
bare, and Vejdovsky ('90. p. 132) maintains the same for Microplana,
the cilia in the latter cases being confined to the ventral surface or sole.
It seems to me, however, that Moseley ('74, p. 118) long ago offered a
satisfactory explanation of the condition, by saying that in Bipalium the
cilia on the dorsal surface of land planarians, being weaker through com-
parative lack of function, are consequently more easily destroyed by the
action of the reagents used in the preparation of the material. Consid-
ering the habits of land planarians, and especially the dissimilar condi-
tions to which the dorsal and ventral surfaces are subjected in regard to
moisture, exposure, contact, etc., it is not strange that the conditions
of the cilia of the different surfaces should be unlike. Iijima ('84,
p. 366) states that it is the exception for the edges of Dendrocoelum lac-
teum to be ciliated, and that the almost constant absence of cilia is due
to certain parasites (Trichodina). He also speaks of a species of Geo-
plana from South America in which the cilia of the dorsum are replaced
by a granular crust. I believe that in planarians there is primarily
no localization of the cilia, and that all non-ciliate conditions are
secondary.

I could nowhere find a cuticula. The superficial portion of the cells
of the hypodermis takes a somewhat deeper stain than the body of the
cells, but there is no sharp line of demarcation between the two; the
color of the superficial portion fades gradually into that of the body of
the cell. A true cuticula such as that described by Minot (77, p. 407)
and Loman ('87, p. 69) for Triclads, and by Keferstein ('68, p. 16) for
Eurylepta, is wanting, and there is only a thickening, a condensation,
of the superficial plasma of the hypodermal cells.

The hypodermis has proved to be the most difficult of the tissues to
study, because of the minuteness of its elements, and the enormous
number of dermal rods, or rhabditi, which so obscure the true condi-
tions that it is only after long and patient study of thin sections and of

MUSEUM OF COMPARATIVE ZOOLOGY. 9

macerated material that one can learn what the true characters of this
tissue are.

The hypodermis is thickest on the dorsal surface ; it becomes thinner
toward the edges of the body, and in passing around to the ventral sur-
face it still continues to become thinner as far as the middle line, where,
forming part of the floor of the pharyngeal cavity, it reaches its greatest
attenuation. There are hypodermal thickenings around the oral and
genital openings, and also over two sensory areas on the ventral surface
of the head region, which will be described in another place.

It is almost impossible to find a region where the cells of the hypo-
dermis are not modified by the presence of the dermal rods. In order
to get at the natural appearance of the cells, it is necessary therefore to
study them in young specimens, and in the region where the rods
are fewest; this region I have found to be near the margin, on the
dorsal side. Very thin cross sections of young individuals are the most
favorable ones for this purpose.

The cells are columnar, the height necessarily varying with the thick-
ness of the hypodermis. The nuclei are large, have an irregular or
sinuous outline, and are situated; as a rule, near the bases of the cells
(Figs. 1 and 2). This position is not constant, and depends upon the
number and influence of the rhabditi that are present. There is no
nucleolus proper, the chromatin being scattered through the nucleus
in many large granules. The size of the nucleus does not appear to
depend upon the size of the cells ; for while the cells in different regions
vary to a great extent, the nuclei remain of nearly uniform size.

The cells are finely striated ; the striations are most prominent at the
basal ends of the cells, and cannot be traced to their free ends. Such
radial striations have been described by Bohmig ('86, p. 294) in the
hypodermis of Graffilla, and more recently by Lippitsch ('90, p. 328) in
the epidermal cells of Derostomum. Iijima ('84, p. 369) also alludes
to fine striations in the epidermal cells of Planaria polychroa. The
cells do not " etwa flach anf die Basalmembran aufsitzen," but are con-
nected with it by fine processes " welche etwa kammformig ziemlich dicht
neben einander stehen." These processes he believes to be directly con-
tinuous with the striations of the cells, and to be protoplasmic prolon-
gations of the cells. He traces them through the basement membrane
into the muscles below, thus establishing " eine organische Verbindung
zwischen dem Epithel und den Kurperinnern." His figure (Taf. XX.
Fig. 4) is confusing, and in addition was drawn, as he admits, from
specimen in which the basement membrane exhibited pathological con-

10 BULLETIN OF THE

ditions. Besides the striations in the cells, there appear creases or folds
resulting from the pressure of the rhabditi.

In thick sections through regions where the rhabditi are numerous,
the epidermal cells have the appearance of being joined to the basement
membrane by foot-like processes. This appearance at first led me to
believe in a condition like that described by Iijima, and it was only
after studying sections of material in which the rhabditi had been
removed (Fig. 3) that I understood their relations to the cells.

The rhabditi do not lie in the hypodermal cells, but between them.
Kennel ('79, p. 126) and Braun ('81, p. 305) are the only observers who
have described them as having an intercellular position. It will be seen
from the following description of their development in Phagocata, that
such a position is the only natural one. The presence of these rods
between the cells produces a crowding, and the pressure is so great that
it causes the cells to become displaced and much modified in shape.
The nuclei may be pushed out to the free ends of the cells, or crowded
down to their bases, and the cells themselves may be so reduced as to
appear like mere filaments (Fig. 3). Kennel ('79, p. 126) describes' the
epidermal cells of Rhynchodesmus after the removal of the rhabditi, as
"feine Fadcheu ... so lang als die Epidermis dick ist." Regarding
their intercellular position, Braun ('81, p. 305) states for Bothrioplana
that the rhabditi "nicht allein zwischen den Zellen stehen, sondern auch
das Protoplasma der Zellen durchbohren." In Phagocata, as in Rhyn-
chodesmus, the rhabditi are so numerous that the hypodermis appears at
first to be entirely composed of them. As Kennel expresses it, " ausser
den feinen, fadenformigen Zellen kaum etwas anderes Platz zwischen
ihnen hat." It is in thick sections, where the epidermis is many layers
deep, that the bases of these compressed cells present an appearance as
if the hypodermis were connected with the basement membrane by fine
foot-like processes. This appearance is only seen where the rhabditi are
most numerous. At the lateral edges of the body, where there are few,
and where consequently the cells retain their primitive cylindrical form
(Fig. 2), the latter are applied to the membrane by their broad bases.
It is in these regions also that the striations previously spoken of are
most distinctly seen.

Moseley ('74, p. 118) says, "The epidermis here [land planariansj is
seen to be made up of large gland-cells and cells containing rod-like
bodies and a certain amount of vertical filaments." "The irregular fila-
ments which fill up the interspaces between the gland-cells and rod-like
bodies appear to be the remains of the cell-walls and rod-like bodies."

MUSEUM OF COMPARATIVE ZOOLOGY. 11

He further says, " Tue substance of the epidermis is probably made up,
in the living condition, of cells resembling the gland-cells described, but
of various dimensions, and of cells containing rod-like bodies."

Since the " rod-like bodies," or rhabditi, are really modified glands,
Moseley's statement amounts to saying that the epidermis is composed
entirely of gland cells, a conclusion which it is not easy to adopt. More-
over, I believe that Moseley's "gland-cells" are only rhabditi that have
been modified by the action of the reagent which he used for their
demonstration. Kennel ('79, p. 126) obtained similar conditions by the
action of chromic and acetic acids on the rhabditi of land planariaus. I
have found that in Phagocata by the use of picric acid the dermal rods
become swollen and granular, resembling the "gland-cells" described
and figured by Moseley. " The vertical filaments " were undoubtedly
the true epidermal cells, reduced to a filamentous condition by the
influence of the many rhabditi lying between them.

I cannot find any organic connection between the cells of the hypo-
dermis and the deeper tissues, such as has been described by Iijima.
Although appearances like those described by him do occur, they are
secondary conditions, dependent on the presence of the rhabditi and the
development of their mother cells. The basement membrane is every-
where traversed by fine tubular processes of the mother cells of the
rhabditi, which lie imbedded in the body parenchyma. This fact,
together with striations of the cells of the hypodermis and the ultimate
reduction of these cells to filaments, might easily lead to conclusions
such as those of Iijima. His sections were thick (10-20 /a) both abso-
lutely and in propoi'tion to the length of his largest specimens (20 mm.),
whereas my sections were only 5-10 fj. in thickness, although the worm
attains the length of 35 mm. ; moreover, isolation preparations were
studied in connection with these sections.

The hypodermis consists of the hypodermal cells and the rhabditi
that lie between them. There are no unicellular glands in it. Lang
('84, p. 49) described in Polyclads a granular "interstitial tissue" con-
taining nuclei and pigment which arises, according to his conjecture,
from a coalescence of indifferent epithelial cells. Such conditions I
cannot find, nor can I detect any cement (" Kittsubstance "), such as
that described by Graff ('82, p. 44) for Rhabdoeoeles.

The dermal rods or rhabditi are defined by Graff ('82, p. 49) as "die
stark lichtbrechende glasartige homogenen Stabchen, welche weder
einen Faden noch einen Nadel einschliessen und durch ihre glatte Ober-
fl'ache, regelmassige Gestalt und ihren Glanz auflfallen."

12 BULLETIN OF THE

In Pbagocata the rhabditi are found in almost every portion of the
hypodermis, there being only one region from which they are altogether
absent, viz. around the gonopore, where they are gradually replaced
by many subcutaneous glands, which open to the exterior in a broad
circular area surrounding that oriiice. They are present around the
oral opening, even up to the aperture, where they abruptly cease.
They are most abundant in the middle line on the back, becoming
gradually fewer toward the sides and anterior end, but they are again
abundant on the ventral surface. They are found over the eyes, and in
the epithelium of the two anterior sense organs, where they are well
developed but few in number. Iijima ('84, p. 371) has stated that they
are wanting in this region in the case of D. lacteum, but are present in
Planocera polychroa and Polycelis tenuis. He has also shown that in
the case of I), lacteum they are unusually abundant in the region of the
genital orifice, both in the epithelium and in the parenchyma, and sup-
poses that they have a sexual significance as urticating organs, the
" Liebesfeile " of Schneider ; but their absence in this region in Phago-
cata precludes the assumption that they have in this species any such
function.

The rhabditi are all of one kind, but they vary in size. The varia-
tions are not local, different sizes occurring wherever rhabditi are found.
Some are as long as the hypodermis cells, while others are comparatively
short; they vary from 1.5 /a to 16 /a in length. There is an interesting
correlation between the thickness of the hypodermal layer and the size of
the largest rhabditi ; those of the thin hypodermis of the ventral surface
are invariably smaller than those of the dorsal side. Each is spindle-
shaped, and the outer end is slightly more pointed than the deep end.
They stain intensely in the carmine dyes, and then appear perfectly
homogeneous ; but when stained in Orth's picrocarminate of lithium
with an excess of the picric acid, they take on a bright yellow color,
and appear more or less swollen and distorted, according to the length
of time the dye is allowed to act. Often they have the appearance of
hollow capsules filled with granules, or containing a few irregular re-
fractive lumps (Fig. 9). It was probably the swollen and altered rhab-
diti that Moseley mistook for gland cells. The peripheral portion of the
substance of the rhabditi is not affected by the reagent as the contents
are. This outward unaltered portion presents the appearance of a cap-
sule, or thick membrane, with a double contour. Moseley says of his
glandcells, "The cell appears to have a double wall, for an irregular
crumpled membrane is seen often within it."

MUSEUM OF COMPARATIVE ZOOLOGY. 13

The rhabditi which lie between the hypodermal cells are not parallel,
but are somewhat inclined toward each other, the outer ends generally
converging about centres so as to form groups or packets. The small
ones lie out near the free surface of the hypodermis ; and the largest
may reach the basement membrane (Fig. 1). Usually the long axes of
the rhabditi are approximately perpendicular to the surface of the epi-
dermis, but they may assume almost any angle with each other ; small
rods are sometimes seen lying at right angles to neighboring ones.

It was first shown by Oscar Schmidt ('48, p. 6), in 1848, that in the
case of Rhabdocoeles the rhabditi are developed in subcutaneous flask-
shaped cells. Since that time similar conditions have been discovered
in all the Triclads. Up to the present time, the development of these
cells, "Stabchenbildungszellen," has not been traced. My studies seem
to throw some light on their genesis, and also to show how the rods
find their way out between the cells of the epidermis. I first rec-
ognized the parent cells in isolation preparations, and saw them in
sections only after depigmenting and staining the sections on the slide.
Later, I obtained a fresh supply of material, and was able to demonstrate
them in abundance, and in all stages of development. They are more
easily to be seen on the ventral side of the animal, where they are less
obscured by pigment. In their fully developed condition they lie in
the body parenchyma immediately beneath the longitudinal muscles.
On the ventral side, where the muscle layer is very thick, they may be
found in between the strands of the muscles as well as below them.
The parent cells have the form of flasks with greatly elongated narrow
necks tapering off into long tubular processes, which are traceable out-
ward through the muscles to the basement membrane, and, traversing
this, are seen to open out between the cells of the hypodermis. Thus
the deep-lying parent cells are in direct communication with the outer
world (Figs. 1, 6, and 10). It isby means of these tubular processes
that the rhabditi find their way to the exterior, and at length come to
occupy positions between the hypodermal cells. I have previously
pointed out that the rhabditi in the epidermis lie in groups or packets;
presumably each of these groups was at one time contained in a single
parent cell.

The connection of the parent cells with the epidermis is a primitive
one, for they are only modified cells of the hypodermis, which never
cease to retain their connection with that layer. In the earliest stages
of development that I have found, they appear like small sacs im-
bedded in the superficial portion of the longitudinal muscle band, close

14 BULLETIN OF THE

to the basement membrane, with which they are connected by short
necks or tubes (Fig. 4). The cell at this stage contains a single very
large nucleus, in which there is no nucleolus, since the chromatin exists,
as in the other cells of the hypodermis, in the form of fine particles
scattered uniformly through the nucleus. £ater, the cell begins to sink
deeper into the tissue below the hypodermis, and the tubular neck
increases correspondingly in length. The cell contents become finely
granular, and appear to grow at the expense of the nucleus, which no
longer fills so completely the sac, but becomes smaller and occupies
the bottom of the cell (Fig. 7). In the protoplasm surrounding the
nucleus, there appear small, round, highly refractive particles that stain
deeply. These increase in number and in size, and soon become elon-
gated, taking on the spindle shape so characteristic of the rhabditi (Fig. 6,
rhb.). During these stages of formation the cell conies to lie in the
body parenchyma below the muscle bands, but still retains a connection
with the hypodermis by means of its long tubular process. The cells
are at length filled with rods, and the nucleus is crowded to the bottom
of the cell (Figs. 1, 5, and 10).

The fully developed rods are guided to the exterior by means of the
tubular prolongations of the parent cell, and finally make their way
through the basement membrane and come to lie between the cells of
the hypodermis. The rhabditi, so long as they are contained in the
parent cell, are not hard and rigid, but possess a certain amount of
plasticity, as can be seen by the manner in which they are bent when
many are packed in one cell. This plastic condition of the rods facili-
tates their passage through the basement membrane. I have been able
to find a number of cases such as that represented in Figure 8, where I
have shown one of the rods in the act of passing through the membrane.
The rods possess this pliability until they leave the deeper tissues, and
they attain their definite shape only after they reach the hypodermis,
where they become hard and inflexible. After the discharge of the
rhabditi. the parent cells become absorbed and disappear.

Anton Schneider ('73, p. 87) says concerning the parent cells, "Sie
haben mehrere nach der Haut gehende Auslaufer, deren Epithelzellen
reichlich damit gefiillt sind." According to Moseley ('74, p. 119),
" The parent cells of the rod-like bodies are arranged beneath the exter-
nal longitudinal muscular layer at a tolerably even depth ; they are, in
spirit specimens, of an elongated oval form, with the upper extremity
drawn out in a point or long filament, which in some cases may be seen
to reach up to the basement membrane." In another place ('74, p. 120)

MUSEUM OF COMPARATIVE ZOOLOGY. 15

he says, " On treatment with potash, the cells of Bipalium swell up, are
seen to contain rod-like bodies, and the fine filament at the upper
extremity appears like a duct leading to the surface of the basement
membrane."

Hallez.and Iijima do not make mention of any processes of the sub-
hvpodermal parent cells, but believe that the cells are ruptured, and that
the rhabditi make their way to the epidermis through the tissues of the
body. Hallez ('79, p. 6) says : " Jai ete temoin une seule fois de la rup-
ture d'une cellule productrice chez Mesostomum tetragon u rn ; il m'a ete
impossible de retrouver dans cette cellule rompue la moindre trace du
noyau." Iijima ('84, p. 371) writes as follows: "Die Bildungszellen
sind rundlich und mit einem ausserordentlich feinkornigen Inhalt ver-
sehen." And again : " Haben die Rhabditen ihre definitive Grbsse er-
reicht, so durchbrechen sie die Zellenwand, w T elche schlieslich absorbirt
zu werden scheint und wandern durch den Bindgewebe und die Basal-
membran entweder einzeln oder in Gruppen nach aussen in die Epidermis-
zellen, in denen sie definitiv verbleiben."

Not all of the rhabditi that are developed in the parent cells of the
sub-hypodermal tissue find their way to the exterior. Many of the cells
apparently lose their connection with the hypodermis, and their rhab-
diti are discharged into the body parenchyma; only on this assumption
can one explain the presence of the numerous rhabditi that are found
scattered in the sub-hypodermal tissues. This condition is not the
normal, or at least not the original one. These often occur in large
numbers in the zone immediately inside of the longitudinal muscle
bands, which is occupied by the mother cells, where they lie in no
definite positions, and with their axes directed at all angles.

Rhabditi of all sizes may be developed in the same parent cell.
Those of different sizes are not confined to special cells, as found by
Schneider ('73, p. 83) and Graff ('74, p. 128) for Mesostomum. Be-
sides the fully developed rhobditi there are in the cells particles that
have no constant form, but have the same optical appearance and stain
the same as the rhabditi (Figs. 6, 10). These bodies may be either
residual matter, disintegrating rhabditi, or incipient rods. They never
occur in the epidermis, but are left behind after the discharge of the
rhabditi, and by the absorption of the wall of the parent cell they fine
their way into the body parenchyma, where, with the rods previously
referred to, they lie scattered about. Lang ('84, p. 52) found similar
bodies along with the rhabditi in Polyclads, and speaks of them a:
" junge kugelige Stabchen." I am inclined to regard them as residua
secretions.

16 BULLETIN OF THE

To my mind it is unquestionable that the parent cells of the rhabditi
are of ectodermic origin, as first suggested by Hallez ('79, p. 7). It is
only in Triclads and in Rhabdocoels that the mother cells lie in the
deeper tissues, and we know so little about the embryology of these
groups that we cannot tell just how the passage from the exterior takes
place. I have endeavored to show that the cells have a connection with
the hypodermis in the earliest stages of their development, long before
they show any traces of rhabditi, but whether the cells pass from the
hypodermis through the basement membrane, or are separated from the
hypodermis before the formation of such a structure, I cannot say. The
epidermis of embryos of Mesostomum was found by Graff ('82, p. 5G)
to be filled with rhabditi, while he could find no traces of the sub-hypo-
dermal parent cells so prominent and abundant in the adult. In Poly-
clads, the development of the rhabditi is in my opinion identical with
that in Triclads ; but in the former the parent cells lie permanently in
the hypodermis, whereas in the latter they sink down below that layer,
where greater opportunity for growth is afforded. The condition found
in Polyclads, therefore, I believe to be the primitive one.

Another mode of origin of the parent cells of the rhabditi has been
proposed by Loman ('87, p. G9), who considers them to be modified
connective-tissue cells that migrate from their original positions in the
mesenchyma and pass bodily through the basement membrane, and
come to lie eventually between the cells of the hypodermis ; or, in the
words of the author, "Nach meiner Meinung sind die Stabchenzellen
mesenchyinatose Gebilde, die eine factische Wanderung durch dass sie
umgebende Bindgewebe unternehmen, wahrend ihr Inhalt sich zu den
fadenfbrmigen Stabchen ausbildet. Endlich treten sie durch die Basal-
membran (wovon spater die Rede sein wird), drangen sich zwischen die
Zellen der Oberhaut," etc. Thus according to Loman the parent cells
form a part of the hypodermis, and only differ from the conditions
found in Polyclads in that their epidermal position is a secondary one.
Loman presents no evidence, and in the face of the facts here presented
his position is untenable.

Rhabditi are being constantly discharged from the epidermis during
the life of the individual, and provision must be made for their renewal.
Parent cells are therefore being continually produced to supply the
steady demand of the epidermis for rhabditi. The evidence of this lies
in the fact that in individuals of all ages these cells are found in all
stages of development. Iijima ('84, p. 373) says "es sicher scheint,
dass die Rhabditen nicht ausgestossen werden." If the rods are not dis-

MUSEUM OF COMPARATIVE ZOOLOGY. 17

charged from the hypoderruis, why are they being continually devel-
oped throughout the lifetime of the individual? Something must be-
come of them, or there would be an accumulation too great to find
room in the hypodermis.

Kennel ('88, p. 474) says, relative to this subject : " L'asst man sie
[planarians] aber in Uhrochalchen mit Wasser langere Zeit unbehelligt,
so dass sie sich festsetzen, und stort sie dann plotzlich, so ziehen sie sich
stark zusammen, machen heftige Bewegungen und suchen zu entfliehen.
An der betr. Stelle aber findet man bei schneller Unter?uchung Massen
von Rhabditen in alien Stadien der Auflosung, und wenn man das Wasser
schnell ausgiesst, findet man dort ein Kliimpchen zahen Schleim, — die
Stiibchen Ibsen sich in Schleim auf." I have often repeated the experi-
ment of Kennel, and have always found rhabditi in large numbers in
the slime secreted by the worm when placed on a glass plate.

We may now consider the question of the morphological and physiologi-
cal meaning of the rhabditi. Two interpretations of the morphological
vtilue of the dermal rods have been given by naturalists. The larger
number of observers consider them homologous with the nematocysts 1 of
Ccelenterat.es ; whereas the more recent investigators believe them to be
the morphological equivalents of gland secretions. I coincide with the
latter explanation, and offer the following arguments in its support. The
parent cells are unicellular glands, and the rhabditi, their secretions,
like the secretions of other dermal glands, are voided from the body of
the individual. The rods cannot function as organs of touch in lending
resistance to the epidermis, as suggested by Max Schultze and main-
tained by many others, for they do not lie in the epidermis cells, but
between them. The insensible gradations that exist between rhabditi
and the secretions of glands, as exemplified in the so called " Pseudo-
rhabditen," " Schleimstabchen," " Schleimblockchen," and " Kbrner-
driisen," have been to me one of the most striking evidences of the
glandular significance of the rhabditi. The dermal rods of Phagocata,
when acted on by reagents, present conditions resembling all the varie-
ties of dermal bodies figured by Lang, and. as I have said elsewhere, I
believe that the epidermal "gland-cells " of Moseley were only rhabditi
modified by acids. Sub-hypodermal glands and the mother cells of the
rods never occur together. Where rhabditi are absent, their place is
taken by glands, and vice versa. This is illustrated in the region of the

1 According to Camillo Schneider ('90, p 375) even the nematocysts are to be
considered only as highly specialized secretory cells derived from simple gland
cells.

VOL XXI. — NO 1. 2

18 BULLETIN OF THE

gonopore and at the edges of the body. Another proof consists iu
the fact that the reaction with stains is always the same for both
glands and rhabditi. With picrocarmine the effect is most striking.
All the tissues of the body take the carmine except the rhabditi and
the glands, both of which, owing to their yellow color, stand out in
contrast to the rest of the body.

Keferstein ('68, p. 15) was the first to speak of the rhabditi as gland-
ular secretions, and he called the parent cells " Stabchendrusen," and
the rods "geformte Schleimmassen." More recently this view has been
confirmed by Lang ('84, p. 52) and Kennel ('88, p. 474). The secre-
tions both of the slime glands and of the accessory sexual glands often
appear as rod-shaped bodies, and it was evidently this appearance of
the secretions occurring around the sexual organs that led Jensen ('78,
p. 11) to consider them rhabditi, and to speak of them as urticating
organs functional during copulation, — the theory first suggested by
Anton Schneider. Similar rod-shaped secretions are figured by Graff,
who calls them " Schleimpropfehen."

If we are to consider the parent cells as glands, what part do the
rhabditi play in the economy of the worm 1 I must agree with Kennel,
that the rhabditi are of use to the worm in securing food, and, I may
add, serve also for protection. Phagocata, like all planarians, is car-
nivorous, and observation of its feeding habits has shown me that rhab-
diti are cast out of the body in large numbers, and that fhis condensed
secretion helps to entangle and disable the prey. If one of the worms
be placed on a glass plate with a very little water, it soon becomes
hopelessly entangled in its own secretions, and when in this condition
placed in abundant water, some minutes elapse before it can free itself
and regain its activity. If some of the slime be examined with high
powers of the microscope, it will be seen to contain many rhabditi, in
all stages of dissolution. The rhabditi dissolve slowly in water, and it
is by reason of this slow disintegration that the slime retains a thickness
and tenacity that impedes the movements of an organism in contact with
it long enough for the worm to lay hold of it with its many pharynges.

The conditions found in parasitic Turbellarians may be mentioned as
evidence that this is the function of the rhabditi. Only four parasitic
species have been studied histologically, three of which belong to the
Ehabdocoeles and one to the Triclads. In all of these forms rhabditi
are absent, but in their stead are found sub-hypodermal glands which
resemble the parent cells of rhabditi, and like them open to the exte-
rior, — another illustration of the complementary occurrence of rhabditi
and glands.

MUSEUM OF COMPARATIVE ZOOLOGY. 19

Von Ihering ('80, p. 149) states that in the case of Graffilla murici-
cola, from the kidneys of Murex, concretions aud rhabditi are altogether
wanting in the epidermis. Their function, he says, is one of protection,
and hence they are not needed in a parasite. Lang ('80, p. 108) says of
Graffilla tethydicola, from the foot of Tethys, that there are no rhabditi,
but " Unmittelbar unter den Haut liegt eine grosse Anzahl eizelliger,
birnformiger, sich hauptsachlich mit Picrocarmine intensiv farbender
Driisen." Graff ('82, p. 375) says, concerning the same species, " Ueber-
dies finden sich hier unter der Haut zahlreiche einzellige birnfdrmige Drii-
sen." Anoplodium parasitica, a parasite in the body cavity of Holothuria
tubulosa, also possesses no rhabditi : " Ich habe weder an frischen noch an
conservirten Exemplaren von stabchen formigen Kbrpern oder von irgend
einem Pigmente etwas wahrnehmen kdnnen." (Graff, '82, p. 376.)

In Planaria limuli, a Triclad ectoparasitic on Limulus polyphemus, I
have been unable to find any trace of rhabditi, but have found in
abundance sub-hypodermal glands that resemble the parent cells of
rhabditi, and like them send long ducts to the epidermis. Graff ('79,
p. 203) states regarding this species that there are no true rhabditi ;
but he speaks of certain " Haftorgane," which he compares to rosettes
of rod-like bodies, and then adds: "Die dieselben zusammensetzenden
Stabchen (Haftstabchen) bilden sich im Innern des Kdrpers in beson-
deren Driisen und farben sich ausserst intensiv in Carmine uud Hema-
toxylin," — but I could not find these organs.

Thus we see that in parasitic Turbellarians there are no rhabditi,
their place being taken by many sub-hypodermal glands. Assuming
that the rhabditi are condensed secretions used in securing prey and
for protection, the conditions present in parasitic forms are in every
way consistent with our conclusions. The only other possible function
for the rhabditi is that assumed by Graff ('82, p. 58) and stated by him
as follows : " Die plausibelste Anschaunng ist auch heute noch die von
Schultze gegebene und von Stein auch fur die Stabchen der Infusorien
adoptirte, wonach die Stabchen indem sie dem aussern Drucke einen
Widerstand entgegensetzen, in ahnlicher Weise befordernd auf der
feinere Tastgeftihl der Haut einwirken, wie der Nagel auf Tastver-
mogen der Fingerspitze." I have shown that on account of their inter-
cellular position the rods probably cannot have such a function ; but
even if this evidence were considered insufficient to disprove their sup-
posed office, one would have to encounter the objection that so important
a function would not be likely to be entirely lost in parasites, particu-
larly in such active ectoparasites as P. limuli, where the parasitism is of

20 BULLETIN OF THE

such a nature that sensory organs would still be of great importance in
the animal's economy.

To summarize, then, the dermal rods are to be considered as condensed
secretions arising in sub-hypodermal unicellular glands of ectodermic origin.
All gradations exist between rhabditi and the secretions of normal glands.
Tlie rhabditi are being continually cast out of the body, and replaced by new
ones developed in new parent cells within the body parenchyma. The con-
nection of the parent cells with the epidermis is a primitive one, and the
rods pass to the exterior by means of the tubular ducts formed by the neck
of the elongated cells. The rods lie between the cells of the epidermis; they
are slotvly soluble in water, and are used by the animal in securing food
and for protection.

The basement membrane is a homogeneous layer immediately under the
hypodermis, the cells of which are directly connected with it. It varies
in thickness in different individuals and in different parts of the same
individual ; 1 p. and C.5 jx are the extremes that I have found. It
stains deeply in all of the carmine dyes, and always takes a darker color
than the underlying muscles. A granular condition, such as is men-
tioned by Iijima ('Si, p. 375), does not exist, nor is there any appear-
ance of the fibrous structure described by Lang ('84, p. G3) for Polyclads.
Minot ('77, p. 408) states that the basement membrane is composed of
circular fibres. The only appearance in Phagocata approaching that
described by Minot is seen in surface views of bits of the membrane
occurring in isolation preparations, where on one surface there appear
parallel markings ; but these are no doubt due to the intimate contact
of the membrane with the circular muscle fibres. The membrane is
closely applied to the muscle fibres, and in longitudinal sections, where
the circular muscles are cut across, the inner contour appears uneven,
owing to the projecting ridges which it sends, into the intermuscular
spaces (Figs. 4, 6, 7, and 10) ; stated in another way, the circular mus-
cles may be said to indent the basement membrane, leaving their
impression in the form of parallel grooves on its under surface. In
cross sections of the worm, the inner border of the membrane appears
perfectly smooth, and parallel to the circular muscles (Fig. 2). The
only departure from homogeneity is caused by the fine channels occupied
by the processes of the parent cells of the rhabditi (Figs. 1, 4, 6, 7, and
10), and these are only transitory, soon becoming obliterated. Occa-
sionally pigment granules find their way through these openings, and
may become caught in the basement membrane.

MUSEUM OF COMPARATIVE ZOOLOGY. 21

There can be little doubt that the basement membrane is a product
of the hypodermis. There is a direct relation between its thickness and
that of the latter ; hence it is thickest on the dorsal, and thinnest on
the ventral surface of the animal. It is true that the hypodermis is
easily separated from the membrane, but on the other hand the intimate
relation between the two structures is evident from the manner in which
the cells of the former remain attached to the latter after the rhabditi
have been removed by partial maceration (Fig. 3) ; and even when
the hypodermis has been entirely removed, the outer contour of the
membrane in regions where, in consequence of the presence of many
rhabditi, the hypodermal cells have become much compressed, appears
irregular, the uneven projections representing the points of attachment
of the hypodermal cells. In those regions where the rhabditi are few
or absent, the basement membrane presents a comparatively smooth
surface. There is no evidence in Phagocata that the membrane is an
independent cellular tissue, as in Polyclads, since no traces of structure
could be demonstrated, the membrane appearing homogeneous with all
of the stains that were employed. In my opinion, therefore, the base-
ment membrane is of hypodermal origin.

The pigmeyit in Phagocata occurs in the form of fine granules, of an
irregular outline and of a dirty greenish color. It lies principally in
the longitudinal bands of muscles between the fibres, so that, when a
worm is put under pressure and viewed with moderate powers, the
pigment appears as if arranged in parallel rows running lengthwise of
the animal. In the deeper tissues, below the muscle bands, the pigment
occurs in patches and streaks (Fig. 1). No pigment occurs normally
in the hypodermis. There are no special pigment cells ; the pigment
occurs in the form of distinct separate granules, which are intercellu-
lar in position, never intracellular. The origin of pigment as isolated
granules might be explained by some such theory as that of Eisig ('87,
p. 765), by which it is to be considered as a product of the excretory
system, — a kind of utilized excreta.

There are only three systems of muscles: the circular, the longitudinal,
and the sagittal or dorso-ventral. As compared with the complicated
musculature of other fresh-water planarians, that of Phagocata is much
simplified, and in this respect it agrees with Gunda sementata (Lang,
'81 a , p. 193) and Planaria abscissa (Iijima, '87, p. 344). The circular
muscles form a single layer immediately under the basement membrane,
to which, as we have seen, they are closely applied. The longitudinal
muscles form a thick band inside of the circular layer, and are much

22 BULLETIN OF THE

thicker on the ventral side (Fig. 10) than on the dorsal (Fig. 1). In
cross sections the longitudinal muscles appear separated into bundles,
between which the ends of the dorso-ventral fibres are seen passing to
- the basement membrane, into which they are inserted. I have not been
able to find ja nucleus in or on either the circular or longitudinal mus-
cle fibres. The nucleus of the dorso-ventral fibres is eccentric, as in the
muscles of Planarta torva, figured by Ratzel ('69, p. 275, Taf. XXIII.
Fig. 26). In cross sections both circular and longitudinal fibres have
an irregular outline and show a differentiation into an outer highly re-
fractive contractile portion and an inner feebly refractive axis (Plate I.).
Branching ends were observed oidy in the sagittal fibres.

A reticulate mesenchyma constitutes the greater portion of the sub-
stance of the body, occupying all the spaces between the organs. The
spaces left by the irregular network formed by the branching cells
(Plate II. Fig. 18) are connected with one another, and are to be con-
sidered as a kind of pseudocode ; they are filled with a granular peri-
visceral fluid. The sagittal muscle fibres in some places appear to be
directly continuous with branches of the mesenchyma cells (Plate II.
Fig. 18, mu. sag.), so that by contraction of the muscles the sizes of the
spaces would be altered, and thereby the perivisceral fluid would be set
in motion, thus establishing an irregular circulation in the pseudocode.
Lang ('84, p. 83) maintains that in the case of Polyclads these spaces
are intracellular in their origin, and that the so called perivisceral fluid
is the result of a liquefaction of the plasma of the connective-tissue cells,
which thus become vesicular, and finally, by the breaking through of
their thin walls, form a network. But if the pseudocoslar spaces were
intracellular in their origin, as claimed by Lang, it would be more diffi-
cult to understand the intimate relation between the muscles and the
processes of the reticulated parenchyma cells ; it would not, however, be
in any way an exceptional condition for muscle fibres to be attached to
the prolongations of stellate connective-tissue cells, more especially when
we consider that the muscle fibres and mesenchyma cells have a common
origin. As is well known, the Hertwigs have produced evidence to
show that the mesenchymatous muscles of the Pseudocoelous animals
are " besonders differenzirte Zellen der Bindesubstanz " ('81, p. 98).
Moreover, the mode of origin maintained by Lang is not founded, as far
as I understand it, on evideuces from embryonic conditions. Graff ('82,
p. 72) was unable, from the evidence found in Rhabdocceles, to estab-
lish " a distinction between muscle fibres and connective-tissue fibres " ;
and Hamann ('85, p. 96) has shown that in Echinoderras the connective-

MUSEUM OF COMPARATIVE ZOOLOGY 23

tissue cells are in direct continuation with the muscle fibres of mesen-
chymatous origin. From the study that I have made of the conditions in
Phagocata I am convinced that they are like those found in Rhabdocoeles.
Imbedded in the mesenchyma are the parent cells of the rhabditi and
also the glands that open at the surface in different regions. There are
two large accumulations of glands that open to the exterior, one around
the gonopore, the other on the ventral surface of the head region. A
smaller accumulation exists near the posterior end of the body. The
glands that occur in the head region afford important evidence of the
morphological equivalence of rhabditi-producing cells and ordinary
dermal glands. The deep ends of these glands are located behind the
brain, between it and the ovaries, and in passing over the brain they
run downward and forward till they open out on the ventral surface of
the head close to its anterior margin. They are numerous, and occur in
two bundles or groups, one on either side of the median plane of the
body. They appear as long sinuous tubes with enlargements or swellings
occurring at intervals (Plate II. Fig. 17), but without any evidence of
branching, and it has not been possible to distinguish between the gland
proper and its duct. Not being uniformly distributed, the finely gran-
ular contents of the tubes cause the irregular enlargements referred
to. Nuclei could not be detected in any portions of the ducts. The
two bundles of glands begin immediately in front of the ovaries, and as
they pass forward converge, so that when they pass over the commis-
sure of the brain they are in contact with each other ; but they soon
diverge again, and make their way to the surface as already described.
These two bundles of glands I believe to be the homologues of the
" Stabchenstrassen " found in Rhabdocoeles, and most prominently in
the Mesostomidse. Both the position and the course of the glands in
Phagocata are identical with those of the "Stabchenstrassen" in Rhabdo-
coeles, and the " wiederholte Anschwellungen" (A. Schneider, '73, p. 83)
in the latter correspond to the repeated enlargements in the former.
The glandular organs of Rhabdocoeles differ from them only in the
nature of their contents. Furthermore, the almost complete absence of
rhabditi in the head region of Phagocata strengthens this conclusion.
One has only to compare Leuckart's ('52, p. 23) description of Mesosto-
mum and the figures given in Graff's great monograph with the condi-
tions present in Phagocata, at once to recognize the probable equivalence
of these structures. A similar but smaller accumulation of glands is
found at the posterior extremity of the body in Phagocata, and it is
worthy of note that there is likewise in Rhabdocoeles an accumulation

24 BULLETIN OF THE

of rhabditi-secreting organs in the same region. The slime-secreting
glands at the extremities of the body are used in Phagocata as a means
of attachment, for it is principally by its extremities that the worm
fastens itself to objects, as can be seen when one attempts to remove it
from the side of the aquarium.

The other glands that are imbedded in the mesenchyma are those
which open around the genital orifice. Together with their ducts they
resemble in form the parent cells of the rhabditi ; they also react like
the glands of the head region with all stains. A portion of one of
these glands from an isolation preparation is represented in Plate IV.
Figure 41.

The digestive apparatus of Phagocata is like that of other Triclads,
except in regard to the number and arrangement of the pharynges,
which form such a striking feature of this species. The form, position,
relations, etc. of these pharynges have already (p. 4) been described,
and it has also been stated that at the junction of the three main tracts
of the intestine there is one pharynx which is larger and more promi-
nent than the rest (Plate II. Fig. 20, phy. m.), and that this is the homo-
logue of the single pharynx of other Triclads. There is no difference in
histological structure between this median pharynx and those which con-
nect with the lateral tracts. In a cross section of a pharynx (Plate II.
Fig. 12) the following layers can be distinguished, beginning from the
outside: (1) the fine cilia covering the external surface, (2) the
external epithelium, (3) a single layer of longitudinal muscle fibres,
(4) a single layer of circular muscles, (5) a wide zone occupied by con-
nective-tissue cells and salivary ducts and traversed by radial muscle
fibres, (6) a single layer of longitudinal muscle fibres, (7) a broad band
of circular muscle fibres, (8) the internal epithelium, and (9) the cilia
lining the lumen (compare also the longitudinal section shown in Fig. 1G).
The external covering of cilia disappears at a region about two thirds
of the distance from the free end of the pharynx toward its insertion
on the intestine, and the epithelium there loses its smooth appear-
ance, becoming wrinkled and creased. The cilia that line the lumen
of the pharynx are more restricted in their distribution, and are lost at
about one third of the way from the extremity, where the internal
epithelium also becomes longitudinally folded, many of the folds pro-
jecting far into the lumen of the pharynx (Plate II. Fig. 12, e'th. ?'.). In
this portion of the epithelium there are many nuclei, whereas in the cili-
ated region nuclei cannot be seen. Compare Figures 12 and 16, Figure 12
being a cross section which passes thi'ough the non-ciliated portion of

MUSEUM OF COMPARATIVE ZOOLOGY. 25

the internal epithelium. There are no nuclei anywhere in the external
epithelium of the fully developed pharynx, except near its proximal end.
By the use of silver nitrate, however, I have heen able to demonstrate
that the layer is a true epithelium. Isolated pharynges were killed
with hot 1 ct silver nitrate. By using the solution hot, the pharynges
were killed in an extended condition. A tangential section through
material treated in this way is represented on Plate IV. Fig. 47. I
have been unable by any method of staining to demonstrate the presence
of nuclei in these cells, the boundaries of which are so plainly brought
out by impregnation with silver.

In young pharynges (Plate II. Fig. 14), where the tissues are not
fully differentiated, nuclei are to be seen in both the external and inter-
nal epithelial coverings, although no trace of them can be found later
on. It is not difficult to find pharynges in different stages of develop-
ment, since the number increases with the age of the individual. The
young pharynx begins as a solid bud of tissue projecting into a cavity
hollowed out of the mesenchyma. The cavity is lined with a layer ot
flattened cells, which is continuous with the cell layer covering the young
pharynx (Plate II.. Fig. 11). The cavity is at first closed on all sides,
but eventually communicates with the common pharyngeal chamber.
The lumen of the pharynx is formed by an infolding of its free end,
which projects into the cavity. Although I have not been able to trace
directly all the steps in the invagination, I have seen specimens where
the lumen was lined throughout with an epithelium, and where there
was as yet no connection with the intestine. The epithelium lining the
lumen is continuous with that covering the outer surface of the young
pharynx, and hence with that lining the pharyngeal cavity, and it pre-
sents the same histological conditions as the latter (Plate II. Fig. 11).
Figure 14 represents a cross section of a young pharynx somewhat
advanced in development, where the cellular structure of both the inner
and outer epithelium is still evident ; there are as yet no cilia, and no
traces of the longitudinal muscles. I expect to describe in another
paper the changes by which the mass of indifferent cells composing the
young pharynx is converted into the ultimate histological structures of
the mature pharynx.

The outer layer of the pharynx has never been described as possessing
a distinctly cellular structure. Moseley ('74, p. 131), in speaking of
land planarians, describes "an epithelium in which no definite cell
structures could be observed ; but it appeared transparent, and marked
by vertical lines which might represent separation into cellular ele-

26 BULLETIN OF THE

merits." Iijima ('84, p. 389) also saw "eine senkrechte Streifung."
Lang ('81, p. 196, and '84, p. 109) speaks of it as a " euticulaanliches
Epithel " with flattened nuclei which it was difficult to see, and Minot
('77, p. 426) gives to it a well defined basement membrane. It is
obvious from the description that I have given of the young pharynx
that the outer layer, though ultimately much modified in appearance,
is nevertheless an epithelial layer.

I could not demonstrate the presence of a cuticula with pore canals
such as has been described by Iijima ('84, p. 390) ; neither could I dis-
cover anything answering to the nerve plexus described for other forms,
nor could I detect any nerve tissue. From the automatic movements
of the isolated pharynges, one would expect to find a complicated system
of nerves, and perhaps one or more ganglionic centres.

In the mature pharynges the radial muscle fibres run from tne outer to
the inner epithelium, to both of which they are attached by their finely
branched ends (Figs. 12 and 1G). These muscles no doubt act antago-
nistically to the broad band of circular muscles in dilating the lumen of
the pharynx, and by means of these two systems the peristaltic motions
displayed by the pharynges are accomplished. Between the radial fibres
there is a network of connective-tissue cells, and in the outer half of this
middle zone occur the salivary ducts (Figs. 12 and 1G, dt. sal.), which
run the whole length of the pharynx and open at the edge of its lip. In
the meshes of the connective-tissue network are seen fine granulations;
these spaces are undoubtedly in communication with the pseudocode
of the body mesenchyma, and it is to the coagulation of the perivisceral
fluid which has made its way out into the tissues of the pharynges that
is due the granular appearance seen.

I have little to add to what has been written concerning the histology
of the intestine, my observations agreeing in the main with those of
Iijima. The structure is the same in the principal tracts and in the
smaller branches ; there are no differentiated gland cells. During the
periods of most active digestion the intestinal cells are filled with highly
refractive oil-like globules, of different sizes (Plate IV. Fig. 43), — the
food matter absorbed by the cells. In this condition the cells are large,
and protrude into the lumen, so that in the smaller branches of the
intestine the latter has entirely disappeared. The contents of the cells
are eventually absorbed by the neighboring tissues, and the intestinal
cells themselves then appear vacuolated.

I have not been able to trace out the course of the excretory canals.
Although I have endeavored many times to study them, I have never

MUSEUM OF COMPARATIVE ZOOLOGY. 27

seen more than a few loops in the head region, and these were seen only
when the animal was put under great pressures-resulting in disintegra-
tion of the tissues.

The nervous system of Phagocata agrees in the main with the. descrip-
tions given by Lang ('81, p. 53) and Iijima ('87, p. 349) for other plana-
rians. The longitudinal nerve trunks unite near the anterior end of the
body in a well developed brain mass (Plate III. Figs. 25 and 33), and
posteriorly are connected with one another by fine commissures. Larger
commissures unite the trunks to one another throughout their whole
length, either running straight from trunk to trunk, or branching in
their passage (Plate IV. Fig. 38). The latter condition may be regarded
as closely related to one in which two commissures are united to each
other by means of a connective, a condition that often occurs. There
is no fixed relation between the number of transverse commissures and
the lateral diverticula of the intestine, but lateral nerves are usually
given off from the main stems at points opposite to the union of the
latter with transverse commissures (Plate IV. Fig. 38). The main nerve
trunks are prolonged anterior to the brain. They diminish rapidly in
size, and give off several lateral branches, which are directed obliquely
forwards and outwards (Plate III. Figs. 25 and 36), and they finally
break up into minute branches which form a network. The lateral
nerves from the main trunks run, sometimes with, sometimes without
branching, to the margins, where they unite with a second pair of finer
longitudinal nerves, — the marginal or peripheral nerves (Plate IV.
Fig. 37, n.pi'ph.). The marginal nerves form the lateral edges of a
great nervous network, which lies near the ventral surface just inside the
sheet of longitudinal muscles. Figures 37 and 38 represent portions of
two successive horizontal sections close to the ventral surface. The
sections are 30 p thick, and pass through the floor of the pharyngeal
chamber ; the light areas show where the knife has cut through the wall
into the pharyngeal cavity. The animal having been sectioned from
the ventral side, Figure 38 is the deeper (i. e. more dorsal) section.
The position of the oral opening (o) indicates that the portions of the
sections shown are from the same region of the body. In Figure 38 are
seen the main nerve trunks (n. Vp.) together with transverse commis-
sures {com. t.) and lateral nerves (n. I.). It may be seen from Figure 37
how the median branches from the peripheral nerves (n. pi'ph.) break up
into a network or plexus, which is distributed to the muscles (plx. mu.).
This network covers the whole of the ventral surface, and at the extreme
anterior end of the body is continuous with finer ramifications of the

28 BULLETIN OF THE

anterior longitudinal trunks. I could find no trace of a similar plexus
in connection with the less developed muscles of the dorsal side.

The nervous system of planarians may be readily understood, it
seems to me, if we regard it as composed of two more or less distinct
parts, — a deep-seated and a more superficial portion. The deep-seated
and more central part is present in all planarians hitherto investigated,
and consists of the brain, longitudinal nerve trunks, their commissures,
and the lateral nerves given off from them. The superficial portion
consists of a nerve plexus which lies just underneath the longitudinal
muscles, and may be confined to one or the other of the two surfaces, or
may be wholly wanting. A conspicuous part of this superficial system,
whenever it exists, is the marginal nerve. The connection between the
deep and superficial portions of the nervous system is effected by means
of vertical nerves running between the two, and, as I have found in
Phagocata, the marginal nerve also serves in an indirect way the same
purpose ; for on the one hand it is connected with the lateral nerves of
the central system, and on the other it forms the marginal terminus
of the superficial system.

Lang ('81, p. 72) has described in Gunda a marginal nerve directly
connected with the lateral nerves given off from the main trunks, but
has been unable to find any other evidence of a plexus. In Rhyncho-
desmus, according to the same author ('81, p. 62), there are both dorsal
and ventral plexuses, which are in contact with the deep surfaces of the
longitudinal muscles, and are connected with the central system by
vertical branches from the main trunks, from the lateral nerves, and
from the transverse commissures, but there is no peripheral nerve. Lang
('81, p. 57) also finds a plexus in connection with the deeper longitu-
dinal muscles in Planaria torva. Iijima ('84, p. 426) has likewise found a
dorsal plexus in a similar position in PI. polychroa and in D. lacteum, and
Loman ('87, p. 76) has found the same conditions in Eipalium suma-
trense and B. javanum. In Gunda ulvae and PI. abscissa there exists,
according to Iijima ('87, p. 349), a second, dorsal pair of longitudinal
stems, giving off" branches that break up into a plexus and unite with
the plexus from the lateral branches of the main trunks, the whole form-
ing a " Nervenschlauch." He says that, a "Randnerv" is present, but
he does not state what are its relations to the plexuses

From this brief survey it is obvious that Gunda represents one
extreme, and Rhynchodesmus the other; since in the former there are
no superficial plexuses, and in the latter there is a superficial plexus on
both dorsal and ventral surfaces in addition to the parts found in Gunda,

MUSEUM OF COMPARATIVE ZOOLOGY. 29

except that Rhynchodesmus has no marginal nerve. Both Phagocata
and Planaria abscissa are intermediate between these extremes, PI.
abscissa possessing only the dorsal portion of the superficial system (in
which a special dorsal longitudinal nerve has arisen), and Phagocata
having only the ventral portion of that system. Both possess, however,
the marginal nerve found in Gunda, and I believe that it probably sus-
tains in PI. abscissa the same relations to the deep portion of the ner-
vous system that I have found to exist in Phagocata.

It is evident, I think, from what I have shown in Phagocata, that the
marginal nerve is to be regarded as one of the means of communication
between the central and superficial parts of the nervous system ; or per-
haps rather as a differentiation of that portion of the superficial system
which is put in connection with the deep system by means of the lateral
branches from the main trunks.

It may perhaps be reasonable to suppose that the more concentrated
condition in Gunda has been brought about by a process of centraliza-
tion from the more diffuse and more primitive (1) condition shown in
Rhynchodesmus.

The brain is formed on the same plan as that of Gunda (Lang, '81,
p. 67 ; '81 a , p. 213). I find two commissures, a larger anterior commis-
sure which Lang calls in Gunda the sensory, and a smaller posterior one
which he calls motor (Plate III. Figs. 23, 33, and Plate IV. Figs. 39, 46).
The posterior commissure lies somewhat behind and below the anterior
one. It directly connects the longitudinal nerve trunks, since it lies in
the same ventral plane with them, while the anterior commissure, occu-
pying a higher plane, is only indirectly united to these ; viz. by means
of the lateral masses of the brain from which vertical commissural fibres
(the motor-sensory commissures of Lang) extend to the nerve trunks.
Lang describes four pairs of nerves as arising from the lateral sensory
masses of the brain. I cannot discover that there is any fixed number
in Phagocata. The only well defined one is the optic nerve (Plate IV.
Fig. 40, n. opt.). A great sheet of fine nerves is given off from the
lateral surface of the brain, and, spreading out fan-like, runs forward to
the anterior margin of the body (Plate III. Figs. 25 and 34, ».). It is
from these nerves that the " Tastorgane " of this highly sensitive portion
of the body receive their nerve supply.

A comparison of the figures will make clear the relation of the differ-
ent parts of the brain. Figures 26 to 31 are from cross sections through
the region of the brain taken at intervals of 60 /x. Figures 32 to 36 are
consecutive sections in the horizontal plane, Figure 32 being the most
dorsal of the series.

30 BULLETIN OF THE

Lang ('81, p. 79) speaks of a " Zellenbeleg von wirklichen Ganglien-
zellen " around the brain of Triclads. Iijima ('87, p. 353) describes these
cells as being unipolar with extremely delicate processes. I also find a
layer of closely packed cells with large nuclei around the brain, more
especially about the so called sensory portions (Plate III. Figs. 26-31,
Plate IV. Figs. 39 and 40), but I cannot saj* that these are ganglionic
cells. They resemble in every way connective-tissue cells ; they react
like them with stains, and are more prominent only on account of their
compact arrangement. The nuclei of the two large " Substanzinseln" in
the lateral masses of the brain (Plate III. Figs. 28, 33, Plate IV. Fig. 39,
cont. tis.) are both identical and continuous with the nuclei surrounding
the brain, and those found in the main nerve trunks cannot be distin-
guished from them. The ganglionic cells occurring in the nerve tissue
are not as large, nor do their nuclei stain as deeply, as those occurring
around the brain mass. I therefore believe that the latter belong to the
mesenchyma, and that the " Substanzinseln " are only intrusive con-
nective tissue. 1 Aside from this, I can add nothing to the observations
of Iijima on the finer structure of the nervous tissue. The longitudinal
nerve trunks in some places appear to be double for a considerable dis-
tance, being split, as it were, by the ingrowth of mesenchymatous tissue
(Plate III. Figs. 33 to 36, and Plate IV. Fig. 38). All sudi openings,
as pointed out by Lang ('81, p. 56), occur between the points of origin
of the lateral nerves.

The testes are numerous, and are found lying close together through-
out the whole length of the animal. Their development takes place
before that of the yolk glands. While the latter are still in an early
stage of development, spermatogenesis has been completed, the testes
have disappeared, and the spermatozoa are found filling the vasa def-
erentia. The testes first appear as spherical clusters of cells, which
by division increase in number and arrange themselves in the form
of hollow spheres. Some of the peripheral cells divide rapidly into
small spherical cells, that come to lie in the cavity of the testis. These
cells become elongated or pear-shaped, and are then differentiated into
two portions, a deeply stainable thickened end, and a tapering tail por-
tion (Plate II. Fig. 24). Further elongation takes place, until the form

1 Since I came to these conclusions in regard to the mesenchymatous character
of the so called " Substanzinseln," I have been gratified to learn that my conclusions
agree with those of Loman ('87, p. 77). In Bipalium, then, as well as in Phago-
cata, the " Substanzinseln " present in all particulars the same differences from
ganglionic cells.

MUSEUM OF COMPARATIVE ZOOLOGY. 31

of the adult spermatozoon is reached. Many stages of development can
be seen in the same testis. The different stages occur in distinct groups,
each group probably being the pruduct of one of the pareut cells. The
wall of the testis, when the spermatozoa first begin to develop, is com-
posed of many cells, most of which by division go to form spermatozoa ;
a few of the cells, however, are differentiated into flattened epithelium,
which constitutes the wall of the capsular testis (Plate II. Fig. 24, eth.).

I have not succeeded in ascertaining the exact manner in which the
spermatozoa find their way into the vasa defe'rentia, but Iijima's state-
ment ('84, p. 408) that they do not wander through the spaces of the
mesenchyma is certainly incorrect. The testes give rise to tubular
prolongations, but whether these are directly connected with the vas
deferens or first unite into one or more vasa efferentia, I cannot say.
The testicular canals appear to be direct outgrowths of the wall of
the testis. Their walls and those of the vasa deferentia have the same
simple structure (compare Plate II. Figs. 23 and 24), being composed
of a single layer of thin epithelium. The nuclei in the walls of the
tubes often occur in pairs, and thus suggest that the cells to which they
belong have recently undergone division (Fig. 24).

According to Moseley ('74, p. 139), the testes in land planarians open
directly into the vasa deferentia ; Minot ('77, p. 432), on the contrary,
speaks of fine testicular canals that unite to form larger tubes. Kennel
('79, p. 137) states that the testes, arranged in rows, fuse to form the
vasa deferentia.

The anterior ends of the vasa deferentia in Phagocata lie on either
side of the pharyngeal chamber in the region of the mouth opening.
They have the form of large elongated sacs (Plate IV. Fig. 42, x) which
open into comparatively narrow tubes (va. df.), which are of an even
calibre, and much convoluted and twisted. They run backward parallel
to each other until near the base of the penis ; they then turn at right
angles toward the middle plane, where they unite to form a single tube
which terminates at the apex of the penis. The spermatozoa when ripe
leave the testes by the testicular canals previously described, and pass
into the vasa deferentia, which become filled from their enlarged blind
ends up to a point beyond that where they unite to enter the penis.
Here the spermatozoa remain stored until arranged into spermatophores,
in which form they pass into the vagina of another individual. After
the spermatozoa have found their way to the vasa deferentia, all traces
of the testes disappear.

Physiologically considered, the vasa deferentia of Triclads are to be

32 BULLETIN OF THE

considered as vesiculse seminales. In Polyclads and in Rhabdocceles a
vesicula seminalis is present. This organ has been described for land
planarians by Moseley ('77, p. 278) and Loman ('87, p. 81), and Kennel
('88, p. 4G0) speaks of " mehrfach gewundenen Samenblasen " in Pla-
naria alpina. There can be no doubt that the terminal enlargements
found in Phagocata are a provision for the storage of a great number of
spermatozoa, as their size is found to vary in different individuals and
on different sides of the same individual according as the number of
spermatozoa is large or small.

The penis or intromittent organ is a highly muscular plug-like structure
(Plate IV. Fig. 42, pe.) that lies in the genital atrium or penis sheath.
It is covered with a flattened epithelium, under which there are alter-
nating layers of circular and longitudinal muscles, five of each, form-
ing a thick zone. Immediately outside the epithelial lining of the tube
there is a band of circular muscles, and between these and the outer
muscles there is a broad zone occupied by a mesh work of muscular fibres,
prominent among which are those having a radial direction. The lumen
of the. penis is not of an even calibre, but consists of a succession of
chambers, or dilatations, lined with a granular epithelium, which is
probably glandular. It is within the lumen of the penis, no doubt, that
the spermatophores are formed. The sheath of the penis is lined with
an epithelium of cylindrical cells, the nuclei of which lie close to the
bases of the cells, and are stained deeply, while the glandular cell
substance is stained only slightly. These cells also may be glandular,
but if so, I can find no explanation for their faint reaction with staining
reagents. In that respect they differ from all other glandular tissue.

The female sexual organs consist of a pair of ovaries with their
oviducts, the vitellarium or yolk gland, the uterus, and the vagina.
The single pair of ovaries is situated in the anterior part of the body
a little behind the brain mass. They are symmetrically placed on
the ventral side of the body just dorsad of the main nerve trunks,
one on either side. They appear as rounded sacs filled with ova
(Plate II. Fig. 21). The wall of the ovary is a delicate membrane,
in which I could detect no sign of cell structure, such as Moseley ('74,
p. 137) found in the ovary of land planarians. Scattered in between
the ova are the nuclei of a connective-tissue network that fills the spaces
between the ova (Plate II. Fig. 21, nl. can't, tis.). Iijima ('84, p. 412)
considers the branching cells between the ova as rudimentary egg cells,
at whose expense the ova develop. I have not yet seen different stages
in the development of the ova.

MUSEUM OF COMPARATIVE ZOOLOGY. 33

Intimately associated with the ovaries are two prominent compact
cell masses with deeply stained nuclei, which may provisionally be called
parovaria (Plate II. Fig. 21, vCm.). They are larger than the ovaries,
and envelop them above, in front, and on the outside ; that is to say,
the ovaries are surrounded on three sides, being partially imbedded, so
to speak, in these cell masses. The latter are present in every indi-
vidual, and their size relative to that of the ovaries varies only with the
condition of the sexual organs. They are smallest during the develop-
ment of the spermatozoa, and are most prominent at the time when the
yolk glands have reached their full development. For a long time these
cell masses puzzled me. I believed them to correspond to the second
pair of rudimentary ovaries described by Iijima ('84, p. 412) for Poly-
celis tenuis, and I at first accepted his interpretation of their significance;
but sections through additional material, where the female organs were
not so advanced, served to show their true meaning ; they are the' organs
tvhich give rise to the yolk glands. At an early stage in the development
of the testes no yolk glands are present, but they begin to appeal" at the
time when the spermatozoa are ripening.

The first traces of the yolk glands are seen in branching chains of cells,
which arise as outgrowths from the parovaria. Each cell has a large
nucleus that, is stained deeply in carmine. In these chains the cells lie
either in a single row, or it may be in several rows (Plate II. Figs. 19
and 19a). The nuclei are large and granular, and occupy the greater
part of the cell. It is to be inferred that the cells are dividing rapidly,
since nuclei are found in all stages of division, and two nuclei are
frequently seen in the same cell ; the division appears to be direct, or
amitotic (Plate II. Figs. 19 and 22). The rudimentary yolk glands oc-
cupy at first the ventral regions around the oviducts, but afterwards
they send branches from there dorsad, until there is formed a dendritic
system of rapidly dividing cells, which ramify through the tissues.
From each of the cell masses around the ovaries is derived one half
of the yolk system, that belonging to its own side of the body. The
cell chains of the young yolk glands are seen to be directly connected
with the parovarial cell masses, and histologically the structure of the
two is identical (compare Fig. 19 with Fig. 22, Plate II.). Furthermore,
at the time of development of the yolk glands there is a very active
division of the cells of the parovarial masses, a condition that does not
exist when the yolk glands have matured. A similarity in the condition
of the cells of the yolk glands and those of the parovarial masses is
evident at all stages of development. The young cells of the yolk

VOL XXI — NO. 1. 3

34 BULLETIN OF THE

glands increase in size, but do not grow as rapidly as the surrounding
protoplasm, and therefore the nucleus becomes smaller in proportion to
the size of the cells. Many highly refractive granules appear in the
protoplasm, and increase in number with the growth of the cells, till
eventually, when the cells have attained their full size, they form a
relatively large proportion of the cell mass (Plate IV. Fig. 45). Cor-
responding to the changes that take place in the yolk cells, there is a
slight increase in the size of the parovarial cells, in which there is also
an accumulation of highly refractive granules (Plate IV. Fig. 44), but
the nuclei retain more nearly their original proportions to the cells
than in the case of the yolk cells. In addition to the identity of
histological structure, a most striking evidence of the derivation of the
yolk glands from the parovarial cell masses is found in the reaction of
both kinds of cells with staining fluids, more especially with picrocar-
minate of lithium. Figures 44 and 45, Plate IV., represent respectively
sections through parovarial cells and mature yolk-gland cells of the same
individual. Figures 19 and 22, Plate II., are sections from another
individual ; Figure 22 is a section of young parovarial cells, and Fig-
ure 19 of incipient yolk -gland cells. Upon comparison of Figures 19
and 22 with Figures 45 and 44, it will be noticed that, in addition to
the appearance of the granules in the protoplasm of the older cells,
there has been an increase in the size both of the yolk cells and of the
cells of the parovarium. It is my belief, then, that the two large
dendritic yolk glands arise by cell proliferation from the parovarial
organs which exist in intimate relation with the ovaries.

lijima ('84, p. 412) describes a pair of structures lying in front of the
ovaries in Polycelis tenuis as being composed of a solid mass of cells,
and as i*esembling young ovaries, so that this species possesses, in his
opinion, two pairs of ovaries, one of which is rudimentary. - On account
of their terminal position, he considers these rudimentary structures,
although not the functional ovaries, as the homologues of the single
pair of ovaries present in other species. His account of the growth of
the yolk glands, as given at p. 417, coincides with my observations, but
concerning the source of the chains of young yolk cells he says (p. 455) :
" Wir diirfen sagen, dass die Dotterstriinge durch Vermehrung einzelner
Zellen, welche in dem Mesenchym sich befinden, ihren Ursprung nehmen."
But his evidence that the " Dotterstrange " arise in situ is not satisfac-
tory. It is to be regretted that he has not given a fuller account of the
so called rudimentary ovaries of Polycelis, which, I am almost certain,
are the equivalents of the parovarial cell masses of Phagocata.

MUSEUM OF COMPARATIVE ZOOLOGY. 35

The absence of yolk glands in Moseley's land planarians can be
accounted for by assuming that in his material they were not yet ripe,
as was probably the case. He states ('74, p. 137), however, that there
is occasionally present in Bipalium, "just externally to the lower ex-
tremities of the ovaries, a small mass of large nucleated cells connected
by a pedicle with the ovary itself." He considers that " it may repre-
sent a yolk-gland in a rudimentary condition." With this I fully agree,
and further believe that this rudimentary yolk gland is the homologue
of the structure which in Phagocata I have called parovarium.

The presence of a vitellogenous organ in Phagocata, together with
the condition found in Polycelis by Iijima and in Bipalium by Moseley,
suggests a discussion of the relations of the ovaries and vitellaria. Yolk
glands have long been considered as resulting from the differentiation of
the ovaries. Gegenbaur, as stated in his text-book ('70, p. 281), con-
siders the yolk glands to be " Theile eines ansehnliches Ovars." Hallez
('79, p. G3) maintains that " le vitellogene n'est autre chose qu'une partie
differenciee de l'ovaire," and according to Lang ('81 a , p. 228), " Die
Keimstocke und Dotterstocke der Tricladen sind einander gleichwerthig.
Sie entstehen aus Zellen, die anfangs nicht von einander unterscheiden
lassen." Among Rhabdocoeles all gradations are found, from an undif-
ferentiated "Keimdotterstock," where ova and yolk cells are developed in
different portions of the same organ, to conditions in which the ova and
yolk cells are produced in distinct and separate oi-gans. The yolk glands,
then, have arisen by a divison of labor from a simple germ gland, as has
already been formulated by Graff ('82, p. 130) in the following words :
" Die Keimdotterstiicke miissen wir uns aus Ovarien durch einfache
Arbeitstheilung hervorgegangen denken ; durch raumliche Trennung
der verschieden functionirenden Abschnitte des Keimdotterstockes ent-
standen schlieslich die Keim- und Dotterstocke." I consider the condi-
tion found in Phagocata to be less differentiated than that exhibited by
PI. tenuis (Iijima), inasmuch as the cells which form it still retain a moro
intimate relation to the true ovary than they do in the latter case.

The union of the yolk glands with the oviducts is a secondary one ; it
takes place at intervals throughout their length. I have not studied
this in detail, but, as far as- I have learned, the conditions agree with
the careful description given by Iijima ('84, p. 415). The oviducts
open into the vagina just above the point where it enters the genital
atrium (Plate IV. Fig. 42).

The uterus (Plate IV. Fig. 42, vt.) is a sac-like organ lying just
anterior to the penis, and has thick walls that are thrown into many

36 BULLETIN OF THE

folds. It is lined with an epithelium of elongated cylindrical or pyriform
cells of a glandular nature. The appearance of the cells varies with the
activity of their secretion ; the protoplasm may be either homogeneous,
or filled with oil-like globules, or it may be vacuolated. The cells rest
upon a fine basement membrane. There is no musculature, and there
are no cilia.

The mouth of the uterus is prolonged into a tube with thick muscular
walls, the vagina (Plate IV. Fig. 42, vag.), which runs backward, pass-
ing above and to the left of the penis and then dipping down toward the
ventral side of the body, where it opens into the genital atrium. Where
the vagina arises from the uterus it is lined with a ciliated epithelium
of low cubical cells, and possesses a musculature of circular and longitu-
dinal fibres. As it passes backward, the cells of the lining epithelium
become taller and cylindrical (Plate II. Fig. 15, eth.), and the nuclei are
elongated. The outer ends of the cells show distinct granulations, and
the contour of the lumen becomes uneven ; the glandular nature of the
cells now becomes apparent. Along with the change in the appearance
of the cells of the lining epithelium there is an increase in the thickness
of the musculature, which now consists of alternating layers of circular
and longitudinal fibres. The musculature of the vagina reaches its great-
est development at the point where it bends toward the' ventral side of
the body ; from this point onward the cells lose their glandular char-
acter, and the musculature diminishes in thickness, till, at the point
where the vagina receives the oviducts, it again consists of only a single
layer each of circular and longitudinal fibres. Moseley ('74, p. 141) and
Iijima ('84, p. 420) speak of radial fibres in the walls of the vagina; but
I could not find any.

The accessory female organs of Triclads have been the subject of
much discussion. There are no other structures about which so many
opinions at variance with each other have been advanced. The organ
which I have called the uterus is regarded by Iijima ('84, p. 419) as a
simple gland whose secretions go to form the cocoon. In his opinion, it
has no function in connection with the union of the sexual elements; he
considers it homologous with the shell gland of Cestodes and Trematodes.
According to Kennel ('88, p. 458), it is to be considered as a receptacu-
lum seminis, and its secretions serve to preserve the spermatozoa. Hallez
('87, p. 24) maintains that fecundation takes place in the uterus, and
that in it the yolk cells join the egg cells. According to Hallez, there is
a division of labor among the cells lining the uterus. The majority of
them secrete the substance of the cocoon, others secrete " uu liquide

MUSEUM OF COMPARATIVE ZOOLOGY. 37

special " to support the vitality of the male elements, and possihly to
aid in fecundation. He states that in PI. polychroa the cocoon is pro-
duced in the uterus, but as regards Dendrocoelum lacteutn he agrees
with Iijima in maintaining that the cocoon is formed in the genital
cloaca or atrium. In Phagocata I have found ova as well as spermatozoa
in the uterus, and believe that fecundation takes place there. The
spermatophores are deposited in the vagina and from there the sperma-
tozoa make their way into the uterus. I believe also that a portion of
the contents of the cocoon are secreted by the uterus, but that the
substance of its wall, the shell, is produced from the glandular lining of
the vagina, so that in Phagocata at least the " uterus " cannot be re-
garded as homologous with the shell gland of Cestodes. It is my pur-
pose to discuss at length these questions, together with that of the
formation of the spermatophore, in a subsequent paper on the embryology
of this species.

No organ comparable with the "muskulosen Drusenorgan " of Iijima
('81, p. 422), or the " vesicule (bourse) copulatrice" of Hallez (79, p. 57,
and '87, p. 20), is present.

Summary.

Phagocata differs from all known Triclads in possessing, besides a
pharnyx which opens into the intestine at the junction of its three
main trunks, many additional pharynges which are joined to the two
lateral trunks of the intestine. The lateral pharynges are histologically
identical with the median one ; they differ from the latter only in size ;
the moi'e remote they are from it, the smaller they are.

The rhabditi or dermal rods lie between the cells of the hypodermi:,
not in them. They are developed in cells lying in the sub-hypodermal
mesenchyma ; the cells are connected with the hypodermis by fine
tubular prolongations. The connection of the parent cells of the
rhabditi with the exterior is a primitive one, and the rods enter the
hypodermis ,by emergence along these prolongations. The rhabditi are
ultimately discharged from the hypodermis, and new ones are constantly
being developed in new parent cells. They are slowly soluble in water,
and are used for securing prey and for protection.

The parent cells of the rhabditi are unicellular glands, and the rods
are their condensed secretions.

The " Stabchenstrassen " of Rhabdocoeles are homologous with the
slime glands in Phagocata.

38 BULLETIN OF THE

The basement membrane is a product of the hypoderniis. It is struc-
tureless.

The pigment is intercellular, occurring in the form of scattered
granules.

The pseudocoelar spaces of the mesenchyma are intercellular in ori-
gin, and sagittal muscles are directly continuous with processes of the
mesenchyma cells.

The nervous system consists of a deeper and a superficial portion ; a
marginal nerve indirectly connects the two. The condition in Phagocata
may be intermediate between that of Gunda and lihynchodesmus.

The brain presents two commissures, an anterior and a posterior one,
uniting the longitudinal nerve trunks. The so called " Substanzinseln "
are intrusive connective tissue.

The testes give rise to tubular outgrowths, the vasa efferentia. The
vasa deferentia have terminal enlargements and function as vesioulse
seminales.

The yolk glands arise by cell proliferation from two cell masses, the
parovaria, which are in immediate contact with the ovaries. The intimate
connection of the parovaria with the ovaries indicates the differentiation
of the ovary and vitellarium from a common gland.

The so called uterus is not only a gland ; it is a place in which the sex-
ual elements are brought together, and where fertilization consequently
takes place.

Cambridge, August, 1890.

It was not until this paper had gone to press that I had access to the
recent work of Bohmig ('91) on Rhabdocoeles. It was then too late for
any detailed review. I am gratified to observe, however, that he has
arrived at conclusions from his studies of Rhabdocceles which agree in
many points with those which I have expressed in the foregoing paper,
especially in his statements as to the fate and significance of rhabditi.

MUSEUM OF COMPARATIVE ZOOLOGY. 39

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EXPLANATION OF FIGURES.

All the figures are from camera drawings of Phagocata gracilis, Leidy.

ABBREVIATIONS.

atr.

Genital atrium.

mu. r.

Radial muscles.

Cll.

Cilia.

mu. sag.

Sagittal 'muscles.

el. rhb.

Parent cells of the rhabditi.

n.

Sensory nerve.

cl. sp'z.

Parent cells of the sperma-

n.l.

Lateral nerve.

tozoa.

n. l.'a.

Anterior longitudinal nerve.

com. a.

Anterior commissure of the

nl. con't.

(1*8. Nucleus of connective tissue.

brain.

nl. e'th.

Nucleus of epithelium.

com. p.

Posterior commissure of the

n. I.' p.

Posterior longitudinal nerve.

brain.

nl. rhb.

Nucleus of parent cells of

com. t.

Transverse commissure.

the rhabditi.

con't. tis.

Connective tissue.

n. Of it.

Optic nerve.

dt. sal.

Salivary duct.

n. pi'ph.

Peripheral (marginal) nerve.

e'th.

Epithelium.

O.

Mouth opening.

e'th. ex.

External epithelium.

oc.

Eye.

e'th. i.

Internal epithelium.

ov'dt.

Oviduct.

e'th. phy.

Epithelium of pharynx.

pe.

Penis.

go'po.

Gonopore.

phy. m.

Median pharynx.

ft' drm.

Hypodermis.

phy. 1.

Lateral pharynx.

h'drm.'

Aborted cells of hypoder-

plx. mu.

Nerves to muscular plexus.

mis.

rhb.

Rhabditi.

in.

Intestine.

sec.

Secretions which do not

mb. ha.

Basement membrane.

form rhabditi.

ms'chy.

Mesenchyma.

sp'z.

Spermatozoa.

mu. crc.

Circular muscles.

trn. i. a.

Anterior trunk of intestine.

mu. crc. ex

. External circular muscles.

trn. i. 1.

Lateral trunk of intestine.

mu. crc. i.

Internal circular muscles.

ut.

Uterus.

mu. 1.

Longitudinal muscles.

va. df.

Vasa deferentia.

mu. 1. ex.

External longitudinal mus-

vag.

Vagina.

cles.

vt'm.

Parovarium (vitellarium).

mu. 1. i.

Internal longitudinal mus-

X.

Enlarged ends of vasa def-

cles.

erentia.

PLATE I.

Fig. 1. Portion of a longitudinal section of the dorsal wall of the body, showing
the parent cells of the rhabditi and the position of the rhaluliti in the
hypodermis. X 900.

" 2. Cross section near the lateral margin of the dorsal side, in a region
where there were no rhabditi, showing the hypodermis in its primitive
condition. X 000.
In Figures 1 and 2 the basement membrane did not take the stain.

" 3. Longitudinal section through a region where there were many rhabditi
which have been removed by partial maceration, showing the modi-
fied condition of the hypodermal cells due to the crowding of the
rhabditi. X 000.

" 4. Longitudinal section of ventral wall of body, showing a young parent cell
of the rhabditi, the nucleus almost filling the cell. The hypodermis
removed. X 900.

" 5. Two parent cells of the rhabditi, from macerated material. X 960.

" 6. Longitudinal section of ventral wall showing two stages in the develop-
ment of the parent cells of the rhabditi. Two small rhabditi have
already been secreted in the larger cell. The hypodermis removed.
X 900.

" 7. Stage in the development of the parent cells of the rhabditi next older
than that shown in Figure 4. The cell has sunk deeper into the
tissues, and the nucleus is smaller in relation to the size of the cell.
Ventral wall of body, the hypodermis being removed. X 900.

" 8. Longitudinal section of ventral wall showing one of the rhabditi in the
act of passing through the basement membrane. The hypodermis re-
moved. X 900.

" 9. Showing the appearance of the rhabditi after having been acted upon by
picric acid. X 900.

" 10. Longitudinal section of the ventral wall showing one of the parent cells
of the rhabditi filled with the rods. The remnants of another cell
represented by the nucleus and three rhabditi are seen close by The
hypodermis has been removed. X 900.
Owing to a mistake of the lithographer, the nuclei of the parent cell
(nl. rhb.) in Figure 10 are not represented as being granular, as they
should be.

•v

m

m

Woodworth — Phagocata.

PLATE II.

Fig. 11. Cross section of an individual in the region of a young budding pharynx.

Its connection with the intestine has not yet been established. X 300.
" 12. Portion of a cross section through one of the lateral pharynges. X 320.
" 13. A worm feeding on an Annelid ; five of the pharynges are visible. Killed

with hot corrosive sublimate while feeding. X 10.
" 14. Portion of a cross section through young pharynx, showing the nucleated

epithelia. The other tissues are not jet differentiated. X 300.
" 16. Portion of a cross section through the vagina in the region where the

musculature reaches its greatest development. X 120.
" 16. Longitudinal section of the wall of one of the smaller pharynges. X 500.
" 17. Portion of a longitudinal section through the slime glands in the head

region, where they pass over the brain. X 300.
" 18. Portion of a cross section of the body to show the reticulated mesenchyma

and its relation to sagittal muscles. X 500.
" 19 and 19a. Portions of the incipient yolk glands, in Figure 19 the nuclei are

seen in process of division. X 820.
" 20. A partial reconstruction of the whole worm showing the pharynges and

their relation to the intestinal tract. X about 20
" 20a. Outline to show the appearance of the living worm while in progres-
sion. X 9.
" 20b and 20c. Outlines showing forms assumed by the worm when at rest. X 6.
" 21. Longitudinal section through the ovary and parovarium showing their

relation to each other X 300.
" 22. Section through a parovarium at the time when the yolk glands are

beginning to develop. From the same individual as Figures 19

and 19a. X 820.
" 23. Cross section through the vas deferens. X 300
" 24. Portion of a section which passes through one of the testicular sacs,

showing its tubular outgrowth, — vas efferens X 3o0.

I I

-.• •->

* HI,

PLATE III.

Fig. 25. Horizontal section through the head region showing the brain and sensory
nerves, and the relation of the anterior longitudinal nerve to the mar-
ginal nerve (n. pi'[>h.). The right-hand side of the section is a little
more dorsal than the left. X 52.
" 26-31. From a series of cross sections through the brain region. The sections
are taken at intervals of 00^. Figure 26 is the most anterior. X 52.

• " 32-36. From a series of horizontal sections through the brain region, cut
from the dorsal side. The sections are consecutive, and 30 n in
thickness. X 52.

nfn'ph

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ftvnn
nl

nip
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h i

ret:

i>H.

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{w *$k?, ■■■:■ ..,.-■■ - %8& $ * ,y

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hunt.

29

30

31.

loin a tfi/it ii

'Hi' m,,,

fiii i it

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Woodworth. — Phagocata.

PLATE IV.

Figs. 37 and 38 Two consecutive horizontal sections (30 p. in thickness) from the
ventral side passing through the floor of the pharyngeal chamber.
Figure 37 is the more ventral, and shows the marginal nerve ; the re-
lation of the latter to the longitudinal trunks is evident upon comparing
Figures 37 and 38. X 27.

" 39 and 40. Two longitudinal sections, parallel with the sagittal plane, through
the brain region. X 52.

" 41. From an isolation preparation, showing one of the sub-hypodermal glands
from the region of the gonopore. X 700.

" 42. A view of the sexual organs showing their relations to one another. The
figure was accidentally inverted by the lithographer, thus bringing
the posterior end uppermost. Partially diagrammatic, X 35.

" 43. Portion of a cross section of one of the lateral branches of the intestine.
X450.

" 44. Portion of a section through a parovarium of an individual in which the
yolk glands were fully developed. X 680.

" 45. Section through a portion of a yolk gland from the same individual as
Fig. 44. X 2G0.

" 46. Sagittal section through the brain, showing the two commissures. X 60.

" 47. Portion of a tangential section of one of the pharynges, to show the cell
boundaries of the external epithelium. From an isolated pharynx
killed in hot silver nitrate. X 350.

■ rom t

or

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46.

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No. 2. — The Compound Eyes in Crustaceans. By G. H. Parker. 1

Table of Contents.

Page

I. Introduction . 45

II. The Retina ....... 47

III. Arrangement of the Ommatidia 60

IV. Structure of the Ommatidia . . 66

1. In Amphipoda 68

2. In Phyllopoda 73

3. In Copepoda 77

4. In Isopoda 84

Page

6. In Cumacea? 99

7. In Schizopoda 99

8. In Stomatopoda .... 104.

9. In Decapoda 108

V. Ommatidial Formulae . . .115

VI. Innervation of the Retina . .116

VII. Theoretic Conclusions . . .118

VIII. Bibliography 131

5. In Leptostraca 98 1 IX. Explanation of Figures . . .141

Introduction.

Some four years ago, at the suggestion of my instructor, Dr. E. L.
Mark, I began the investigation of the compound eyes in Crustaceans.
Iu order to familiarize myself with the subject, I determined to study
at first in detail the structure of th^ eyes in a single species, and for
this purpose I turned my attention to our common lobster, Homarus
americanus. My results were published in a paper entitled " The His-
tology and Development of the Eye in the Lobster." Since the publica-
tion of that paper, I have had the opportunity of examining the eyes in
a number of other Crustaceans, and my observations and conclusions
concerning these eyes are contained in the following pages.

The material which I have used in the present study was in part sup-
plied to me through the kindness of several friends, and in part collected
by myself. Of that which I obtained myself, some was gathered in
the immediate vicinity of Cambridge, but much of it came either from
Wood's Hull, Mass., or from Newport, R. I. The material which I
obtained at Newport was collected at the Newport Marine Laboratory
during the summer of 1890, and consisted of specimens of Idotea,
Evadne, and Pontella ; that which I got at "Wood's Holl was collected
at the United States Fish Commission Station during a brief period

1 Contributions from the Zoological Laboratory of the Museum of Comparative
Zoology, under the direction of E. L. Mark, No. XXV.
vol. xxi. — no. 2.

46 BULLETIN OF THE

which I spent there in the summer of 1889, and included much of
the material which I used in studying the eyes of Decapods. For the
opportunities of collecting, both at Newport and Wood's Holl, I am
indebted to Dr. Alexander Agassiz. I also desire to express my thanks
to Prof. M. McDonald, the United States Commissioner of Fish and
Fisheries, for many courtesies shown me while at the government
station at Wood's Holl.

Essentially the same methods as those which I used in investigating
the eyes in the lobster were employed in studying the eyes in other
Crustaceans. As these methods have been described at some length in
my paper on the lobster's eye (Parker, '90*, pp. 3, 4), further mention
of them in this connection is unnecessary.

Before proceeding to an account of the eyes in Crustaceans, a few
Statements should be made concerning the use of terms. In the fol-
lowing anatomical descriptions, I have very generally adhered to the
older and more established terms. It must be admitted that some of
these, on account of their derivation, are not entirely satisfactory, but
because of their general acceptance I have chosen to retain them rather
than to attempt to replace them by new ones.

The term retinula, the use of which varies with different writers, was
introduced by Grenacher ('77, p. 17), who employed it to designate the
rhabdome and the group of cells by which this structure is surrounded.
Subsequently, Patten ('86, p. 544) used the same term as a name for
a single cell of the group to which Grenacher gave the name retinula.
In my paper on the eyes of the lobster I followed Patten's usage, but
in the present paper I have decided to employ the term as originally
defined by Grenacher, and to designate the individual cells in the
retinula as retinular celh, — a translation of the term already used for
this purpose in many German publications.

The greater part of the present paper is taken up with descriptions
of the eyes in different Crustaceans. The amount of detail thus col-
lected is considerable, and might appear at first sight to include many
unimportant particulars ; but the number of observations recorded is
justifiable, I believe, on the ground that the majority of them bear
more or less directly upon the solution of the principal question dealt
with in the paper.

The following statements will make clear the character of this ques-
tion. It is now well recognized that the retina in compound eyes is
composed of a number of similar units or ommatidia, and that each
ommatidiuin consists of a cluster of cells regularly arranged around a

MUSEUM OF COMPARATIVE ZOOLOGY. 47

central axis. With very few exceptions, the different ommatidia in the
retina of any given Crustacean agree with one another in the numher
and arrangement of their cells ; in other words, in a given retina any
ommatidium is the structural duplicate of any other. This uniformity
suggests the idea of a structural type, and already a number of such
types have been described. Some of these find representatives appar-
ently only in the ommatidia of a single species, but more frequently- the
type characterizes a genus, family, or even a sub-order. Types differ
from one another, either in the number of their cells or in the arrange-
ment of these cells. Of these differences, the one which involves a
variation in the number of cells is the more fundamental. This dif-
ference, however, has probably arisen by the gradual modification of
an ancestral type, and, granting this, it follows that the ommatidia of
one type are genetically connected with those of other types. This
leads directly to the statement of the principal question, namely, What
are the means by which ommatidial types are modified, and what is the
significance of the changes through which these types pass ?

This question, although easily stated, is not so easily answered ; the
facts presented in the following pages cannot be said to settle it, and
yet they seem to me to increase materially the possibilities of its
solution.

A partial answer to at least the first portion of the question has al-
ready been suggested (Parker, '90 a , pp. 56-58) ; it can be briefly stated
as follows. There is reason for believing that those ommatidia which are
composed of a small number of cells more closely resemble the ancestral
type than those composed of many cells. Granting this statement, one
would naturally expect that the more complex ommatidia had been de-
rived from the simpler ones by an increase in the number of their ele-
ments. Perhaps the most natural method by which this increase could
be accomplished would be by the further division of the cells already
forming the ommatidium. Consequently, cell division in this sense
seemed to me to afford a sufficient means for the modification of om-
matidial types. In the present paper it is in part my purpose to show
precisely to what extent cell division can be said to have modified om-
matidia, and to determine whether any other factors have been involved
in this process.

The Retina.

The retina in those Crustaceans in which its development has been
studied originates as a thickening in the superficial ectoderm. At least

48 BULLETIN OF THE

three types of retinal structure can be distinguished, depending upon
the ultimate form which this thickening assumes.

The first type which will be described is in several particulars
the simplest, and probably represents a primitive form from which
the other two are derived. This type is characteristic of the eyes
in Decapods, Schizopods, Stomatopods, Isopods, the Nebalise, and the
Branchiopodidae, and is represented by a simple thickening in the super-
ficial ectoderm.

Branchiopodidae. — In the eye of adult specimens of Branchipus the
retina is a lenticular thickening occupying the inner concavity of the
distal end of the optic stalk. Near its edges the retina is directly con-
tinuous with the adjoining hypodermis. Its proximal face is bounded
by a basement membrane which is also continuous with the corre-
sponding membrane of the hypodermis, and its distal face is closely
applied to the inner surface of the superficial cuticula. Thus the retina
in the adult has in every respect the appearance of a simple thickening
in the hypodermis.

The way in which the retina originates in Branchipus confirms the
opinion that this organ has the simple structure suggested in the fore-
going paragraph. The development of the retina in this genus bus been
studied by Claus ('86, p. 309), whose account can be summarized as
follows. In that part of the head from which the optic stalks eventu-
ally arise, the ectoderm becomes considerably thickened; this thickening
is subsequently divided into a superficial and a deep portion ; the latter
sinks into the head and becomes a part of the central nervous system ;
the former retains its external position and is converted into the retina.
In Branchipus, therefore, the retina originates as a simple ectodermic
thickening which retains its superficial position throughout the life of
the individual. This method of origin, and the position permanently
retained by the retina, are the two principal characteristics of the first
retinal type.

Isopoda. — In adult specimens of Idotea irrorata, as sections perpen-
dicular to the external surface of the eye show (Plate V. Fig. 49), the
retina bears the same relation to the hypodermis as it does in Branchi-
pus. Similar structural relations occur also in the eyes of Idotea ro-
busta and of young specimens of Serolis Schythei.

The development of the retina in Isopods has been observed by Dohrn
and Bullar. As early as 1867, Dohrn ('67, p. 256) described the eye in
Asellus as originating in connection with a thickening in the lateral
wall of the head, presumably in the ectoderm of that region. The de-

MUSEUM OF COMPARATIVE ZOOLOGY. 49

tails of the development of this organ were not followed on account of
the continual increase of pigment. Bullar ('79, pp. 513, 514) in a paper
on parasitic Isopods described the development of the retina in Cymothoa.
His account is substantially as follows. In the course of the develop-
ment of the cerebral ganglion, wheu this structure is separated from the
superficial ectoderm, the latter remains on the exterior of the embryo as
a layer of considerable thickness. From this superficial layer is devel-
oped the retina, i. e. all parts of the eye which in the adult lie between
the basement membrane and the corneal cuticula.

I have studied a few stages in the development of the eyes in Idotea
robusta. The retina in this species originates as a simple thickening in
the superficial ectoderm, in essentially the same manner as Bullar has
observed in Cymothoa.

The retina in Isopods, both in respect to its method of development
and its general structure in the adult, is unquestionably a representative
of what I have called the first type of retinal structure.

Nebalice. — In Xebalia, as the figures given by Claus ('88, Taf. X.
Figs. 8 and 17) show, the retina and adjoining hypodermis are directly
continuous, and the former presents all the characteristics of a simple
thickening in the hypodermis.

Stomaiopoda. — In an adult specimen of Gonodactylus which I ex-
amined, the relation between retina and hypodermis was the same as in
Nebalia.

Nothing is known, I believe, of the development of the retina in either
the Xebalia? or the Stomatopods. The structure of the eyes in the adults
of both groups, however, shows very conclusively that their retinas belong
to the same structural type as those of Branchipus.

Schizopoda. — In describing the development of Mysis chamelio, Nus-
baum ('87, pp. 171-185) states that the retina arises from a thickening
in the superficial ectoderm, and adds that its formation, so far as his
observations extended, was not complicated by an involution.

In Mysis stenolepis, a Schizopod whose eyes I have studied, the
retina and hypodermis in the adult are directly continuous, as in Bran-
chipus. This relation is what would be expected from the method of
development described by Xusbaum.

Decapod'/. — Carriere ('85, p. 1G9). in his account of the eyes in Asta-
cus, showed very clearly that in the adult the retina and hypodermis
formed a continuous layer. This relation was subsequently observed by
me in Horaarus (Parker, '90 a , p. 5), and I have since seen the same con-
dition in Gelasimus, Cardisoma, Cancer, Hippa, Palinurus, Pagurus,
vol. xxi. — xo 2. 4

50 BULLETIN OF THE

Carabarus, Crangon, and Palsemouetes. There is, therefore, considera-
ble ground for the support of CarrieWs generalization, that the relation
of the retina to the hypodermis as shown in Astacus is characteristic
of all Decapods.

The development of the retina has been more fully studied in Deca-
pods, perhaps, than in any other group of Crustaceans. Nevertheless,
the accounts given by various writers are by no means in agreement, but
differ in several important particulars. In a former paper (Parker, '90%
pp. 31—43), I devoted considerable space to the discussion of these
accounts, and I shall therefore not reopen the subject here. Suffice
it to say, that since the publication of the paper referred to nothing has
transpired to alter my belief that the retina in Decapods originates as
a simple thickening in the superficial ectoderm.

In a recent preliminary communication by Lebedinski ('90) on the
development of a marine crab, Eriphya, a brief description of the origin
of the eye is given. This description, however, is so very much con-
densed that it is not easily understood, and since the author himself
confesses that, on account of the complexity of the subject, a descrip-
tion without figures must be almost unintelligible, it would be unwise
to hazard a presentation of his views. I shall therefore pass over this
paper without further comment.

The evidence advanced in the course of the preceding paragraphs
leaves no doubt in my mind that the retinas in the Branchipodida?, the
Nebalia?, the Isopods, Stomatopods, Schizopods, and Decapods, belong to
the same structural type, and that this type is represented by a thick-
ening in the external ectoderm (hypodermis), which retains permanently
its superficial position.

The second retinal tyi>e is more complicated than the first, and
differs from it in that the retina does not retain its position at the
surface of the body, but becomes buried beneath a fold of integument.
Our knowledge of this type is largely due to the researches of Grobben
('79). The type is represented in the eyes of the Apusidre, the Estheridae,
and the Cladocera.

Estheridce. — In adult specimens of Limnadia Agassizii the two lat-
eral eyes are rather closely approximated, and occupy a position in
the ventral anterior portion.of the animal's body (Plate IV. Fig. 33).
The relation of the eye to the surface of the body can be seen most
satisfactorily in sagittal sections. In such a section (Fig. 35) the eye
has the appearance of a stalked structure which projects anteriorly into
a cavity, the optic pocket (brs. oc.) ; this pocket communicates with the

MUSEUM OF COMPARATIVE ZOOLOGY. 51

exterior by means of a small opening (po. brs.), the optic pore. The
free surface of the stalked portion of the eye is covered with a delicate
cuticula, which, after being reflected from the base of the stalk over the
inner surface of the wall of the pocket, becomes continuous at the pore
of the pocket with the superficial cuticula. The retina (Fig. 35, r.)
occupies the greater portion of the optic stalk. Its distal face is bounded
by the delicate cuticula already mentioned, and its proximal face is lim-
ited by a basement membrane (mb. ba.). This membrane becomes indis-
tinct as the base of the stalk is approached, but the retina itself is
apparently continuous in this region with the layer of cells which rests
on the cuticular wall of the optic pocket, and which finally unites at the
pore of the pocket with the superficial hypodermis. Thus the retina
may be said to be continuous with the hypodermis.

The structure of the eyes in Limnadia Agassizii is such that they
can be described as stalked eyes which have been surrounded by a fold
of the integument, so as to become enclosed within a space, the optic
pocket, which communicates with the exterior only by means of the
optic pore.

An eye of essentially this structure has been described by Grobben
('79, p. 255) in Limnadia Hermanni, Limnetis brachyurus, and Estheria
ticinensis, and in the last genus enough of the development of the eye
was observed to indicate that the optic pocket was formed by the growth
of a fold of integument over the optic stalk.

Apusidce. — In Apus, according to Grobben ('79, p. 256), the plan of
the eye is essentially similar to that in the Estheridre. The eyes pro-
ject into an open pocket, the cavity of which permanently communi-
cates with the exterior. Judging from the figure given by Claus ('86,
Taf. VII. Fig. 11, compare p. 366), the right and left retinas in Apus
are not so close to one another as in the Estheridpe (compare Plate IV.
Fig. 34).

Cladocera. — The structure and development of the retina in the
Cladocera has been carefully studied by Grobben. My own observa-
tions on this group have been limited to a single genus, Evadne, and
as this genus is not very favorable for the determination of the general
relations of the retina I must rely almost entirely upon Grobben's
descriptions.

In the development of Moina, according to Grobben ('79, p. ,253), the
retinal thickening is covered by a fold of the integument in such a
manner that an open optic pocket is produced, as in Limnadia. By the
closure of what corresponds to the optic pore, this pocket eventually

52 BULLETIN OF THE

loses its connection with the exterior, and becomes reduced to a closed
sac on the distal face of the retina. With the closure of the sac, the
continuity of the retina with the superficial hypoderniis becomes in-
terrupted.

In other Cladocera, especially the genera Sida and Daphnia, Grobben
has found evidence to believe that the eyes are of essentially the same
structure as in Moina. In a majority of the Cladocera the two com-
pound eyes coalesce even more completely than in Limnadia.

In the development of Moina, as the preceding description indicates,
the eye passes through a phase which closely resembles the permanent
condition in Limnadia. The eye in the latter may therefore be inter-
preted as representing a stage in the phylogeny of the eye in Moina.

In accordance with the facts presented in the foregoing account, the
second retinal type can be described as one in which the retiua.does not
retain its primitive external position, but sinks below the surface of the
animal and becomes covered by a fold of the integument. The optic
pocket thus formed may remain permanently open, as in the Apusidse
and Estherida?, or may become closed and partially obliterated, as in
the Cladocera. The right and left retinas either remain separated, as in
the Apusidse, or become closely approximated, as in the Estheridse, or
fused, as in the Cladocera.

The minor modifications which this retinal type presents are not with-
out importance. Bearing in mind the general statement that the com-
pound eyes in Crustaceans are separate, paired, superficial structures, it
is evident that the eyes in the Apusidse, in which the retinas are sepa-
rate and the optic pocket permanently open, depart only slightly from
the primitive condition. In the Estheridse, in which the two retinas
are closely approximated, the eye is farther removed from the original
type ; but not so far as in the Cladocera, in which not only the two
retinas are fused, but the optic pocket is closed and partially obliterated,
thus entirely disconnecting the retina from the hypodermis. The three
groups — the Apusidse, the Estheridse, and the Cladocera — may con-
sequently be taken to represent a series in the differentiation of the
second retinal type. That this series is a natural one, and that it cul-
minates in the Cladocera, is shown from the fact that in the develop-
ment of Moina, and perhaps many other Cladocera, the eyes pass
through stages which reproduce the essential features of the perma-
nent condition in the Apusidse and Estheridse.

In the third retinal type, as in the more dif rentiated form of
the second, the retina is completely separated from the hypodermis.

MUSEUM OF COMPARATIVE ZOOLOGY. 53

The method by which the separation is here accomplished is not by
the closure of an involution, as in the second type, but by a process
the nature of which will be described in the following pages. The
third type is represented by the eyes in Amphipods, and possibly in
Copepods.

Amphipoda. — The peculiar relation which the retina bears to the
hypodermis in Amphipods can be easily seen in Gammarus. In this
genus, as Carriere ('85, pp. 156-160) has clearly demonstrated, the
retina lies immediately below the hypodermis, and is separated from the
latter by a well defined structure, the corneo-conal membrane (Fig. 1,
mb. crncon.). This membrane, although visible with perfect clearness,
is nevertheless extremely delicate, and has the appearance of a single
lamella. I believe, however, that its structure is more complex, and
that it is composed of two very intimately united membranes, one of
which is produced by the retina, the other by the corneal hypodermis.
This belief is based upon the fact that at the edge of the retina the
apparently single membrane separates into what may be considered its
two constituents. One of these becomes the basement membrane of
the general hypodermis, and the other, which I have called the cap-
sular membrane, passes over the edge and proximal face of the retina,
and is finally reflected over the optic nerve (Fig. 1, mb. n. opt.). In
addition to the capsular membrane, the eye in Gammarus possesses
still another membrane (Fig. 1, mb. ba.). This is a delicate lamella,
which is approximately parallel to the deep face of the eye at a level
between the rhabdomes and retinular nuclei (compare Fig. 2), and which
consequently divides the space within the capsular membrane into two
chambers, a larger distal and a smaller proximal one. At its periphery
this intercepting membrane unites with the capsular membrane.

The corneo-conal and capsular membranes in Gammarus show no evi-
dence of being perforated, but together constitute a closed capsule, which
separates the retina from all adjoining tissues except the optic nerve.
Both membranes are composed apparently of a homogeneous substance,
in which I have never been able to distinguish any trace of cells. It
is therefore probable that these membranes are cuticular.

The intercepting membrane, unlike either the capsular or the corneo-
conal membrane, is pierced by a great number of holes, through which
the pi-oximal ends of the retinular cells project. This membrane, there-
fore, has the form of a mesh work. According to Carriere ('So, p. 158)
it is composed of numerous connective-tissue cells, but this statement
is not confirmed by my own observations. In depigmented sections of

54 BULLETIN OF THE

the retina the intercepting membrane had the appearance of a delicate
lamella, in which I was unable to find any trace of cells. i\'ot un fre-
quently the nuclei of certain accessory pigment cells (Fig. 2, nl. tidrm.)
appear to touch the membrane, and even at times to lie with their long
axes parallel to it, but in no case could these nuclei be said to be in the
membrane. In sections of the retina from which the natural pigment
had not been removed, it was often difficult to decide whether a given
nucleus was in the membrane or only next to it. Possibly appearances
such as these have led Carriere to believe that the membrane was cel-
lular. My own opinion is, that the intercepting membrane, like the
other two membranes, is a cuticula, and does not contain cells.

From the foregoing account, it will be seen that in an adult Gammarus
the retina lies immediately under an undifferentiated corneal hypoder-
mis, and is enclosed, excepting where the optic nerve emerges from it,
by a non-perforated cuticular capsule. The space within this capsule
is divided by a perforated cuticular membrane into a large distal and a
small proximal chamber.

In Hyperia, judging from the figure given by Carriere ('85, p. 1G1
Fig. 123), the retina has essentially the same structure as in Gammarus.
The intercepting membrane is in a position proximal to the rhabdomes
and distal to the retinular nuclei. The layer of pigment cells, which
Carriere ('85, ]>. 101, Fig. 124) apparently considers the intercellular
membrane itself, in my opinion marks only approximately the position
of that membrane. Probably in Hyperia, as in Gammarus, these cells
rest on the distal face of the intercepting membrane.

In Phronima each side of the head is occupied by two eves, instead of
one, contrary to the condition in the more typical Anrphipods. Of the
two eyes, one is dorsal, the other lateral. This difference in position
affords a convenient means of distinguishing them. The lateral eye pre-
sents all the essential structural features of the single eye in Gammarus
(compare Carriere, '85, Figs. 125 and 121). The dorsal eye, although
differing considerably in shape from the lateral one, is nevertheless con-
structed upon the same morphological plan. Its most important pecu-
liarity is the shape of its intercepting membrane and the adjoining
structures. In the dorsal eye the intercepting membrane, instead of
lying in a plane nearly parallel with the external surface of the retina,
as in the lateral eye, is cone-shaped. The axis of this cone corresponds
to the axis of the eye : its apex is near the brain, and its base faces the
external surface of the eye (compare Clans, '79, Taf. III. Fig. 20, and
Taf. VII. Fig. 58). The ommatidia are arranged approximately parallel

MUSEUM OF COMPARATIVE ZOOLOGY. 55

to its principal axis ; distally, they terminate in the region of its base ;
proxhnally, they end either at its apex or on its lateral walls near the
apex. The rhabdomes lie within the cavity of the cone, i. e. they are
distal to the intercepting membrane, as in other Amphipods. The retin-
ular nuclei cover the apical portion of the external surface of the cone,
i. e. they are proximal to this membrane. These nuclei are covered ex-
ternally by a second cone-shaped membrane, which separates them from
the surrounding tissue. This membrane" occupies the position of the cap-
sular membrane of other Amphipods, and is unquestionably homologous
with it.

The fact that both the lateral and dorsal eyes in Phronima are con-
structed upon the same plan as the single eye in Garamarus, supports
the view that these two eyes have arisen by the division of a primitively
single retina into two parts, and the subsequent independent differentia-
tion of each part.

As the preceding account shows, in all Amphipods whose eyes have
been studied carefully, the retinas conform to one structural type well
exemplified by Garamarus. In this type the retina is characterized by
two peculiarities : first, it is not continuous with the hvpodermis, but
lies immediately below that layer ; and secondly, it possesses what appear
to be two basement membranes, the capsular and the intercepting mem-
branes. The significance of these peculiarities will be discussed in the
following paragraphs.

The separation of the retina from the hypodermis is characteristic of
only the more mature conditions of the eye in Amphipods ; for as Pereyas-
lawzewa ('88, p. 202) has shown in Gammarus, and Rossiiskaya ('89,
p. 577, and '90, p. 89) has demonstrated in Orchestia and Sunamphitoe,
the retina originates as a thickening in the superficial ectoderm, in the
same manner as in the majority of Crustaceans. So far as I am aware,
however, no one has observed the detachment of the retina from the
hypodermis, a process which must take place before the adult condition
is reached. In the figure of the developing eye in Gammarus given by
Pereyaslawzewa ('88, Plate VI. Fig. 120), the distal portion of the retinal
thickening contains almost nothing but developing cones. In sections of
my own from a corresponding region in a young specimen of Gammarus,
the distal portion of the retina contains not only developing cones, but
also isolated nuclei, which occasionally lie between the cones, but more
frequently occur in positions distal to them. These nuclei are as numer-
ous in the centre of the distal face of the retina as on its edges, and at
this stage can always be easily distinguished from the nuclei of the cone

56 BULLETIN OF THE

cells. I believe they represent the nuclei of the corneal hypodermis.
The retina proper is probably separated from this hypodermis by delami-
uation ; at least, the corneo-conal membrane is formed at a stage slightly
older than that last mentioned, and, judging from the appearances at
this stage, its formation is not accompanied by any folding of the hypo-
dermis or retina, but is the result of a differentiation in place. Unfor-
tunately, none of the specimens which I studied showed any steps in the
formation of the corneo-conal membrane, and I am therefore uncertain as
to the exact method of its growth.

Of the two membranes in the basal portion of the retina of Gammarus,
presumably only one corresponds to the basement membrane of other
Crustaceans. The position occupied by the two membranes, as well as
their structure, serves to indicate which is the true basement membrane.
At first sight one might suppose that the capsular membrane, at least
in its proximal portion, corresponds to the basement membrane, but this
interpretation is not probable, for the reason that the capsular mem-
brane is not pierced by the fibres of the optic nerve, a characteristic of
the true basement membrane of the eye. I therefore believe that the
intercepting membrane, since it is perforated by these fibres, is the homo-
logue of the basement membrane, and that that portion of the capsular
membrane which might be regarded as a basement membrane is in
reality merely the cuticular sheath of the optic nerve.

So far as I can foresee, the only objection to be urged against this
interpretation of the intercepting membrane is found in its relation to
the retinular nuclei. These nuclei in the eyes of almost all other Crusta-
ceans lie on the distal side of the basement membrane. Granting that
the intercepting membrane is the basement membrane, one must admit
that in Amphipods they lie on the proximal side of this membrane.
This admission might at first sight appear to offer an obstacle to the
homology which I have suggested ; but it can be made with consistency,
I believe, provided one can show that the position of the retinular nuclei
is not necessarily fixed. That such is the case is evident from the fol-
lowing facts. In Decapods the retinular nuclei usually occupy a position
in their cells distal to the rhabdome. In Porcellio, as Grenadier ('79,
Taf. IX. Fig. 96) has shown, they have a more proximal position, lying
in the same transverse plane as the rhabdome itself. In Serolis they
are midway between the rhabdome and the basement membrane. These
conditions show, I believe, that the retinular nuclei ma}^ occupy very
different positions in their cells, and that the step from the condition
shown in Decapods to that shown in Serolis is not greater than that

MUSEUM OF COMPARATIVE ZOOLOGY. 57

from Serolis to the Amphipods. It seems to me, therefore, that the
objection suggested at the beginning of this paragraph is almost without
weight. This conclusion, moreover, is supported by the fact that in
Idotea (Plate V. Fig. 49) the retiuular nuclei lie proximal to the base-
ment membrane, whereas in the majority of other Isopods they are
distal to that membrane.

From the preceding discussion, I conclude that the retina in Amphi-
pods originates as a simple thickening in the superficial ectoderm, and
that this thickening subsequently becomes separated, probably by a pro-
cess of delamination, into a deeper portion, the retina proper, and a
more superficial portion, the corneal hypodermis. The latter alone re-
tains its original connection with the adjacent hypodermis. Of the two
membranes present in the basal portion of the eye in Amphipods, that
which I have called the intercepting membrane is homologous with the
basement membrane of the retina in other Crustaceans, and that which
has been designated as the capsular membrane is in large part the
cuticular sheath of the optic nerve.

Copepoda. — The retinas in the Branchiura and Eucopepoda, the two
divisions of the Copepods, present such different structural conditions
that for purposes of description it is better to consider them separately.

Branchiura. — In adult specimens of Argulus, the retina is completely
separated from all surrounding tissue, excepting the optic nerve, by an
intervening blood space (Plate II. Fig. 11, coel.). This peculiar condi-
tion was first clearly described by Leydig ('50, p. 331), although as early
as 1806 Jurine ('06, p. 447) l'emarked that the eye in this genus was
contained in a transparent membranous sac, which apparently contained
a fluid, and Midler ('31, p. 97) some twenty-five years later described the
retina as separated from the "cornea" by an intervening space filled
with fluid. It remained, however, for Leydig to determine the extent
of this space, and to demonstrate that the* fluid which it contained was
blood. The more essential features of Leydig's description have since
been confirmed by Clans ('75, pp. 254-256).

The development of the eye in Argulus- has not been studied with
sufficient fulness to allow one to determine the relation of its retina to
the hypodermis. But from the strong resemblance which the eye in the
adult bears to that in Amphipods, it is probable that the course of
development in the two cases is not. unlike. Probably the retina in
Argulus originates as a thickening in the superficial ectoderm, and subse-
quently not only suffers delamination, as in the Amphipods, but becomes
actually withdrawn from the superficial layer (corneal hypodermis).

58 BULLETIN OF THE

If this course of development really takes place, the various structures
in the eye of an adult Argulus can be easily homologized with those
in Amphipods. Thus the corneal hypodermis and corneal cuticula of
Ainphipods would probably be represented by the hypodermis and cu-
ticula dorsal to the eye in Argulus (Fig. 11). The basement mem-
brane of this hypodermis would correspond to the corneal component
of the corneo-conal membrane of Amphipods, and the conal constituent
would be represented by what is called the preconal membrane in Argu-
lus (Fig. 11, mb. pr'con.). Proximally, the preconal membrane becomes
continuous with the sheath of the optic nerve (Fig. 11, mb. n. opt.),
the equivalent of the capsular membrane of Amphipods. The basement
membrane of the retina in Argulus, as in Amphipods, is the membrane
pierced by the fibres of the optic nerve (Fig. 11, mb. ha.).

Grobben ('79, p. 2.">t>) has suggested that possibly the eve in Argulus
is of the same type of structure as in Phyllopods, but I do not share
in this opinion for the following reasons. In Estheria, the delicate
cuticula which covers the optic stalk is morphologically a portion of
the outer surface of the body, and, as I hope to show subsequently, is
subtended by a true corneal hypodermis. There is no corneal hypo-
dermis beneath the preconal membrane of Argulus. Moreover, there
is nothing in the eye of Argulus to correspond to the optic pocket of
the Estherida?, or to the optic sac of the Cladocera, except the circum-
retinal blood space, and it seems to me very improbable that this space
was once a cavity in communication with the exterior, and afterwards
became converted into a blood space. I therefore believe that the
plan of the eye in Argulus is not similar to that in the Phyllopods,
but rather that it represents a modification of the type presented by
the Amphipods. The satisfactory determination of this question can
be settled, however, only by embryological evidence.

Excopepoda. — In adult specimens of those true Copepods which
possess rudiments of the lateral eyes, — the Pontellida? and Corycaeidse,
— the retina is apparently separated from the hypodermis. In the
Corycseidce it usually lies at- some considerable distance from the hypo-
dermis, and in Pontella the two structures, although near one another,
are nevertheless not continuous.

The development of the lateral eyes in the Corycaiidte and Pontel-
lidae has not been studied, and consequently it cannot be stated with
certainty whether the retinas in these Crustaceans originate from the
hypodermis or not. In the metanauplius larva of Cetochilus, a Copepod
which as an adult has no lateral eyes, Grobben ('SO, p. 262) has de-

MUSEUM OF COMPARATIVE ZOOLOGY. 59

scribed a pair of thickenings, which extend from the superficial ectoderm
of the antero-lateral part of the head to the brain. These thickenings
are present only in the early stages of development, and represent the
unsevered connection between t he brain and the superficial ectoderm.
They closely resemble the developing lateral eyes of Branchipus, and
Grobben has therefore very justly considered them rudiments of the
lateral eyes. If the rudiments of the lateral eyes in Cetochilus de-
velop from the superficial ectoderm, it is probable that the lateral eyes
in other Copepods have a similar origin.

To which of the three retinal types already described the eyes in
Copepods belong is not easily decided. The absence of any indication
of an optic pocket, either in the development of what Grobben con-
siders the rudiments of the lateral eyes in Cetochilus, or in the fully
formed eyes in other genera, seems to me to preclude the possibility of
these eyes belonging to what I have described as the second type.
The separation of the retina from the hypodermis prevents them from
being classed with the first type, and, especially in the case of the
Branchiura, brings them into close relation with the third type. It is
my opinion, that, if the lateral eyes in Copepods are not representatives
of a fourth type, essentially different from the three already described,
they must be considered members of the third retinal type.

Certain species of Cumacese, Ostracods, and Cirripeds possess optic
organs which probably represent the compound eyes of other Crusta-
ceans ; but so far as I am aware, the relation of these structures to the
hypodermis is unknown. It is therefore impossible to state whether
those eyes represent other retinal types, or belong to one of the three
already described.

According to the preceding account, three retinal types can be dis-
tinguished in the compound eyes of Crustaceans. In the first of these
the retina is a simple thickening in the superficial ectoderm (hypo-
dermis). This type is characteristic of the eyes in Isopods, the Bran-
chiopodidse, the Nebalise, "Stomatopods, Schizopods, ami Decapods. In
the Isopods, the eyes are sessile ; in the other groups of the first type,
they are borne on the distal ends of movable optic stalks.

In the second type, although the retina, as in the first type, originates
as a thickening in the superficial ectoderm, it ultimately becomes en-
closed within an optic pocket. This may remain permanently open, as
in the Apusidse and Estheridse, or it may become closed, "as in the
Cladocera. In the Apusidaj, so far as I am aware, the eyes are not

60 BULLETIN OF THE

capable of motion, and in the Estherida? they are, if at all, only slightly
movable. In the Cladocera, where the second type probably reaches its
greatest differentiation, the retina is remarkable for the freedom of its
motion.

In the third type the retina originates from thickened hypodermis,
which subsequently separates into two layers, the corneal hypodermis
and the retina proper (a layer of cones and retinulae). This separation
is accomplished either by the formation of a corneo-conal membrane, as
in Ampliipods, or by what I believe to be an actual withdrawal of the
retina proper from contact with the hypodermis, as in Copepods. Only
in the representatives of the extreme modification of this type, the Cope-
pods, are the eyes movable.

The course of development taken by each of the three types very
clearly indicates their mutual relations. Evidently the first type is a
primitive one, and since the first steps in the development of the second
and third reproduce the permanent condition of the first, these two may
therefore be considered derivatives from the first. It is interesting to
observe that in the simpler condition of each type the retina is fixed,
whereas in the more differentiated form it has become movable. The
sinking of the retina into the deeper parts of the body, as represented in
the second and third types, may have been induced by the protection
thus obtained for the eye. After the three types were differentiated,
each one seems to have been modified in a special way to give rise to a
movable retina.

Arrangement of the Ommatidia.

The ommatidia in the retinas of some Crustaceans are so few in num-
ber that they can scarcely be said to be grouped according to any system.
"Where they are numerous, however, they are arranged upon one or the
other of two plans. These may be designated the hexagonal and tetrago-
nal plans of arrangement. In the hexagonal plan the imaginary outline
of the transverse section of an ommatidium is a hexagon, and each
ommatidium, excepting those on the edge of the retina, is surrounded by
six others. In the tetragonal arrangement the ideal transverse section
of an ommatidium is a square. Each of the four sides of this square
is occupied by one of the four faces of an adjoining ommatidium.

The arrangement of the ommatidia can usually be determined by
a careful inspection of the external surface of the eye ; this determina-
tion is considerably facilitated by the presence of a facetted cuticula.
Sometimes the form of a single facet is sufficient to indicate the plan of

MUSEUM OF COMPARATIVE ZOOLOGY. 61

arrangement. Thus, hexagonal facets have never been observed except
in connection with the hexagonal plan of arrangement. Circular facets
are likewise known to occur only with this method of grouping. Square
facets, on the other hand, may accompany either the hexagonal or te-
tragonal arrangement of deeper parts.

The hexagonal arrangement is apparently characteristic of the om-
matidia in all Crustaceans, 1 except the Decapods. In the Decapods, as
will be shown presently, the ommatidia are arranged either upon the hex-
agonal or the tetragonal plan. Before proceeding, however, to a descrip-
tion of the arrangement of the ommatidia in Decapods, it would be well
perhaps to call attention to the rather peculiar grouping of these struc-
tures in Gonodactylus, a Stomatopod.

For a clear understanding of the arrangement of the ommatidia in
this Crustacean, it is necessary to have some previous knowledge of the
shape of its optic stalk. In Gonodactylus the stalks are elongated cyl-
inders, the distal ends of which are rounded. In alcoholic specimens
the stalks in an undisturbed position rest with their longitudinal axes
approximately parallel with the chief axis of the animal, and with their
distal ends directed forward. The retina occupies the free end of the
stalk. Dorsally it extends over the distal half, ventrally over only the
distal third of the stalk.

The ommatidia in Gonodactylus are of two kinds, large and small,
which are always easily distinguishable from each other, although they
differ in no essential respect except size. The large ommatidia are defi-
nitely arranged in six rows, which extend as well defined bands from
the dorsal posterior edge of the retina anteriorly over its rounded distal
end, and posteriorly over its ventral surface to its ventral posterior edge.
This band thus occupies both dorsally and ventrally the median portion

1 Judging from the figures as well as the statements marie by the authors
quoted, the hexagonal arrangement is characteristic of the ommatidia in the fol-
lowing Crustaceans (exclusive of the Decapods) : Branchipus (Burmeister, '35,
p. 531, Spangenberg, '75, p. 30), Neballa (Claus. '89, Taf. X. Fig. 10). Gammartis
(Sars, '67, p. 62), Orchestia (Frey und Leuckart, '47, p. 204), Phronima (Claus, '79,
Taf. VI. Fig. 48), Cymothoa (Miiller, '29, Tab III. Figs. 5, 6, Bullar, '79, p 514),
Lygidium (Lereboullet, '43, p. 107, Planche 4, Fig. 2 b ), Serdis (Owen, '43, p. 174),
Arcturus (Beddard, '90, Plate XXXI. Fig. 4), Aneetis (Hesse, '58, pp. 100 and 103,
Dohrn, '70, Taf. VIII. Figs. 33, 34), Stpnlla (Milne-Edwards, '34, p. 117, Will, '40,
p. 7, Frey und Leuckart, '47, p. 204, Leydig, '55, p. 411), and Mysis (Sars, '07,
Planche III. Fig. 7, Grenadier, '79, Taf. X. Fig. 112). I have observed the hexag-
onal arrangement in the following genera: Aims. Branchipus, Estheria, Evadne,
Argulus, Gammarus, Caprella, Talorchestia, Idotea, Serolis, Porcellio, Sphceroma,
Mysis, and Gonodactylus.

62 BULLETIN OF THE

of the retina, and separates the remaining retinal surface into two parts,
one on either side of the stalk. In alcoholic specimens this median band
is readily visible with the aid of a hand lens, and a little closer scrutiny
shows that it is composed of six lines. These lines, of course, correspond
to the six rows of ommatidia previously mentioned. The smaller om-
matidia, on either side of the median band, are also arranged in lines
parallel to those in the band ; but, on account of their smaller size, the
lines formed by them are not visible with an ordinary lens.

The smaller ommatidia in Goniodactylus are arranged upon the typi-
cal hexagonal plan (see the left half of Fig. 93, Plate VI II.). The
larger ones have a somewhat similar grouping, although the fact that
they are in six longitudinal rows rather obscures their hexagonal ar-
rangement. (See the right half of Figure 93, in which three rows, and
a part of a fourth, of large ommatidia are shown.) The hexagonal
arrangement is not disturbed, as might be expected, on the line which
separates the larger from the smaller ommatidia, but both kinds form
parts in a common system. That this is true can be seen from Figure
93, where it will be observed that the centres of any two small ommatidia
lying in the same vertical line are as far apart as the centres of the cor-
responding larger ommatidia. Moreover, as I have demonstrated by
actually counting the ommatidia of long parallel series, a vertical band
which contains twenty-five large ommatidia has the same length as one
composed of a corresponding number of small ones. The apparent differ-
ence in numbers at first sight presented by lines of the two kinds of
ommatidia is principally due to the fact that the larger ommatidia are
arranged in distinct rows, whereas the smaller ommatidia are so grouped
that the individuals in one row are slightly interpolated between those
of the two adjoining rows (compare Fig. 93).

In Decapods the ommatidia are arranged either upon the hexagonal
or tetragonal plan. In the Brachyura, 1 as well as in three families of the
Macrura, the Hippidre, Paguridie, and Thalassinida:, 2 the arrangement

1 The presence of hexagonal facets has been recorded in the following genera
of Brachyura: Port,n»is (Will, '40, p. 7) ; Ilia (Will, '40, p 7, Leydig, '55, p. 411) ;
Cancer; Maja; Carpllius (Frey una" Leuckart, '47, p. 204) : Herbslia, Dorippe, and
Lambrus (Leydig, '55, pp. 407, 410, and 411, respectively). This form of facet is
present only when the ommatidia are hexagonally arranged. Leydig ('55, p. 411)
states that the outline of each facet in Dromin Rumphii is square, but, as his
description clearly indicates, the facets are arranged upon the hexagonal plan.
As my own observations show, the ommatidia in Cardisoma Gvanhumf, Latr.,
Cancer irroratus, Ray, and Gelasimus pugilator, Latr., are hexagonally grouped.

- The outline of the corneal facets is stated to be hexagonal in the following
genera: Pagurus (Swammerdam, '52, p. 88, Cavolini, '02, p. 130, Milne-Edwards,

MUSEUM OF COMPARATIVE ZOOLOGY. 63

of the ommatidia is invariably hexagonal. In the remaining macrurous
Decapods 1 the ommatidia are grouped on the tetragonal plan. This last
statement, however, is not without exceptions, for in Typton, and at
times also in Galathea, 2 the hexagonal arrangement appears to prevail.
An explanation of these exceptions will be offered in a subsequent
paragraph.

Before attempting this explanation, however, it will be well to gain a
precise idea of the relation of the hexagonal and tetragonal methods of
arrangement. At first sight, it might appear that these two methods
had no definite relations, and were simply characteristic of different
Decapods. Such, however, is not the case; for, as the development of
the lobster shows, the ommatidia in a single animal can be arranged at
first according to one plan, and afterward according to the other. In
the lobster the hexagonal arrangement characterizes the earlier stages
of development, and is replaced only subsequently by the tetragonal
grouping. A similar change also occurs in the spiny lobster. Thus,
in Phyllosoma, the larva of either Palinurus or Scyllarus, the hexagonal
facets observed by Milne-Edwards ('34, p. 115) afford unquestionable
evidence of the hexagonal arrangement at this stage. In the adult con-
dition, however, both of Palinurus and of Scyllarus, according to my own
observations, the ommatidia are tetragonally grouped. In the common
lobster and the spiny lobster, then, the hexagonal arrangement of the
early stages is replaced by the tetragonal one in the adult. These ob-

'34, p. 117, Will, '40, p. 7, Frey und Leuekart, '47, p. 204, Chatin, '78, p. 8);
Calllanassa ; and Gebbia (Milne-Edwards, '34, p. 117). In Pagurus longicarpus,
Say, and Hippa talpolda, Say, I have observed a hexagonal arrangement of the
ommatidia.

1 Judging from the figures given by various authors, the ommatidia of the fol-
lowing genera are characterized by the tetragonal arrangement ■• Galathea (Will,
'40, Fig III. c); Astacus (Muller, '26, Tab. VII. Fig. 13, Leydig, '57, p. 252,
Fig. 134, Keichenbach, '80, Taf. XIV. Fig. 22G, Huxley, '57, p. 353) ; Homarus
(Newton, '73, Plate XVI. Fig. 3, Parker, '90 a , p. 8) ; Pahvmon (Grenadier, '79,
Taf. XI Fig. 118 A, Patten, '80, Plate 31, Fig. 115); Penmis (Patten, '86, Plate
31, Fig. 75). As my present observations have shown, the tetragonal arrangement
is characteristic of the ommatidia in Palinurus Argus, Gray, Cambarus Bartonii,
and Pahemonetes vulgaris, Say.

2 According to Chatin (78, p 13) the outline of the facet in Tgpton is hexagonal.
Presumably the arrangement of the ommatidia in this genus is upon the hexagonal
plan. In Galathea, according to the figures given by Patten ('80, Plate 31, Fig.
110), the ommatidia are hexagonally arranged, although it must be borne in mind
that Will's ('40, Fig. Ill e.) figure of the facets in Galathea strigosa affords unmis-
takable evidence of a tetragonal arrangement.

64 BULLETIN OF THE

servations appear to me to afford considerable evidence in favor of the
view that the hexagonal arrangement is phylogenetically more primitive
than the tetragonal.

Granting this conclusion, a number of otherwise exceptional observa-
tions can be explained. Thus, as long ago as 1840, Will ('-±0, p. 7)
called attention to the fact that in Astacus, where the ommatidia are
normally arranged . upon the tetragonal plan, facets near the edge of the
retina are often irregularly hexagonal. The edge of the retina is well
known to be the last part produced, and therefore it is probably the
part least differentiated. Admitting the hexagonal arrangement to be
a primitive one, it is only natural to expect that, if it persists at all,
it will persist in the less modified portion of the retina. Hexagonal
facets also occur on the periphery of the retina in Ilomarus, and are to
be explained, I believe, in the same way.

On the assumption that the hexagonal plan is primitive, the occur-
rence of a few genera with ommatidia hexagonally arranged, in a group
in which the tetragonal arrangement is the rule, can also be explained.
InTypton, for instance, the hexagonal plan obtains, although in almost all
Crustaceans closely related to it the tetragonal system prevails. This
condition may be explained, however, by the fact that the eyes in Typton
show evident signs of degeneracy, due in all probability to tJie parasitic
habits of the Crustacean. If the hexagonal arrangement represents an
early ontogenetic phase in the development of Decapods related to Typ-
ton, it would be natural to expect that in Typton itself, where the normal
development of the eyes is interrupted by parasitism, this arrangement
would persist permanently.

In Galathea, as I have already mentioned in a note on page 63, the
ommatidia according to Will are arranged tetragonally ; according to
Patten, hexagonally. At first sight these observations might appear
to be irreconcilable, but such is not necessarily the case. So far as I
have been able to ascertain, Patten does not mention the name of the
species which he studied. Possihly he may have examined some other
than G. strisrosa, the one from which Will's figures were drawn. In
such an event, a difference in the arrangement of the ommatidia may
have been characteristic of the two species, although, if both possessed
well developed eyes, this difference would be somewhat anomalous. If
this is not the true explanation, it is stdl possible that the specimens
studied by Patten were somewhat immature, in which case the hexagonal
arrangement might very naturally he present. From what has been said,
I think it must be evident that the apparent contradiction in Will's and

MUSEUM OF COMPARATIVE ZOOLOGY. 65

Tatten's statements is not so serious as might at first be supposed, and
that, admitting the relations already mentioned between the two plans
of arrangement, the observations of these two writers can be explained
without supposing either of them to be wrong.

The probable method of rearrangement by which the hexagonal plan
is converted into the tetragonal has been suggested in a previous paper
(Parker, '90 a , p. 50). It involves two changes: the conversion of the
hexagonal outline of the ommatidium, as seen in the corneal facet, into
a square one, and the slipping of the rows of ommatidia one on the other,
so that the lines which bound the four sides of each facet finally form
parts of two series of lines which cross each other at right angles.

A condition somewhat intermediate between the hexagonal and tetrag-
onal arrangement is shown in the retina of Crangon (Plate X. Fig. 123).
In this genus the outlines of the ommatidia as seen in the facets are
square, although their arrangement suggests the hexagonal type. The
permanent grouping of the ommatidia in Crangon represents a stage
slightly in advance of the condition seen in some young lobsters (com-
pare Parker, '90% Plate IV. Fig. 55), and the particular features in
which this advance is shown are two. First, the distal retinular nuclei
in Crangon (Fig. 123) are grouped in pairs, more as they are in adult
lobsters, and not in circles of six, as in young ones (compare Parker,
'90 a , Figs. 5 and 55). Secondly, the arrangement of the ommatidial
centres in reference to the hexagonal plan is more symmetrical in the
young lobster than in Crangon, where the rows of ommatidia have ap-
parently slipped somewhat upon one another so as to resemble more
nearly the condition in the adult lobster.

I have been unable to determine with certainty what occasions the
change from the hexagonal to the tetragonal arrangement. Apparently
it accompanies an excessive growth on the part of the individual omma-
tidia. In the lobster, for instance, the ommatidia rearrange themselves
between the times when the young animal is one inch and eight inches
long. During this period the ommatidia increase about ten times in
length and about five times in breadth. The increase is especially
noticeable at their distal ends, and particularly in the cone cells. In
young lobsters of one inch in length (Parker, '90 a , Plate IV. Fig. 55),
the space between the cones of adjoining ommatidia is considerable ; in
adults, it is proportionally very much less (compare Parker, '9()\ Plate I.
Fig. 5), and the cones are crowded against one another. Under these
conditions, the hexagonal arrangement apparently gives way to the te-
tragonal. So far as I am aware, the tetragonal arrangement occurs only

VOL xxi. — no. 2. 5

66 BULLETIN OF THE

in connection with this crowding of the cones, a condition found for the
most part only in macrurous Decapods.

In accounting for the rearrangement of the ommatidia, the eyes in
the Stomatopod Gonodactylus afford some important evidence. As I
have previously mentioned, the ommatidia in this genus are of two sizes.
The larger ones have several of the peculiarities characterizing the tetrag-
onal arrangement : their facets are generally square ; they are arranged
in single lines, and these lines, so far as the relations of the individual
ommatidia are concerned, show evidences of having slipped upon one
another. The smaller ommatidia have hexagonal facets, and are clearly
arranged according to the hexagonal plan. The larger ommatidia are
rather closely packed; the smaller ones are arranged with more open
space between them (compare Plate VIII. Fig. 93). In this genus,
then, as in the lobster, the tetragonal arrangement occurs in connec-
tion with the crowding of the ommatidia.

How an increase in size, accompanied by a crowding of the retinal
elements, can induce the change in arrangement which seems to follow
it, I am at a loss to explain. Nevertheless, the two phenomena ap-
pear to be in some way connected.

From the pi'eceding discussion concerning the arrangement of the
ommatidia, the following conclusions can be drawn. The ommatidia,
when numerous enough, present one of two plans of arrangement,
the hexagonal or the tetragonal. The hexagonal plan is phylogeneti-
cally the older, and is characteristic of the eyes of all Crustaceans
except some families of the macrurous Decapods, especially the Gala-
theidse, Palinuridse, Astacidse, and Carididse. In these the hexagonal
arrangement is usually replaced by the tetragonal; but in the adults of
some species, especially those in which the eyes are partially rudi-
mentary, the hexagonal arrangement persists. The change from the
hexagonal to the tetragonal arrangement is connected apparently with
an increase in size, and consequent crowding, of the ommatidia.

The Structure of the Ommatidia.

Each ommatidium, as I have previously mentioned, consists of a
cluster of cells more or less regularly arranged about a central axis.
The crreatest number of kinds of cells which an ommatidium is known
to contain is five. These are the cells of the corneal hypodermis, the
cone cells, the proximal and distal retinular cells, and the accessory
cells.

MUSEUM OF COMPARATIVE ZOOLOGY. 67

The cells of the corneal hypodermis are usually arranged in a very
thin layer, and constitute the most superficial tissue in the retina.
They either present no definite arrangement, as in Amphipods, or they
are regularly grouped in pairs, one pair for each ommatidium, as in
the majority of Crustaceans. On their external faces they produce the
corneal cuticula. This is unfacetted in those Crustaceans in which the
corneal cells are not regularly arranged and facetted when they are
grouped in pairs.

The cone cells in each ommatidium are united to form the cone, a
transparent body which extends from the corneal hypodermis proximally
through the ommatidium at least as far as the rhabdome. The cone
occupies the axis of the distal portion of the ommatidium.

The proximal retinular cells are usually limited to the proximal por-
tion of the ommatidium. They are definitely arranged around the
axial structure of that region, the rhabdome, and together with it form
a single body, the retinula. The optic nerve fibres terminate in the
proximal retinular cells.

The distal retinular cells are present in only the more differentiated
ommatidia. They are two in number, and invest the sides of the cone
distal to the plane at which this structure emerges from the retinula.
When distal cells are present, the remaining cells of the retinula will be
distinguished as proximal cells ; when the distal cells are wanting, the
other cells will be called simply retinular cells.

The accessory cells fill the space between the elements of an omma-
tidium, or between separate ommatidia. Their number is apparently
inconstant, and they present a variety of forms. They may or may
not contain pigment. Depending upon their source, two kinds can be
distinguished, ectodermic and mesodermic.

In describing the ommatidia, I shall consider them according to the
groups of Crustaceans in which they occur. Under each group the
elements comprising the ommatidium will be described in the order
in which they have just been mentioned.

My object in the following account is to determine, as far as possible,
what the different kinds of ommatidial types are, and to define these
types by a brief statement of the number and kinds of cells which char-
acterize them.

Compound eyes are known to occur in some Ostracods, and in the
larva' of some Cirripeds, but their histological structure, I believe, has
never been studied. I am therefore compelled to dismiss these two
groups without further comment, and proceed with the description of

G8 BULLETIN OF THE

the onimatidia in other Crustaceans. The order in which the groups
will be considered is one which is intended to emphasize their relations
only in so far as the structure of their ommatidia is concerned. Natu-
rally, this order will vary somewhat from the one usually given in sys-
tematic treatises. I shall begin with the Amphipods.

Amphipoda.

Within recent years the more important types of eyes in the Amphi-
pods have been studied with such care that the structure of their om-
matidia is perhaps better known than that of any other large group of
Crustaceans. My own observations do little more than confirm the
accounts already published.

The species of Amphipods whose eyes I have examined are Gammarus
ornatus, M. Edw., Talorchestia longicornis, Say, and an undetermined
species of Caprella. Of these the specimens of Gammarus and Caprella
were collected at Nahant, Mass., where I also obtained several sets of
eggs representing stages in the development of the former. Examples
of Talorchestia were kindly supplied me from the collections in the
Museum.

The corneal hypodermis in Amphipods was first satisfactorily described
by Claus ('79, p. 131) in his account of the eyes in Phronima. It is
represented in this genus by a layer of undifferentiated cells lying be-
tween the corneal cuticula and the membrane which limits the distal
ends of the cone cells. A corneal hypodermis similar to that in Phro-
nima has likewise been described by Mayer ('82, p. 122) in Caprella and
Protella, by Carriere ('85, p. 15G) in Gammarus, by Claus ('87, p. 15) in
the Platyscelidse, by Delia Valle ('88, p. 94) in the Ampeliscidse, and
by Watase ('90, p. 295) in Talorchestia. I have also identified this struc-
ture in Gammarus, Caprella, and Talorchestia.

In Gammarus, as Carriere ('85, p. 15G, Fig. 121) has clearly shown,
the corneal hypodermis at the edges of the retina is directly continuous
with the general hypodermis. According to my own observations this
condition is not only met with in Gammarus, but also in Caprella and
Talorchestia.

In Phronima, according to Claus's figures ('79, Taf. YI. Figs. 48 and
49, Ma Z.), the arrangement of the cells in the corneal hypodermis
bears no definite relation to the subjacent cones ; the distal end of each
cone presents an area which is covered by about a dozen hypodermal
cells. In Gammarus I have observed (Plate I. Figs. 2 and 3) an essen-
tially similar distribution of the hypodermal cells ; as in Phronima, the

MUSEUM OF COMPARATIVE ZOOLOGY". 69

number of cells which cover the area of each cone is about twelve. A
corneal hypoderrais of this same character also occurs iu Talorchestia,
although iu this instance the number of cells over a cone is only about
nine.

According to Watase ('90, p. 295), in the species of Talorchestia
which he studied there were only two cells in the corneal hypodermis
opposite each cone, or, as he expresses it, under each facet. When com-
pared with the results recorded in the preceding paragraph, this observa-
tion appears somewhat striking, and the more so since two, the number of
cells recorded, is the usual number found under each facet in other Crus-
taceans. If Watase's observation be correct, the relation which would
thus be established between this Amphipod and other Crustaceans would
be an interesting: one. The desirability of confirming Watase's observation
must, therefore, be evident ; but unfortunately he has not given the name
of the species of Talorchestia which he studied, and I have therefore
not been able to verify his statement. Iu the only species of this ge-
nus which I have examined, viz. T. longicornis, the arrangement of the
cells in the corneal hypodermis is very different from that described
by Watase.

The conclusions which I draw from the preceding account are, that
in'the eyes of Amphipods a corneal hypodermis is present, and the cells
composing it are usually not arranged with regularity.

The peculiar bodies observed by Schmidt ('78, p. 5) in the membrane
between the corneal hypodermis and the retina proper in Phronima, and
considered by Claus ('79, Taf. VI. Figs. 48, 49, B. mi.) as nuclei, are
apparently not represented in other Amphipods. Their significance is
still a matter of doubt.

The corneal cuticula in Amphipods has been described by almost all
observers as unfacetted. 1 According to Delia Valle ('88, p. 94), how-
ever, in some of the Ampeliscidse this cuticula is facetted, and Watase
('90, p. 295) has also observed facets in Talorchestia. But with these
two exceptions the corneal cuticula of Amphipods has been described

1 An unfacetted corneal cuticula has been recorded in the following genera of
Amphipods : Amphithoe (Milne-Edwards, '34, p. 116) ; Caprella (Frey und Leuck-
art, '47 a , p. 103) ; Cynnvis (Miiller, '29, p. 58, Frey und Leuckart, '47, p. 205) , Gam-
marus (Miiller, '20, p. 57, Frey und Leuckart, '47, p. 205, Pagenstecher, '61, p. 31,
Sars, '67, p. 61, Leyriig, '78, p. 235, Grenacher, '79, p. 109) ; Hyperia (Gegenbaur,
'58, p. 82, Grenadier, '79, p. Ill, Carriere, '85, p. 160) ; Phronima (Pagenstecher,
'61, p. 31, Schmidt, '78, p 5, Claus, '79, p. 131 ) ; Talitrus (Grenacher, '79, p. 109) ;
and the Plntjisce.hdai (Claus, '87, p. 15). I have observed an unfacetted corneal
cuticula in Gammarus, Caprella, and Talorchestia longicornis.

70 BULLETIN OF THE

as smooth. The absence of facets from Amphipods is naturally ac-
counted for by the absence of a definite arrangement among the cells
of the corneal hypodermis.

In the genus Tenais, the systematic position of which is probably
somewhere between the Amphipods and Isopods, the corneal cuticula is
stated by Muller ('64, p. 2) to be facetted, at least in the males. Ac-
cording to Blanc's ('83, p. 63.3) more recent observations, however, it is
claimed to be unfacetted.

The cones in Amphipods have long been known to be segmented.
The number of segments of which each cone is composed has been dif-
ferently stated, however, by different observers. According to Clapa-
rede ('60, p. 211), the cones in Hyperia are each composed of four seg-
ments. This also is the number given by Sars ('67, p. 61) and by
Leydig ('79, p. 235) for Gammarus. Both Hyperia and Gammarus
have since been carefully studied, and these observations are now
known to be inaccurate. Claparede was perhaps influenced in his
statement by his belief that all cones were composed of four cells.
Sars was probably misled by the supposed fact that in Gammarus the
cone is surrounded by four bands of pigment, which sometimes give it
the appearance of being divided into four segments.

The actual number of segments in the cone of Amphipods is two.
This number was first recorded by Pagenstecher ('61, p. 31) for the
cones of Phronima. Pagenstecher believed, however, that the cones
in this Crustacean increased in numbers by division, and that they
showed no indication of being composed of two segments except when
they were undergoing this process. I need scarcely add that subse-
quent investigations have not confirmed Pagenstecher's belief. Cones
composed of two segments have been observed in some six or seven
genera of Amphipods. 1

The retinula in Amphipods is stated by different observers to consist
of either four or five cells. Five have been seen by Grenacher ( ; 74,
p. 653) and Carriere (85, p. 160) in Hyperia; by Grenacher ('79,
p. 11.2), Claus ('79, Taf. VIII. Fig. 65), and Carriere ('85, p. 164) in
Phronima; and by Mayer ('82, p. 122) in Caprella.

In Gammarus, Sars ('67, p. 61) observed that the cone had four

1 In Caprella (Mayer, '82, p. 122), in Gnmmarus (Grenacher, '70, p. 110, Car-
riere, '85, p. 156), in Hyperia (Grenadier, 74, p. 652), in Oxj/cephalus (Claus, '71,
p. 151), in Phronima (Schmidt, '78, p. 5, Grenacher, '79, p. 112, Claus, '73, p. 130),
in Talorchestia (Watase, '90, p 296), and in the Piatyscclidic (Claus, '87, p. 15).
In Gammarus omatus, Talorchestia longicornis, and Caprella, each cone is composed
of two cells.

MUSEUM OF COMPARATIVE ZOOLOGY. 71

longitudinal bands of pigment on it. Grenacher ('79, p. 110) took
this as an indication that there were at least four retinular cells in
the ommatidium of this genus, but he was unable to satisfy himself as to
whether there were a greater number or not. Carriere ('85, pp. 156,
157) easily identified the four cells first seen by Sars, and in favor-
able cases observed what he thought might be indications of a fifth cell.
In Gammarus ornatus, as the present observations show, the retinula
is certainly always composed of five cells, one of which, as Carriere
observed, is usually much smaller than the other four (compare d. rt/iJ,
Figs. 4-7).

In Talorchestia, according to Watase ('90, p. 296), the retinula is
composed of only four cells. I have studied T. longicoruis with the
purpose of determining the number of retinular cells, and I find that,
although there are four large retinular cells, there is also one small one,
which is even more reduced than in Gammarus. Hence I conclude that
the total number of retinular cells in an ommatidium of Talorchestia
is five, not four.

Claus's statement ('71, p. 151), that in Oxycephalus the retinula is
usually composed of four cells, is probably inaccurate, as Grenacher
('79, p. 114) suggests; and the same is perhaps true of Delia Valle's
('88, p. 94) observation, that in the Ampeliscidte the retinulae contain
only four cells each. It is therefore probable that the retinula in all
Amphipods is composed of five cells, although possibly in some excep-
tional cases the number may be four.

The retinular cells in Gammarus envelop the sides of the cone, as
Carriere suspected, and extend distally as far as the corneal hypodermis
(Plate I. Fig. 2). In Hyperia and Phronima, according to the descrip-
tion and figures given by Carriere ('85, p. 161, and Fig. 128, p. 165),
these cells appear to be limited to the proximal part of the retina.

The rhabdome in Amphipods, first described by Pagenstecher ('61,
p. 30) as the cylindrical element in the eye of Phronima, presents a
very simple structure. In Hyperia, according to Grenacher ('77, p. 31),
it is a simple rod-like body, composed of five rhabdomeres, one for each
retinular cell. In Phronima, as Claus ('79, p. 128) has shown, the
rhabdome is a tubular structure with five sides. Each side of the tube,
as can be seen in the figure given by Carriere ('85, p. 165, Fig. 128),
corresponds to a rhabdomere. In Gammarus locusta, Grenacher ('77,
p. Ill) has shown that, in transverse section, the distal end of the
rhabdome is cross-shaped. In G. pidex, according to Carriere ('85,
p. 157), the distal end of the rhabdome in section shows four rays, the

72 BULLETIN OF THE

proximal five. In Carriere's opinion, these rays indicate the five rhab-
doineres. In Gainmarus ornatus, the species which I have studied, the
rhabdume (Plate I. Fig. 6, rhb.) is cross-shaped in transverse section
throughout its length. Each rhabdomere has the form of an elon-
gated plate, which is folded on its longest axis, so that its halves are
at right angles to each other. In the rhabdome, the four rhabdomeres
lie so that their folded edges occupy the axis of the ommatidium.
Each of the four large retinular cells rests in the furrow produced
by the folding of a rhabdomere (compare Fig. 6). The fifth retinular
cell always lies at the end of one arm of the cross-shaped rhabdome.
The two rhabdomeric constituents of that arm usually separate slightly,
so as to allow the small retinular cell to slip in between them. Possi-
bly this cell produces a small rhabdomere, as the corresponding cell in
G. pulex does ; but if such is the case, the rhabdomere must be a very
small one, for I have not been able to discover it. A rhabdome of
essentially this structure occurs in Talorchestia.

As the preceding account shows, the rhabdome in Amphipods always
presents some indication of the number of rhabdomeres of which it is
composed. This number is usually five, although it is possible that in
Gammarus it may be only four.

In addition to the cells which have thus far been described as entering
into the composition of the retina in Amphipods, certain other cells may
be present. These may be embraced under the one head of accessory
pigment cells.

In Gammarus, as Carriere ('85, p. 159) has shown, the space between
the ommatidia is filled with rather large cells, the nuclei of which are
usually visible with ease (Fig. 2, nl. h'dnn.). These cells extend from
the basement membrane very nearly, if not quite, to the corneal hypo-
dermis. In the fresh condition they contain a whitish opaque pigment.
On account of their having no definite arrangement, it is difficult to esti-
mate their number, but there are probably two or three for each omma-
tidium. Cells similar in position to these have been described by Watase
('90, p. 296) in Talorchestia.

In Hyperia there are apparently three kinds of accessory pigment
cells. One kind occurs in the region of the basement membrane (Car-
riere, '85, p. 161, Fig. 124, m.) ; another kind surrounds the proximal por-
tion of the cones (Carriere, '85, p. 161) ; a third kind is applied to the
retinulae, and, according to Carriere, exactly equals in number the cells
of the retinula itself. Possibly the cells which Grenacher ('79, p. 112)
described as lying at the distal end of the retinula in Hyperia belong

MUSEUM OF COMPARATIVE ZOOLOGY. 73

to this third kind, although, as must be remembered, Grenacher states
that there are only two such cells for each ommatidinm.

These three kiuds of accessory pigiueut cells, with the possible excep-
tion of those which surround the retinula, occur in the lateral eyes of
Phronima (Carriere, '85, p. 1 64).

Almost nothing is known about the source of the accessory pigment
cells in Amphipods. Those in Gammarus have no resemblance to the
loose mesodermic tissue which lies in the neighborhood of the eye, and
they are probably derived from the original ectodermic thickening which
gave rise to the retina. Although some of the accessory pigment cells
in Hyperia and Phronima have been called connective-tissue cells (Glaus,
'79, p. 125, Carriere, '85, p. 160), a name which might be taken to im-
ply that they have come from a mesodermic source, nothing is really
known about them which would be inconsistent with an ectodermic.
origin.

From the foregoing account of the ommatidia in Amphipods the follow-
ing summary can be made: cells of the corneal hypodermis not definitely
arranged, from about nine to twelve, — possibly two to each ommatidium ;
cone cells, two ; retinular cells, five, — possibly in some cases four; ac-
cessory pigment cells (ectodermic]) present. Of these last there may be
only one kind, as in Gammarus and Talorchestia, or there may be three
kinds, as in Hyperia.

Phyllopoda.

The ommatidia in the eyes of Phyllopods present at least two struc-
tural types, one of which obtains in the Branchiopodidas and Apusidse,
the other in the Estheridaa and Cladocera. On account of the greater
convenience, the eyes in the Apusidse and Branchiopodida? will be con-
sidered first, then the eyes in the Estherida?, and finally those in the
Cladocera.

Branchiopodidce and Apusitbe. — The ommatidia in these two families,
and especially in the Branchiopodidee, have been carefully studied by a
number of competent investigators ; their structure is consequently
well known.

The material which I used in studying these eyes consisted of speci-
mens of Branchipus, probably B. vernalis, Verrill, which I had collected
in the neighborhood of Philadelphia, and which had been preserved
for some time in strong alcohol. Through the kindness of Dr. W. A.
Setchell, I was also able to examine a specimen of Apus lucasanus,
Packard.

74 BULLETIN OF THE

A corneal hypodermis has been described by Claus ('86, pp. 321, 322)
in Branchipus and Apus. In Branchipus torticornis, according to Claus,
tbe nuclei of the hypodermal cells are arranged around the distal end of
each cone in circles of six ; each nucleus participates in three circles, so
that there are in reality only twice as many hypodermal cells as there
are ommatidia. The corneal hypodermis in the eye of Branchipus ver-
nalis (Plate IV. Fig. 30, nl. h'drm.) is similar to that described by Claus
in B. torticornis. According to Patten ('80, p. 6-15), a corneal hypoder-
mis is present in Branchipus Grubii, but the cells, instead of being
regularly placed, as in either Branchipus torticornis or B. vernalis, are
stated to be indefinitely arranged.

The corneal cuticula in Apus is described as unfacetted by Midler
('29, p. 5G), Burmeister ('35, p. 533), Zaddach ('41, p. 40), and Frey
und Leuckart ('-17, p. 205). In Branchipus stagnalis the cuticula is
smooth according to Spangenberg ('75, p. 30), marked by concavo-
convex facets according to Grenacher ('79, p. 114). and smooth exter-
nally but facetted internally according to Leydig ('51, p. 29.">). This
difference of opinion is probably due to the fact that in this species the
facets are so poorly developed that their form can be determined only
with difficulty. In Branchipus vernalis, although the corneal cuticula
is facetted, the facet is not thickened in its centre, but has the form
of a simple concavo-convex elevation, as described by Grenacher in
B. stagnalis. In Branchipus paludosus according to Burmeister ('35,
p. 531), in B. torticornis according to Clans ('8G, p. 320), and in
B. Grubii according to Patten ('8G, p. G45), the corneal cuticula is
unfacetted.

The cone in Branchipus, as Spangenberg ('75, p. 30) first demon-
strated, is composed of four segments. Thus observation has since been
confirmed by Grenacher ('79, p. 115), Claus ('8G, p. 320), and Patten
('SG, p. G4.">). In Branchipus vernalis (Fig. 31, con.) the cone, according
to my observation, consists of four segments. The cellular nature of
each segment was first clearly stated by Grenacher. Each cone in
Apus, according to both Grenacher ('79, p. 115) and Claus ('8G, p. 321),
is composed of four cells.

The retinula in both Apus and Branchipus consists of five cells. This
number has been seen in both genera by Grenacher ('74, p. Goo) and by
Claus ('86, p. 319). Spangenberg, however, ('75, p. 31) counted four
nuclei in the retinula of Branchipus. Since these unquestionably rep-
resent the nuclei of the retinular cells, and since these cells are usually
five in number, Spangenberg's enumeration is probably inaccurate. Pos-

MUSEUM OF COMPARATIVE ZOOLOGY. 75

sibly he was influenced when counting the nuclei by his belief that the
number four was characteristic of many structures in the ommatidium.
In Branchipus vernalis (Plate IV. Fig. 32, cl. rtn.') the retinula contains
five cells.

The rhabdome in Apus is short ; in Branchipus (Fig. 30, rhb.) it is
relatively long. In transverse section (Fig. 32, rhb.) it is circular, or at
times squarish, but never pentagonal, as might be expected from the
fact that it is surrounded by five retinular cells.

The retina in B. vernalis contains no other cells than the three kinds
already described. According to Clans (SG, p. 319), blood corpuscles
may make their way into the base of the retina of B. torticornis.

From the preceding account, the number of cells in the ommatidia of
the Branchiopodidae and Apusidse can be stated as follows : cells of the
corneal hypodermis, usually two, possibly variable in number in some
species; cone cells, four; retinular cells, five. In Branchipus torticornis
the interommatidial space may contain blood corpuscles.

Estheridce. — The species which I studied as a representative of this
family was Limnadia Agassizii, Packard. This species can usually be
obtained in great abundance during summer in small fresh-water pools
in the neighborhood of Wood's Holl, Mass., where my material was
kindly collected for me by Mr. W. M. Wood worth.

The external surface of the retina in Limnadia, as I have mentioned
in my account of the general structure of the eye in this genus, is cov-
ered with an extremely delicate corneal cuticula. This cuticula does
not show the least trace of facets.

Immediately below the corneal cuticula are numbers of small nuclei
(Plate IV. Fig. 37, nl. cm,). These, from their position, are probably
to be regarded as the nuclei of the corneal hypodermis. They are not
regularly arranged, and, although they sometimes lie between the cu-
ticula and the distal end of a cone, they more frequently occur next
to the cuticula in the spaces between the cones.

As a rule, each cone in Limnadia is composed of five cells (Plate IV.
Figs. 37 and 38). In this respect it resembles the cones in Fstheria
californica and E. tetracera described by Lenz ('77, p. 30). In Lim-
nadia Agassizii, however, cones composed of four cells are not infre-
quently met with (compare Figs. 37 and 38). Grube's ('65, p. 208)
observation that the cone in Estheria is composed of two segments is
probably erroneous, but Claus's ('72, p. 300) statement that in Limnadia
the cone consists of four segments may be accurate, contrary to the
opinion of Lenz.

76 BULLETIN OF THE

The reticular cells in Limnadia cover the greater part of the sides of
the cones, and completely hide the rhabdome (Plate IV. Fig. 3G). Their
number can be determined in transverse sections in the region of the
rhabdome. In such sections each rhabdome is surrounded by five retin-
ular cells (Fig. 39, cl. rtn.'). Occasionally nuclei can be distinguished
in the pigment about the base of the cone. These are probably the
nuclei of the retinular cells.

Besides the elements thus far enumerated, the retina in the Estheridae
is not known to contain other kinds of cells. The cells in the omma-
tidia of this family are, therefore, as follows : cells of the corneal hypo-
dermis, not regularly arranged ; cone cells, usually five, sometimes four ;
retinular cells, five.

Cladocera. — The extreme minuteness of the ommatidia in the eyes of
the Cladocera renders their study especially difficult. In an undeter-
mined species of Evadue which I have studied, the ommatidia are
comparatively large, and in this respect are especially favorable for in-
vestigation. In the particular specimens which I used, however, I was
entirely unsuccessful in all attempts to differentiate the nuclei. Al-
though I tried a number of dyes and reagents, I was never able to make
these structures visible. In consequence of this, there are several impor-
tant questions concerning the eyes in the Cladocera which I have not
been able to answer.

It is reasonable to believe that a corneal hypodermis much like that
in Limnadia is present in Evadne, but, probably on account of my inabil-
ity to stain the nuclei, I have seen no traces of it.

The cones in Evadne are very clearly composed of five segments (Plate
IV. Figs. 41, 42). At their distal ends the cone cells are expanded so
that their peripheral membranes (Fig. 41, mb. jji'ph.) are in contact with
one another. At this level, however, the substance of the cone proper is
collected about the axis of the ommatidium. Proximally the peripheral
membranes of each cone contract, and under these circumstances the
cavity of each cone cell is apparently filled completely with the differen-
tiated material of the cone itself (Fig. 42).

A cone composed of five segments has been observed in a considerable
number of Cladocera. Thus it is known to occur in Bythotrephys
(Leydig, '60, p. 245, Clans, '77, p. 144), Daphnia (Spangenberg, '76,
p. 522, Grenacher, '79, p. 117), Polyphemus, Evadne (Claus, '77, p. 144),
Podon (Grenacher, '79, p. 117), and Leptodora (Carriere, '84, p. 678).
Weismann's assertion ('74, p. 36' 1 :) that the cone in Leptodora is com-
posed of four segments is disproved by Carriere's later observations, and

MUSEUM OF COMPARATIVE ZOOLOGY. 77

Claus's statement ('76, p. 37*2) that the same number of segments oc-
curs in the cone of Sida is probably erroneous. There is, therefore,
reason to believe that the cones in the Cladocera are always composed
of five segments.

The composition of the retinula in Cladocera, so far as I am aware, has
never been fully worked out. In Evadne, on account of tbe relatively
large size of the ommatidia, the number of cells in the retinula can be
determined. At the proximal end of the cone, this structure is sur-
rounded by four distinct masses (Fig. 43). The regularity with which
these masses occur leaves no doubt as to their number. Each one prob-
ably represents a retinular cell. In transverse sections made through
the rhabdome (Plate IV. Fig. 45), this structure is surrounded by five
bodies, each one of which I take to be a retinular cell. It is therefore
probable that the retinula of Evadne is composed of five cells, four of
■which approach nearer the surface of the eye than the fifth.

In Evadne I have seen no evidence of the existence of other cells than
those belonging to the cone and retinula. According to Carriere ('84,
p. 678). the interommatidial space in Leptodora contains a number of
cells which envelop the cones more or less completely. These are proba-
bly to be regarded as accessory pigment cells.

From the foregoing account the following general statement can be
made for the ommatidia in the Cladocera : corneal hypodermis, not
observed ; cone cells, five ; retinular cells, five (in Evadne) ; accessory
pigment cells present (in Leptodora).

Copepoda.

I have studied the lateral eyes' in Pontella and Argulus, as representa-
tives of the Copepods. As is well known, the eyes in these two genera
differ greatly in structure, and I shall therefore describe them separately,
beginning with the eyes in Pontella.

Eucopepoda. — The species of Pontella which I. studied was extremely
abundant at Newport in August, 1890. This animal was so transparent
when living, that the general structure of its eyes could be ascertained
by a simple microscopic inspection of it. In addition to its median eye,
which occupies a ventral position, it possesses a pair of lateral eyes
(compare Clans, '63, Taf. III. Fig. 5) situated one on either side of the
sagittal plane at the antero-dorsal angle of the head.

Each lateral eye in Pontella, as Clans ('G3, p. 47) has already stated,
is provided with a spherical lens (Plate II. Fig. 18, Ins.), which is usu-
ally firmly attached to the superficial cuticula. Immediately behind

78 BULLETIN OF THE

this lens, and in fact covering much of its proximal face, is a rather
irregular mass of cells, the retina. In the living animal the cells of the
retina contain a great quantity of black or reddish black pigment. This
coloring matter, however, is so readily soluble in alcohol, that in speci-
mens preserved in that fluid all traces of it disappear. The optic nerve
(«. opt., Fig. 18), an imperfectly defined bundle of fibres, emerges from
the retina near its posterior dorsal edge, and passes directly backward to
the brain.

The lenses of the two lateral eyes in Pontella are so near each other
that their median faces are almost in contact (compare Plate III. Fig.
29). The retinas of the two eyes, as Claus ('63, p. 47) has observed,
are united with one another on their median faces, and so intimately
that they are apparently incapable of independent motion.

The two retinas together may be rotated on their lenses through an
angle of about forty-five degrees. The plane of this rotation corresponds
to the sagittal plane of the body, and the rotation is accomplished by
two pairs of muscles, (me for each retina (compare Claus, 'G3, Taf. III.
Fig. 0). One pair of these muscles is shown in Figure 18. They occupy
a plane approximately parallel to the sagittal plane of the body, and
the effects of their contractions must be apparent from their positions.
When both muscles are relaxed, the retina occupies a position substantially
as shown in Figure 18. By the contraction of the posterior muscle, the
retina may be drawn upward and backward over the surface of the lens,
till its axis, instead of pointing dorsally, is directed forward and upward
at an angle of about forty-five degrees with its original position. The
retina is not usually held for any great length of time in this position, but
is soon returned by the contraction of the anterior muscle to its normal
place. The backward motion of the retina is accomplished with such
rapidity that the animal has the appearance of winking. The forward
motion is rather slower.

Each lens in Pontella is composed of concentric laminae (Plate III.
Fig. 29, lux.). A considerable portion of its distal surface is intimately
connected with the superficial cuticula (Plate II. Fig. IS), although a line
of demarcation between lens and cuticula can always be distinguished.

"When the anterior half of the body of Pontella is boiled in a strong
aqueous solution of potassic hydrate, and afterwards subjected to the
action of concentrated nitric acid, all the soft parts are dissolved, and
only the very resistant chitinous structures remain. In specimens
treated in this way, the lenses retain their firm connection with the
superficial cuticula, and differ in appearance from those in the living ani-

MUSEUM OF COMPARATIVE ZOOLOGY. 79

mals only in that their concentric lamellae are somewhat more distinct.
The fact that the lens is composed of concentric layers indicates that
it is secreted, and the resistance which it offers to reagents is weighty
evidence in favor of its chitinous nature. In my opinion, therefore, the
lens in Pontella is a chitinous secretion.

The development of the lens in Pontella is rather peculiar. Appar-
ently a new lens is formed with each moulting of the general cuticula ;
at least, in a rather large proportion of the numher of individuals exam-
ined, the lenses were abnormally small, having a diameter of one third
or even one fourth of that shown in Figure 18. Moreover, in all such in-
dividuals the superficial cuticula was correspondingly thin and delicate,
and when the animal was subjected to boiling potash, the segments of
its body and appendages separated with a readiness never observed in
specimens with large lenses. There can be no doubt, I believe, that the
small lenses are always accompanied by thin cuticula, a relation which
is to be explained by the immature condition of both structures.

The smaller lenses differ from the larger ones in only one important
particular besides that of size. They are 7iot in contact with the super-
ficial cuticula. This relation can be determined better in optical sec-
tions than in actual ones, for in the latter the position of the lens is
usually somewhat changed by the resistance which it offers to the knife.
The centre of the small lens occupies a position relatively the same as
that of the large lens, the space between the surface of the small lens
and the external cuticula being filled with a cellular mass. This mass,
as seen in optical sections, apparently envelops the lens on all sides,
and is undoubtedly composed of the cells which secrete that structure.
As the lens increases in 6ize, the cells are probably excluded from the
region between it and the cuticula, and as they retreat cement the lens
to the cuticula. Upon the completion of the lens, the cells which have
shared in producing it probably withdraw slightly from it to form the
hvpodermal thickenings which occur beneath the adjoining cuticula
(Plate II. Fig. 18, and Plate III. Fig. 29, h\lrm.). These thickenings
are rich in nuclei, and often have delicate strands of protoplasm stretch-
ing to the surface of the lens (Fig. 18).

I believe that these facts justify the opinion that the lenses in the
lateral eyes of Pontella are composed of chitin, that they are produced
unconnected with the superficial cuticula, and that they are secondarily
cemented to it. Like the cuticula itself, they are products of the hv-
podermis, a new lens being generated in all probability with each new
formation of cuticula.

80 BULLETIN OF THE

Lenses similar in position to those in Pontella have been identified
in the lateral eyes of several other genera of Copepods. Gegenbaur
('58, p. 71) described such lenses in Sapphirina, and Leuckart ('59,
p. 250) observed similar ones in the lateral eyes of Corycaeus and
Copilia. In all these genera the lenses, although biconvex, are not
spherical, as in Pontella. Gegenbaur ('58, p. 71), following Leydig's
generalization, believed that in Sapphirina the lenses were thickenings
in the cuticular covering of the body, and Clans ('59, p. 271) considered
them morphologically equivalent to a single corneal facet. Leuckart
('59, p. 250), without definitely committing himself as to the nature
of the lens, states that in Copilia and Corycaeus the lens is implanted
in the superficial cuticula, and further describes it in Corycaeus as com-
posed of two parts, an outer and an inner. According to Grenadier
('79, p. 67), both parts can be identified in the lens of Copilia ; the
outer part is a portion of the superficial cuticula ; the inner part, both
in its optical properties and its behavior toward reagents, is unlike the
cuticula. The inner part, however, contains no traces of cells, but is
composed of a homogeneous substance, probably secreted. This view of
the duplicity of the lens contrasts with the older idea of its origin as a
thickening in the superficial cuticula.

It is possible that the lenses in the Pontellidae and Corycaeidae are not
homologous structures, but on account of their similarity I am inclined
to consider them as such. Since in Pontella both parts are derived
from the cuticula, 1- believe that a similar origin will be demonstrated
for these parts in the Corycaeidae. The differences which Grenadier
has pointed out between the two parts of the lens in Copilia do not
necessarily oppose this view. It is possible that the cuticular secretion
which forms the proximal part of the lens may originate separately
from the other cuticula, as in fact it does in Pontella ; and it may also
be true, although this is not supported by the condition in Pontella,
that the two parts, although both secretions of the hypodermis, may
differ enough in their substance to account for all the peculiarities ob-
served by Grenadier.

The retina and lens in Pontella are not separated by an intervening
space as in the Corycaeidae, but are in immediate contact. The retina
is composed of a mass of cells, the number and arrangement of which
can be seen in the figures on Plate III. These figures represent a
series of consecutive sections cut in planes transverse to the axis of
the eye, i. e. parallel to the horizontal plane of the animal (compare
Fig. 18, Plate II.). The series is complete in that it represents all

MUSEUM OF COMPARATIVE ZOOLOGY. 81

the sections which pass through the retina. The most ventral section
is shown in Figure 20, the most dorsal in Figure 29.

Immediately below the lens the central part of the retina is occupied
by a roundish granular mass (Fig. 18, con.), which in the living animal
is the only part without pigment. In trausverse sections this mass is
seen to consist of two bodies (cl. con. 1, and cl. con. '2, Fig. 25), which
extend as far as to the lens (compare Figs. 25-27). Each body con-
tains a nucleus (nl. con., Figs. 25 and 27) and consequently represents
a cell. From the position which the mass occupies, and from the fact
that it contains no pigment, it represents, I believe, a cone, and the two
cells of which it is composed are its two segments.

Claus ('63, p. 47) states that in Pontella each retina is provided with
six or more small crystalline cones, but my own observations do not
confirm this statement. The body which, on account of its position, I
have described as the cone in Pontella, is probably homologous with
what Dana ('50, p. 133) first described as the inner lens in Corycseus,
and with what subsequent investigators have called the crystalline cones
in Sapphirina (Gegenbaur, '58, p. 71) and Copilia (Leuckart, '59, p. 252).
^Nothing, I believe, is known of the cellular composition of the cone in
these genera.

The arrangement of the elements in that portion of the retina which
surrounds the cone in Pontella is not easily made out. The most con-
spicuous structures in this region are rod-like bodies, which probably
represent rhabdomeres. Eight of these, arranged in three groups, are
present in each retina. The largest group, composed of five rods, lies
directly beneath the cone. The rods of this group have been numbered
from one to five in the retina to the left in Figures 21, 22, and 23.
Posterior to this group, in the same retina, is the sixth rod, seen in
Figures 24, 25, and 26. Anterior to it are the seventh and eighth
rods, seen in Figures 26, 27, 28, and 29.

The outlines of the cells to which these rods belong cannot alwavs be
distinguished ; that there is a cell for each rod is evident from the fact
that near each rod there is a large nucleus. The nucleus belonging to
the cell from which the eighth rod was produced is shown in Figure 28
(id. rtn.') ; those belonging to the cells from which the sixth and seventh
rods arose are indicated in Figure 26 (nl. rtn'.), and those belonging to
the cells from which the central group of five rods came are seen, four in
Figure 24 and one in Figure 25 (nl. rtni).

In addition to these nuclei, which judging from their positions and
number are unquestionably the nuclei of the cells to which the rhab-

vol. xxi. — no. 2. 6

82 BULLETIN OF THE

domeres belong, the retina contains a number of smaller nuclei (Fig. 21,
nl. Ii dnii.). These nuclei have been drawn in the figures of the various
sections in which they occur, and probably represent undifferentiated cells.

To what extent the retina of Pontella can be resolved into omma-
tidia may be seen from the foregoing account. Evidently the two
cone cells, the subjacent groups of five retinular cells, and probably
seme of the undifferentiated cells, are the equivalent of one omma-
tidium. The sixth cell, with its rod, is probably the representative of
a second ommatidium, and the seventh and eighth cells are probably
representatives of one, or perhaps two, more.

If this interpretation be correct, the cells in the one complete omma-
tidium in Pontella would be as follows : corneal hypodermis, undifferen-
tiated ; cone cells, two; retinular cells, five; undifferentiated pigment
cells (ectodermic V) present.

Each retina in Sapphirina, according to Grenacher ('79, pp. 69, 70),
contains one group of three rhabdomeres. These are accompanied,
as in Pontella, by an equal number of large nuclei. The body desig-
nated at y, and perhaps some of those marked x, in Grenadier's figure of
Sapphirina (Fig. 43), may also represent isolated rhabdomeres. In Co-
pilia, Grenacher believes that the number of rhabdomeres in each retina
is three. Possibly in this genus, as in Sapphirina, the body marked x
bv Grenacher (Taf. VI. Fig. 40) may represent an isolated rhabdomere.
Grenacher's observations, when coupled with what I have seen in Pon-
tella, show that in Copepods the number of retinal elements is open to
considerable variation, and that what would correspond to the retinula
in Sapphirina, and perhaps in Copilia, consists of a cluster of only three
cells, instead of five, as in Pontella.

Branchiura. — The ommatidia in Argulus are rather small, and their
structure is consequently imperfectly known. The specimens of this
Crustacean which I studied were, obtained from an aquarium in which
the common Killifish, Fundulus heteroclitus, had been kept. I have not
been able to determine the species to which these specimens belong.

The corneal hypodermis in Argulus is separated from the retina proper
by a space filled with blood (Plate II. Figs. 11, 12, ccrd.). The cells in
this layer (Fig. 12, h'drm.), as in the corneal hypodermis of Amphipods,
are not arranged in groups, but are irregularly scattered. On their
distal faces they produce the corneal cuticula (Fig. 12, eta.), -which, as
Muller ('31, p. 97) observed, is without facets. Proximally they are
separated from the- blood space by the delicate corneal membrane (Fig.
12, mb. crn.).

MUSEUM OF COMPARATIVE ZOOLOGY. 83

The distal face of the retina proper in Argulus is bounded by a deli-
cate preconal membrane (Figs. 11-13, mb. ])r'con.) and its proximal face
is limited by the basement membrane (Figs. 11-13, mb. ba.).

The most conspicuous objects in the retina are the cones (Fig. 11,
eon.), which lie with their distal ends usually somewhat below the
preconal membrane (Fig. 13). Each cone, as Claus ('75, p. 256) has
observed, is composed of four segments (Fig. 14). The segments corre-
spond to cells, and although the cone itself terminates proximally before
reaching the rhabdome, the cone cells form an axis free from pigment
and extending from the cone to the rhabdome (compare Fig. 12). In
depigmented sections the peripheral membranes of the cone cells (Fig.
13, mb. pi'ph.) can be distinguished as sharply marked lines which ex-
tend from the sides of the cone to the sides of the rhabdome. The
intercellular membranes of the cone cells in the region between the cone
and rhabdome are apparently marked by thickenings which appear in
both longitudinal and transverse sections (compare Figs. 13 and 15).
At the distal end of the rhabdome the four cone cells separate, and, after
passing partly around the rhabdome, become lost in the adjoining tissue
(Fig. 16, cl. con.). I have not been able to discover the nuclei of the
cone cells.

It is difficult to determine the number of cells in the retinula of Argu-
lus. Slightly below the proximal end of the rhabdome, the retinula is
divided into five distinct pigmented masses (Fig. 17, cl. rtn.'). Since the
rhabdome (Fig. 16, rhb.) is composed of five rhabdomeres, it is highly
probable that the retinula consists of five cells; but I have not been able
to determine with precision the outline and extent of these cells.

The nuclei which are visible in the retina of Argulus closely resemble
one another. They are limited for the most part to two regions (Fig.
13), one near the level of the cones, the other near the basement mem-
brane. Apparently there are no nuclei immediately below the preconal
membrane. Those which are near the cones (Figs. 13, 14, nl. h\lrm.),
judging from their arrangement and position, probably represent inter-
ommatidial pigment cells. Those near the basement membrane (Fig.
13, nl. rtn.') may be the retinular nuclei, as their position seems to indi-
cate. For some distance proximal to the basement membrane, nuclei
(Fig. 13, nl. WdrmJ) occur among the nerve fibres. Possibly they repre-
sent scattered cells in this region, but the strong resemblance which
they have to the nuclei on the distal side of the membrane induces mo
to believe that they too are retinular nuclei, which, as in the Amphi-
pods, have migrated to a position below the basement membrane.

84 BULLETIN OF THE

The cells in the ommatidium of Argulus are as follows : cells of the
corneal hypodermis, not arranged in definite groups ; cone cells, four ;
retinular cells, probably five ; accessory pigment cells probably present.

Isopoda.

The material which I used in studying the eyes in Isopods came from
several sources. I collected specimens of Asellus and Porcellio in the
neighborhood of Cambridge, and the two species of Idotea which I
studied were obtained at Newport. Specimens of Serolis Schythei,
Liitken, and of an undetermined species of Sphseroma, were kindly fur-
nished me from the collections in tbe Museum.

The ommatidia in Isopods present two types of structure : one of
these is characteristic of the eyes in a majority of the members of this
group ; the other, so far as is known, is represented only in the genus Se-
rolis. These two types will be considered separately, and the one which
is common to the greater number of Isopods will be described first.

The corneal hypodermis in the more common of these two ommatidial
t)pes was first identified by Grenadier. In Porcellio, according to this
author ('79, p. 107), the proximal surface of each facet is covered with
two comparatively thin cells. These are the cells of the corneal hypo-
dermis. Bellonci ('81*, p. 98, Tav. II. Fig. 11 n.) figures similar cells
in the ommatidium of Sphaeroma, and Beddard ('90, p. 3G8) concludes
justly, I believe, that, of the four nuclei found near the distal end
of the cone in Arcturus, two represent cone cells and two cells in the
corneal hypodermis. In Idotea irrorata I have identified two cells in
the corneal hypodermis for each ommatidium. The nuclei of these cells
lie very near the nuclei of the cone cells (compare nl. con. and nl. cm. in
Figs. 50 and 51, Plate V.). In an ommatidium of Porcellio, Grenacher
('79, pp. 107, 108) observed that the plane which separates the two
cone cells also separates the two cells in the corneal hpyodermis. In
Idotea, also, both kinds of cells are separated by a single plane.

The facetted condition of the corneal cuticula of Isopods was observed
as early as 1816 by G. R. Treviranus ('16, p. 64), in wood-lice, and
subsequently in the same animals by Lereboullet ('43, p. 107, '53,
p. 119). The shape of the facets in different Isopods has given rise to
some difference of opinion. According to Miiller ('29, p. 42), in Cymo-
thoa each has the form of a biconvex lens. Leydig ('64% p. 40) states,
however, that in Oniscus the facets are concavo-convex with their hollow
faces innermost. In Asellus, according to the figure given by Sars
('07, Planche VIII. Fig. 14), they are plano-convex with their flat faces

MUSEUM OF COMPARATIVE ZOOLOGY. 85

innermost. These differences, although at first sight somewhat con-
tradictory, are not matters of great importance, for it is probahle that
each time an Isopod sheds its cuticula and a new one is formed, the
lens assumes, at successive stages of its growth, outlines which coincide
very closely with those recorded by the different observers. Thus, an
early stage would be represented by the concavo-convex lens described
by Leydig, an intermediate stage by the plano-convex lens figured by
Sars, and the final condition by the biconvex lens mentioned by Miiller.
Either this is the explanation of the differences, or the observations of
Leydig and Sars are probably erroneous, for the results of the more
recent investigations point to the conclusion that the facets in Isopods
have the form of a biconvex lens. Facets of this shape have been seen
by Grenacher (77, p. 29) in Porcellio, and by Eellonci ('81 a , p. 98) in
Sphseroma. According to my own observations, they also occur in
Idotea, Asellus, Porcellio, and, as I shall show subsequently, in Serolis.
In the four genera mentioned the inner face of each facet is distinctly
convex; this is also true of the outer face in Asellus and Porcellio. In
Senilis and Idotea (Plate V. Fig. 50), however, the outer face is so
slightly curved that it is difficult to decide whether its curvature is that
of the general corneal cuticula or one peculiar to the facet itself.

That the cone in Isopods is composed of two segments was first ob-
served by Leydig ('64 a , p. 41, and '64, Taf. VI. Fig. 8) in Oniscus. Ac-
cording to this author, each segment is spherical. Each ommatidium,
therefore, contains two spheres, and these, as Leydig's figure shows, are
placed side by side immediately below the corneal facet.

It is now well known that in many Tsopods, especially in the wood-
lice, the cone itself is nearly spherical, and its two segments would con-
sequently be hemispheres, not spheres as figured by Leydig. How Ley-
dig's statement of the spherical shape of the segments can be accounted
for, is not apparent. Since the two spheres described by him occupy
the same relative positions as the hemispherical segments of a normal
cone, there is not much question in my mind that they represent these
segments. Possibly their separation and spherical form may have been
due to the swelling action of some reagent which Leydig may have used
to make the tissue transparent. A cone composed of two segments has
been observed by Sars ('67, p. 110) in Asellus, by Leydig ('78, p. 256)
in Ligidium, by Grenacher ('77, p. 29) in Porcellio, by Bellonci ('81 a ,
p. 98) in Sphseroma, by Sye ('87, p. 23) in Ja?ra, and by Beddard
('90, p. 368) in Arcturus. In the three genera which I have examined,
Idotea, Asellus, and Sphseroma, each cone consists of two segments.

86 BULLETIN OF THE

These observations naturally lead to the conclusion that in all Isopods
each cone is composed of two segments. To this general statement,
however, there are two noteworthy exceptions, one recorded by Sars, the
other by Beddard. Sars ('G7, p. 110) has shown that, of the four om-
matidia in each eye of Asellus aquaticus, three have cones composed
each of two segments; in the fourth, however, the cone is divided into
three nails. This observation has been confirmed by Carriere ('85,
p. 155). It is important to observe that in the figure given by Sars
('67, Planche VIII. Fig. 12) the three parts of the cone are not of
equal size; one is about as large as a single segment in the cones of
the other three ommatidia, whereas the remaining two are each about
half as large. In the eyes of the species of Asellus found about Cam-
bridge, the ommatidia are usually twice as numerous as in the European
species, A. aquaticus, and, so far as I could observe, the cones in the
American species were always composed of only two segments. In
Arcturus, according to the figures given by Beddard ("JO, Plate XXXI.
Figs. 1 and 4), cones of three segments are occasionally met with.

The cellular composition of the retinula in Isopods was first made out
by Grenadier ('74, p. 653), who found that in Porcellio this structure
consisted of seven cells. Distally these cells surround the cone; proxi-
mally they are continuous with the optic-nerve fibres. A retinula con-
sist 'iiil: of seven cells has also been demonstrated by Puller ('70, p. 513)
in Cymothoa, and by Beddard ('88, p. 413) in .Ega and Ligia. As
Beddard ('88, Plate XXX. Fig. 13) has shown, the seven cells in the
retinula of .F-a pass through the basement membrane and become con-
tinuous with the nerve fibres. In Porcelho, as I have observed, the
fibrous ends of the seven retinular cells not onty can be identified as nerve
fibres below the basement membrane, but each cell contains a well de-
veloped fibrillar axis (Plate V. Fig. 40, ax. n.), and I therefore conclude
that in Porcellio all seven cells are functional as nervous elements.

In Idotea robusta, transverse sections of the retinula in the region
where the rhabdome is thickest present the outlines of what seem to be
i retinular cells (Plate V. Fig. 48). In positions either distal or
proximal to this, however, only six cells appear. These six cells pass
through the basement membrane and taper into nerve fibres, and their
nuclei, unlike the corresponding nuclei in other Isopods, occur in that
part of the cell which is proximal to the basement membrane (Figs. 49
and 50, nl. rtu'.). The seventh body (Fig. 48, cl. rud.), in those sections
in which it occurs, has in all essential respects the same appearance as
any one of the adjoining six cells. It differs from these, however, in that

MUSEUM OF COMPARATIVE ZOOLOGY. 87

it is usually somewhat smaller, and I therefore conclude that it is a
rudimentary cell. It does not appear to contain a nucleus ; granting,
however, that it is a rudimentary retinular cell, one would look for its
nucleus, not in the region ahout the rhabdome, but in the region of
the nuclei of the other retinular cells, i. e. proximal to the basement
membrane. Owing to the irregularity with which the fibrous ends of
the retinular cells are arranged in this region, I have not been able
to identify any nucleus with this rudimentary cell. Neither have I
found any fibrous projections reaching from the rudiment of the cell
toward the basement membrane such as might be expected provided
the nucleus and a part of the rudimentary cell persisted below the
membrane. Nevertheless, I believe, for the reasons already stated,
that the retinula in Idotea robusta is composed of seven cells, one of
which is extremely rudimentary.

In Idotea irrorata (Plate V. Figs. 53, 55) the retinula consists of only
six cells, all of which possess fibrillar axes, and are therefore probably func-
tional as nervous structures. In one retina of the several pairs of eyes
which I examined, there was a single ommatidium with seven functional
cells ( Fig. 54). With this one exception, however, I have not been able to
find any trace of the seventh cell in Idotea irrorata. In Arcturus, accord-
ing to Beddard ('90, p. 3(38), the retinula is also composed of six cells.

In SphsBroma, Bellonci ('81, p. 98, Tav. II. Fig. 12) has figured and
described a retinula consisting of five cells. These cells alternate with
five other cells, which probably represent accessory pigment cells. If Bel-
lonci's statement is correct, it must be admitted that the number of cells
in the retinular of Isopods may be as few as five. My own observations,
however, do not. confirm Bellonci's account. In the species of Sphreroma
which I have studied, there are seven cells in the retinula, four of which
are large and three small (Plate V. Fig. 58). All these cells pass through
the basement membrane ; all the large ones, and certainly some of the
small ones, are also connected with nerve fibres.

These observations indicate that in the Isopods the retinula is com-
posed of either six or seven cells. If Bellonci's statements prove to be
correct, this structure may be composed in some cases of only five cells,
but my own observations are opposed to this view.

The rhabdome in Isopods presents two types of structure, one of
which has been well described by (Irenacher ('77, p. 30) for Porcellio
scaber. In this species the rhabdome is composed of seven rhabdomeres,
each of which remains in connection with the retinular cell which pro-
duced it. In transverse section the rhabdome has the form of a seven-

88 BULLETIN OF THE

pointed star, a ray corresponding to a rhabdomere. Each ray projects
into its retinular cell, not between two cells. My own observations on
Porcellio confirm Grenadier's statements. A second representative of
this type of rhabdome has been described by Bellonci ('81, p. 98) for
Sphseroma. Here, however, the rays, although they agree in number
with the retinular cells, project between the cells, not into tbem.

The second type of rhabdome is well represented in the eye of Arc-
turns furcatus. In this species, according to Beddard ('90, pp. 3G8,
3G9), the distal portion of the rhabdome, although surrounded by six
retinular cells, is bounded by four perpendicular sides. Each of the six
cells appears from its position to contribute to the formation of the
rhabdome, and yet in the greater part of this structure segments cor-
responding to rhabdomeres are not visible. In its proximal portion,
however, the rhabdome, according to Beddard, is divided into six rhab-
domeres, each of which is applied to its proper retinular cell. In Idotea
robusta the rhabdome (Plate V. Fig. 48, rhb.) is nearly square in trans-
verse section. So far as I have been able to discover, it does not show
at its proximal end any indication of rhabdomeres.

Of these two types of rhabdome, the one in which the rhabdomeres
are evident is probably more primitive than the one in which their in-
dividuality is almost, if not completely h>st.

The retinas of Isopods may contain, in addition to those already
mentioned, two other kinds of cells. Of these the one most frequently
met with fills the space between ommatidia. Cells of this kind have
been identified in Porcellio by Grenadier ('79, p. 107), and it is probable
that the pigment cells described by Bellonci ('81, p. 99) as intervening
between the retinular cells in Sphseroma belong to this class. I have
observed interommatidial cells in Idotea ; here they contain few or no
pigment granules, but are easily recognized by means of their nuclei
(Plate V. Fig. 54, vt. h'drm.).

The source of these cells is not definitely known, but there appears to
be no evidence in favor of their having been derived from outside the
retina. Grenadier believed that those in Porcellio are undifferentiated
hypodermal cells ; this interpretation probably holds good for those in
Sphseroma and Idotea.

The hyaline cells, the second kind of accessory cells, have been iden-
tified by Beddard ('87, p. 235, '88, PI. XXX. Fig. 9, A.) in Mga. and
Cirolana. Since these cells are best developed in the eyes of Serolis, a
full description of their structure will be deferred until the account of
the eyes in that genus is given.

MUSEUM OF COMPARATIVE ZOOLOGY. 89

The cells which characterize the ommatidia in Isopods (except Serolis)
are as follows : cells of the corneal hypodermis, two ; cone cells, two ;
retinular cells, seven, six, or possibly five. Undifferentiated hypodermal
cells are sometimes present, and hyaline cells occur in a few genera.

The structural peculiarities of the ommatidia in Serolis were first de-
scribed by Beddard ('84, pp. 339-341) about seven years ago. Recently
Beddard's observations have for the most part been confirmed by Watase
('90), and it -must now be admitted without question that the ommatidia
in Serolis differ in several important respects from those of many other
Isopods.

The material which I used in studying the eyes in this Crustacean
consisted of advanced embryos and matured individuals of Serolis
Schythei, Liitken. This material was collected in Patagonia by the
Hassler Expedition, and was preserved in strong alcohol. Fortunately,
it was in good histological condition, and sections prepared from it
showed very clearly the finer structure of the eyes. My observations,
as the following account will show, differ in no very important respects
from those of Beddard and Watase.

Although Patten's generalization, that a corneal hypodermis was to be
found in the compound eyes of all Crustaceans, led Beddard ('88, p. 447)
to look for it in Serolis, he was not able to identify it. Watase ('90,
pp. 290 and 293) was more fortunate, and succeeded in finding under
each facet two cells in the corneal hypodermis. I have not been as
successful as Watase was in determining the exact number of hypo-
dermal cells in an ommatidium, but I have seen enough to convince me
that such cells are present. In sections approximately tangential to the
external face of the adult retina, one occasionally finds nuclei (Plate
VI. Fig. 60, nl. crn.) between the distal ends of the cone cells and the
corneal cuticula. These represent unquestionably the cells of the cor-
neal hypodermis, and are not to lie confused with the nuclei of the cone
cells, which lie in a deeper plane. In making sections, the corneal
cuticula splintered so irregularly that the tissue immediately below it
was completely disarranged. It was therefore possible to get only ir-
regular fragments of the tissue in this region, such as Figure 00 shows,
and these fragments were always too small to admit of an accurate
determination of the number of hypodermal cells under a single facet.
I have also been equally unsuccessful in my attempts either to isolate
these cells or to study them in situ on the corneal cuticula.

The eyes in the adinu, owing to the thickness of the cuticula, are
unfavorable for the study of the corneal hypodermis ; but in embryos of

90 BULLETIN OF THE

even an advanced stage, the cuticula is so thin that the hypodermis can
be studied with comparative ease. An ommatidium from the eye of au
advanced embryo is seen in Figure 65 ; the ommatidium is viewed from
the side. Distal to the cone (con.) four nuclei can be seen ; one (ill. cm. 1)
is superficial in position, three are deep. The relation of these nuclei to
the ommatidium can be satisfactorily studied in sections transverse to
the axis of the ommatidium. A series of three such sections is seen
in Figures 66, 67, and 68. Of these, the most distal is that shown in
Figure 66. This includes only the most superficial layer of the retina,
and contains two nuclei (compare nl. cm. 1, in Figs. 65 and 66). These
nuclei, as their position clearly indicates, represent cells of the corneal
hypodermis. In the plane of the section which includes the three deeper
nuclei of Figure 65, four nuclei are in reality present (Fig. 67) ; two of
these (nl. con.) are large, and lie directly below the superficial ones in
the corneal hypodermis; two are small (nl. cm. 2) and lie between the
ends of the deeper large nuclei. Of the deep nuclei, the two large ones
(nl. con.) rest one above each segment of the cone ; in fact, as a section
in a slightly deeper plane shows (Fig. 68, nl. con.), these nuclei coincide
so closely with the segments of the cone that they must be regarded as
the nuclei of the cone cells.

It is difficult to state what nuclei in the adult correspond to the
smaller of the four deep ones in the embryo. The number of these
nuclei (two) in the embryo equals the number of pigment cells which
Watase ('90, p. 294) has described as surrounding the cone ; but that
these nuclei do not belong to such cells is evident from the fact that in
the embryo, the nuclei of the pigment cells can lie identified in a posi-
tion somewhat proximal to that in which the smaller of the four nuclei
occur (compare nl. dst. in Figs. 65 and 69.) Possibly the cells repre-
sented by these small nuclei in the embryo become in the adult the
small interommatidial pigment cells, or it may be that they retain
their relatively superficial positions, and, while occupying the space be-
tween the corneal facets, perhaps produce the cuticula of that region.
In the fragments of the adult retina, from immediately below the cor-
neal cuticula, small nuclei are not unfrequently met with in the spaces
between the ommatidia. These are possibly derived from the smaller
deep nuclei of the embryo.

It will thus be seen that my conclusions concerning the corneal hypo-
dermis agree in the main with those of Watase ; namely, that for each
ommatidium there are two cells in this layer. Besides these, however,
it is possible that the hypodermis may contain an equal number of other

MUSEUM OF COMPARATIVE ZOOLOGY. 91

cells, which occupy positions immediately under the cuticula and be-
tween the ommatidia.

The facets in the corneal cuticula of Serolis, when viewed from the
exterior, are irregularly circular in outline, often approaching a six-sided
form. As I have already observed, they are arranged on the plan of the
hexagonal type. The distal face of each facet is flat, or only slightly
convex ; the proximal face is decidedly convex. The curvatures of the
two faces and the thickness of the cuticula in the facet of S. Schvthei
was about the same as that figured by Watase ('90, Plate XXIX. Fig. 1)
for the species which he studied.

The cone, as Beddard ('84, p. 340) first demonstrated, and as "Watase
('90, p. 290) afterwards confirmed, is composed of two nearly hemi-
spherical segments, which correspond to the two cone cells. The proto-
plasmic material of each cone cell covers the curved surface of the seg-
ment to which it belongs, and contains a nucleus in its distal portion.
These relations have been well shown by Watase ('90, Plate XXIX.
Fig. 1).

From the condition presented even in advanced embryos (Fig. 65)
it is evident that the part of the cone earliest formed, is the one which
is nearest the applied faces of the two cone cells, and that from this as
a centre the cone has continued to increase outwards. Although at this
stage the outline of the cone itself is sharply marked (Fig. 65), the ex-
ternal limits of the cone cells are only approximately indicated by the
distribution of the pigment granules, which have begun to form in the
surrounding pigment cells.

In Serolis, as in Porcellio and Idotea, the cone cells and the cells of
the corneal hypodermis are separated by the same perpendicular plane.
There are some complications in the structure of the cone cells which
can be discussed subsequently with greater clearness.

The retinula in Serolis, as Beddard ('84, p. 340) first observed, is
peculiar in that it is composed of only four cells. My own observations
add almost nothing that is new to the previous accounts of this structure.
The figure which Watase has drawn ('90, Plate XXIX. Fig. 1) of the
characteristic form of the retinular cell when viewed from the side and
its relation to its rhabdomere, reproduces very closely the structural
conditions which I have observed in S. Schvthei.

The rhabdome in Serolis has been carefully studied by Beddard ('88,
pp. 448-450). Owing to the complexity of its structure, one meets with
difficulties in attempting to interpret its parts in terms of the relatively
simple rhabdome of many Crustaceans. The peculiarities of this struc-

92 BULLETIN OF THE

ture can be approached most satisfactorily perhaps from the side of its
adult anatomy.

In a transverse section of the distal end of the rhabdome, five struc-
tures can be observed (Fig. 61). Four of these (Fig. 61, r/ib'm.) are
squarish pieces confluent on one side with a retinular cell, and in contact
with one another only at their angles The sides of these pieces which
are directed towards the axis of the ommatidium are convex, and to-
gether bound a central area which contains the fifth or axial structure
(cl. con.). Each of the squarish pieces also exhibits a line slightly concave
towards the axis of the ommatidium. This line, which might be taken
for the separation between the axial and peripheral structures, is in real-
ity entirely within the latter. That these are five separate structures
is indicated by the fact, that in transverse section, when for any reason
the elements have been broken apart, the separation almost always occurs
on the lines which 1 have described as the limits of the different pieces.

Evidently the squarish masses (r/ib'm.) on the axial faces of the retinu-
lar cell correspond to the rhabdomeres of other Crustaceans, and like these
structures are produced by the cells to which they are attached. It is
more difficult to explain the axial element, for it shows no indication of
having been produced by the surrounding retinular cells, nor are there
other colls in the neighborhood to which its production could be
referred.

"When the longitudinal extent of these structures is considered, the
difficulty of explaining the axial portion is increased. In S. Schythei
the rhabdomeres extend only a short distance distally and proximally,
but throughout the whole of that distance they are closely applied to
the axial face of the retinular cells. This condition has been well figured
by Watase ('90, Plate XXIX. Fig. 1), and supports the statement
alread\' made that these bodies correspond to the rhabdomeres in other
Crustaceans. I have never observed a rhabdomere, such as that figured
by Beddard ('87, p. 234), in which the proximal half of the structure
is not in contact with the retinular cell. The axial part has a much
more considerable extent in a longitudinal direction than the rhab-
domeres. Apparently it is continued proximally into a fibrous bundle
which stretches towards the basement membrane, where according to
Beddard ('88, p. 449) it may terminate as a single fibre.

From what has just been stated it must be evident that the so called
rhabdome of Serolis consists of two sets of structures, one of which
includes the four rhabdomeres and the other the axial part with its prox-
imal fibrous prolongation.

MUSEUM OF COMPARATIVE ZOOLOGY. 93

The development of these structures has been studied by Beddard
('8$, p. 450). In the youngest embryos which he examined, the axial
portion was already formed, and at that stage it was closely invested by
the four retinular cells and two other cells, the hyaline cells. Judging
from their positions, Beddard believes that both kinds of cells may con-
tribute to the formation of the axial structure, although the fact that
this body is squarish in transverse section leads him to conclude that the
four retinular cells play the more important part in its formation. Bed-
dard regards the axial body as the rhabdome of the immature eye. In his
opinion, the rhabdome in the adult is produced by subsequent secretions
from the retinular cells, and presents the form of the four rhabdomeres
already described. Although these rhabdomeres form the principal part
of the rhabdome in the adult eye, he believes that the rhabdome of the
earlier stages persists as the axial fibrous structure in the later stages,
and constitutes perhaps the greater part of its distal continuation
between the rhabdomeres.

Unless some such explanation of the origin of the axial part of the
rhabdome as that proposed by Beddard be accepted, it is difficult to
understand how the fibrous portion could arise as a secretion ; for in the
adult the proximal portion of it is touched by neither retinular nor
hyaline cells.

Granting for the moment the adequacy of Beddard's explanation of
the origin of the axial part, we are still confronted by what appears to
me to be unparalleled in the structure of the eyes in Arthropods, namely,
an ommatidium which produces two distinct rhabdomes. This may not
be an impossibility, but if it occurs at all, it is certainly exceptional.

I believe, however, that the so called axial part of the rhabdome in
Serolis is capable of another interpretation, against which the objections
already suggested cannot be urged. That the axial portion terminates
proximally on the basement membrane has been fairly well established
by Beddard. The distal termination of it, however, has not been so
clearly made out. It is my belief that the axial structure is directly
continuous distally with the cone cells; in other words, that this struc-
ture is to be regarded as a proximal extension of the cone cells, not as
a part of the rhabdome. The termination at the basement mem-
brane of this prolongation of the cone cells, as observed by Beddard,
is perfectly consistent with the interpretation which I have suggested,
and makes the condition in Serolis similar to that in Homarus, where
the fibrous ends of the cone cells also terminate on the basement mem-
brane. That the fibrous structure should be present in the embryo of

94 BULLETIN OF THE

Serolis before the formation of the rhabdome proper is rather in favor
of my interpretation than opposed to it. The direct evidence that the
axial body is a proximal extension of the cone cells is not as conclusive
as could be desired. The condition which most favors this view is as
follows. In longitudinal and transverse sections of the ommatidia, both
in adult and embryonic specimens, no line of separation has been observed
between the protoplasm at the deep end of the cone and the substance
which occupies the axial part of the ommatidium proximal to the cone
(compare Fig. 65). In attempting to determine the true relation, it is
important to keep clearly in mind the fact that the proximal end of the
cone, usually bounded by a sharply marked line, is not the proximal end
of the cone cells ; but; as Watase ('9(3, Plate XXIX. Fig. 1) lias well shown,
the cone is surrounded proximally as well as laterally by the protoplasmic
material of its cells. It is this material, not that of the cone proper,
which forms the proximal elongation.

I had hoped that by isolating the elements of the retina I could ob-
tain more conclusive evidence of the connection of these parts, but my
efforts were of no avail. My ill success was due, I believe, not to any
want of connection between the structures treated, but to the fact that
the material at my disposal had been kept so long in strong alcohol that
it had become unfit to serve for isolation. This conclusion seems to me
to be confirmed by the fact that I was unable even to isolate satisfac-
torily the retinuhe, structures which are usually separable with ease in
the fresh retinas of most Crustaceans.

If the view which I have set forth in the foregoing paragraphs con-
cerning the interpretation to be put upon the axial part of the so called
rhabdome of Serolis be correct, it follows that the true rhabdome of this
Crustacean must be considered as composed of four rhabdomeres, each
of which is applied to the axial face of its appropriate retinular cell,
and that these four rhabdomes are prevented from uniting with one
another by a proximal extension of the cone cells which occupies the
axis of the ommatidium from the cone to the basement membrane.

Beddard ('84% p. 21), in bis account of the eye in S. Schythei, states
that the cone is " enclosed in a sheath of deep black pigment cells," and
Watase ('90, p. 294) has observed that in tins genus there are two such
cells for each ommatidium. I believe that the number has been given
correctly, fir although I have not satisfactorily isolated the cells, I feel
confident that I have identified their nuclei, and the number of these is
twice that of the ommatidia.

The nuclei of these pigment cells are most satisfactorily seen in ad-

MUSEUM OF COMPARATIVE ZOOLOGY. 95

vanced embryos (compare nl. dst., in Figs. Go and G9). In transverse
sections at this stage (Fig. G9) each cone is surrounded by a circle of
six nuclei. Each nucleus, however, participates in three adjoining cir-
cles, consequently there are only twice as many nuclei as ommatidia.
In the adult the nuclei of these pigment cells (Fig. 60, nl. dst.) occupy
the same relative positions as in the embryo ; in the latter, however, they
are usually somewhat hidden by the pigment which surrounds them.

In the embryo the nuclei of the pigment cells surrounding the cone
resemble very closely, except in point of size, the nuclei of the retinular
cells (compare nl. dst. and nl. px. in Fig. 65). In the nuclei of the
retinular cells there is usually one distinct nucleolus, sometimes two, but
as a rule no finer particles. This condition also obtains in the nuclei of
the pigment cells. 2s ot only are the nuclei of these two kinds of cells
similar in the embryo, but they are also much alike in the adult (com-
pare nl. dst. in Fig. 60 with nl. rtnJ in Fig. G3).

Because of this resemblance, I believe that the pigment cells which
surround the cone can be fairlv considered to be modified retinular cells,
which have lost their sensory function in precisely the same way as in the
case of the distal retinular cells in Decapods (see Parker, '90 a , p. 57). If
this interpretation of the pigment cells be accepted, it follows that in
Serolis, as in Decapods, two kinds of retinular cells are present, proximal
and distal, and that the primitive ommatidium from which that of Serolis
was derived probably contained six retinular cells functional as nervous
structures. It need scarcely be added, that this number is characteristic
f>r the ommatidia of many Isopods.

The retinnla in the species of Sphseroma which I studied presents
an appearance which suggests the differentiation of simple retinular cells
into proximal and distal cells. In Sphgeroma there are seven retinular
cells (Plate V. Fig. 58) ; three of these are considerably reduced ; the
remaining four are large, and recall the four retinular cells of Serolis.
In transverse sections it can be shown that the four large cells in Sphse-
roma not only resemble in appearance the four proximal cells in Serolis,
but that they occupy the same relative positions in the ommatidium.
In Serolis the plane which separates the two cone cells of any given
cone, when extended, separates the four proximal retinular cells into two
groups of two cells each (compare Plate VI. Fig. G8 with Figs. 71 and
72). The plane of separation in the cone of Sphseroma divides the retin-
nla by passing through the single small retinular cell shown in the lower
part of Figure 58 (Plate V.) and betiveen the two small cells on the oppo-
site side, thus separating the four large retinular cells into two groups,
as in Serolis.

96 BULLETIN OF THE

The change which would convert an ommatidium like that in Sphse-
roma into one like that in Serolis is easily imagined. It would consist
in the complete abortion of one of the three small retinular cells, and the
conversion of the other two into the pigment cells surrounding the cone.

In addition to the elements which have already been described in the
ommatidium of Serolis, there are certain small pigment cells which oc-
cur for the most part in the region of the retinulse. Beddard ('84 a ,
p. 21) describes these as long branching "connective-tissue cells," a
name which might imply that they originated from the mesoderm, and
were therefore intrusive. AVatase ('90, p. 293, Plate XXIX. Fig. 1) has
also described and figured these cells, but distinctly states his belief that
they are reduced ectodermic cells. In the adult I have observed in the
region of the cones, as well as near the retinulre, certain small nuclei
which are usually surrounded with more or less black pigment. These,
I believe, represent the cells described by Beddard and "vYatase. In the
embryo certain scattered nuclei (nl. h'drm., Figs. 65 and 70) occur in
the spaces between the ommatidia. It is probable that these nuclei are
ectodermic in origin, and I am at a loss to know what has become of
them in the adult, unless they form the pigment cells already men-
tioned. I am therefore inclined to believe, with Watase, that the small
additional pigment cells are reduced ectodermic cells.

The presence of the hyaline cells in the ommatidium of Serolis is, as
Beddard has pointed out, almost a unique feature. These cells, usually
two in each ommatidium, till the space immediately below the rhabdome.
They are bladder-like (Fig. 62, cl. hyl.) and contain each a large gran-
ular nucleus. Although it is stated that there are usually two of these
cells in each ommatidium, I never found more than one to an ommatid-
ium in the several eyes of S. Schythei which I examined. This circum-
stance, however, is not surprising ; for, as Beddard ('84 a , p. 22) has
remarked, the number of these cells is subject to variation, there being
sometimes one, sometimes two, for each ommatidium. In S. Schythei
the single hyaline cell envelops more or less completely the distal part
of the fibrous portion of the cone cells, so that this part seems to pierce
the hyaline cell. A closer inspection, however, will usually show two
lines extending from the fibre to the periphery of the hyaline cell (com-
pare Fig. 62), and these lines indicate, I believe, the two walls of the
cell which have been infolded by the presence of the fibre during the
growth of the hyaline cell.

The source of the hyaline cells is not definitely known. Their nuclei
(Fig. 65, nl. hyl.), as Beddard ('88, p. 450) has observed, are present

MUSEUM OF COMPARATIVE ZOOLOGY. 97

in the retinas of embryos; and, although the cells may possibly be
intrusive, the evidence on the whole favors the view that they are
ectodermic in origin.

Several functions have been attributed to the hyaline cells. Their
close connection with what Beddard took to be the proximal extension
of the rhabdome led him ('88, p. -±50) to suspect that they might be
rudimentary retinular cells, but, as he (p. 451) further remarks, the fact
that no nerve fibres are connected with them opposes this view. Their
transparency suggested to him ('84 a , p. 22) that they might form a part
of the dioptric apparatus ; but it is difficult to understand, consider-
ing their position, precisely what that function would be. I am inclined
to believe, with Watase ('00, p. 293), that they are chiefly concerned
with the support of the structures occupying the basal portion of the
retina.

In the retina of S. Schythei many of the open spaces between the
cones and the basement membrane contain free non-pigmented cells
(Fig. 61, cp. sng.). These have a distinct nucleus, finely granular pro-
toplasm, and a sharply marked outline. On account of the extreme va-
riations in form which the different cells present, it is probable that when
living they exhibited amoeboid motion. In appearance they correspond
exactly to the blood corpuscles of the body spaces, and as they occur not
only in the retina, but also in the rather large openings through the
basement membrane (compare Fig. 6-i), and in the space proximal to
this membrane, I am of opinion that they are blood corpuscles.

The peculiarities which have led me to consider the ommatidium
in Serolis separately from that of other Isopods, are two : the posses-
sion of one or more hyaline cells, and the pi - esence of only four
retinular cells. The latter peculiarity, as I have already shown, is not
fully established : for in this genus, as in many other Isopods, the om-
matidium really contains six cells, although two of these, the distal ones,
are probably no longer functional as nervous structures. The other
peculiarity, the possession of hyaline cells, is not a very important char-
acteristic, for, as Beddard ('87, p. 235) has shown, these cells also occur
in iEga ; and it is probable, moreover, that they must be regarded as
abnormally enlarged elements, specialized from among those cells which
in other Isopods fill the spaces between the ommatidia. What dis-
tinguishes the ommatidium in Serolis from that of other Isopods is,
therefore, not so much the possession of hyaline cells as the fact that
its retinular cells are differentiated into two sets, proximal and distal.

vol. xxi — xo. 2. 7

98 BULLETIN OF THE

In accordance with the facts already presented, the number of cells
contained in the ommatidium of Serolis can be stated as follows : cells
of the corneal hypodermis, two, with possibly two others interomma-
tidial in position ; cone cells, two ; retinular cells, six, two distal and
four proximal ; hyaline cells, one or two ; a variable number of small
pigment cells of ectodennic (?) origin.

Leptostraca.

The histological structure of the ommatidia in the Nebaliie has been
investigated, so far as I am aware, only by Claus ('88, pp. G5-84). I
have had no material for the study of the eyes in these Crustaceans,
and I can therefore only present, in the form of a summary, the more
important results of Claus's exhaustive study.

In Nebalia there is a corneal hypodermis (Clans, '88, pp. 68 and 69),
the cells of which are grouped in pairs. As in many of the higher
Crustaceans, there is one pair of these cells for each ommatidium. The
corneal cuticula is facetted ; the outlines of the facets are circular, and ad-
joining facets are separated from one another by a small amount of inter-
vening cuticula (Claus, '88, Taf. X. Fig. 10). The cones are composed
of four segments (Claus, '88, p. 69). The structure of the retinula is
somewhat complex. The greate'r part of the rhabdome is surrounded
by seven retinular cells. Distal to these cells, however, are seven pig-
ment cells, which enclose the proximal prolongation of the cone cells and
the distal end of the rhabdome. Such a relation between pigment cells
and retinular cells is not of common occurrence among Crustaceans, and
it is possible that the bodies which Claus has taken for pigment cells are
really the distal ends of the retinular cells. Claus describes and figures
what he believes to be the nuclei of both kinds of cells, but I think
his figures fail to show that these nuclei are within the limits of the
cells to which they are said to belong. It seems to me quite possible
that what he has described as two circles of seven cells each may
be merely one circle seen at two different levels, as the correspondence
in numbers suggests. This single circle would be of course composed
of retinular cells, the nuclei of which are probably the distal ones of
the Iwo sets described by Claus. The proximal nuclei, which, accord-
ing to Claus, belong to the retinular cells, occupy positions not unfre-
quently taken by the nuclei of accessory pigment cells, and I am inclined
to think that such is their real nature. This interpretation would be
more in accordance with the conditions found in ommatidia which have
seven retinular cells than is the one given by Claus ; but as I have not

MUSEUM OF COMPARATIVE ZOOLOGY. 99

had the opportunity of studying the eyes in Nebalia, I can offer it
merely by way of suggestion.

Probably two kinds of accessory cells are present in Nebalia ; one of
these extends from the corneal cuticula to the basement membrane, the
other, the presence of which is not so fully established, probably occurs
near the basement membrane.

Cumacece.

Excepting what is contained in Burmester's ('83, pp. 35-37) account
of the degenerate eyes in Diastylis (Cuma) Rathkii, nothing, I believe,
is known of the finer structure of the eyes in the Curnacea^. The speci-
mens at my disposal for the study of these eyes proved upon examina-
tion to be blind. At least, the optic plates of all the individuals which
I examined, both when studied from the exterior and when examined
in sections, showed no evidence of eyes. My material consisted of
specimens of Diastylis quadrispinosa, G. 0. Sars, and of three other un-
determined species, two of which belonged to the genus Diastylis and
one to Eudorella. These were kindly sent me by Prof. S. I. Smith.

Schizopoda.

The species of Schizopod the eyes of which T have studied is My sis
stenolepis, Smith. Specimens of this Crustacean were kindly collected
for me at Wood's Holl, Mass., by Mr. C. B. Davenport. I am also
under obligations to Dr. H. V. Wilson, of the United States Fish Com-
mission, who at my request sent me specimens of this species freshly
preserved in Midler's fluid.

In several of the previous accounts of the eye in Mysis the nuclei
of the corneal hypodermis, although recognized, have been described as
Semper's nuclei, i. e. as nuclei of the cone cells. The differences between
the hypodermal nuclei and those of the cone cells can be easily seen in
Mysis stenolepis (Plate VII. Fig. 73). In this species the hypodermal
nuclei (nl. cm.) lie in a plane somewhat nearer the external surface of
the eye than the nuclei of the cone cells (ill. con.). In transverse sec-
tions at the proper levels, each ommatidium will be seen to contain two
elongated nuclei (Fig. 75, nl. cm.) belonging to the corneal hypodermis,
and two oval nuclei (Fig. 76, nl. con.) in the cone. The hypodermal
nuclei occupy such positions that the plane of separation between their
cells would be at right angles to that between the cone cells (compare
Figs. 75 and 7G). The group of four nuclei, two belonging to the corneal

100 BULLETIN OF THE

hypodermis, and two to the cone cells, correspond without much doubt
to the so called four Seiuper's nuclei mentioned by Claparede ('CO,
p. 194) in Mysis flexuosa, and described by Sars ('07, p. 33) in M. ocu-
lata. Nusbaum ('87, p. 179) also observed four similar nuclei in the
developing eye of Mysis chameleo, and Grenadier ('79, p. 118) described
the same number in Mysis vulgaris. In the last named species, accord-
ing to Grenadier, the four nuclei are grouped in two pairs, one of which
occupies a more distal plane in the ommatidium than the other. The
more superficial pair undoubtedly belongs to the corneal hypodermis, the
deeper pair to the cone cells.

It must be evident, then, that the nuclei of the cone cells and corneal
hypodermis have not always been carefully distinguished. In all cases
where they have been separated, the corneal hypodermis has been shown
tb possess two nuclei for each ommatidium.

The corneal cuticula in Mysis, as Frey and Leuckart ('47*, p. 113)
first pointed out, is facetted, and the outline of the facet is a circle.
In Mysis stenolepis the circumference of the facet is tangential to the
circumferences of six adjoining facets (Fig. 74). In Mysis vulgaris,
Grenadier ('79, p. 118) has shown that the facet is not lens-like, but is of
uniform thickness throughout. In M. stenolepis. however, the cuticula
is often slightly thicker at the middle of the facet than at its edges (Fig.
73, eta.). In this respect, therefore, different species probably vary.

The cones in Mysis vulgaris, according to Grenadier ('79, p. 118), are
composed of two segments. The same number is also present in the
cones of M. stenolepis (compare Figs. 7G-78, con.). In longitudinal sec-
tions the cone (Fig. 73, con.) appears to consist of a uniformly and finely
granular substance enveloped in a delicate but distinct membrane.
Near the distal end of the cone the material which composes it becomes
more coarsely granular ; in this the nucleus of the cone cell is usually
lodged. Cones (Fig. 92) which have been isolated in macerating fluids
are plumper and apparently not so contracted as those which have been
subjected to the process of cutting. The nuclei also are rounder and fuller.
The cone proper (Fig. 92 con.) occupies a more central position in the
cone cells, and is surrounded by a finely granular material, which is es-
pecially abundant at the proximal end. The difference between the cone
proper and this granular material was not generally observable in sections
of the cones. In all of the many cones which I succeeded in isolating,
the proximal ends invariably had a broken appearance. Consequently, I
believe that I have never completely isolated a pair of cone cells. The
question of the proximal extent of the cone I shall recur to later.

MUSEUM OF COMPARATIVE ZOOLOGY. 101

The retinular cells in Mysis are of two kinds, proximal and distal.
The proximal cells extend from the basement membrane distally to
the level at which the cone rapidly contracts. The pigment which they
contain is for the most part concentrated around the rhabdome, and
their nuclei occupy a distal position in the cell (Fig. 73, nl.px.).

In Mysis the number of cells comprising the retinula is at least seven
(Figs. 85-87). Possibly, as I have elsewhere suggested (Parker, '90 tt ,
p. 55), the total number of cells in this retinula, as in that of Homarus,
may be eight.

In order to determine this question, I have counted the number of
nuclei in several retinuloe of Mysis. The enumeration of these can be
easily followed in Figures 79 to 82. These figures represent successive
transverse sections through four ommatidia, in the region occupied by
the proximal retinular nuclei. The axis of each ommatidium is marked
by the fibrous portion of the cone cells (cl. con.), and the same omma-
tidium is designated in different sections by the same Roman numeral.
The nuclei in ommatidium II. can be counted the most readily. In
Figure 79, which represents the most distal section of the series, the
cone in ommatidium II. is surrounded by a circle of six nuclei, which
have been numbered from 1 to 6. Each of these nuclei, however, par-
ticipates in three circles (compare nucleus 5), and hence only two of the
six can be referred to ommatidium II. Two similar circles occur, one
in the sections shown in Figure 80, and one in that shown in Figure 81.
As in the former instance, two nuclei in each circle belong to omma-
tidium II. In these three circles, then, there are in all six nuclei to be
allotted to ommatidium II. In addition to these nuclei, it will be no-
ticed that to the right of the cone in Figure 80 there is one more
nucleus (No. 7), and still another in a similar position in Figure 82.
These two nuclei, when added to the six already summed up for om-
matidium II., make a total of eight nuclei for this ommatidium.

The same number of nuclei occurs in each of the other three omma-
tidia, but their arrangement is not quite so regular as in the one just
counted. From this I conclude that the number of nuclei in a retinula
of Mysis is eight.

The different nuclei in this retinula usually present a very uniform
appearance. The most proximal one differs somewhat from the others
in being more elongated (compare Figs. 73 and 82). The seven distal
nuclei, on account of their general resemblance, belong, I believe, to the
seven functional retinular cells. The single proximal nucleus probably
represents an eighth rudimentary cell. The position of this nucleus,

102 BULLETIN OF THE

proximal to the other retinular nuclei, is similar to that occupied by the
nucleus of the rudimentary retinular cell in Homarus (compare Parker,
'90 a , pp. 20,21).

The rhabdome in Mysis stenolepis lies ir the proximal portion of the
retina. It is rather stout, blunt at its distal end, but sharper proxi-
mally (Fig. 90). Its surface is marked with coarse corrugations. In
transverse section, its outline is a square ; this is subdivided by two
lines into four smaller squares, a condition already observed by Grena-
dier ('79, p. 119) in M. flexuosa. The relation of the retinular cells
to these divisions of the rhabdome can be clearly seen in Figure 87.

According to Grenadier's account ('79, p. 118), a rod-like structure
extends, in Mysis vulgaris and M. flexuosa, through the axis of the
ommatidium from the distal end of the rhabdome to the region of the
proximal retinular nuclei. "Whether this rod be a proximal continuation
of the cone, or a distal extension of the rhabdome, Grenadier found it
difficult to decide. He is inclined, however, to the former opinion.

A similar structure occurs in the ommatidia of Mysis stenolepis.
Although I have made repeated attempts, I have never succeeded in
isolating the rod in connection with either the rhabdome or the cone
cells. In transverse sections, the distal end of it appears in a position
slightly proximal to the retinular nuclei (Figs. 73 and 83). The cone
cells extend proximally as a transparent axis to this region, and the
most distal indications of the rod are four fibres which lie on the
periphery of what I take to be the proximal end of the cone cells
(Fig. 83). Somewhat deeper than this, the four fibres thicken, and
finally fuse (Fig. 84), producing a body which in transverse section has
the outline of a four-pointed star. In a plane slightly more proximal,
the outline changes to a squarish one (Fig. 85), and this is retained
almost to the proximal end of the rod. Throughout its extent, this
problematic rod is closely surrounded by the seven proximal retinular
cells (Fig. 85). It is separated from the rhabdome by what appears to
be an open space (Fig. 90, at the level of the dotted line 8G). In trans-
verse sections (Fig. 86), however, this space is seen to be divided by
delicate membranes into four compartments.

These facts, however, do not aid much in deciding the relationship
of the rod. The fact that it shows indications of being composed of
four parts suggests its connection with the rhabdome. The four parts
of which it consists do not, however, correspond in position to the seg-
ments of the rhabdome, but fall between them. (Compare Figs. 83 and
87.) On the other hand, if it were an extension of the cone, one would

MUSEUM OF COMPARATIVE ZOOLOGY. 103

expect it to be composed of two, instead of four parts. Its position, bow-
ever, is one which is mure frequently occupied in other Crustaceans by
a slender extension of the cone cells than by a process from the rhab-
dome, and, notwithstanding its division into four parts, I am inclined
to agree with Grenadier, and to regard it as belonging to the cone cells
rather than the rhabdome.

The distal retinular cells in Mysis surround the lateral faces of the
cones (Fig. 73, cl. dst). Apparently they reach the cuticula : their
proximal ends are attenuated and become lost in the region of the
nuclei of the proximal cells. Their pigment is limited to their proximal
halves, and consists of a distal layer of brownish material, proximal to
which is a much more extensive deposit of blackish granules. Each cone
is surrounded by six of these cells, as can be seen from their outlines
(Fig. 78, cl. dst.), and still more satisfactorily from the arrangement of
their nuclei (Fig. 75, nl. dst.). Each cell, however, participates in three
circles ; consequently, there are only twice as many of these cells as
ommatidia.

The axis of each distal retinular cell is occupied by a transparent
rod, which in transverse section has the appearance of a light spot
(Fig. 77). In depigmented sections stained with Kleinenberg's hema-
toxylin, these rods are deeply colored (Fig. 78). I shall recur to their
probable significance.

The pigment which is found in the region of the rhabdomes in Mysis
is of two kinds : blackish granules, and a fine flaky material, white by
reflected light, yellowish by transmitted light. The black granules are
for the most part contained in the retinular cells. The lighter pigment
is always associated with certain nuclei, two of which are shown in
Figure 90 (nl. ms'drm.). These nuclei are closely invested by the pig-
ment, and probably belong to the cells in which the pigment is con-
tained.

The source of the yellowish pigment cells is not easily determined.
Apparently they are not limited to the retina, but also occur in the
spaces below it. At least these spaces contain masses of pigment and
nuclei which in all essential respects are similar to those distal to the
membrane (compare the two nuclei, nl. ms'drm., Fig. 90). In one case
the nucleus of one of these cells was found apparently caught in its
passage through an opening in the basement membrane (Fig. 91). For
these reasons I believe that the yellowish pigment cells on the two sides
of the membrane have had the same origin. The question as to the
source of the yellowish pigment cells in the retina, therefore, appears

104 BULLETIN OF THE

to me to involve that of the origin of the similar cells beneath the
retina. If I am right in this conclusion, all these cells must either have
arisen in the retina, many of them migrating in a proximal direction
out of it, or they must have had some extra-retinal origin, some of them
migrating into it. On account of the considerable numbers in which they
exist in the spaces below the retina, it seems to me much more probable
that they have had an extra-retinal origin than that they have come
from the retina itself. If this is their source, it is evident that those
which are in the retina are intrusive. The nucleus which has already
been mentioned as caught in an opening of the basement membrane
(Fig. 91) has more the appearance of a body which is making its way
into the retina than of one which is moving in the reverse direction,
and may therefore be regarded as confirming to some extent the view
of the extra-retinal origin of these cells. Their source, however, cannot
be stated with certainty. Their power of migration implies amoeboid
activity, and this might be taken as an indication of their mesodermic
origin.

The following cells characterize the ommatidium of Mysis : cells of
the corneal hypodermis, two: cone cells, two; proximal retinular cells,
eight, one of which is rudimentary ; distal retinular cells, two; accessory
pigment cells (mesodermic 1) present.

Stomatopoda.

The material which I have had for the study of the eyes in the Stoma-
topods consisted of two specimens of Gonodactylus chirarga, Latr. These
were kindly given me by Mr. \V. S. Wadsworth, who had collected them
in the Bermudas. One of them had been killed in hot water and pre-
served in alcohol ; the other was both killed and preserved in strong
alcohol ; both were in excellent histological condition.

In Gonodactylus, as I have previously mentioned, there are two kinds
of ommatidia ; these differ in no important respect except size.

Longitudinal sections of both kinds are represented on Plate VIII. ;
the figure of the larger kind (Fig. 94) is taken from a depigmented sec-
tion, that of the smaller one (Fig. 95) from a section containing the
pigment in its natural condition. In the following description I shall
give an account of the structure of the larger ommatidia, alluding to
the condition of the smaller ones ouly when it differs in some important
respect from that of the others.

The corneal hypodermis is represented in the ommatidium of Go-
nodactylus by two cells, the nuclei (Figs. 94-9G, nl. cm.) of which can

MUSEUM OF COMPARATIVE ZOOLOGY. 105

be recognized easily. Directly under the corneal cuticula each pair of
hypodermal cells is in contact with similar pairs belonging to adjoining
ommatidia, so that, the layer here forms a continuous sheet. In a more
proximal plane the neighboring pairs of hypodermal cells are not in con-
tact (compare Fig. 93, a tangential section in which the extreme right-
hand edge represents the condition immediately below the cuticula, while
the parts to the left represent central portions successively more proxi-
mal in position). The only indication of a separation between the two hy-
podermal cells of each pair is seen in the distal projection of the cone
between the two hypodermal nuclei (compare Figs. 94 and 96, con.).

The corneal cuticula in Gonodactylus is facetted, but the proximal and
distal faces of the facets are apparently plane. Over the smaller om-
matidia the facets are hexagonal in outline, whereas over the larger ones
they are rectangular, and their arrangement is often indicative of the
tetragonal system. In Squilla mantis, according to Will ('40, p. 7), the
facets are hexagonal.

The cones in Gonodactylus are composed for the most part of a uni-
formly granular substance. Distally, they are pointed and probably
touch the corneal cuticula; proximally, they terminate at the rounded
end of the rhabdome (Fig. 94). Each cone contains in its distal enlarge-
ment four nuclei (Fig. 97, nl. con.), two of which lie directly proximal
to the nuclei of the corneal hypodermis, while the remaining two alter-
nate with them (compare Figs. 96 and 97). The proximal part of the
cone is divided longitudinally into four segments (Fig. 98). Each seg-
ment, if extended distally, would include one of the four nuclei, and
corresponds to one of the four cells by which the cone was produced.
In Squilla mantis, according to Steinlin ('6$, p. 17), the cone is also
composed of four segments.

The retinular cells of Gonodactylus are of two kinds, proximal and
distal. The proximal cells, constituting the retinula itself, surround the
rhabdome completely, and extend distally only a short distance beyond
it (Fig. 95). They contain only a small amount of pigment, which is
concentrated in two regions, at their distal ends and near the basement
membrane. The rhabdome is surrounded throughout its length by a
thin but rather dense layer of pigment. This layer is more extensive
in the smaller ommatidia (Fig. 102) than in the larger ones. The
nuclei of the proximal retinular cells (Figs. 94 and 95, nl. 2^x.) are
located near their distal ends.

The number of cells in the retinula of Squilla, as described by Grena-
dier ('77, p. 33) and by Hickson ('85, p. 341, Fig. 2), is seven. In

106 BULLETIN OF THE

Gonodactylus (Fig. 101) the retinular cells are certainly as numerous
as in Squilla ; but seven obvious cells in the retinula, as I have already
shown in Mysis, may suggest the presence of eight in all, one of them
being rudimentary. This conditiou is in fact characteristic of Gonodac-
tylus also, as can be seen in the series of ommatidia shown in Fig. 100.
These six ommatidia represent consecutive individuals in one of the
bands of larger ommatidia previously mentioned. The band as a whole
is cut obliquely, and in such a way that the ommatidia from 1 to (i are
cut successively in deeper or more proximal planes. In ommatidium 1
the rhabdome is surrounded by seven retinular cells, four of which are
upon the right side and three upon the left. In addition to these, a
large nucleus (nl. px.) lies close to the rhabdome. Ommatidium 2 lias
essentially the same structure as ommatidium 1. In ommatidium 3 the
nucleus corresponding to the one seen in ommatidium 1 and 2 is no longer
visible, but in its stead there is a small mass of granular protoplasm.
A similar mass is also seen in ommatidia 5 and G. It is usually pres-
ent directly proximal to the nucleus figured in ommatidia 1 and 2, and
is, I believe, the protoplasmic body of the cell to which this nucleus
belongs. In ommatidium 4, the seven nuclei of the seven large (func-
tional) retinular cells can be seen. These nuclei appear very large in
transverse section compared with the cells in which they occur. It is
probable that the cell wall is distended by them, although, owing to the
indistinctness of the cell boundaries, I have not obtained positive evi-
dence of this. In ommatidium G the seven retinular cells are seen in
section at a plane proximal to that in which their nuclei lie. As in
ommatidium 1, three of them arc upon one side of the rhabdome
and four upon the other. In a part of the ommatidium more proxi-
mal than that shown in number G (Fig. 100), the transverse section
of the retinula has the appearance seen in Figure 101. Here the
retinular cells have the same relation to the rhabdome that they do in
ommatidium G (Fig. 100), except in the case of the upper right-hand
cell of that figure. This cell enlarges in its more proximal portion, and
comes to occupy a position directly below the cell whose nucleus is shown
in ommatidium 1 (Fig. 100). The gradual disappearance of this distal
cell as one proceeds in a proximal direction from the plane of number
6, Figure 100, to that of Figure 101, and the gradual shifting in the
position of the cell which replaces it proximallv, can be followed so
easily that there is not the least question as to the accuracy of the
relations described. It is evident, then, that in Gonodactylus, as in Mysis,
the retinula consists of eight cells, one of which is rudimentary.

MUSEUM OF COMPARATIVE ZOOLOGY. 107

The rhabdome (Figs. 94 and 95, rhb.) in Gonodactylus is an elongated
rod-like structure of uniform thickness, which extends from the region
of the proximal retinular nuclei to the basement membrane. It shows
a distinctly toothed edge (Fig. 94), especially in specimens which have
been treated with potassic hydrate. In transverse section it is squarish.
Owing to its small size, the exact relation of the seven surrounding cells
to its four faces cannot be easily determined. The single unpaired cell
(Fig. 101) certainly lies opposite a face, not an angle. In this respect
it agrees with the unpaired cell in Squilla as figured by Grenadier (79,
Taf. XL Fig. 122). Probably in Gonodactylus the remaining six cells
are related to the sides of the rhabdome as the corresponding ones are
in Squilla (compare Grenadier's Fig. 122). In Gonodactylus the retinu-
lar cells and rhabdome are in close contact with one another. The
separation of these elements as figured by Grenadier in Squilla is prob-
ably artificial, as Grenadier himself suggests. In Squilla, according to
both Steinlin ('68, p. 17) and Grenadier ('79, p. 125), the rhabdome
in transverse sections is subdivided into four equal parts, somewhat as
in Mvsis. I have not observed this condition in Gonodactvlus.

The distal retinular cells in Gonodactylus occupy the usual position
near the cones. They contain very little pigment, and their number
can be determined only by that of their nuclei. These agree with the
nuclei of the proximal cells in the possession of a single well defined
nucleolus, which is most readily seen in depigmented sections (compare
nl. dst. and nl. px. in Fig. 94). The distal nuclei, especially in the
region of the larger ommatidia, are arranged in rows which alternate
with the rows of cones (Fig. 99, nl. dst.). Although the nuclei are not
very definitely arranged, they often show a tendency to be grouped in
pairs, and these pairs are so placed that in each row there is evidently
one for each adjacent ommatidium. Moreover, in equal lengths of ad-
joining rows of nuclei and cones, the nuclei are always double the num-
ber of cones. I am convinced by these facts that there are two distal
retinular cells for each ommatidium.

Besides the cells already described, certain others occur in the proxi-
mal part of the retina in Gonodactylus. These are represented by a
few small, elongated nuclei (Fig. 94, nl. ms'drm.), which are very similar
in appearance to certain nuclei occurring in the spaces below the base-
ment membrane. I therefore believe that in Gonodactylus. as in Mvsis.
the proximal portion of the retina is occupied by intrusive cells, which
are probably mesodermic in origin.

The kinds of cells found in the ommatidium of Stomatopods are as

108 BULLETIN OF THE

follows : cells of the corneal hypoderniis, two ; cone cells, four ; proxi-
mal retiuular cells, eight, one of which is rudimentary ; distal retinular
cells, two ; accessory cells (mesodermic ?) present.

Decajwda.

I have studied the eyes of the following species of Decapods : Gelasi-
mus pugilator, Latr. ; Cardisoma Guanhumi, Latr. ; Cancer irroratus,
Say ; Hippa talpoida, Say ; Palinurus Argus, Latr. ; Pagurus longicarpus,
Say; Homarus americanus, Edw. ; Cambarus Bartonii, Fabr; Crangon
vulgaris, Fabr. ; and Pala)monetes vulgaris, Say. I collected much of
this material at the Station of the United States Fish Commission at
Wood's Holl, Mass. The specimens of Cambarus were obtained in the
vicinity of Philadelphia. I am under obligations to Mr. Herbert M.
L'ichards for specimens of Palnemonetes collected by him at Newport,
P. I. A number of eyes of two Crustaceans, Cardisoma and Palinurus,
were kindly obtained for me by Mr. Isaac Holden ; the}- were collected
on the coast of Florida by Mr. Palph Munroe, to whom I am indebted
for the careful way in which they were preserved.

The corneal hypodermis in Decapods was first recognized by Patten
('8G, pp. 626 and 642), who observed it in Fenams, Palsemon, Pagurus,
and Galathea. Since Patten's announcement of the presence of this
layer in Decapods, it has been identified in a number of other genera:
in Crangon by Kingsley ('86, p. 863), in Alpheus by Herrick ('86, p. 43),
in Astacus by Carriere ('89, p. 225), in Cambarus and Callinectes by
Watase ('90, pp. 297 and 299), and in Homarus by myself ('90 fl , p. 6).
More recently I have observed it also in Palrcmonetes (Plate IX. Fig.
103, cl. cm.), Crangon, Cambarus, Palinurus, Pagurus, Hippa, Cancer,
and Cardisoma.

In almost all Decapods in which the arrangement of the cells in the
corneal hypodermis has been observed, these elements have been found
to be grouped in pairs, and so distributed that each pair occupies the
distal end of an ommatidium (compare Figs. 103 and 106, Plate IX.).
This arrangement has been observed, either by others or by myself, in
the genera mentioned in the preceding paragraph, except Callinectes,
in which the exact arrangement of the cells has not been recorded.
Reichenbach's statement ('86, p. 91), that in Astacus there are four
hypodermal cells under each facet, is probably erroneous, as Carriere's
observations show.

Although Patten was the first investigator who clearly demonstrated
the presence of the corneal hypodermis in Decapods, Grenacher, in 1879,

MUSEUM OF COMPARATIVE ZOOLOGY. 109

described, I believe, the nuclei of this layer, without however correctlj
interpreting them. In his account of the ommatidium in Palcemon,
Grenacher ('79, p. 123) mentions two kinds of bodies in what he takes
to be the distal ends of the cone cells. Of these, the more distal ones
(Taf. XL Fig. 117, n.) represent, in his opinion, the nuclei of the cone
cells; the more proximal (Fig. 117, Kk l .) he considers as differentiated
parts of the cone itself. The positions occupied by these bodies in
Palamon, and by certain bodies which I have observed in Pakeinonetes
(Plate IX. Fig. 103), are so similar that I believe the structures in the
two genera to be homologous. In Paleemonetes the distal bodies lie in the
cells of the corneal hypodermis (Fig. 103 cl. cm.), and are the nuclei of
these cells. They represent what Grenacher considered the nuclei of the
cone cells in Palaemon. The proximal bodies in Palsemonetes (Fig. 103,
nl. con.) are unquestionably the nuclei of the cone cells, yet they corre-
spond to what Grenacher considered the four pieces of the distal segment
of the cone. I therefore believe that what Grenacher has described as the
nuclei of the cone cells are really the nuclei of the corneal hypodermis,
and that what he considered distal segments of the cone are the nuclei
of the cone cells.

The corneal cuticula in Decapods, in correspondence with the differ-
entiated condition of the corneal hypodermis, is facetted. The outline
of the facets is either hexagonal or square. The particular genera in
which these different kinds of facets occur have already been mentioned
in dealing with the arrangement of the ommatidia in Decapods. The
faces of the facets in Decapods are usually very nearly plane, but in
Palaemon according to Grenacher ('79, p. 123), and in Palaemonetes
(Plate IX. Fig. 103, cm.) according to my own observations, the facets
are slightly biconvex. In Homarus, as Newton ('73, p. 327) has ob-
served, and in Astacus according to Carriere ('85, p. 167), the distal
surface of the facet is plane, the proximal slightly convex. In even
the most extreme cases, however, the convexity of the facets in Decapods
is not sufficient to make them very effective as lenses.

The facets in Decapods are generally bisected by a fine straight line.
This line, as Patten has suggested, probably represents the plane of
separation between the two subjacent hypodermal cells. In the square
facets this line either divides the facet diagonally, as in Homarus
(Parker, '90 a , Fig. 2), or transversely, as in Palaemonetes (Plate IX.
Fig. 105). In the hexagonal facets it either bisects opposite sides, as in
Cancer (Plate X. Fig. 120), or unites opposite angles, as occasionally in
Galathea (Patten, '86, p. 644, Plate 31, Fig. 114). Leydig's ('57, p. 252,

110 BULLETIN OF THE

Fig. 134) figure of Astacus, in which each facet is suhdivided hy two
diagonal lines into four areas, and Newton's ('73, p. 327) statement
that the same condition occurs in Homarus, are probably incorrect.

The cones in Decapods are composed of four segments. This number
was first observed by Will ('40, p. 13) in Palsemon, and has since been
recorded in many other genera. So far as I am aware, there are no
Decapods in which the number of segments is not four. As Claparede
('60, p. 194) first pointed out in Galathea and Pagurus, each segment
contains a nucleus and represents a single cell. Although the signifi-
cance of these nuclei was without doubt first fully appreciated by
Claparede, it is probable that they were previously seeu by Leydig
('55, Taf. XVII. Fig. 31) in the crayfish.

As a rule, the distal termination of the cone cells is on the proximal
side of the corneal hypodermis. In the lobster, however, and in Palae-
monetes (Plate IX. Fig. 104), the pointed ends of these cells pass
between the two cells of the corneal hypodermis, and probably come
in contact with the corneal cuticula near the middle of a facet.

It is difficult to determine with accuracy the proximal termination of
the cone cells. They can be easily traced to a region immediately distal
to the distal end of the rhabdome. In this region, as Schultze ('G8,
Taf. I. Figs. 9 and 11) has clearly demonstrated in Astacus, the fibrous
ends of the four cone cells separate, and pass partially around the rhab-
dome. In Homarus, these fibres extend proximally, and finally ter-
minate at the basement membrane. A similar method of termination
also occurs in Palinurus. In the other genera which I have studied, the
fibres, although visible near the distal end of the rhabdome, are lost in
the adjacent tissue, and I do not know whether they terminate in this
tissue without special attachment, or whether they make their way as
excessively fine fibres to the basement membrane. The separation of
the fibrous ends of the cone cells, near the distal end of the rhabdome,
has been observed by Steinlin ('GG, p. 93) in Palrcmon, and by Schultze
('G7 and '68) in several other Decapods. The statement made by many
of the older investigators, and recently reaffirmed by Patten, that the
cone and rhabdome are parts of one continuous structure, is without
doubt incorrect.

The resolution of the retinula into its cellular constituents was first
attempted in Decapods by Leydig ('55, p. 408), according to whom the
retinula of Herbstia contains four cellular bodies, the nuclei of wdiich
can be distinguished in the distal part of the structure. A somewhat
similar condition was described by Newton ('73, p. 333) for Homarus ;

MUSEUM OF COMPARATIVE ZOOLOGY.. Ill

in this genus, as in Herbstia, it was maintained that there were only
fuur cells. Subsequent investigators have not confirmed this conclusion.
In transverse sections of the retinula of Palaemon, Grenacher ('77, p. 32)
has demonstrated that the rhabdome is surrounded by seven retinular
cells. He also ('77, p. 33, and '79, p. 125) observed the same number
in the retinulae of Astacus and Portunus. Since the publication of
Grenadier's observations, a retinula containing seven cells has been
seen in Astacus by Carriere ('85, p. 169), in Penseus, Palsemon, Gala-
thea, and Pagurus by Patten ('86, pp. 630 and 643), and in Cambarus
by Watase ('90, p. 299).

In Homarus, as I ('90 a , p. 21) have already shown, the retinula con-
tains, in addition to the seven functional retinular cells, an eighth rudi-
mentary one, which is little more than a nucleus. In order to ascertain
the presence or absence of this eighth cell in other Decapods, I have
been careful to record the number of retinular nuclei, as well as the
number of functional retinular cells. In some genera, such as Cardisoma
and Hippa, I have not been able, on account of the unfavorable condition
of the tissue, to make this determination ; but in Palasmonetes, Palinurus,
Cambarus, Crangon,,and Cancer, I have succeeded in ascertaining the
number both of the functional cells and of the nuclei in the retinula).

In Paloemonetes each rhabdome is surrounded by at least seven re-
tinular cells (Plate IX. Fig. 114, cl. px.). The nuclei of these cells
usually lie slightly distal to the rhabdome (Fig. 104, nl. px.). Their
arrangement is shown in Figures 110, 111, and 112, which represent a
series of consecutive sections through the region occupied by the prox-
imal retinular nuclei of five ommatidia. The nuclei of the different
ommatidia are arranged upon the same plan, and the corresponding
nuclei in the different sets have been marked by the same number. In
several instances, nuclei have been cut in two, and their parts are found
in consecutive sections ; in such cases the separate portions have been
marked with the same number. As can be seen in these figures,
the number of nuclei in the distal portion of each retinula is seven.
But in addition to these, there is also another one, which occupies a
position near the rhabdome. This nucleus resembles the others in all
respects except that it is somewhat longer and narrower. It is drawn
in Figure 103 at the level marked 114, and in Figure 114 one can see
the regularity with which it occurs. This nucleus is the eighth in the
retinula of Pakx-monetes, and since it differs somewhat in structure from
the other seven, and occupies a more proximal position, I believe it rep-
resents a rudimentary retinular cell.

112 BULLETIN OF THE

la the distal portion of the retinula in Cambarus there are eight
nuclei. The arrangement of these, as seen in successive transverse
sections, is shown in Plate X. Figs. 118 to 122. In Figure 118, which
represents the most distal section of the series, there are four nuclei,
and these are so arranged that there is evidently one for each omma-
tidium. 1 In the next section (Fig. 119) there are seven nuclei, none
of which were seen in Figure 118 ; the place for an eighth is indicated
by an open area, and the eighth nucleus itself is seen somewhat out of
place in Figure 120 (x). Four of the eight nuclei belonging in Figure
119 are arranged in a manner similar to those in the preceding sec-
tion, but are not to be confounded with them. The remaining four
are so placed that there are two for each ommatidium. Hence in this
plane there are, as a whole, three times as many nuclei as there are
ommatidia. In the next section (Fig. 120), omitting the nucleus
marked x, which has been recorded as belonging to the preceding
section, there are four nuclei, so arranged that there is one for each
ommatidium. In the following section (Fig. 121) the nuclei, omit-
ting the one marked x, which will be considered as belonging to the
next following section, are so arranged that there are two for each
ommatidium. In the last section (Fig. 122), the nuclei are not so
regularly grouped as in the previous section, but when taken with the
nucleus marked x in Figure 121, they constitute a group of four, the
arrangement in which is such that each nucleus is intermediate between
four groups of cone cells rather than between two, and therefore in the
plane of this section there is one nucleus for each ommatidium. From
this enumeration it is evident that the total number of retinular nu-
clei is eight ; namely, one in the first section, three in the second, one
in the third, two in the fourth, and one in the fifth. The structure

1 The nuclei shown in Figures 118 to 122 are arranged upon either the plan
shown in Figure 118 or that in Figure 121 (omitting nucleus x). Imagine the
arrangement in Figure 118 extended over a large surface. The groups of four
cone cells could then he regarded as forming lines in the direction of the length
of the plate. These lines would alternate with lines of nuclei, and as the nuclei
in an}' line would alternate with the groups of cone cells in an adjoining line, the
number of nuclei must equal exactly the number of groups of cone cells ; i. e. in
this arrangement there is one nucleus for each ommatidium. In a similar way,
alternating vertical lines may be constructed from the arrangement in Figure 121.
One line would be composed entirely of nuclei situated one opposite each group
of cone cells ; the other, of alternating nuclei and groups of cone cells. In the
former, as well as in the latter, there would be as many nuclei as groups of cone
cells. Hence, in this arrangement the nuclei are twice as numerous as the groups
of cone cells ; i. e. there are two nuclei for each ommatidium.

MUSEUM OF COMPARATIVE ZOOLOGY. 113

of these nuclei affords no clue as to which oue belongs to the rudi-
mentary cell.

In Palinurus (Plate X. Fig. 125, nl. px.), the eighth nucleus is regu-
larly present and easily seen. In Cancer (Fig. 129, nl. px. 8) it occu-
pies a position between the adjacent retiuulae. It can also be identified
in Crangon.

The retinula? in Decapods, according to all recent observers, contain
seven functional cells. In Homarus, Palinurus, Cambarus, Crangon,
Pakemonetes, and Cancer, the retinulae contain, in addition to the
seven nuclei of the functional cells, an eighth nucleus, which repre-
sents, I believe, a rudimentary cell. It is probable, therefore, that in
all Decapods each retinula really contains eight cells, one of which is

rudimentary.

The rhabdome in Decapods presents a very uniform structure. It is
usually an elongated body, pointed both at its distal and its proximal end,
and completely covered, except at its distal tip, by the proximal retinular
cells. In those Decapods in which it is large enough to be conveniently
observed, its transverse section is squarish, and usually subdivided by
two straight lines into four smaller squares (Plate IX. Fig. 113). As
Grenacher ('77, pp. 31, 32) first demonstrated in Palaemon, the retinular
cells are rather peculiarly arranged around the rhabdome. One of its
four sides is flanked by one cell, the other three by two cells each. This
arrangement can be seen in Palsemonetes (Fig. 113), and probably obtains
for all Decapods.

In Palinurus Argus (Plate X. Fig. 124) there appears to be no rhab-
dome, unless the translucent axial portion of each retinular cell can
be said to represent segments of it. The fibrous ends of the cone cells
(cl. con.) can be easily identified between the retinular cells, but the
centre of the retinula is filled with pigment, and shows not the least trace
of a rhabdome. This peculiarity of Palinurus was noticed as early as
1840 by Will ('40, p. 15), who described the ommatidium in this genus
as being without a transparent mass (= rhabdome).

Although the distal retinular cells in Decapods were collectively rec-
ognized by Midler ('26, pp. 355, 356) some sixty years ago as a definite
pigment band in the distal portion of the retina in the crayfish, they
were not identified as separate cells until quite recently. The first in-
vestigator to observe them was Carriere ('85, p. 169), who described
them in Astacus as a pair of pigment cells flanking each cone. In Cam-
barus, Crangon, and Homarus, they also cover the sides of the cone, and
in the last named genus they are produced proximally into long fibres,

vol. xxi. — no. 2. 8

114 BULLETIN OF THE

which perhaps pass through the basement membrane. In Palocmonetes
(Plate IX. Fig. 108, cl. dst.) and in Cancer (Plate X. Fig. 127, cl. dst.)
they are reduced to pigmented threads, which, starting from comparatively
large- bases, twine around the lateral surfaces of the cones.

The arrangement and number of the distal retinular cells can be most
readily determined from their nuclei. In Cancer (Plate X. Fig. 128)
the cells are arranged in circles of six around each group of cone cells ;
each cell, however, participates in three circles, and consequently there
are in reality only twice as many cells as ommatidia. This arrangement
of the cells also occurs in Cardisoma, Hippa, and Pagurus. In Crangon
(Fig. 123), as I have previously remarked, the nuclei of the distal retinu-
lar cells are arranged in rows alternating with the rows of cones. There
are twice as many nuclei as cones ; hence I conclude that here also
there are two distal cells for each ommatidium. In Homarus, Palinurus,
Cambarus, and Pala;monetes (Plate IX. Figs. 103 and 109, nl, <lst.) the
nuclei are grouped distinctly in pairs, one pair for each ommatidium.

Each cone in Penajus, according to Patten ('8G, p. G34), is surrounded
by two pairs of pigment cells, and Watase ('90, p. 299) states that in
Cambarus the dioptric part of the ommatidium is sheathed by four pig-
ment cells. In Cambarus Bartonii I have been able to find only two
such elements, the pair of distal retinular cells already described, and in
the other Crustaceans which I have studied I have observed nothing
which supports Patten's statement concerning the four pigment cells in
Penseus. I am therefore inclined to doubt the accuracy of these two
observations.

The interommatidial space in the basal part of the retina in Pala;-
monetes contains a light pigment similar to that described in the retina
of Mysis. Like this the pigment in Palaemonetes is white by reflected
light, and yellowish by transmitted light (compare Plate IX. Fig. 115).
It is apparently contained within cells (Fig. 103, cl. ms'drm.) whose out-
lines are very irregular, and whose nuclei (Fig. 104, nl. ms'drm.) are
small and somewhat variable in form. These cells occur on both sides of
the basement membrane. As in Mysis, they have probably migrated
into the retina, and are perhaps mesodermic in origin. They have been
seen by Carriere ('85, p. 169) in Astacus, by Patten ('86, p. 636) in
Penseus, and by myself ('90 a , p. 25) in Homarus. I have also recently
observed them in Crangon, Cambarus, Cardisoma, Pagurus, and Pali-
nurus, as well as in Paleemonetes.

From what has preceded it is evident that the ommatidium in Deca-
pods contains the following elements : cells of the corneal hypodernus,

MUSEUM OF COMPARATIVE ZOOLOGY.

115

two ; cone cells, four ; proximal retinular cells, eight, one of which is
rudimentary; distal retinular cells, two ; accessory cells, mesodermic (1)
in origin, often present,

Table of Ommatidial Formulae.

I have now concluded my account of the structure of the ommatidia
in Crustaceans, and for the purpose of presenting in a condensed form
its more important features I have devised the following table. This
consists of a series of ommatidial formulae constructed upon the plan
which I have described in the Introduction. The figures indicate the
numbers of particular kinds of cells present in the ommatidium of a
given group. The abbreviation pr. (present) marks the presence of any
kind of cell when the number of that kind is not constant for diflerent
ommatidia in the same individual.

Table showing the Cellular Composition of the Ommatidian

Crustaceans.

Groups of Crustaceans.

Cells of
Corneal

Ilypo-
dermis.

Cone
Cells.

Retinular Cells.

Accessory
Cells.

Undiffer-
entiated.

Differentiated.

Proxi-
mal.

Distal.

Amphipoda,

pr.

2

5

pr. (ect.'')

Branchiopodidaa and Apusidae,

2

4

5

Estheridae,

pr.

5(4)

5

Cladocera,

?

5

5

pr. (ect.?)

Copepoda : Pontella,

Sappliirina,
Argulus,

pr.
i

pr.

2
i

4

5
3
5

pr. (ect. ?)
i

Isopoda : Idotea,

Porcellio,
Serolis,

2

2

2(+1)

2
2
2

6

7

4

2

pr. (ect.?)
pr. (ect. ?)
pr. (ect. ?)

Nebaliae,

2

4

7

pr. (ect.?)

Schizopoda,

2

2

7 + 1

2

pr. (mes. ?)

Stomatopoda,

2

4

7 + 1

2

pr. (mes. ?)

Decapoda,

2

4

7 + 1

2

pr. (mes. "*)

A few features in the table require explanation. Among the number
of cells recorded for the Estheridas, the figure within the parenthesis

116 BULLETIN OF THE

under the head of Cone Cells indicates the occasional occurrence of cones
containing only four cells, although the usual number is five. In the
line fur Serolis, under the head of Corneal Hypodermis, the parenthesis
and included signs are intended to indicate the possibility of there being
more than two cells in the corneal hypodermis for each ommatidium.
In the Schizopods, Stomatopods, and Decapods, the number of prox-
imal retinular cells is expressed in the form of 7 + 1 instead of 8, be-
cause one of the cells is rudimentary.

The Innervation of the Retina.

The innervation of the retina in the compound eyes of Crustaceans is
chiefly interesting, because of its importance in relation to physiological
questions. As this paper deals with a morphological topic, it would be
obviously irrelevant to enter upon any extended discussion of this sub-
ject. Nevertheless, the innervation of the retina is not without some
bearing on the general question which I have set for myself, and I shall
therefore not pass it by, but put in as brief a form as possible what I have
observed concerning it.

In my account of the retina in the lobster, I described the optic-
nerve fibres as terminating in the proximal retinular cells. Near the
ganglion each fibre consists of a bundle of fibrils, simply enclosed
within a sheath, but as it approaches the retina it becomes coated with
pigment. The pigment increases in quantity and the fibre correspond-
ingly enlarges till it finally becomes continuous with the deeply pig-
mented retinular cell. The fibrillar axis can be distinguished in the
pigmented portion of the fibre as a transparent axial structure, and it
can also be traced distally through the pigment of each retinular cell
till it breaks up into its ultimate fibrillar, which are spread over the dis-
tal half of the rhabdome. This is the method of nerve termination in
the lobster, and points very conclusively to the rhabdome as the termi-
nal organ.

What I have seen of the termination of the nerve fibres in other
Crustaceans confirms the account which I have already given for the
lobster. In some species which I have studied, owing to the small size
of the retinal elements, 1 was unable to determine the cells with which
the nerve fibres connected. The termination of the fibres in the cells
of the retinula was observed, however, in the following genera : Bran-
chipus, Limnadia, Pontella, Gammarus, Talorchestia, Idotea, Porcellio,
Spheeroma, Serolis, Gonodactylus, Mysis, Palaemonetes, Crangon, Cam-

MUSEUM OF COMPARATIVE ZOOLOGY. 117

barus, Palinurus, Pagurus, Cancer, and Cardisoma. In the majority of
these, a fibrillar axis could be distinguished. 1 In Cambarus, as in Homa-
rus, the nerve fibrillar spread over the distal portion of the rhabdome.

In Serolis an exceptionally interesting condition is presented. At the
level of the basement membrane each retinular cell contains a large fibril-
lar axis (Plate VI. Fig. 64, ax. n.). This becomes somewhat subdivided
in the more distal portion of the cell, and in the region of the retinular
nucleus it is represented by a cluster of several smaller axes (Fig. 63).
At the level of the hyaline cell, these however cannot be distinguished
(Fig. 62), but the scattered condition of the pigment granules in this
plane is probably to be accounted for by the presence of many separate
fibrils in the substance of the cell. In the region of the rhabdome an
immense number of fine lines can be seen extending from the retinular
cell into the substance of each rhabdomere (Fig. 61). These, I believe,
represent the fibrils of the nervous axis. They have been previously
observed in Serolis by Watase ('90, p. 291), and are so readily visible
that there can be no question as to their presence. Each fibril is per-
pendicular to the longitudinal axis of the ommatidium, and extends
through the rhabdomere to its axial surface. Before reaching this,
however, the fibril passes through what seems to be a delicate mem-
brane. When closely examined, this membrane often has the appearance
of a row of dots instead of a line, and in several cases I have been unable
to discover any traces of it. What its significance is, I am at a loss to
say. As I have previously observed, when the elements of the retinula
are separated the rhabdomere shows no tendency to break along this line.
Since the structure is pierced by the fibrils, and does not appear to be
a natural plane of rupture, and since sometimes it is apparently absent,
I believe it may be considered, from a morphological standpoint at least,
as a secondary and rather unimportant modification within the rhabdo-
mere itself.

If I am correct in maintaining that the nerve fibrils in Serolis terminate
in the rhabdomere, it is probable that they have a similar method of
ending in all other Crustaceans, and in such instances as Homarus,
where they have been traced only to the surface of the rhabdome, their
actual termination has probably not been seen.

1 A definite fibrillar axis was traced from below the basement membrane to the
region of the rhabdome in Gammarus (Plate I. Figs. 6-S), Porcellio (Plate V. Fig.
46), Idotea (Plate V. Figs. 53 and 55-57), Mysis (Plate VII. Figs. 87-80), Gono-
dactylus (Plate VIII. Figs. 101, 102), Palsemonetes (Plate IX. Figs. 116, 117), Cam-
barus, Pagurus, Cancer (Plate X. Figs. 130 and 131), and Cardisoma.

118 BULLETIN OF THE

The termination of the fibrillac of the optic nerve in the rhabdome
supports Muller's belief that the nerve fibres terminate in a region near
the proximal ends of the cones, and Grenacher's more specific view that
they are connected with the retinular cells, and that the rhabdome is the
terminal organ. This method of termination is not consistent with the
opinion of Gottsche and Leydig, that the cone is the terminal organ,
nor with Patten's rather similar belief that the ultimate nerve fibrilhe
are distributed to the cone. I am therefore compelled to think that
these authors are mistaken in their conclusion.

Theoretic Conclusions.

In attempting to account for the variation in the number of cells in
different types of ommatidia, two courses naturally suggest themselves.
Either the different kinds of ommatidia vary in the number of cells
which they contain, because they have had separate origins, or they are
different because in some or all of them the ancestral ommatidium has
suffered modification. An examination of the table on page 115 shows
conclusively, I think, that in Crustaceans even the most extreme types
are so little removed from one another that it is much more probable
that the different kinds of ommatidia are genetically connected, than
that they have been produced independently. Granting this statement,
the question naturally arises, What are the means by which the primi-
tive ommatidium was modified? I believe that a close scrutiny of the
cellular structure of the ommatidia in living Crustaceans will disclose
some of the factors in this process. There are at least three of these to
be distinguished : the differentiation of cells, the suppression of cells,
and the increase in the number of cells by cell division.

By the differentiation of cells, I do not mean the process by which
hypodermal cells have become converted into retinular or cone cells,
but that by which an element already differentiated in the ommatidium
is secondarily modified to subserve another function. The only instance
of this kind with which I am acquainted occurs among the retinular cells.
In the majority of the simpler Crustaceans, the sides of the cones are
covered with pigment, which is almost always contained in the distal ends
of the retinular cells. In Serolis, among the Isopods, and apparently
in all the genera of Ptomatopods, Schizopods, and Decapods, the cones
are surrounded by special pigment cells. These are always twice as
numerous as the ommatidia, and represent, I believe, retinular cells
which have become differentiated for the special purpose of sheathing

MUSEUM OF COMPARATIVE ZOOLOGY. 119

the cones. The way in which this differentiation may have occurred
has already been suggested in my paper on the lobster ('90 a , p. 57).

Although I have expressed the opinion that these cells are to be re-
garded as modified retinular cells, it might be maintained that they are
merely enlarged accessory pigment cells, such as occur in the inter-
ommatidial space of many Crustaceans. But I believe such an interpre-
tation of these cells would be erroneous, for the following reason. In
Serolis the nuclei of the pigment cells which surround the cone (Plate VI.
Fig. 65, nl. dst.) possess one, and sometimes two, well marked nucleoli,
but no fine chromatine granules. In this respect they closely resemble
the nuclei of the proximal retinular cells (jiLpx.), and differ consider-
ably from those of the accessory pigment cells (nl. h'dnn.). The nu-
clei of the last named cells contain only fine granules. So far, then,
as their nuclei are concerned, the distal retinular cells bear a much
closer resemblance to the proximal cells than to the accessory pigment
cells. Each retinula in Serolis contains, moreover, only four cells, and
in this respect differs considerably from other Isopods, where the number
of retinular cells is either six or seven. On the supposition that the
pigment cells surrounding the cone in Serolis are accessory pigment
cells, one would be called upon to account for the exceptionally small
number of cells in the retinula of this genus ; whereas, if the cells
around the cone are regarded as modified retinular cells, they may be
taken to indicate for Serolis a primitive retinula composed of six cells,
a number characteristic of the retinula; in other Isopods. This inter-
pretation of the condition of the retinula in Serolis is borne out by
what is known of the retinula in Sphseroma, where, it will be remem-
bered, a transition between the condition in Serolis and that in other
Isopods was distinctly indicated.

In the Stomatopods, Schizopods, and Decapods, if my observations
are correct, there are no ectodermic accessory pigment cells. Conse-
quently, a comparison between these cells and what I have called the
distal retinular cells cannot be drawn. In Mysis (Plate VII. Fig. 73),
Gonodactylus (Plate VIII. Fig. 9-t), and Palsemonetes (Plate IX. Fig.
103), as well as in all other Decapods which I have examined, the resem-
blance between the nuclei of the retinular cells and those of the pigment
cells which surround the cone is as striking as in Serolis, and suggests
the origin of these cells from retinular cells rather than from any other
source. In Homarus, the pigment cells around the cone present a con-
dition of some interest in this connection. Each pigment cell is extended
proximally as a long fibre, which certainly reaches nearly to the base-

120 BULLETIN OF THE

ment membrane, and probably passes through it in company with the
fibrous ends of the retinular cells (compare Parker, '90 a , pp. 17-19).
Admitting that these cells are merely modified accessory pigment cells,
such a condition as this is quite unintelligible to me ; but granting
them to be differentiated retinular cells, their fibrous extensions can
be easily explained as the rudiments of the fibrous portion of the cell
with which the nerve fibre was once connected. A somewhat similar
case occurs in Mysis, where the centre of each of the pigment cells
which surround the cone contains a small transparent axis. This axis
in every respect except that of connection with a nerve fibre corresponds
to the fibrillar axes described in the functional retinular cells of this
Crustacean (compare Plate VII. Figs. 77, 78, and 87). Consequently,
the axis in the distal cells either represents a rudimentary nervcis axis,
in which case the cell containing it must be regarded as a retinular cell,
or it is something for which I can suggest no explanation.

These facts lead me to conclude that the pigment cells which sur-
round the cone in Serolis, the Stomatopods, Schizopods, and Decapods,
are to be regarded as modified retinular cells, and I have therefore
described them under the name of distal retinular cells, in contrast to
proximal retinular cells, or those which retain their primitive position
around the rhabdome. In the differentiation of a group of simple
retinular cells into proximal and distal cells, the latter necessarily
change their function from that of terminal nervous organs to that of
screens chiefly concerned in excluding the light from the sides of the
cones. Wherever the distal retinular cells occur, they afford evidence,
I believe, that the structure of the ommatidium has undergone a modi-
fication from the primitive ommatidial condition.

The second method by which the structure of ommatidia may be
changed, namely, the suppression of cells, is perhaps the one whose
presence is most easily detected because of the frequent persistence of
the partially reduced cells. These rudimentary cells can be identified
most readily in the cases where they belong to groups in which the
number of elements is constant for different ommatidia. I know of no
evidence of suppression among the groups of cells in the corneal hypo-
dermis or the cones. Among the retinula3, however, it seems to be of
rather common occurrence. The first indication of this process is natu-
rally a diminution in the size of the cell to be suppressed. Such a step
is perhaps shown in the retinula of Gammarus (Plate I. Fig. 6), where
one of the five cells, although evidently functional, is nevertheless con-
siderably reduced. Without much doubt, the body described in the

MUSEUM OF COMPARATIVE ZOOLOGY. 121

retinula of Idotea robusta represents, for reasons already stated, the
seventh cell present as a functional structure in Porcellio. In Idotea
irrorata the retinula?, with very few exceptions (Plate V. Fig. 54), contain
only six cells, showing no trace of the seventh cell. This condition, I
believe, is to be interpreted as one in which a cell has been completely
suppressed. In Stomatopods, Schizopods, and Decapods the retinula?
have been shown to contain, in addition to the nuclei of the seven func-
tional cells, an eighth nucleus, which may represent a rudimentary cell.

In all of the cases thus far cited, it might be maintained that what I
have considered rudimentary cells are really cells newly acquired by the
ommatidia, and not old cells gradually undergoing suppression. The con-
dition in Idotea, however, where the body in question apparently contains
no nucleus, would be difficult to explain on this assumption, whereas, if
it be considered a cell undergoing reduction, its condition can be easily
accounted for. In Stomatopods, Schizopods, and Decapods, the con-
stancy in the number of cells and in the position of the eighth nucleus,
the small amount of protoplasm which surrounds it, and the striking
resemblance which it has to the other retinular nuclei, are facts difficult
to explain on the assumption that it represents a newly acquired cell,
but easily accounted for on the supposition that it is the remnant of a
partially suppressed cell. For these reasons, I believe that the instances
cited are valid cases of partial suppression, and that this must be regarded
as one of the actual means employed in the modification of ommatidia.

That ommatidia have been modified by an increase in the number
of their cells by cell division, is a proposition not easily established.
The difficulty of obtaining conclusive evidence on this point can be
made clear by an example. Let it be assumed that cones composed
of two cells are converted by the division of the cells into cones com-
posed of four cells. This step, even when first taken, would probably
be accomplished during the embryonic growth of an animal, and there-
fore before the cones themselves had begun to be differentiated. . What
would actually happen would probably be this : the two cells, the
homologues of which in all previous animals had given rise to two cone
cells, would in this case each divide, thus producing a group of four
cells, which ultimately would form a cone of four segments. If we
could compare the adult animal in which such a process had occurred
for the first time with its immediate ancestors, the only important
difference that would be observed would be in the number of the
cells in each cone, and if the genetic relations of the two individu-
als were not known, it could not be stated with certainty whether in

122 BULLETIN OF THE

one case we were dealing with an animal which had lost two cone cells
or in the other, with one which had gained two ; in other words, it
would be impossible to determine which of the two conditions was the
primitive one. The importance of embryological evidence in determin-
ing this question must therefore be apparent. But evidence from even
this source might not be conclusive. Thus in the development of the
lobster I have traced in detail the steps by which the ommatidia are
formed, and although in this Crustacean the considerable number of
cells in each ommatidium would warrant one in expecting some evidence
of increase by division, the division of the cells in the retina is entirely
accomplished some time before these elements show any grouping into
ommatidia. Hence, the exact method of origin of the cells of the om-
matidium cannot at present be given. I have observed that the same
is also true in Gammarus ; cell division is completed before the cells are
grouped into ommatidia. Perhaps in the development of some other
Crustaceans evidence of the kind which I have sought may be obtained,
but in the few species which thus far have been studied the evidence
has not been produced.

Although the supposition that ommatidia may increase the number
of their cells by the division of those which they already possess is not
supported by any direct observations with which I am acquainted, there
are some facts recorded which are indirectly confirmatory of it. Thus,
in Phyllopods, an increase in the number of cone cells appears to accom-
pany a progressive differentiation of the retina itself. In this group, as
I have already pointed out, the simplest condition of the retina is found
in Branchipus and Apus. From the retina of Apus that of the Estheridse
can be easily derived, and the retina in the Estherida) represents a con-
dition from which the retina of the Gladocera may have arisen. That
this series of retinas, from Apus through the Estherida) to the Cladocera,
is a natural one is abundantly proved by the course taken in the develop-
ment of the eye in these groups. If we regard the condition of the
cones in these Crustaceans, we shall find that in the most primitive
retina, that of either Branchipus or Apus, they consist of four cells;
that in the more complex retina of the EstheridaB they are usually com-
posed of five cells, although cones of four cells are not unfrequent occur-
rences ; and finally, that in the Cladocera they are always composed of
five cells. Apparently in this series the development of the retina is
paralleled by a corresponding development in the cones, whereby one
composed of four cells is ultimately converted into one with five cells.
Since the resemblance between any two of the cells in a cone composed

MUSEUM OF COMPARATIVE ZOOLOGY. 12

Q

of five elements is quite as close as that between the cells in cones con-
taining only four elements, I believe that the additional cell, which has
increased the number of segments from ftnxr to five, has been derived by
the division of one of the original four cone cells, and not from an extra-
ommatidial source.

Another instance of this kind occurs among the Isopods. The cones
in this group, it will be remembered, are usually each composed of two
segments. According to Beddard's figures ('90, Plate XXXI. Figs. 1
and 4) in Arcturus, however, they occasionally consist of three segments,
and in Asellus aquaticus, according to Sars ('67, p. 110), although three
of the four cones in each eye are composed of only two segments each,
the fourth regularly contains three. The size of the segments in the
fourth cone differs ; two are small, and together their bulk about equals
that of the third, and the last is approximately of the size of a segment
in one of the other cones. If we attempt to explain the condition of the
cone composed of three segments by supposing it to have been produced
by adding to the normal pair of cone cells a single cell from some source
external to the ommatidium, we are met with the difficulty, that what
is apparently the added cell — the larger one — resembles more closely
a segment in the other cones than do either of the two remaining cells,
although the latter must on this assumption represent the original seg-
ments. If, however, we imagine the small segments to have arisen by
the division of a single larger one similar to the large one which remains
in the cone, the relation of the resulting segments both in size and num-
ber is a perfectly natural one. This explanation, therefore, seems to me
to be more probable than the former. For these reasons, I believe that
an increase in the number of cells in an ommatidium takes place by the
division of the cells already forming a part of that ommatidium, rather
than by the importation of new elements hitherto foreign to the om-
matidium.

The conclusion which I would draw from the preceding discussion is,
that there are at least three means of modifying the numerical formulae
of ommatidia, all of which involve only the cells primitively belonging
to the ommatidium, and therefore do not necessitate the introduction
of new cells from extra-ommatidial sources. They are cell differentia-
tion, cell suppression, and cell multiplication.

Having now determined the means by which the cellular structure of
the ommatidia in living Crustaceans is modified, we are prepared to ap-
proach the question of the structure of the primitive ommatidium. If
it could be shown that ommatidia were modified only by increasing the

124 BULLETIN OF THE

number of their elements, it would naturally follow that those com-
posed of the fewest cells would more nearly resemble the ancestral type
than those which consist of many cells. On the other hand, if the sup-
pression of cells were the only means employed in modifying structure,
the ommatidia containing the greatest number of elements would most
nearly approach the primitive type. Since, as I believe, both means
are employed in the Crustacea, the determination of the structure of
the ancestral ommatidium is evidently a difficult problem. Perhaps
the most satisfactory way of attempting its solution is to consider sep-
arately the different categories of cells which enter into the formation of
an ommatidium, and, after reviewing the conditions presented by each
in different Crustaceans, to determine, if possible, which of these condi-
tions is the most primitive. The conclusions thus arrived at concerning
each kind of cell will afford the necessary grounds for the construction
of an hypothetical formula of the ancestral ommatidium. Although it
is not necessary that this ommatidium should be represented in any liv-
ing Crustacean, for the ommatidia in all these may have suffered modifi-
cation, yet it is possible that a representative of it may still exist.

Turning now to the consideration of the different groups of cells, we
find that the corneal hypodermia presents two conditions; one in which
its cells are not regularly arranged, and another in which they are
grouped in pairs, each pair lying at the distal end of an ommatidium.
The latter condition is characteristic of the Decapods, Schizopods, Sto-
matopods, NebalisB, Isopods, and some Branchiopods; the former, so far
as is known, occurs in the Amphipods, the Branchiura, and in some
Branchiopods (Limnadia and some species of Branchipus). In view of
the fact that the corneal hypodermis is a part of the retina which re-
tains the function of the general hypodermis but slightly modified, and
that in the latter the cells do not present a regular arrangement, it
is probable that a corneal hypodermis in which the cells are not regu-
larly arranged is of a more primitive character than one in which they
are definitely grouped.

The number of cells in the individual cones of Crustaceans varies
from two to five. Cones composed of two cells occur in Eucopepoda,
Amphipods, Isopods, and Schizopods; cones of three cells are present
only exceptionally in Isopods ; cones of four cells are found in the
Decapods, Stomatopods, Xebalise, Branchiura, and some Branchiopods ;
cones of five cells characterize the Cladocera and some Branchiopods.
I have already given reasons for regarding the cones composed of three
cells as having been derived from those containing two, and cones com-

MUSEUM OF COMPARATIVE ZOOLOGY. 125

posed of five cells from those possessing four. Since there is no evidence
of degenerate cells in any of the cones composed of two segments, I am
convinced that cones with four cells are derived from those with two cells,
and not the reverse. On these grounds, I conclude that the most primi-
tive form of cone in living Crustacea is that consisting of two cells.

The retinular cells in Crustaceans are subject to considerable varia-
tion. As I have previously shown, an onimatidium may contain one or
two kinds. When there is only one kind, all the cells are grouped
around the rhabdome, and are known simply as retinular cells. When
there are two kinds, one occupies a position around the rhabdome, and
the other around the cone ; the former I have called proximal retinular
cells, the latter distal retinular cells. Proximal and distal retinular
cells occur in Serolis, the Stomatopods, Schizopods, and Decapods ;
simple retinular cells apparently characterize the ommatidia of all other
Crustaceans. I have already presented reasons for considering the
distal retinular cells as modified simple retinular cells, which, in the
separation of the cone from the rhabdome by the elongation of the
ommatidium, have lost their connection with the nervous element, but
have retained their place next the dioptric one. A group of retinular
cells in which this differentiation has occurred is not so primitive in its
structure, therefore, as one in which all the retinular cells retain their
original position around the rhabdome, as in the groups of Crustacea
which possess simple retinular cells.

The number of simple retinular cells in Crustacean ommatidia varies
from five to seven. In Nebalia, and some Isopods, the retinula con-
tains seven cells ; in other Isopods it is composed of six cells, and in
the Branchiopods, the Cladocera, some Copepods, and Amphipods it
consists of five cells. It is difficult to state which of these numbers
represents the primitive condition. In the Isopods, as I have previ-
ously indicated (pp. 8G and 87), there is considerable evidence to
show that a retinula composed of six cells has been produced from one
containing seven by the suppression of one cell. Possibly in this way
the retinula with five cells was derived from that with six, but I know
of no observations which favor this supposition.

A small amount of indirect evidence on this question is to be ob-
tained from the other structural peculiarities of the ommatidia con-
taining retinulae with five, six, or seven cells. These retinulse occur
in connection with two kinds of rhabdomes, — one in which the rhab-
domeric segments are easily distinguishable, and the other from which
they are apparently absent. Of these two kinds, the one in which the

126 BULLETIN OF THE

segments persist is evidently more primitive than the one in which their
outlines are obliterated.

Probably in Nebalia, in which the retinula is composed of seven cells,
and certainly in Idotea, where it consists of six, the rhabdome shows
no indication of being composed of rhabdomeres, but in Porcellio the
seven retinular cells surround a rhabdome composed of a corresponding
number of rhabdomeric segments. In Brauchipus, the retinula consists
of five cells, but the rhabdome is apparently not composed of separable
rhabdomeres, whereas in Pontella, Argulus, Gammarus, Talorchestia,
Hyperia, and Phronima the five retinular cells are each represented by
a rhabdomere. The more frequent occurrence of a primitive condition
of rhabdome with the retinula having five cells than with that having
seven, favors indirectly the idea that the retinula with the smaller
number of cells is the more primitive of the two. The types of cones
associated with the two kinds of retinula) offer almost no evidence on
the question in hand. Thus, a retinula of seven cells is associated with
a cone of four cells in Nebalia, and with one of two cells in Porcellio,
and a retinula of five cells is combined with a cone of four cells in
Brauchipus and Argulus, and with one of two cells in Amphipods. The
relation of the two kinds of retinula) to the corneal hypodermis affords
some slight evidence in support of the opinion that the retinula of five
cells represents the more primitive type ; for although the differentiated
type of corneal hypodermis — the one in which the cells are regularly
arranged — may occur with either type of retinula, the undifferen-
tiated hypodermis — in which the cells are not regularly grouped — is
known to be associated only with retinula) containing five cells (some
Branchiopods, Argulus, and Amphipods). The evidence drawn from
these various sources is obviously very slight ; but such as it is, it indi-
cates that the retinula with five cells, rather than that with a greater num-
ber, represents the more primitive condition. This conclusion receives
some additional support from the fact that the retinula composed of
five cells characterizes the ommatidia in a number of not otherwise very
closely related Crustaceans (Pontella, Argulus, the Branchiopods, and
Amphipods), whereas the type possessing seven cells occurs only among
certain Isopods and in the Xebalise. I believe, therefore, that all the
evidence at present deducible from the condition of the simpler retinula)
indicates that the one which contains five cells is more primitive than
that composed of six or seven cells.

In the present argument I have purposely omitted any mention of the
condition of the retinula in the Corycaadae, those Copepods in which the

MUSEUM OF COMPARATIVE ZOOLOGY. 127

lateral eyes present a highly modified condition. I have done this be-
cause I believe that the lateral eyes in many Copepods are degenerate,
and that therefore the evidence to be drawn from them cannot be as
trustworthy as that from other sources. That the lateral eyes in Cope-
pods are degenerate, is shown from the fact that in many members of
the group the eyes are entirely absent, and that in those in which they
do occur, their structure is subject to considerable variation. Thus iu
Pontellathe retina contains, besides one group of five retinular cells, three
isolated nervous cells, whereas in Sapphirina there is a group of three
retinular cells, and at least one isolated nervous cell. In Pontella, Sap-
phirina, Corycseus, and Copilia each retina is provided with a single lens,
but in Irenseus, according to Claus ('63, Taf. II. Fig. 1), there are two
lenses in each eye. These variations, including the total disappearance
of the organ in some members of the group, lead me to believe that the
lateral eyes in the Copepods are degenerated, and therefore are organs
in which the suppression of cells may have reduced them to even a
simpler condition than that presented by the ancestral ommatidium.

The conclusion which I draw from the preceding argument is, that the
type from which the ommatidia in all living Crustaceans are probably
derived would exhibit the following structures : a corneal hypodermis in
which the cells are not regularly arranged, and consequently an un-
facetted corneal cuticula ; a cone composed of two cells ; a retinma com-
posed of five retinular cells and having a rhabdome which consists of
five rhabdomeres. The retina of the primitive eye, a simple thickening
in the superficial ectoderm, would be composed of ommatidia of this
type arranged upon the hexagonal plan. None of the Crustaceans
with which I am acquainted possess an eye of exactly this structure.
The one in which this condition is most nearly represented is perhaps
Gammarus. In this animal all the requirements of the hypothetical
eye are fulfilled, except that the form of the retina as a whole is some-
what disturbed by the separation of the corneal hypodermis from the
layer of the cones and retinuloe by a corneo-conal membrane, and by the
partially disguised condition of the basement membrane.

If my conclusions be correct concerning the structure of the primitive
ommatidium and the means by which it has been modified, it follows
that the principal types of ommatidia have been produced mainly by
increasing the number of cells in the primitive type, and that, of the
three means of modifying the structure of ommatidia, cell division has
been the most influential.

Although the hypothetical ommatidium which has been described in

128 BULLETIN OF THE

the preceding paragraphs has been spoken of as ancestral, it is not to be
supposed that the condition which it presents must be regarded us necessa-
rily its simplest form. I feel tolerably confident, however, that the prim-
itive ommatidium must have been at least as simple as I have assumed
it to be. Possibly its retinula may have been composed of less than five
cells, as is that seen in some Copepods ; although, as I have previously
remarked, the condition of the lateral eyes in these Crustaceans is
probably influenced by degeneration, and therefore may not represent a
primitive stage. What might be regarded, however, as a more primitive
form of ommatidium than that which I have described, may be seen
in the eye of the Chaetopod Nais (Carriere, '85, pp. 28, 29). In this
worm the eye lies in the hypodermis on the side of the head, and con-
sists of a few relatively large transparent cells, the proximal faces of
which are in part covered by pigment cells. It is probable that the
transparent cells are merely dioptric in function, and that the pigment
cells are nervous. The transparent cells may therefore be looked upon
as the forerunners of cone cells, and the pigment cells at their bases as
retinular cells not yet differentiated into a retinula. It is not difficult
to imagine the origin of an ommatidium from a single one of the trans-
parent cells and its accompanying pigment cells, and, by an increase in
the number of such groups, the production of a retina like that of the
compound eye of Arthropods.

This view of the orig'n of the ommatidia in Arthropods is irreconcila-
ble with that recently advanced by Watase ('90), according to whom
each ommatidium is to be regarded as a pit formed by an involution of
the hypodermis. The supposed cavity of this pit occupies nearly the
whole length of the axial portion of the ommatidium, and is filled by
the secretions of the cells constituting its wall. The secretion in the
deeper part of the pit forms the rhabdome ; that which is produced
nearer its mouth, the cone. During the formation of the pit, the hypo-
dermal cells are believed to retain such mutual relations that their moi*-
phologically distal ends lie next its cavity ; hence the secretions produced
by these ends, the rhabdome and cone, are to be regarded as modifica-
tions of the chitinous cuticula of the outer surface of the body.

Ingenious as this theory is, I have not been able to convince myself of
its tenability. It may be urged against the assumption that the retinu-
lar cells occupy a proximal position and the cone cells a distal one on the
wall of a hypodermal pocket, that in Gammarus the retinular cells extend
from the distal to the proximal face of the retina, and that in Homarus
the cone cells have a corresponding extent ; these conditions show that

MUSEUM OF COMPARATIVE ZOOLOGY. 129

it is possible to interpret the cells in an ommatidium as elements in a
thickened epithelium, all of which originally extended from one face of
the layer to the other, and the grouping of which is not even now in-
terfered with by any process of involution. But granting that the ret-
inal cells are thus arranged, it must be admitted that the surface on
which the rhabdomeres are produced corresponds to the sides of the cells
rather than to their distal ends. This interpretation of the position of
the rhabdome is not, so far as I am aware, contrary to any well estab-
lished facts, and indeed it is rather more in accordance with the condi-
tion seen in the eyes of some Arthropods than that implied in Watase's
theory. Thus, in the lateral eyes of scorpions the retinal cells are ar-
ranged as in an ordinary epithelium, and the lateral wall of each cell is
in part occupied by a rhabdomere. In this instance, then, it must be
admitted either that the rhabdomeres are produced on the sides of the
retinal cells, or that each cell has independently rotated upon itself, so
as to bring its morphologically distal end into a position corresponding
to the side of an ordinary epithelial cell. But there is neither direct
evidence to show that this rotation of single cells has occurred, nor, in
this case, can there be any motive assumed which might have induced
the rotation of single elements. I therefore believe that in the lateral
eyes of scorpions the rhabdomes are on the sides of the retinal cells in
the strictest morphological sense j and if they can occur in this position
in the eyes of scorpions, I can see no reason why they might not occur
in similar positions on the retinal cells of compound eyes. Hence it
seems to me as reasonable to interpret the retina in compound eyes as a
liver of modified epithelium unaffected by involutions, as it is to con-
sider it a layer in which each ommatidium represents an infolding.

When, moreover, an attempt is made to show how a particular omma-
tidium has arisen by involution, some difficulties are encountered. Thus
in Gammarus, in which the ommatidium is of a primitive type, each om-
matidial pocket would involve seven cells/two of which, the cone cells,
must be imagined as forming the neck of the involution, while the re-
maining five, the retinnlar cells, would constitute the deeper portion of
the pocket. The mechanical difficulty which would accompany the forma-
tion of an involution involving so small a number of cells must be obvi-
ous, and offers, I believe, an obstacle to the successful operation of the
process assumed in Watase's theory.

The one instance in which Watase has described an actual involution
to form the eyes in Arthropods is the lateral eye of Limulus. These eyes
consist of a cluster of hypodermal pits, over each of which there is a cu-

VOL. xxi — no 2 9

130 BULLETIN OF THE

ticular lens. Although there cannot be the least doubt that in this case
each pit is a hypodermal involution, the belief that each one is homolo-
gous with an ommatidium is by no means so well founded. In structure
the wall of the pit differs considerably from that of an ommatidium ; it
contains no cells which can be definitely denominated, either as cone
cells or as cells of the corneal hypodermis, and it does contain a large
ganglionic cell, which is only questionably homologous with any element
in an ommatidium. In most respects in which these pits differ from
ommatidia, they resemble simple eyes, and I therefore regard them as
such, rather than as representatives of an early condition in the forma-
tion of an ommatidium.

When to the objections raised in the preceding paragraphs the state-
ment is added, that in both Homarus and Gammarus — representatives
of the extremes of organization — the ommatidia are developed without
showing any trace of infolding, Watase's theory of the formation of om-
matidia by means of involutions appears in a still less favorable light.
I therefore regard ommatidia, not as the result of involutions, but as
differentiated clusters of cells in a continuous unfolded epithelium.

I have not observed anything that would lead to the conclusion re*
cently expressed by Patten ('90), that an ommatidium is a hair-bearing
sense bud. I believe, on the contrary, that they have had a very differ-
ent origin.

In conclusion, I may add, that if my idea of the origin of ommatidia
be correct, it supports Grenadier's opinion, that compound eyes are
not derived directly from aggregations of simple eyes, but from groups
of optic organs which were even more primitive in their structure than
simple eyes. Possibly such primitive organs were the antecedents of
both the compound and simple eyes of Arthropods, as Grenadier sug-
gests ; but possibly the two kinds of eyes may have had totally different
origins.

MUSEUM OF COMPARATIVE ZOOLOGY. 131

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EXPLANATION OF FIGURES.

All the drawings were made with the aid of an Abbe camera. Unless otherwise
stated, the specimens from which the drawings were made were stained in Czokor's
alum-cochineal and mounted in benzol-balsam. The reagent used in depigmenting
sections was an aqueous solution of potassic hydrate \%.

ABBREVIATIONS.

(/.

Anterior.

mb. i'cI.

Intercellular membrane.

ax. n.

Axis of nerve fibrillae.

ml>. n. opt.

Membrane of optic nerve.

brs. oc.

Optic pocket.

mb. pi'ph.

Peripheral membrane.

cl. con.

Cone cell.

mb. pr'con.

Preconal membrane.

cl. crn.

Cell of corneal hypodermis.

mn.

Muscle.

cl. (1st.

Distal retinular cell.

n.fbr.

Nerve fibre.

cl. Injl.

Hyaline cell.

nl. (On.

Nucleus of cone cell.

cl. ms'ilrm.

Mesodermic cell.

nl. crn.

Nucleus of cell in corneal hy-

cl. px.

Proximal retinular cell.

podermis.

cl. rtn.'

Retinular cell.

nl. cist.

Nucleus of distal retinular cell.

cl. rucl.

Rudimentary retinular cell.

nl. h'drm.

Nucleus of hypodermal cell.

cnch.

Shell.

nl. Injl.

Nucleus of hyaline cell.

cod.

Body cavity.

nl. ms'diin.

Nucleus of mesodermic cell.

con.

Cone.

nl. px.

Nucleus of proximal retinular

cp. sng.

Blood corpuscle.

cell.

crn.

Corneal cuticula.

nl. rtn.'

Nucleus of retinular cell.

eta.

Cuticula.

n. opt.

Optic nerve.

d.

Dorsal .

oc.

Eye.

dsc.

Sucking disk.

omm.'

Ommateurrt.

dx.

Right.

/'•

Posterior.

(/}l. opt.

Optic ganglion.

po. brs.

Pore of optic pocket.

h'drm.

Hypodermis.

r.

Retina.

hp.

Liver.

rhb.

Rhabdome.

in.

Intestine.

rl J' m.

Rhabdomere.

Ins.

Lens.

rtn.'

Retinula.

mb. ba.

Basement membrane.

s.

Left.

nib. crn.

Corneal membrane.

v.

Ventral.

mb. cm' con,

Corneo-conal membrane.

en snij.

Blood-vessel.

Such other abbreviations as have been used are explained in the description of
the figures with which they occur.

Parker. — Compound Eyes in Crustaceans.

PLATE I.

Gammarus.

Fig. 1. A section of the right eye in a plane transverse to the chief axis of the
body and through the central part of the retina. X 115.

2. A section lengthwise of an ommatidium. The numbers at the left of

the figure correspond to the numbers of the six following figures
of transverse sections, and mark the levels at which the latter were
taken. X 475.

3. A transverse section in the plane of the corneal hypodermis. X 475.

4. A transverse section through the distal ends of the retinular cells and
cone. X 475.

5. A transverse section through the proximal portion of the cone and
through the adjoining retinular cells. X 475.

6. A transverse section through the retinula in the region of the rhabdome.
X 475.

7. A transverse section through the retinular cells somewhat proximal tp
the basement membrane. X 475.

8. A transverse section through a single retinular cell in the region of its
nucleus. X 475.

9. The proximal portion of a retinular cell viewed from the side. (Compare

Fig. 2.) Isolated in Muller's fluid. Not stained. X 475.

10. A cone isolated in Muller's fluid and viewed from the side. Not stained.
X 475.

■

'

•

:•

(fin

■■ htfrm

cl.rfn.

"

•lib 'ill m\

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P.vrker. — Compound. Eyes in Crustaceans.

PLATE II.

Argnlus.

(Figs. 11-17.)

Fig. 11. A section in a plane transverse to the chief axis of the body and through
the right eye. Depigmented. X 140.

" 12. A longitudinal section of an ommatidium. X 475.

" 13. A longitudinal section of an ommatidium which had been depigmented.
The numbers at the left of the figure correspond to the numbers
of the four following figures of transverse sections, and mark the
levels at which the latter were made. X 475.

" 14. A transverse section through the distal end of a cone and the surround-
ing pigment cells. X 475.

'•' 15. A transverse section through the proximal portion of a group of four
cone cells. The intercellular membranes of the cells present four
thickened regions. X 475.

" 1G. A transverse section through the rhabdome. Depigmented X 475.

" 17. A transverse section through the retinula somewhat proximal to the
rhabdome. X 475.

Pontella.

Fig. 18. The left lateral eye seen from the left side. The section is an optical
one; its plane is very nearly parallel to the sagittal plane of the
body. Depigmented in alcohol (see p. 78). X 275.
" 19. A transverse section of the optic nerve from a region immediately poste-
rior to the retina. The sagittal plane divides the nerve into sym-
metrical halves ; the fibres in each half belong exclusively to the
lateral eye of the corresponding side. X 400.

Parker - Crustacean Eye

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Parker. — Compound Eyes in Crustaceans.

PLATE III.

Pontella.

Figs. 20-29. A complete series of ten consecutive sections through the right and
left retinas in planes parallel to the horizontal plane of the animal.
The sections are viewed from their dorsal faces. Figure 20 represents
the most ventral section ; Figure 29, the most dorsal. The plane of
Figure 25 is approximately indicated by the incomplete dotted line
mu.con.'m Figure 18 (Plate II.). In the sections on the present plate
the different bodies in the left retina have been designated by appropri-
ate letters and figures. The eight rhabdomeres have been indicated
simply by numbers; the same number always refers to the same
rhabdomere. For the sake of distinction, the two cone cells have
been marked cl. con. 1 and cl. con. 2. Some of the nerve fibres
(n.fbr. 7 and n.fbr. S) have been numbered in reference to the par-
ticular rhabdomeres with which they are associated. X 400.

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Parker — Compound Eyes in Crustaceans.

PLATE IV.

Branchipus.

(Figs. 30-32.)

Fig. 30. A longitudinal section of an ommatidium. X 400
" 31. A transverse section through the distal end of four cones. X 400.
32. A transverse section through the middle portion of a retinula. X 400.

Lnnnadia.

(Figs. 33-39 )

Fig. 33. A section through the anterior part of the body, including the eye, in a
plane transverse to -the chief axis. X 25.

" 34. An enlarged portion of a section from the same series as that from which
Figure 33 was drawn, but in a position slightly anterior to the lat-
ter. X 115.

" 35. A section through the eye cut in the sagittal plane of the animal. De-
pigmented. X 90.

" 36. A lateral view of an ommatidium. The numbers at the left of the
figure correspond to the numbers of the three following figures
of transverse sections, and mark the levels at which the latter were
taken. X 475.

" 37. A transverse section through the corneal hypodermis and distal ends of
the cones. X 475.

" 38. A transverse section through four cones at the level where they are
thickest. X 475.

" 39. A transverse section through the central portion of four retinulse. X 475.

Evadne.
(Figs. 40-45.)

Fig. 40. An optical section through the eye and adjoining structures in a plane

approximately parallel to the sagittal plane of the body, but lying

somewhat to the right of it. X 140.
" 41. A transverse section through the distal ends of the cones. X 475.
" 42. A transverse section through the proximal end of a cone. X 475.
" 43. A transverse section through the distal ends of three groups of retinular

cells. In each group the corresponding cells have been designated

by the same number. X 475.
" 44. A transverse section through the central part of four rhabdomes. X 475.
" 45. A transverse section through a retinula. Depigmented. Kleinenberg's

alum-ha?matoxylin. X 475.

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Parker. — Compound Eyes in Crustaceans.

PLATE V.

P or cell to.

Fig. 46. A transverse section through a retinula in a plane slightly distal to the
basement membrane. The single, light, central spot represents the
proximal end of the rhabdome. X 475.

Idotea robusla, Kroyer.

(Figs. 47, 48.)

Fig. 47. A transverse section through the distal end of a retinula. The bodies,
one of which is marked x, are spheres of coagulated material which
occur in the interommatidial spaces, and which have been brought
into prominence by the action of the hardening reagent. X 475.
" 48. A transverse section through three ommatidia in the region of their
rhabdomes. X 475.

Idotea irrorata, M. Edws.

(Figs. 49-57.)

Fig. 49. The anterior face of a section transverse to the chief axis of the body,
and passing through the eye on the right side of the head. X 140.

" 50. A longitudinal section of an ommatidium. The numbers at the left of
the figure correspond to the numbers of the following six figures
of transverse sections and mark the levels at which the latter were
taken. X 475.

" 51. A transverse section through the distal ends of the cones. X 475.

" 52. A transverse section through the middle region of a cone. X 475.

" 53. A transverse section through the middle of a retinula. Near the centre
of each cell can be seen a small axis of nerve fibrillae. X 475.

" 54. A transverse section through a retinula composed of seven cells instead
of six. This section was cut approximately at the same level as that
shown in the preceding figure. X 475.

" 55. A transverse section through a retinula near its proximal end. Each
fibrillar axis is much larger at this plane than in that shown in Fig-
ure 53. X 475.

" 56. A transverse section of several groups of retinular cells immediately
proximal to the basement membrane. X 475.

" 57. A transverse section of four retinular cells at the level in which their
nuclei occur. The axis of nerve fibrillae in the plane of this section
and in that of the preceding one (Fig. 56) are smaller than they
are at the base of the retina (compare Fig. 55).

Sphceroma.

(Figs. 58, 59.)

Fig. 58. A transverse section of a retinula at a level slightly distal to the base
ment membrane. X 475.
" 59. A transverse section of the fibrous ends of the cells from a single re-
tinula. The plane of section is slightly proximal to the basement
membrane. The only indication of an axis of nerve fibrillae is the
more transparent condition of the central part of the cells, due to the
partial absence of pigment granules. X 475.

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Parker. — Compound Eyes in Crustaceans.

PLATE VI.

Serolis.

Figures 60 to 64 inclusive represent the structure of the ommatidium in the
adult. Figures 65 to 72 are drawn from sections of ommatidia in well ad-
vanced embryos. All figures are magnified 475 diameters.

Fig. 60. A tangential section through the most distal portion of the retina. This
section includes a portion of a cone and the tissue lying between it
and two adjoining cones.

" 61. A transverse section ofaretinula in the region of its rhabdome. The
arrangement of the pigment granules and nerve fibrillae is indicated
in only one of the four cells. Of the two lines which appear to
separate the cone cell6 (cl. con.) from the rhabdomere (rhb'm.), the
one nearer the axis of the ommatidium is the real line of separa-
tion ; the other lies within the substance of the rhabdomere itself
(compare p. 92).

" 62. A transverse section through a retinula proximal to the rhabdome and
in, the region of the hyaline cell. As in Figure 61, the pigment
granules are drawn in only one of the retinular cells.

" 63. A transverse section through a single retinular cell in the region of its
nucleus. The axis of nerve fibrillae is represented by several small
axes in the substance of the cell at one side of the nucleus.

" 64. A transverse section of the fibrous ends of the cells of one retinula in
their passage through the aperture in the basement membrane.
Each cell shows a well marked fibrillar axis, the centre of which is
often occupied by a core of pigment. The basement membrane is
viewed from its distal face. The irregularly oval body in the upper
left-hand corner of the figure is probably a nucleus. It lies on the
proximal face of the membrane through which it is seen.

" 65. A longitudinal section through the ommatidium of an advanced embryo.
The numbers at the left of the figure correspond to the numbers of
the six following figures of transverse sections, and indicate the
levels at which the latter were taken. Figure 68 represents a sec-
tion so nearly in the same plane as that shown in Figure 67 that its
number has been omitted.

" 66. A transverse section at the level of the corneal hypodermis.

" 67. A transverse section through the distal end of a cone.

" 68. A transverse section made in a plane only slightly proximal to that
shown in Figure 67.

" 69. A transverse section through the region of the distal retinular nuclei.

" 70. A transverse section through the proximal ends of the cones.

" 71. A transverse section through the retinula in the region of the rhabdome.

" 72. A transverse section at the level of the proximal retinular nuclei.

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Pakker. — Compound Eyes in Crustaceans.

PLATE VII.

Ilfysis.

Fig. 73. A longitudinal section of an ommatidium. The numbers at the left of
the figure indicate the levels at which the sections for Figures 76-89
were taken. X 475.

" 74. The distal face of a corneal facet, cleaned in potash and examined in
water. X 475.

" 75. A transverse section of three ommatidia in the plane of the corneal hy-
podermis. X 475.

" 76. A transverse section through the distal end of a cone. X 475.

" 77. A transverse section through the proximal end of a cone and the adjoin-
ing distal retinular cells. X 475.

" 78. A transverse section similar to that shown in the preceding figure,
except that it is depigmented and stained in Kleinenberg's alum-
haematoxylin. X 475.

Figures 79 to 82 inclusive represent consecutive transverse sections through the
region of the proximal retinular nuclei of four adjacent ommatidia. The
centre of each ommatidium is indicated by the group of cone cells (cl. con.),
and the corresponding ommatidia in different sections are designated by the
same Roman numeral. The nuclei around ommatidium II. have been num-
bered in Figures 79-81. Figure 79 represents the most distal section, and
Figure 82 the most proximal one of the series.

Fig. 79. The bodies marked x and y are portions of nuclei the rest of which are
correspondingly marked in Figure 80. X 475.

" 83. A transverse section of the four fibres at the distal end of the rod (com-
pare p. 102). Depigmented, and stained in Kleinenberg's alum-haem-
atoxylin. X 615.

" 84. A transverse section of the rod at a slightly more proximal level than
that shown in Figure 83. Depigmented, and stained in Kleinenberg's
alum-hsematoxylin. X 615.

" 85. A transverse section of the retinula somewhat distal to the distal end
of the rhabdome (compare Fig. 90). Depigmented, and stained in
Kleinenberg's alum-haematoxylin. X 615.

" 86. A transverse section from the region between the distal end of the rhab-
dome and the proximal end of the rod (compare 86 in Fig. 90). De-
pigmented, and stained in Kleinenberg's alum-hasmatoxylin. X 615.

" 87. A transverse section through the rhabdome and surrounding retinular
cells. X 615.

" 88. A transverse section, at the level of the basement membrane, through
the nerve fibres from a single retinula. Depigmented, and stained
in Weigert's hematoxylin. X 615.

" 89. A transverse section through the fibres of the optic nerve at a level mid-
way between retina and optic ganglion. Preparation as in Figure
88. X 615.

" 90. A longitudinal section through the basal portion of one and parts of two
adjoining ommatidia. Depigmented, and stained in Kleinenberg's
alum-haematoxylin. X 615.

" 91. A section cut in the same plane as that shown in the previous figure,
but including only the proximal ends of two rhabdomes. Prepara-
tion as in Figure 90. X 615.

" 92. A cone viewed from the side. Isolated in Muller's fluid and studied in
water. X 475.

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Parker. — Compound Eyes in Crustaceans.

PLATE VIII.

Gonodactylus.

Fig. 93. Part of a tangential section through a superficial portion of the retina.
The extreme edges of tiie section both right and left are immediately
beneath the corneal cuticula ; the central portion is farthest from the
cuticula. At the right of the middle line are seen the ends of the
larger ommatidia ; at the left, those of the smaller. X 275.

" 94. A longitudinal section of a large ommatidium. The numbers at the left
of the figure correspond to the numbers of six figures of transverse
sections (Figs. 96-101), and mark the levels at which the latter were
made. Depigmented. X 275.

" 95. A longitudinal section of a small ommatidium containing its natural
pigment. X 275.

;< 96. A transverse section through the cells of the corneal hypoderruis and the
distal end of the cone in a large ommatidium. X 275.

" 97. A transverse section through the distal part of a cone in a large omma-
tidium. X 275.

" 98. A transverse section through the middle of a cone from a large omma-
tidium. X 275.

" 99. A tranverse Section through a number of cones at the level of the distal
retinular nuclei in the large ommatidia. X 275.

" 100. A transverse section through six retinula? of the large ommatidia in
the region of the proximal nuclei. Each retinula is numbered. The
plane of this section is slightly oblique, so that retinula 1 is cut at a
relatively higher level than any of the others, and retinula 6 at the
lowest level. X 475.

" 101. A transverse section of a retinula from one of the larger ommatidia, in a
plane not far from the basement membrane. Depigmented. X 475.

" 102. A transverse section of a retinula from one of the smaller ommatidia cut
in a plane nearly corresponding to that of Figure 101. X 475.

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Parker. — Compound Eyes in Crustaceans.

PLATE IX.

Palcemonetes.

In all Figures on this plate the magnification is 475 diameters.

Fig. 103. A longitudinal section of an ommatidium. The numbers at the left of
the figure correspond to the numbers of nine of the following fig-
ures of transverse sections, and mark the levels at which the latter
were taken.

''" 104. A longitudinal section of an ommatidium which has been depigmented.
The bodies marked x resulted from the action of the depigmenting
reagent.

" 105. A facet from the corneal cuticula ; cleaned in strong potassic hydrate,
and examined from its distal side in water.

" 106. A transverse section through the region of the corneal hypodermis.

" 107. A transverse section through the distal end of a cone in the region of
the nuclei of the cone cells.

" 108. A transverse section through the middle of a cone.

" 109. A transverse section through parts of four ornmatidia in the region of
the distal retinular nuclei.

Figures 110-112 represent three successive transverse sections, each through
five ornmatidia, in the region of their proximal retinular nuclei. Only the
outlines of the nuclei and the five groups of cone cells (cl. con.) are drawn.
The nuclei in each ommatidium are numbered from 1 to 7, and as their plan
of arrangement is the same in the different ornmatidia, corresponding nuclei
have been designated by the same number. In some cases the nuclei were
cut in two, and consequently appear in two adjoining sections. In such
cases the two parts have been marked with the same number. Figure 110
is the most distal of the series; Figure 112, the most proximal.

Fig. 113. A transverse section of the retinula near the distal end of the rhabdome.

Depigmented.
" 114. A transverse section of four retinulae at the level of the eighth retinular

nucleus.
" 115. A transverse section through four retinulae in the region of the accessory

pigment cells; viewed by reflected light. The retinula? appear as

dark masses embedded in a whitish field composed for the most part

of the substance of the accessory pigment cells.
" 116. A transverse section through a retinula at about the same level as that

shown in Figure 115. Depigmented.
" 117. A transverse section through the optic nerve fibres at a level slightly

proximal to the basement membrane. Depigmented.

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Pakker. — Compound Eyes in Crustaceans.

PLATE X.

In all Figures on this plate the magnification is 475 diameters.

Canibarus.

Figures 118-122 represent a series of five successive transverse sections through one
and parts of four adjoining ommatidia in the region of their proximal
retinular nuclei. Figure 118 represents the most distal section in
the series, Figure 122, the most proximal. In these figures, only
the outlines of the nuclei and the groups of cone cells are drawn.

Crangon.

Fig. 123. A transverse section through a number of ommatidia in the region of
t^eir distal retinular nuclei.

Pahnurus.

Fig. 124. A transverse section througli a retinula in its middle region. The out-
lines of the retinular cells cannot he distinguished; the position of
each cell is marked by an irregular light mass in its centre.
'■' 125. A transverse section through a retinula in the plane of its eighth
nucleus. Depigmented.

Cancer.

(Figs. 126-131.)

Fig. 126. A corneal facet viewed from its distal surface. The cuticula from which
this facet was drawn was cleaned by being boiled in a strong aqueous
solution of potassic hydrate. It was examined in water

" 127. A transverse section of the distal end of a cone.

" 128. A transverse section through three ommatidia at the level of the distal
retinular nuclei. The pigment granules have been indicated in only
the lower circle of cells.

" 129. A transverse section through the distal region of four retinula?. In the
two on the right, the pigment granules have not been drawn

" 130. A transverse section through a retinula near the base of the retina.

" 131. A slightly oblique section through the basement membrane. The upper
part of the figure represents the retinula? as seen in transverse sec-
tion distal to the basement membrane; the part marked mb. ba.
represents the region in which the membrane itself appears in section,
and the lower half of the figure shows the cut fibres of the optic
nerve. The pigment granules are omitted from the right side of the
figure. The transition from the retinular cells to the nerve fibres
is evident in passing over the section from top to bottom.

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Xo. 3. — On some Points in the Anatomy and Histology of
Sipunculus nuclus, L. By Henry B. Ward. 1

I Introduction

Methods

II. External Anatomy ....

1. Introvert

2. Tentacular Fold ....
III. Histology

1. Body Wall

a. Cuticula and Hypodermis

b. Cutis

c. Pigment Cells ....

d. Dermal Bodies ....
a. Bicellular Glands . .
p. Multicellular Glands .
■y. Sense Papillae . . .

e. Muscular Layers . . .

Contents.

Page

143
144
145
145
147
149
149
149
150
150
152
152
155
157
159

IV.

2. Tentacular Fold . .

a. Oral Wall ....

b. Migratory Corpuscles

c. Musculature . . .

d. Vascular System .

e. Aboral Wall . . .

3. Nervous System . .

a. Brain

a. Ganglionic Cells
p. Internal Structure

b. Cerebral Nerves .

c. Ventral Nerve Cord and
Plexi

4. Cerebral Organ . .
Conclusions

Bibliography 180 | Explanation of Figures

Page
159
159
160
101
162
164
165
165
166
169
170

171
172
176

183

I. Introduction.

Some two years ago, while working on Sipunculus nudus in the zoo-
logical laboratory at Gottingen under Prof. E. Ehlers, my attention was
attracted by a peculiar organ in the region of the dorsal ganglion ; and
although it was a prominent feature of all transverse sections, no men-
tion of its presence was found in the literature on Sipunculus. The ob-
servations made at that time interested me so much that the opportunity
afforded by a short stay at the Xaples Zoological Station last spring,
for which I am indebted to the great kindness of Prof. A. Weismann and
the Cultusministerium of Baden, was embraced to procure new, carefully
preserved material. A study of the literature on Sipunculus revealed
such lack of agreement between authors that a more general studv
of the form seemed likely to yield results, and, on the advice of Prof.

1 Contributions from the Zoological Laboratory of the Museum of Comparative
Zoology, under the direction of E L. Mark, No. XXVI.
vol. xxi. — no. 3.

144 BULLETIN OF THE

E. L. Mark, a more particular consideration of some moot anatomical
and histological points was undertaken. This was unfortunately limited
by the material on hand, which consisted merely of the anterior portion
of the body, corresponding in general to the introvert of recent writers.
As this contains, however, nearly all of the important organs of the
nervous system to which especial attention has been paid in this paper,
and as its separation from the rest of the body at the time of killing
insured good preservation, it is hoped that the conclusions reached may
not be without value, in spite of their incompleteness. The histological
structure of the body wall and of the nervous system has been treated
in detail, and from the results an attempt has been made to throw some
new light on the systematic position of the Sipunculids.

Methods.

The material used in these investigations was preserved with especial
care, and every effort was made to procure a method of killing which
should afford a clear idea of anatomical and histological relations under
normal conditions, since many of the contradictory statements of va-
rious writers have been undoubtedly the result of studying specimens
in a distorted state, due to muscular contraction, or have followed
the examination of tissues poorly preserved. The thick impermeable
cuticula, and the wealth of muscular tissue in the body wall, render it a
difficult matter to avoid at the same time both evils. The method
finally adopted as yielding the best results is as follows.

After remaining some time in clean sea-water to clear tentacles, body
wall, and oesophagus of adhering sand, the animals were brought into
a shallow dish of sea-water, and 5of alcohol was allowed to flow gently
over the surface, forming thus a thin film, which disseminated itself
gradually, and produced in the animals a complete relaxation of the
body muscles. It did not seem to answer equally well when the alcohol
and water were mixed at the start, as has been recommended for some
animals. The length of time necessary for the attainment of com-
plete narcosis cannot be exactly given. It varies greatly with different
individuals ; but if, after lying some four to eight hours, the animals make
no contractions on being gently probed with a dull instrument, they may
be regarded as sufficiently stupefied, and transferred to 50^, alcohol.
After a short stay in this, the introvert was cut off, and this alone sub-
jected to treatment with higher grades of alcohol, which insured the pene-
tration and consequent good preservation of the tissues. The only point
in the process which requires especial care, and which often produces

MUSEUM OF COMPARATIVE ZOOLOGY. 145

a disappointing failure, is the transfer from the salt water and its added
alcohol to 50$ alcohol. If the animal is but partially narcotized, the
muscular contraction induced by the transfer will spoil the specimen.
If, on the other hand, it be left too long, the weaker parts of the body
wall, especially the upper smooth zone of the introvert, swell out quite
rapidly (through osmosis?), and not only the external form but the his-
tological elements as well are badly distorted. The golden mean be-
tween these two extremes yields specimens as excellent for histological
work as for the study of external relations. Material preserved in this
way may be well stained by all methods. Where any stain has been
of especial value in the study of particular organs or tissues, it will be
noted under the topic in question. In this place I wish to express my
thanks to Prof. E. Ehlers of Gottingen and to Prof. A. Dohrn of Naples
for past favors, and to Mr. A. Agassiz, Prof. E. L. Mark, and Prof. E. B.
Wilson for more recent kindnesses in supplying me with material for
this study.

II. External Anatomy.

Selenka ('83, p. 92) has given a full description of the external char-
acters of Sipunculus nudus. There are however numerous points of in-
terest which first appear in a well expanded specimen, and which deserve
especial attention. The body consists of a large posterior region covered
by the quadratic integumentary areas (Hautfelder) and of a portion
anterior to these, which is called the introvert.

1. Introvert.

This includes on the average one sixth of the entire length of the
animal, and has in general the shape of a truncated cone (Fig. 1), the
anterior base of which, only a little less in diameter than the posterior,
is surmounted by a wreath of .tentacles which nearly encircle the mouth.
This region is ordinarily found entirely, or for at least two thirds of its
length, invaginated into the following portion of the body, and is only
rarely seen extended. In the latter condition it measures from three to
four centimeters in length. The circular muscle bands, which are sepa-
rate in the posterior part of the body, are here fused into an unbroken
sheet of muscular tissue. The fusion .takes place abruptly, and causes
the immediate cessation of the integumentary areas (Hautfelder) due to
the banded musculature, thus fixing a definite posterior boundary to
the introvert. On the latter one can distinguish (Fig. 1) four regions :

VOL. xxi. — no 3. 10

146 BULLETIN OF THE

(1) a posterior papillate zone 1 (z.pap.p.), (2) a smooth zone {z. lev.'),
(3) an anterior papillate zone (z. pap. a.), and (4) the tentacular
crown (j)li. ta.).

The posterior papillate zone occupies the posterior half of the intro-
vert, and shows a posterior portion, which is thickly studded with papillae,
and is dark brown in alcoholic specimens, and an anterior part much
lighter in color, where the papillae are somewhat scattered. The lighter,
almost translucent appearance of the anterior portion of this zone, which
permits the central mass of the oesophagus and retractors to shine
through as a dark band, is due to the great diminution in thickness of
the muscular layers. The line of demarcation between the lighter and
darker portions of this zone is somewhat definite, and is marked inter-
nally by the fusion of the longitudinal muscles into a continuous sheet,
and by the entrance into the body w r all of the first large composite nerve
given off from the ventral nerve cord (cf. infra).

The papilla? of this region are all shaped like the bowl of a spoon with
the concavity directed toward the body and the tip posteriad. Adjacent
to the integumentary areas they are closely crowded, and overlap like
the shingles of a roof, so as to hide the skin completely. They vary in
size and shape, but are in general broadly pointed, measuring on the
average .25 mm. in length, and .65 mm. in breadth. 1 Passing forward,
this general form is preserved until the point of transition from the dark
to the light portion of this posterior papillate zone is reached. Here the
papilla? grow abruptly smaller in absolute size, though relatively longer
and narrower, until the characteristic mammiform papilla of the light
region is reached. These only are represented in Figure 1. They are
much lighter in color, and much less crowded, than the posterior papillae,
and leave irregular patches of skin entirely free. In breadth such a
papilla measures .25 mm. ; in length, .37 mm. I am unable to confirm
the statement of Andreae ('81, p. 205), that they are arranged "in
gleichen Abstariden " ; for the relative distances are extremely variable,
being from 70 to 300 fx in the anterior portion of this zone. I was also
unable to find the arrangement in a double spiral reported by Vogt und
Yung ('88, p. 381). There seemed to be in fact no regular arrangement
common even to a majority of the specimens examined.

Passing forward, the papillae grow ever sparser, and finally terminate
along a well defined line, which marks the beginning of a smooth zone
(z. lev., Fig. 1) entirely free from papillae. It measured 7 mm. in breadth
in a specimen which had an introvert of 4 cm. total length. Anterior to

1 The posterior half of the posterior papillate zone is not shown in Figure 1.

MUSEUM OF COMPARATIVE ZOOLOGY. 147

this is a zone (z. pap. a.) with small papillae; this measured 3 mm. in
breadth in the same specimen. The papillae of this zone appear super-
ficially as minute discoidal elevations of the skin. In well expanded
specimens, the tentacles droop over and nearly cover this zone, which is
not separated from their base by any definite line, since the papillae
extend forward a short distance over the aboral surface of the tentacles,
becoming gradually less frequent. They are indeed met with occasion-
ally on the whole of this surface, but are entirely wanting on the oral
aspect of the tentacles. In all well expanded specimens these regions
are as well defined as in the one which has served as the basis for this
description, and the zones have the same relative size as in the measure-
ments given.

2. Tentacular Fold.

The tentacles (Tentakelmembran) originate in the larva as two folds
of the oral margin, — " lippeuartige Falten," Hatschek ('83, p. 115), —
separated dorsally but continuous ventrally, and lying right and left of
the median line. Starting, then, from this primitive condition, the form
found in the adult would be reached, if it be supposed that these flaps
of skin are plaited radially to the oral centre, and that the growth is
more rapid on the oral surface as well as toward the margin, thus ne-
cessitating a reflection of the flaps back upon the aboral surface. For
a careful examination shows that in well expanded specimens the so-
called tentacles consist of a thick fold of skin surrounding the terminal
oral orifice with numerous plaits and folds arranged radially. This
continuous flap may be called the tentacular fold, in preference to
membrane, since the latter suggests a false idea of its nature, and its
subdivisions may conveniently be termed the radial plaits.

The general form of the tentacular fold, as viewed from above (Fig. 2),
may be said to be that of a horseshoe with the smaller dorsal curvature
interrupted on the middle line. The external or ventral semi-circum-
ference is reflected over the superior portion of the introvert, whereas the
internal or dorsal portion makes a ventral flexion over the mouth, and lies
higher than the other half of the tentacular fold. The superior height
of the dorsal portion of the flaps in the larva caused Hatschek ('83, p. 115)
to regard this as the " Anlage " of the first pair of tentacles. He knew
nothing, however, of the further development of this portion, which
probably represents the origin of the dorsal horns, since separate tenta-
cles do not exist. In the adult, at any rate, this region shows two horns
(Figs. 1 and 2, crmi. d.) projecting ventrad over the oral aperture, and

148 BULLETIN OF THE

forming together the dorsal curve of the horseshoe. Brandt ('70, p. 22)
assigned a horseshoe shape to the crown of tentacles, but this has been
declared false by later investigators.

This normal hippocrepian form is often distorted when the introvert
is only partially extruded, or when there is undue muscular contraction
within the soft mass of the fold itself, and it is always more or less
disguised by the secondary radial plaits into which the fold is thrown.
The relation of these parts will be easily understood by comparing
Figures 1, 2, and 3. It will thus be seen that the reflection of the ten-
tacular fold, with its deep radial plaits, brings into prominence regions
— the "triangular tentacles" of some writers — which alternate with
retreating portions, so as to impart to the margin the appearance of
being cut or toothed, especially if the contraction of the muscular ele-
ments in this soft fold has drawn it somewhat out of shape. In fact,
the description uniformly given by systematic writers has represented
the tentacles as a membrane with numerous marginal incisions. This
error is due in part to distorted specimens ; the true form may be said
to be crenate.

Therefore one can speak of the formation of tentacles only in a gen-
eral sense. But the fold may be regarded perhaps as the simpler form,
from which, by the development of certain areas alternating with regions
of reduction, the more highly specialized digitate tentacles might be de-
veloped. Only the main folds are represented in Figure 2. These may
be much complicated by the appearance of subordinate plaits, until the
general plan is confused by a mass of detail. The more simple forms
proved, on microscopic examination, to have been the most successfully
killed, in that the muscular elements were in a more perfectly relaxed
condition. The aboral surface of the tentacular fold is concave, except
in the dorsal horns, where it is convex ; it has the same radial folds as
the oral surface with which it is approximately parallel. Numerous low
circular ridges traverse the aboral surface, and bear in varying number
the small papillae already mentioned. These ridges are not regular in
course or size, and evidently vary with the convexity of the tentacular
fold. In the midst of these, on the dorsal median line, can be found on
careful examination a small oval opening (Figs. 2, 3, can. o. ceb.). It is
often so hidden in the ridges of the aboral surface as to make its dis-
covery a matter of some difficulty. The opening measures about 1 by
0.5 mm., with its long axis transverse, and is surrounded by an evident
marginal ridge. This is the opening of the canal of the cerebral organ,
to be described later.

MUSEUM OF COMPARATIVE ZOOLOGY. 149

III. Histology.

1. Body Wall.

In the body wall may be found the following layers, beginning with
the surface : (1) a cuticula, (2) a hypodermis, (3) a cutis, (4) the mus-
cular system, covered internally by (5) a delicate peritoneal membrane.

a. Cuticula and Hypodermis.

The cuticula consists of a substance optically like chitin, but differing
from this, as has often been pointed out, in being soluble in boiling
KOH. It is further aberrant in the absence of cellulose, which has
been shown by Ambronn ('90) to be characteristic of true chitin. Tests
with chloriodide of zinc showed neither any trace of blue nor the sub-
sequent pleochroismus described by that author for true chitin. This
layer is undoubtedly the product of the underlying hypodermal cells,
which are everywhere found in a single layer, and normally display a
sac-like form, although, as mentioned by Vogt und Yung ('88, p. 383),
they may by contraction or compression of the body wall be drawn out
into the form of spindles. This has given rise, as they mention, to the
erroneous interpretation of such groups of elongated cells as being
sensory organs. In contradistinction to these authors, I do not find
the proximal ends of these cells ordinarily continuous with fibres which
extend to the muscular layer, and cannot agree with them in regarding
the entire mass external to the muscles as one layer. For if one exam-
ines a transverse section of the body wall as seen in Figure 5, the major-
ity of the hypodermal cells are seen to be clearly marked off from the
underlying tissue by the cell wall. The fibres of this subjacent tissue, to
be described later, often extend up to the bases of the hypodermal cells ;
but close examination in favorable 'regions shows the connection to be
merely apparent. Often when these cells are crowded and distorted by
near-lying glands, one is inclined to believe in an actual continuity of
cell and fibre which cannot be demonstrated, and which, so far as I could
find, is not present in less confused regions.

Lying partly in the hypoderm, but mostly below it, are the dermal
bodies (Hautkbrper), which are of three sorts. A description of these
will be given in the account of the cutis, with which they are most
closely associated. No further specialized cells of any kind were found,
neither sensory cells nor peculiar nerve endings of any sort, and I am
inclined to regard the claims of their presence as founded upon the ex-

150 BULLETIN OF THE

animation of poorly preserved material. Several times it was observed
that delicate filaments, branching from some nerve fibre of the skin, pro-
ceeded to the hypodermis and penetrated apparently undifferentiated
cells ; certainly the distal surface of these cells bore no sensory hair or
bristle. But the exact manner of termination of the nerve filament
remained in doubt.

b. Cutis.

In placing a cutis in the list of the layers of the body wall, I am not
unaware that the two most recent publications on Sipunculus deny its
presence. As already mentioned, Vogt und Yung ('88) regard the entire
extra-muscular layer as hypodermal, while Andrews ('90) evidently dis-
credits the existence of a cutis by omitting the name altogether. What,
then, is the actual condition of affairs 1 In sections one finds (Figs. 4, 5)
between the hypodermis and the muscular layers a mass of gelatinous
tissue, traversed in all directions by fibres, and containing not only
glands of various sorts, but nerve fibres and pigment cells as well.
Thus, though varying greatly in thickness in different regions of the
body, it may properly be regarded, in the light of the characters men-
tioned, as a true cutis. The principal part of this layer is the connect-
ive-tissue jelly, homogeneous in its consistency and forming the matrix
in which the nerves and dermal bodies lie. It is traversed in all direc-
tions by a multitude of the finest connective-tissue fibrils, which anas-
tomose but rarely. Occasionally a minute nucleus can be observed in
the course of a fibre. Scattered nuclei of a larger size, connected with
nerve fibres or amoeboid cells, are not infrequent in this mass, and
have been erroneously regarded as belonging to the connective tissue.
Irregular amoeboid cells with but one nucleus and of a different refractive
index from the general mass are found, sometimes in considerable num-
bers, and are perhaps similar in nature to the leucocytes of the tentacles,
to be described later.

c. Pigment Cells.

Besides these elements one finds multinuclear cells of irregular out-
line more or less filled with granules of a highly refractive character.
These are the pigment cells, so characteristic of this group that they de-
serve special consideration. Andreae ('81, p. 209) has given almost the
only description of these peculiar structures. He represents the pig-
ment granules as closely packed in meshes of connective tissue on which
nuclei may be observed. This appearance is no doubt due to poorly

MUSEUM OF COMPAEATIVE ZOOLOGY. 151

preserved material ; the true nature of the cells, as well as the process
of deposition of the pigment, can clearly be understood from a section
such as is shown in Figure 5. The cutis contains here a group of irregu-
lar amoeboid (?) cells, distinguishable from the surrounding mass by their
refractive power, and containing from five to many deeply stained nuclei
3 /jl in diameter. The cells are all without any proper membrane, though
often surrounded by an envelope of connective fibres, and enclose a
varying number of highly refractive granules distinguished by indiffer-
ence to any coloring matter but picric acid, which they take up with great
avidity. Their natural color by transmitted light is a greenish yellow ;
by reflected, however, a dull brown or yellow. That the process of for-
mation is gradual becomes evident on the examination of a section like
Figure 5. In some cells are seen only a few such granules, or they are
confined to one part of the cell ; and all stages are present from this up
to a mass of closely packed granules in which neither cell plasma nor
nuclei are visible. Even in such cells the nuclei could be demonstrated
by prolonged staining and thin sectioning. The plasma of these cells
shows at first some slight affinity for hcematoxylin, which disappears as
the granules become more crowded. In the first stages of deposition
the granules are mere bright dots too small to be measured ; in the more
thickly crowded cells they have reached often twice or thrice the size of
a nucleus, and alongside of these are also granules as minute as those of
the earlier stage. Such cells are present not only in the cutis, but also
in all other organs of the body. They are not always as numerous as
shown in Figure 5 ; in the tentacles they are quite rare, whereas the
nervous system contains especially large numbers in all its parts. Some-
what similar cells were found by Burger ('90) in the nervous system of
.Nemertines. "Wherever these cells are found in Sipunculus they dis-
play the same structure, except that elsewhere than in the cutis they
are only found well filled with granules. Whether a migration actually
takes place, as is suggested by their evidently amoeboid character, I was
unable to determine. It is to the presence of large numbers of these
cells that the papillae of the posterior zone and the walls of the cerebral
canal owe their dark color. The pigment cells are present in much
greater numbers in large than in small specimens, i. e. in older than in
less mature ones. I can confirm the statement of Vogt und Yung ('88,
p. 38G) that fasting rapidly decreases their number. It is not a neces-
sary conclusion that this is to be regarded as reserve material. For
even waste may, under the pressure of failure in the food supply, be
drawn into the system and worked over again.

152 BULLETIN OF THE

d. Dermal Bodies.

Various opinions have been held by different authors as to the mor-
phological value of the dermal bodies. Keferstein und Elders ('61) de-
scribed them as glands, Leydig ('61) regarded them as sensory organs;
but later writers have inclined to the former view. Andreae ('81) de-
scribed three varieties of these organs, whereas Vogt und Yung ('88)
made the claim that the sensory organs, Andreae's third variety, do not
exist, and that all of the glands are merely modifications of one sort. As
to the first statement, they are undoubtedly correct ; but to the latter
view I am unable to assent. The transition from one sort of gland to
the other,, though plausible from surface views such as given by those
authors, is only apparent. For if one examines carefully prepared sec-
tions, the seeming similarity gives way to a well marked difference. Not
one of the glands is actually unicellular, as claimed by Vogt und Yung,
and the multicellular contain never less than five cells, which serves
to separate them clearly from the other kind, which is always bicel-
lular. Moreover, their behavior toward staining fluids is very differ-
ent. For while the bicellular glands take up hematoxylin with such
rapidity as to become almost black in a few seconds, the multicellular
are but little affected by this reagent. Carmine solutions stain the two
about equally, but bring out the nuclei, which are invisible in a hema-
toxylin stain. And, finally, the morphological elements of the two sorts
are essentially different, as will be shown. The old classification of bi-
cellular and multicellular glands will therefore be retained, and the
structure of each will be examined more in detail.

The bicellular glands, when viewed, even in the living animal, directly
from above, display a clear zone along the line of the partition wall be-
tween the cells. This is invisible if the gland be viewed from the side,
or at a considerable angle, and gives rise to. various images if the line of
sight be more or less nearly perpendicular to the surface.' As the pa-
pillae which contain the glands have sloping sides, never exactly alike,
it is easy to understand how views of the glands from many different
directions may be had from a surface inspection, and how the various
images may give the appearance of a series from the bicellular to the
multicellular gland. If one examines, however, sections of the skin per-
pendicular to the surface (Figs. 4, 5), the bicellular glands appear at
once as a distinct type. Ordinarily spherical, they may often be found
mutually flattened where several lie closely pressed together. They
vary in diameter from 40 to 50 fi, and present very different appearances

MUSEUM OF COMPARATIVE ZOOLOGY. 153

according to the stain employed. The greatest number of structu-
ral details are obtained from those lightly stained with hscmatoxylin.
Sections thus stained are represented in Figures 6, 7, and 8. Though
evidently differentiated hypodermal cells, they lie almost entirely in the
cutis, enveloped by a delicate coat of connective tissue, in which can be
found occasional flattened nuclei. The distal half of each cell is occu-
pied in great part by a large vacuole, directly continuous with that of
the adjoining cell. The space thus formed measures 12 X 15 X 25 /x, and
communicates with the exterior by means of a narrow canal opening
simply on the surface of the cuticula. The duct measures 6-8//. in di-
ameter, and at the distal end of the cell does not lie in the centre of
the neck (Fig. 9). The connective-tissue envelope does not i metrate
between the cells, which in consequence are separated only by their own
membranes (Fig. 6 or 1 1,*), and these, continued over or under the distal
vacuole, appear, if the cell be viewed along the plane of the partition,
to bisect the vacuole (Figs. 6, 10), the latter suffers, however, a slight
constriction along this line, so as to impart to it in transverse section a
biscuit-shaped appearance (Fig. 7). Its longitudinal section is cordi-
form, as shown in Figure 6. The two large clear spherical nuclei, 9 /x
in diameter (Figs. 10, 11), may be differentiated with carmine or saf-
franin, and then appear in the lower half of the cell, usually nearly
symmetrical to the dividing membrane. Each displays a single central
deeply stained nucleolus, and many minute chromatine granules. If the
plane of the section pass transversely below the vacuole (Fig. 11), the
cells are seen to possess a hemispherical form, and the dividing mem-
brane to make an S-shaped curve.

Whether active or resting, a clear zone of plasma forms the periphery
of the cell on all sides, and is therefore adjacent to the vacuole, as well as
to the external surface of the cell. This zone is traversed radially by
delicate fibrils, the beginnings of the plasma reticulum which fills the cell,
but which ordinarily is easily seen only in this clear zone. In every sec-
tion one finds a few cells of this sort, which, besides an empty vacuole,
exhibit this reticulum very plainly throughout the entire faintly tinted
cell body (Fig. 8). 1 They are evidently the functionally inactive or
resting cells. The first stage in secretion is seen in the accumulation
of numerous granules in the basal portion of the cell (Fig. 6), which
are stained deeply with hajmatoxylin, and by continual aggregation

1 Strictly speaking, Figure 8 represents the last phase in secretion. The first
differs only in the absence of matter in the vacuole, and of the few granules just
below it.

154 BULLETIN OF THE

finally obscure the reticulum, and impart to the entire cell, save its
marginal zone, an appearance almost opaque (Fig. 7). The secretion
first appears in the vacuole in the form of minute beads at the periph-
eral ends of the reticular fibrils which traverse the clear zone and
terminate at the edge of the vacuole each in a single bead (Fig. 7).
During the formation of the secretion in the vacuole, the mass of
opaque granules moves toward this space ; and the close of the process is
represented in Figure 8, where the vacuole is filled with a homogeneous
mass, displaying in a somewhat lesser degree the affinity for hasma-
toxylin stains which characterized the granules while contained in the
cell substance itself. At the same time, these granules have disappeared,
except a few which are grouped in a zone about the vacuole ; aud the
cell has become thereby so much lighter as to show the reticulum at its
proximal end.

This description of the activity of these organs would seem to place
their glandular nature beyond question. In comparing the two sorts of
glands, it is of great importance to note that the cells do not show in
this case any connection with nerves, whereby they are sharply distin-
guished from the multicellular glands. The space (Spalt) which Andreae
('81, p. 215) describes as existing between the cells of these glands was
found not infrequently in some preparations, but it is evidently due to
shrinkage. The double membrane separating the cells, described by the
same author, was probably produced in the same way.

The distribution of these glands is peculiar. Over the general sur-
face of the body they are found only rarely, and on the introvert they
are present only in the papillce, the interspaces being entirely free from
them. Each papilla of the posterior zone of the introvert shows in
surface views an irregular double or triple row crossing the convex outer
surface near the base, and occupying one half to one third of its entire
breadth. Rarely isolated bicellular glands are found near the tip. This
regular limited distribution allows perhaps a conjecture as to their
possible function. Inasmuch as the behavior of the secretion toward
coloring reagents would seem to mark it as mucine (cf. Hoyer, '90), may
it not be that these glands furnish the lubricant demanded by the con-
stant movements of the two walls of the introvert 1 The papilla? are
especially affected, of course, rubbing against each other in the con-
stant inversion and eversion. They receive, furthermore, the greater
part of the pressure as the animal forces its way through the sand,
in the method described by Andrews ('90, p. 391). The animal does
not advance backward with the "Eichel voran," as maintained by An-

MUSEUM OF COMPARATIVE ZOOLOGY. 155

dreae ('81, p. 220) ! The secretion may also be of use in cementing
the sand grains into a sort of tube noticeable when the animals are dug
out of the sand. 1

The multicellular glands present a type easily distinguishable from that
just described. They are to be met with everywhere, not only in the
papillae, but lying in the interspaces as well, and extending up into the
clear zone of the introvert, where they are the only differentiated hypo-
dermal cells. Never much crowded, they become here sparser, until they
completely disappear at the level of the upper papillate zone ; nor are
they to be found in or above this zone, nor at any point on either sur-
face of the tentacular fold. The multicellular glands may be identified
on surface preparations, but an insight into the histological relations is
first afforded by sections. With hsematoxylin the cell body stains lightly
but uniformly, the mass at the distal end more deeply (Fig. 12), but
with this stain no nuclei can be found either in the cells or in the con-
nective-tissue investment of the gland. Each gland is seen to be made
up of a number of flask-shaped cells, which are separated by thin par-
titions and which unite at their distal ends into a duct piercing the
cuticula and opening upon its surface to the exterior. Andreae ('81,
p. 216) was unable to find any nuclei in these cells. The application
of a carmine stain, however, shows their presence near the proximal ends
of the cells (Fig. 14), where they often lie flattened against the cell
membrane by the crowding of the granules accumulated in the cell
plasma. The same stain demonstrates also (Fig. 13) smaller nuclei at
various points in the connective-tissue investment. There is likewise
seen to be a difference in the cells of any one gland which indicates
alternation in secretive activity. Thus the plasma of some cells is
thickly crowded -with large granules, which are entirely wanting in other
cells. This is most clearly demonstrated in a transverse section of the
gland, as shown in Figure 13. The cells differ in intensity of color to
correspond with the number of granules present, and large distended cells
are found near those which are evidently thinner and poorer. The pro-
duct of these glands is a substance more waxy than fluid, to judge from
its manner of caking in the duct, and breaking up into small fragments,
like sebaceous material. Tts discharge is evidently gradual like its pro-
duction ; for I have never found a gland empty, nor does the total
amount of secretion present vary greatly. This alternation in func-
tional activity between the various cells of one gland and the constancy

1 For this suggestion, and the observation that such a tube exists, at least for
S. Gouldii, I am indebted to my friend, Mr. C. B Davenport.

156 BULLETIN OF THE

of secretion from the gland as a whole stand in strong contrast with the
resting and active stages in secretion as found in the bicellular glands.
The function of the secretion from the multicellular glands is probably
more general, since the glands are so uniformly distributed over the sur-
face of the body. 1

One of the most peculiar points in connection with these glands is
their relation to the nervous system. In almost every instance, a nerve
fibre can be clearly traced from the subdermal plexus to the proximal
end of the gland, and on fortunate sections (glJ" n.flr., Fig. 14) it was
possible in a number of cases to demonstrate an' actual connection be-
tween gland cell and fibre, in that the former was prolonged into a deli-
cate fibril, which, passing out from the glandular cavity in company with
similar fibrils from the adjacent gland cells, entered within the neuroglia
into the substance of the nerve and appeared to make up its fibrillar
structure. This connection of gland cell with nerve fibre is found in
all regions of the body, and is not confined, as Andreae maintained, to
the posterior tip (Eichel) of the animal. In spite of this direct nervous
connection, there seems to be little ground for regarding these struc-
tures as sensory organs, the interpretation put upon them by Leydig
('61) and others after him. A careful examination brought to light
only the single kind of cells, which are in no way comparable with
sensory cells. On the other hand, it may be said that a rich nervous
supply is not without parallel for glandular structures.

The capsules of these glands are very thick, and nuclei are found on
the partitions between the cells, showing that each cell is enclosed in a
separate investment. But the partitions are never as strong as the gen-
eral sheath of the entire gland, which possesses nearly the optical appear-
ance of muscular elements. The variations in size are* so great, being
from 40 X 50 /x, to 90 X 150/t in the same region of the body, that the
probability of a muscular capsule suggests itself strongly.

Allusion has already been made to the relation of the glands to the
papillae. In each papilla of the posterior zone, one finds at its tip an
indefinite crowded mass of multicellular glands, and in an irregular
double or triple row across the basal half, the bicellular variety. All of
these open upon the external convex surface of the papilla. That the
relation of glands to papilla is an intimate one, first appears clearly from
the formation of the latter. As it is evident that new papillae must be
added with the growth of the animal, it is of interest to note the steps
in the formation of these structures. The first indication is an evident

1 See Addendum.

MUSEUM OF COMPARATIVE ZOOLOGY. 157

crowding of the otherwise scattered multicellular glands in the centre
of some interspace ©f more than average size. Then the bicellular glands
make their appearance as a loose double row, and so quickly that no
intermediate stage could be found. They grow more crowded, and soon
after their appearance a shallow furrow may he seen to enclose the
mammiform area which they occupy. The skin seems to be tucked in
on the three sides at once, and as the furrow grows deeper the papilla
becomes more and more prominent. The growth in any papilla is in-
crease in breadth rather than in length, so that the relative dimensions
gradually change, and the older papillae in any region are markedly
wider than those more recently formed, while the length remains nearly
constant throughout the entire zone.

Sense Papil/ce. — The papilla? of the anterior zone are thickenings or
modifications of the hypodermis, rather than typical papillae like the
posterior ones ; they correspond probably to the " Wimperdrusen " of
Vogt und Yung ('88, p. 406). They are externally marked as small
rounded prominences of the skin, varying in diameter from .15 to .40 mm.,
and often exhibit an oval or dumbbell-shaped opening in the centre of the
prominence. Viewed in cross section (Plate II. Fig. 18) they display an
evenly rounded contour, which is surmounted by cilia. These are short
on the lateral margins of the area, but increase in length as they approach
the apex, where they are longest. If one notices the basement mem-
brane, here for the first time well developed, it will be seen that the
prominence is almost entirely due to the increased height of the hypo-
dermal cells, which have changed their form from that of the usual hypo-
dermal elements so as to assume the character of filamentous cells, such
as compose the hypoderm of the tentacles, with which they are identical.
The isolated elements of the latter (Plate II. Fig. 21) might, indeed, an-
swer equally well as types of these cells. In addition to the elongated
nuclei of these cells, some few rounded ones are seen scattered between the
filamentous cells, more usually near the basement membrane. Perhaps
more common than the normal expanded form of the papillae, just de-
scribed, is the retracted condition shown in Figure 17. Such are found
in all degrees of contraction, alternating irregularly with the normal form.
The papilla figured is perhaps fully retracted, and one notes that the ap-
ical area lies sunk in the structure, so as to give the effect of a cavity and
a duct. That this is due in part to the contraction of the cells themselves,
and in part to the retraction of the central portion of the papilla, is clear
from a comparison of Figures 17 and 18. In spite of this, I was unable
to identify any muscular elements connected with the organ, the many

158 BULLETIN OF THE

fibres which are attached to the proximal side of the basement membrane
being, in refractive power and other optical properties, and in the char-
acter of their nuclei, indistinguishable from the other cutis fibres. One
often finds such an appearance as is given in Figure 16. This is evi-
dently a tangential section of a similar organ ; the central clear space
represents the hollow produced by the retracted apical area, and the
apparently round nuclei are merely the elongated forms transsected. The
appearance of the cells suggests no glandular nature, and nothing could
be found resembling a secretion. For this reason I am inclined to ques-
tion the propriety of the name " Wimperdriisen " (Vogt und Yung, '88,
p. 40G), and to regard them as simple sensory organs. The retraction of
the apical area would then be a simple method for protecting the long and
delicate cilia during the advance of the animal through sand, similar to
that reported by other observers for such organs in various groups. I was
unable to discover any nerves connected with these organs, so that their
sensory nature remains unproved, although none the less probable (Eisig,
'87, p. 548). The structures just described are distributed over the
aboral surface of the tentacles in somewhat irregular lines, becoming less
frequent toward the margin of the fold, but are not present on its oral
aspect. They suggest strongly the diffuse sensory organs (Becherorgane)
of Capitellida?, described by Eisig ('87, p. 547), but they are certainly
less highly differentiated in the following respects : — 1. The cilia are not
confined to the apical area (Polfeld), but are more or less diffused over
the entire prominence. 2. There are only a few of the nervous nuclei
(Korner) present in the basal portion. These structures recall the cup-
shaped organs of CapiteUida) most strongly in the character of their ele-
ments, the filamentous cells, in their relation to the general hypodermis,
and in the thin cuticula which covers them. In both cases, connection
with nervous elements remains a matter of conjecture.

Very similar organs have been described by Spengel ('80, p. 465) for
Echiurus, as appears at once from a comparison of the figures given by
that author (Taf. XXIV. Figs. 21, 22). These, however, differ materi-
ally from those in Sipunculus in two respects : first, no cilia were present
(Spengel believes them to have been lost through poor preservation); sec-
ondly, a fact of more importance, a large number of unicellular glands are
found immediately below and in connection with these organs in Echiu-
rus. The latter are certainly not present in Sipunculus. The distribu-
tion of these organs is quite different in the two forms, since there occur
from one to seven on each of the papillse of Echiurus, whereas in Si-
punculus they are confined to the small anterior zone of the introvert.

MUSEUM OF COMPARATIVE ZOOLOGY. 159

e. Muscular Layers.

Of the muscular layers the diagonal is not present in the introvert.
The circular layer, which is banded throughout the rest of the body, fuses
at the end of the integumentary areas into one continuous sheet, and
grows gradually less important anteriad, being almost entirely wanting
in the anterior zones. The longitudinal muscular bands do not fuse
until the middle of the posterior papillate zone is reached. From this
point anteriad they also become reduced so that in the smooth zone the
entire muscular layer measures but 70 to 90 /x in thickness. This rem-
nant passes over into the retractors in a manner to be described in
treating of the tentacular musculature.

2. Tentacular Fold.

A cross section of the tentacular fold shows that it consists of two
layers of skin, which form the oral and aboral walls of an irregular cav-
ity, traversed perpendicularly by numerous trabecular binding the two
sides together (Fig. 3). This cavity is the extension of the so-called
blood system, and is often found more or less filled with a coagulum.
The character of the oral and aboral walls of this cavity differs : the
structure of the oral portion will be considered first.

a. Oral Wall.

The cuticula (Plate III. Fig. 23) is extremely thin, never exceeding
2 /a, and usually appearing as a fine double contour. It is pierced by
many pores for the exit of the fine cilia, which cover this surface from
the apex of the fold down into the mouth. Evidently the inversion of
surfaces in the retracted conditiou of the introvert led Selenka ('83,
p. xvii) and others to regard the oral surface of the tentacles as with-
out cilia, and to maintain that the aboral surface was ciliated, exactly
the reverse of which is true.

The hypodermis (Plate III. Fig. 23) is composed of very high cells,
which are in contact merely by their distal ends. Proximally they are
prolonged into delicate processes, by which they are attached to the base-
ment membrane. These cells are of the type of filamentous cells (Haut-
fadenzellen) described by Eisig ('87, p. 300). Lying nearly in the centre
of the cell is the elliptical granular nucleus, which measures 1 1 by 4 /i.
These cells are exactly similar to those contained in the sensory organs
before described. Some such cells are seen in Figure 21, d, /(Plate II.).
In addition to these there are occasional cells in the hypoderm, the nuclei

160 BULLETIN OF THE

of which are narrower and stain much deeper, which possess a denser,
more highly refractive cell body. Figure 21, a, c, e, represents these
cells, which are seen in situ at cl. sns., Figure 23. These may be sensory
cells ; I was, however, unable to discover the sensory hairs described by
Selenka ('83, p. xvii) as found on the external surface ; these cells
certainly possessed merely such cilia as those adjacent. At the level
of the mouth there is a transition from these filamentous cells to the
columnar cells of the intestinal tract. This serves to fix the level of the
oral opening proper, which would otherwise be indefinite on account of
the various degrees of expansion or contraction of the animal.

b. Migratory Corpuscles.

Between these filamentous cells are found at varying heights highly re-
fractive spherical nuclei 4 fx in diameter. My attention was first called to
them in a preparation stained by Ilamann's carmine (Plate II. Fig. 15),
where they become prominent by reason of their being stained deeper than
the other nuclei. A more careful examination showed that they were
not accidental, as at first surmised, but definite independent structures.
Each is surrounded by an irregular clear zone varying in width from a
mere line to one half the diameter of the nucleus. By means of these
peculiarities, such cells were traced back through the cutis, where they
were most abundant in the spaces just below the basement membrane,
to the blood cavity, and were found to agree precisely in size and optical
character with one kind of blood corpuscle found in the coagulum there.
They may then be regarded as migratory corpuscles or leucocytes,
analogous perhaps to those of vertebrates. Similar cells are often met
■with, though never in such numbers, throughout the body wall.

The thin basement membrane to which the processes of the filamen-
tous cells are attached is not everywhere equally distinct. Owing to the
contraction of the different areas, it maybe thrown into extensive and
complicated folds, which, combined with the basal processes of the fila-
mentous cells, render its identification a matter of difficulty, but in
suitable regions it may be identified beyond a doubt.

Beneath this membrane lies a cutis, very similar to that of the body
wall. It differs chiefly in the scarcity of pigment cells and in the en-
tire absence of glands. The " Wimperdriisen " seen by Vogt und Yung
('88, p. 406) on the oral surface of the tentacles, are merely appearances
due to unequal contraction of certain areas, which produces structures
superficially similar to the sensory organs of the anterior papillate zone
already described. The cutis is further peculiar in the possession of

MUSEUM OF COMPAEATIVE ZOOLOGY. 161

numerous muscular elements, which are primarily arranged about the blood

cavity. The relation of these to the body musculature is of considerable

interest.

c. Musculature.

If a transverse section be made in the plane of the annular mass of
muscle surrounding the pharynx which is produced by the fusion of the
four retractors, there appears only an indehnite mass of confused hbres.
If, however, the section be cut in any longitudinal plane, it will be seen
that the longitudinal fibres which compose this mass divide into two
unequal parts, each of which draws its fibres from all parts of the origi-
nal muscular mass. In such sections each of these portions appears like
a band ; the smaller curves over into the muscularis of the body wall of
the introvert, or rather goes to form the longitudinal muscles of this, its
fibres being directly continuous with those of the predominant longitu-
dinal layer. The other and larger portion ascends into the tentacular
fold ; a few of its fibres follow the aboral surface of the blood cavity, but
by far the greater number are continuous as an apparent muscular band
along the oral side of the cavity immediately adjacent to the latter.
At the base of ttye tentacular fold it is thickest, measuring half the
thickness of the oral wall in which it lies ; but as it advances distad
through the tentacular fold, fibres are continually given off peripherally,
so that they radiate toward the surface. These terminate in the vac-
uolated portion of the cutis in some manner not exactly determined.
In this way the muscular band becomes looser and looser by the gradual
loss of its elements, until at the tip of the tentacle only a few fibres
remain, which attach themselves there. In cross sections the tentacular
fold shows a few circular fibres, immediately adjacent to the blood cav-
ity, which turn into the trabecule and cross into the corresponding layer
of the opposite wall of the fold. In addition to these, the trabecular
have other fibres which cut the muscular band at right angles, and run
from one side of the tentacular fold to the other. At the outside of
the muscular band there can be found usually a few circular fibres. If
one considers that this muscular band, prominent in longitudinal sec-
tions through any plane, thus represents a muscular sheet extending
throughout the whole tentacular fold, and that this lies in the cutis
of the oral, or in an expanded condition convex, surface of the blood
cavity, with its fibres radiating into every fold of the tentacles, — if one
remembers, further that it is at its base connected with the fused re-
tractors, and is in fact merely an extension of them, — then its action
in .the inversion of the tentacles by drawing in and packing together

vol. xxi. — no. 3. 11

162 BULLETIN OF THE

the various folds and plaits becomes at once clear. Furthermore, the
muscles concerned in emptying the blood cavity are primarily the power-
ful trabecule and the longitudinal muscles, whereas the circular muscles,
which are comparatively scanty, are only of sepondary importance.

The cutis of the oral fold contains also numerous vacuoles in groups
near the basement membrane, and these may be seen in transverse sec-
tions filled with the migratory cells previously mentioned. In addition
to these small leucocytes, occasional larger granular cells are found in
the lacunae. These correspond again to the granular corpuscles of the
blood. They do not make their way into the hypodermis. A tissue
which might be homologized with the supporting tissue of Phoronis does
not, according to my observations, exist in this form. 1

Lastly, lining the blood cavity and covering the trabecule is an endo-
thelium of very flat cells with proportionally large nuclei. This en-
dothelial lining is continuous, and is adjacent to a mass of gelatinous
connective tissue, which is without vacuoles, so that the blood corpuscles
could reach the hypodermis only by a definite migration through the
endothelium and the connective tissue. The cavity is often distended
by a coagulum which contains corpuscles that, as various writers have
maintained, actually differ in size from those of the coelomic fluid, so
that a connection between the two cavities was regarded as improbable.
I can confirm the statement of previous writers that no such connection
exists. Yet as the corpuscles are in size between the extremes of those
in the coelomic fluid, and not far from the average (rf. the exact meas-
urements of Brandt, '70, p. 3), it is not improbable, in view of the
migratory tendency of the corpuscles already described, that the coelomic
fluid receives its quantum from the blood system by the active emigra-
tion of the corpuscles which are formed in that system. This was con-
jectured by Brandt ('70, p. 24), who had found, however, no evidence of
such a tendency on the part of the corpuscles. The detailed and careful
account of the vascular system given by him has been overlooked by
many later investigators.

d. Vascular System.

In contradistinction to Sipunculus Gouldii and to Phascolosoma, the
blood cavities of S. nudus are not in the form of regular vessels, but
are indefinite lacunar spaces, traversed by trabeculee at irregular inter-
vals and extending throughout the whole tentacular fold, everywhere
almost equally distant from the exterior.

1 See Addendum.

MUSEUM OF COMPARATIVE ZOOLOGY. 163

Numerous facts have been adduced by Andrews ('90, p. 419) to prove
the branchial nature of the tentacles in S. Gouldii, chiefly the circulation
and the red coloring matter of the corpuscles. Certain structural and
other peculiarities compel me to deny the respiratory nature of this
system in S. nudus. As was pointed out by Brandt ('70, p. 23), the
extreme thickness of the layer of connective tissue in the tentacles
would militate against the opinion that respiration takes place to any
considerable extent in this part. Furthermore, although I have watched
S. nudus in aquaria for considerable periods of time, not only when they
were lying upon glass, but also when they were on the surface of the
sand, and in their burrows wherever these were adjacent to the glass so
as to permit observation, I have seen the tentacles extruded but seldom,
and never for more than a second or two, until the water had become so
impure as to partially narcotize the animals. The respiratory value of
the tentacles when retracted cannot be regarded as very important !

But the greatest objection to assigning a respiratory character to this
system would seem to be the utter inadequacy of the internal mediation
between the vessels and the ccelomic fluid. The possible importance of
this system in a respiratory direction must be seriously questioned when
one considers that the ring canal and the two blind sacs (in S. Gouldii
but one !) buried in the connective tissue of the oesophagus, which at
best expose but one half their surface to the ccelomic liquid, are the
only means of transmitting oxygen from the so-called vascular system
to the general body fluid. The observations of Keferstein ('62, p. 47)
upon living animals — these were made on Phascolosoma elongatum of a
few millimeters in length and fully transparent — showed a constant move-
ment of the fluid, but no passage of it from the vessels into the tentacles,
or vice versa, except under considerable pressure or violent injuries.

The probable lack of respiratory function in the vascular system can-
not be extended to all Sipunculids. In this connection it is of great
interest to notice that various species are provided (Selenka, '83, p. xix)
with several or many branched lateral appendages attached to the blind
sac. Such organs are found in Phascolosoma Semperi, P. maniceps,
Phymosoma asser, Dendrostoma signifer, et al. All of these forms pos-
sess, according to the same author, long thin filamentous tentacles
(cf. his generic descriptions and figures). This peculiarity suggests at
once the probability of a respiratory nature for the tentacles ; and its
occurrence in single species of various genera would indicate that it is a
secondarily acquired function. 1

1 See Addendum.

164 BULLETIN OF THE

The numerous dermal canals close under the hypodermis of S. nudus
are unquestionably of great value in respiration, and the region of the
introvert, which is distinguished by thin cuticular and muscular layers,
actually not so thick as the walls of the tentacular fold, presents a far
greater surface for the transmission of oxygen directly to the coslomic
fluid than the entire vascular system.

Primarily, then, this system is hydrostatic, and this is probably its
chief function in S. nudus. The dorsal and ventral vessels are reser-
voirs into which the fluid is driven by the contraction of the tentacular
fold. Ou the other hand, the muscular walls of these vessels serve to
furce the fluid out into the lacunae of the tentacular fold, and thus to
move and expand the latter. The varying contraction of these two sets
of muscular elements gives rise to the constantly changing form of the
tentacles, as the fluid is driven to and fro. This movement might easily
simulate, or even under certain conditions become, a circulation. More-
over, any method of killing which worked violent contraction would dis-
tort the tentacular fold by driving the fluid into the extreme distal ends
of the lacunae, or by drawing together the entire mass of the tentacular
fold, and forcing the fluid back into the dorsal or ventral vessel. It is
probably in this way that the lobed or cut form was produced, which
has been given as the typical one in all generic descriptions hitherto
published. It is worthy of notice that those animals which were killed
with expanded tentacles showed the walls of both dorsal and ventral
vessels almost in contact, whereas in those which had retracted their
tentacles these vessels were so filled by masses of coagulum as to reach
a considerable diameter. The probable function, then, of the dorsal and
ventral vessels is to receive and hold the fluid forced out of the tenta-
cles at the time of inversion of the introvert.

e. Aboral Wall.

The aboral wall of the tentacular fold differs from the oral chiefly in
the undifferentiated condition of the hypoderm. The latter is here com-
posed of low non-ciliated cells, identical with the hypodermis of the
general body wall, except where it is elevated into the papillae or sensory
organs already described. Sensory cells are wanting. The cutis of the
aboral wall lacks the vacuoles which characterize that of the oral wall,
and there are only very few leucocytes to be found.

The thin cuticula, cilia, and sensory cells of the oral, as well as the
<reneral sense organs of the aboral, wall of the tentacular fold, show it to
be a most important organ of touch. This view is strengthened by the

MUSEUM OF COMPARATIVE ZOOLOGY. 165

large nervous supply it receives from the brain direct. The cilia un-
doubtedly aid in propulsion of food particles into the mouth.

3. Nervous System.

a. Brain.

The supraoesophageal ganglion {g:i. su'ce., Fig. 3) lies dorsal to the
blood sinus, between the two dorsal retractors, and is enveloped by an
investment of connective tissue. The posterior surface (Plate II. Fig.
22) is marked by a considerable incision in the median plane, and the
anterior dorsal margin bears numerous digitate processes, which project
into the ccelomic liquid. In sagittal sections the brain appears nearly
flat on its ventral side, whereas the dorsal aspect is considerably curved.
As seen from transverse sections, however, the dorsal surface is plane,
while a deep median furrow (Plate III. Fig. 25) penetrates the ventral
wall. Posteriorly (Fig. 25) this is continuous with a partition which
divides the brain into two symmetrical lobes. Anteriorly (Fig. 24) the
partition fails, and the division is only indicated by the furrow. On the
antero-ventral surface is the termination of the cerebral canal (cf. infra).

The entire ganglion is covered by a capsule (Fig. 25, cps. eric), the
origin of which can only be determined by the consideration of a series
of transverse sections. Following such a series from a short distance
posterior to the brain, it will be seen that the septum joining the two
dorsal r-etractors is here fused with the dorsal wall of the oesophagus, in
which lies the dorsal vessel. As the posterior extremity of the brain is
reached, this septum rises upon the brain, covering its dorsal aspect, and
still showing laterally the connection with the dorsal retractors. Imme-
diately inferior to the brain lies the dorsal vessel (Fig. 25, va. sng. (/.), or,
anterior to this, the blood sinus, which is separated from the brain only
by its own wall, which thus forms the ventral covering of the brain.
The dorsal and ventral layers of the capsule are continuous on those
parts of the lateral aspect of the brain where there are no outgoing
nerve stems ; but when the latter exist, a neighboring portion of the brain
capsule is reflected over them to form the neurilemma (Fig. 25, co/it. tis.).
This composite capsule is made up of a loosely woven mass of fibres which
often show a plaited arrangement. The discoid nuclei measure 3.5 by 5 /x,
and are deeply stained in all coloring fluids. Inferior to this basketwork
of fibres are found occasional nuclei, which stain very faintly and pos-
sess each a few small nucleoli. These are surrounded by a small amount
of a granular substance, and are very similar to nuclei found in the midst

166 BULLETIN OF THE

of the meshes of the brain itself. Passing inward from this external cap-
sule and directly continuous with its elements, fibres penetrate the brain
in every direction, either in definite strands, or in a delicate network
surrounding the ganglionic cells (Fig. 25). The fibres which make up
these meshes are finer than most of those composing the capsule itself,
and recall strongly the finer elements of the cutis. With these they
also agree in the possession of minute elongated nuclei, although the
clear nuclei previously mentioned are by no means rare. These fibres
surround each ganglionic cell with a definite covering (Plate III. Fig. 32)
of interlacing elements from which others pass oil' tangentially to neigh-
boring cells. (Cf. Kohde, '87, Taf. IV. Fig. 44-68.)

Ganglionic Cells. — None of the many previous writers on Sipunculus
have considered the histological elements of the central nervous system
more than cursorily, so that a more extended description of these may
be of interest, especially for comparison with the recent exact deter-
mination of these elements in many other groups of worms. All the
ganglionic cells which were so situated as to admit of a positive answer to
the question of their polarity were unipolar, though by no means always
unifilar. Such cells as were accurately determined were usually periph-
eral, since the mass of other fibres and the confusion of many cells make
an accurate determination in the case of those cells which are located in
the centre of the nervous mass often impossible. I am inclined to think
that in the latter region there are multipolar cells, although the demon-
stration of these was not wholly satisfactory. The cells are uniformly
without any proper cell membrane. Each lies enveloped in a covering
of delicate connective-tissue fibi-es (Plate III. Fig. 32), which accom-
panies the fibrous processes in the form of a delicate sheath (neuroglia).
These enveloping fibres are a part of the meshwork which has already
been described as arising from its external capsule, and penetrating
through the whole brain.

Of all the ganglionic cells, the smallest (Plate III. Figs. 24, 25, cl. gn.
I.), which usually appear simply as nuclei measuring 6 by 4 //. (Fig. 30),
are the most abundant. They are highly refractive, and show a great
affinity for coloring matter. There is a nuclear membrane which is
stained deeply, as are also the numerous (4 to 1Q) nucleoli ; between the
latter many minute chromatine granules are distinguishable with a
high power. These nuclei seem somewhat irregular in shape, varying
from circular to oval. This variation I regard as due to the direction of
the plane of section, and consider the true form as oval. In most cases
it is impossible to find even a trace of a cell body, and I was at first

MUSEUM OF COMPARATIVE ZOOLOGY. 167

inclined to doubt the presence of any recognizable cell substance, and
consequently to compare them with the " Nervenkerne " described by
ltohde ('87, p. 30). But at length fortunate staining and thin section-
ing showed unmistakably the presence, in many cases at least, of an ex-
tremely small cell body, such as is shown together with the nerve fibre
in Figure 31. It will be noticed that the nucleus is oval in this case,
and that the nerve fibre proceeds from one of the small ends of the oval.
This fact, as well as the variation in form noticed by careful focusing on
the nuclei, would seem to warrant the assumption that these nuclei are
uniformly oval. From the diminutive size and transparency of the cell
body in comparison with the highly refractive nucleus, it is at once evi-
dent that the former can be seen only under the most favorable circum-
stances. Since I was unable to find any difference in position, size, or
optical qualities between these and the other nuclei of similar size and
appearance, I feel justified in maintaining the existence of such a cell
body for all nuclei of the class.

The second sort of ganglionic cell (cl. gn. II) is distinguished by the
presence of a much larger cell body (Plate III. Fig. 29). The nuclei
correspond so exactly to those of the first class that they can hardly be
distinguished from them. I was unable to see that they were either
more or less deeply stained, or that they were, on the average, larger or
smaller than nuclei of the first sort. The great difference is in the cell
bodies, which in this case are several times larger than the nucleus,
measuring 20 by 14 /a, and always evident on account of their slight affin-
ity for stains. One or more vacuoles of non-colored matter, the para-
mitome of recent writers, may always be found, and in favorable cases
there can be seen such a distribution of these as is shown in Figure 32.
The paramitome exists in the form of numerous peripheral vacuoles sub-
jacent to the enveloping connective fibres, and possibly (?) surrounded
by them. The nucleus lies in a zone of clear matter, while the mitome,
or filar substance, appears densest external to this. Between these and
the first sort of ganglionic elements there exists every possible transi-
tion, so that this class is but poorly marked off from the preceding one.
The vast majority of these cells are, however, of approximately uniform
size, and I therefore cannot agree with Nansen ('87, note, pp. 113, 114)
when he maintains that such transitional forms forbid the grouping of
these cells in different classes. Such intermediate forms serve rather to
explain the development of the one type from the other, without detract-
ing from the individuality of either class.

168 BULLETIN OF THE

The third type is that of the large x cells, represented in Figure 28,
Plate III. These vary considerably in size and shape ; the mean longi-
tudinal diameter is 55 /x, the transverse 40 fx. The cell protoplasm
stains rather deeply, and is notably granular. These granules resolve
themselves in the nerve processes into fine lines. Each large ganglionic
cell contains a number of clear spaces, the paramitome, exactly similar
to those already described for cells of the second class ; and these are
often arranged concentrically, and more or less regularly along the
periphery of the cell. The single nucleus, 15 by 12 ^u. in diameter, is
usually found nearer the end of the cell from which the nerve fibre
emeiges, and, in contrast with those already described, is stained only
lightly. A nuclear membrane is very distinct, and there is one large
nucleolus 2-3 /x in diameter. In rare cases two smaller nucleoli were
found, never more. The nucleus also contains numerous fine granules
of chromatine, which are very distinct in the matrix, which remains
completely unstained. Xuclei of this class are not infrequently cres-
centic, with a clear space enclosed by the horns of the crescent, corre-
sponding exactly to such forms as are figured by Rhode ('87, Taf. IV.
Fig. 51 et at.). Although variable, these cells represent a more isolated
type than either of the other classes, and intermediate forms, especially
in nuclear appearance and structure, are rarely seen in the brain.

An examination of the ventral nerve cord in transverse section shows
a preponderance of cells of the second class. The first class is poorly
represented, though the size of the plasmatic portion varies greatly in
different cells. Occasionally one finds cells which in their deep staining
and nuclear appearance recall the large cells of the brain. But meas-
urements showed one such cell to be only 24 by 30 fx and its nucleus
7 by 11 fx in diameter, dimensions which are far smaller than those of
the average of the large cells in the brain. These cells do not seem to
be regularly arranged in the ventral nerve cord, and no grouping could
be found which suggested metamerism. The peripheral nervous plexi
possess very few ganglionic elements, and these few are not reducible to
the types present in the central nervous system, for they are invariably
multipolar, and are situated at the crossing or branching of fibres.

1 It is much better, for the sake of clearness in neurological terminology, to
keep the term "giant cells" (Riesenzellen) for the huge elements in the ner-
vous system of Nemertines, Annelids, et a/., as German writers have done, than,
with Shipley ('90, p. 16) and Andrews ('90, p. 424), to apply the term to such
cells as I have placed in the third class, to which the former are at most only re-
motely homologous.

MUSEUM OF COMPARATIVE ZOOLOGY. 169

The internal structure of the brain shows a strictly bilateral arrange-
ment of the elements. A transverse section through the middle of the
ganglionic mass is represented in Figure 24 (Plate III.). The fibrous
matter is collected into two commissural masses, in which the fibres run
both anteroposteriad, chiefly at the lateral extremities, and laterally,
chiefly in the middle. The real relation of these commissures to each
other is first seen in sagittal sections (Plate II. Figs. 19 and 20), where
the fibrous matter has the form of a > with the apex directed forward.
The dorsal arm of this > is prolonged backwards in two lateral horns,
which are surrounded by ganglionic cells. The tips of these horns, cut
transversely, are seen in Figure 25 (Plate III). The similar ventral
horns are the roots of the circumcesophageal connectives. From near the
anterior apex of the > a small arm of fibrous matter is directed forward,
as seen in a sagittal section of the brain near its left lateral margin (Plate
II. Fig. 19). This becomes, in a median sagittal section, a small commis-
sure cut transversely (corns, a., Fig. 20), and separated from the brain by
the connective-tissue capsule. This commissure is at its right side again
'connected with the brain, as already described, for the left extremity.
Thus it resembles in its form and relation to the main fibrous mass of
the brain the handle of a basket, the handle being directed forward.
It lies, as can be easily seen from the figures, immediately below the
surface of the cerebral organ, and its relation to that structure will be
more fully explained later.

The arrangement of the ganglionic elements in the brain is somewhat
definite. Ganglionic cells of the first sort are found in nearly every
part, and make up all diffuse centres, where, however, transitional forms
render their separation from the second class difficult. The former are
most strongly marked at the tip of the dorsal horn (cl. gn. I., Fig. 25),
where they are very densely crowded. They cover also the lateral and
dorso-lateral aspects of the dorsal commissure (Fig. 24) in similar dense
masses. The anterior face of the fibrous matter is also almost exclu-
sively occupied by cells of the first class ; and from this region they
extend a short distance ventrally. Here one finds a gradual transition
into the ganglionic cells of the second class (cl.gn. IT., Plate II. Fig. 20,
Plate III. Fig. 24), which occupy the entire ventral and posterior as-
pects of the fibrous matter. These cells also fill the space between the
dorsal and ventral commissures, but are found dorsally only between
the two lateral fibrous swellings on the lateral edges of the dorsal
commissure. They are never so crowded as cells of the first class, and
display no particular arrangement into clusters or groups.

170 BULLETIN OF THE

The large ganglion cells of the third class (cl. gn. III.) are present in
a somewhat limited number, and always in a definite position. They lie
in the posterior third of the brain, on its medial posterior boundary
(Figs. 19, 20/25). The large fibres which pass off from these cells are
easily seen to turn toward the opposite side of the body and to make
their way into the veutral commissure, where they are lost to view,
either because they are split up into a number of small ones, or from
some other cause sutler a diminution in diameter. This crossing of
fibres from cells on one side of the body to the connective 1 of the other
certainly does not take place frequently in either of the other two groups
of ganglionic cells. Wherever circumstances permitted the following of
nervous processes in groups I. and II., these were seen to pass off towards
the connective on the same side of the body as the cell itself.

b. Cerebral Nerves.

From either side of the brain two groups of nerves pass off; the an-
terior consists simply of the first tentacular nerve (n. ta. 1, Fig. 22,
Plate II.) ; the posterior contains the second, third, and fourth tentac-
ular nerves and the oesophageal connective. The tentacular nerves
radiate from the brain to the aboral wall of the tentacular fold, and,
splitting there into numerous branches, follow the aboral wall of the blood
cavity toward the distal margin of the fold. The first tentacular nerve
supplies that portion which was designated as the dorsal horn. Follow-
ing the margin of the fold from this region toward the ventral line, its
successive parts are seen to receive their nerve supply from the third,
second, and fourth tentacular nerves successively. Each of these in-
nervates about equal poi-tions of the fold. I was unable to trace the
ultimate termination of the nerves in this region.

The oesophageal connectives give off each three branches : (1) the
splanchnic, (2) the muscular, and (3) the inferior muscular. The
splanchnic is given off ventrally and medially immediately after the
connective leaves the ganglion (n. spl, Figs. 22, 25). It passes diag-
onally forward, — not posteriad, 2 as stated by other writers? — and into

1 I use the word connective in the sense first suggested by Lacaze-Duthiers, to
distinguish the nerve fibres joining ganglionic nerve centres which are on the same
side of the body, reserving the word commissure for such fibres as cross the median
plane of the body.

2 This nerve is turned backward in Figure 22, for the sake of clearness in the
drawing. Normally, it extends forward under the ganglion.

3 This relation is obscured when the introvert is slightly retracted, and even
apparently reversed when the retraction is greater.

MUSEUM OF COMPARATIVE ZOOLOGY. 171

the circular muscles of the pharynx, where it terminates in a distinct
ring at about the level of the middle of the brain.

At the point where the splanchnic nerve forms a ring around the
pharynx one finds a few nerve cells, but they are few in number, and
hardly deserve the name of a ganglion. From this ring it is easy to
trace in serial sections the stems of the intestinal plexus, which are
here large. This plexus lies in the connective tissue of the intestinal
wall, and was first described by Andrews ('90, p. 405) for S. Gouldii.
However, he failed to find a splanchnic ring, or any anterior connection
of the plexus with the central nervous system.

The muscular branch (n. mu. ret.) passes off laterally from the middle
of the oesophageal connective, and divides near the centre of the fused
mass of the dorsal and ventral retractors into two branches, one of
which traverses each retractor. Not far behind this branch there is
upon the connective a small trunk (*, Fig. 22), which passes to the sur-
face of the muscular mass, but which could not be traced farther. It
remained doubtful whether this was a subsidiary muscular branch or of
other value.

c. Ventral Nerve Cord and Plexi.

After the union of the two connectives, the ventral nerve cord thus
formed floats a short distance free in the body cavity, and sends off nu-
merous long nerves to the body wall. The first of these, the composite
nerve of Andreae ('81, p. 248), is by no means always composed of eight
branches in a single sheath, as stated by that author. The number varies
from six to nine, and the size of the different trunks varies as w T ell (Plate
II. Fig. 22, /.). In fact, the later branches, which according to him
consist of two trunks, one from each side of the nerve cord, not only show
great variability in the size of these trunks (Fig. 22, II.), but also at
times only a single trunk can be found, which then comes from but one
side of the nerve cord. All these frequent irregularities point to a lack
of metamerism in the nervous system. On reaching the body wall these
nerves branch in a digitate manner through the muscles of the intro-
vert, the main trunks being longitudinal, and do not form nervous rings
around the body as in other parts of the wall. From these longitudi-
nal stems large trunks pass outwai'd through the musculature to the
dermal plexus.

This dermal plexus lies in the cutis at its plane of union with the mus-
culature, and consists of large longitudinal trunks (plx. n. drm., Plate
I. Fig. 4) with lateral anastomoses. From this network, fibres (rm. gl.)

172 BULLETIN OF THE

pass outward through the cutis to the multicellular glands and to the
hypodermal cells, as already described, as well as inward (rm. mil.)
to the muscles. The existence of such a plexus has already been
shown by Andrews ('90, p. 395) for S. Gouldii. To his description,
which answers equally well for S. nudus, I can only add a few obser-
vations as to the histology of the nerve trunks. Each of these pos-
sesses a well defined sheath or neuroglia (n'gl., Fig. 5), in which discoid
nuclei (rCgl. nl.) measuring 2 by 4.5 by G /x are common. These nuclei
lie either inside or outside of the membrane ; they may be stained deeply,
and contain many nucleoli. The substance inside the neuroglia has a
distinct fibrillar appearance, and when these nerve stems were bent
upon themselves so as to be cut transversely and still extend longitudi-
nally within the same section, the fibrilhe appear in the transverse sec-
tion as dots. These are also the fibres which are connected with the
cells of the multicellular glands (///.'" n.fbr., Plate I. Fig. 14).

The existence of the peritoneal plexus found by Andrews ('90, p'. 395)
in S. Gouldii could not be demonstrated in preserved specimens. No
doubt the examination of fresh material will show its presence in S.
nudus as well.

4. Cerebral Organ.

This interesting structure may be considered under two heads : first,
the canal ; and secondly, the surface next to the brain, or the cerebral
organ proper.

The canal opens, as already described, on the dorsal median line, just
posterior to the tentacular fold (can.o.ceb., Figs. 2 and 3). From this
point it extends posteriad about 1.5 to 2 mm., to the anterior ventral
surface of the brain, where it terminates blindly (o. ceb., Fig. 3). From
the marginal fold which surrounds the opening arise numerous longi-
tudinal ridges, which traverse the entire canal, and give it in transverse
section {ran. o. ceb., Plate III. Fig. 20) a branched appearance. In a sur-
face view the walls of the canal appear thickly spotted with brown, and
further examination shows this to be due to the presence of large num-
bers of the characteristic pigment cells, which arc usually seen crowded
in masses along the summits of the ridges (cl. pig., Fig. 20). It is prob-
ably this canal which was found by Keferstein und Elders ('01, p. 47) in
S. tesselatus. The canal is correctly figured (Taf. VII. Fig. 1, 2, ■«, u'),
but they evidently mistook its true character, since they say : " Ausser-
dem sieht man vora Him zum Tentakelkranz einen aus zwei Hal/ten.
bistehenden, dicken Strang verlaufen, dcr dort endet, und an dem End-

MUSEUM OF COMPARATIVE ZOOLOGY. 173

punkte, wie man bei der Betrachtung von aussen her "wahrnimmt, in
cler Haut von einer Gruppe kleiner Falten nmgeben ist als wenn er eine
R'uhre ware und hier nach aussen mundete." 2 Among recent writers,
Vogt und Yung ('88, p. 404) mention and figure the "cerebral canal,"
without a more particular description of its structure or morphological
relations. 2

The histological study of the canal shows some features of interest.
Its entire surface is lined by an extremely thin cuticula, which appears
under high powers merely as a double contour, pierced by numerous
short cilia. The cells of the ventral wall of the canal have the appear-
ance of ordinary hypodermal cells, except that they bear cilia. The
dorsal wall is made up of similar cells near the mouth of the canal,
but these become higher as the brain is neared, until at the middle of
the canal they have assumed the form of a high columnar epithelium
with large nuclei. This condition is preserved up to the surface of the
brain. When examined more closely, these cells are seen to be filled
with granules of a highly refractive nature, especially at their distal
ends, and may be regarded as the source of the more or less extensive
coagulum always found at the basal end of the canal. We have here,
then, the secretive portion of this organ.

In cross sections of the canal (Plate III. Fig. 26) one sees clearly a group
of muscular fibres which is deflected from the circular layer of the body
wall and encircles the canal in the form of a sphincter (spht.), which,
although most marked at the opening of the canal, is present along its
entire extent. The function is evidently to prohibit the entrance of
extraneous matter during the forward motion of the animal, and to

1 The Italics are not in the original.

2 P. S. — Since writing the above, I have obtained access to a preliminary com-
munication by Spengel (77), and find that in this he has maintained " die Existenz
eines vom Gehirn zur Basis der Tentakeln fiihrenden, offenen Canales." Spengel
was thus the first to arrive at the true form of this structure, but I cannot find that
he has anywhere given a more detailed account of its morphological or physiologi-
cal character. In the same paper he says : " Das Gehirn stellt sich als eine knopf-
artige Verdickung des diesen Canal auskleidenden, mit der Epidermis zusammen-
hangenden Epitheles dar." Against this interpretation it may be said that the
embryological evidence of Hatschek ('83) makes it probable that the canal is sec-
ondarily formed. Furthermore, a histological examination of the parts shows that
the brain is less closely connected with the cerebral organ than appears super
ficially, since the brain capsule separates the two completely, except at the entrance
of the anterior commissure, which furnishes the nervous supply to the organ in
question. A full discussion of these relations follows the histological description
of the cerebral organ which is given later.

174 BULLETIN OF THE

assist in changing the water contained in the canal. In the latter func-
tion it would be assisted by the cilia lining the canal.

At its posterior end the canal widens abruptly into a saucer-shaped
cavity, which lies with its concave surface upon the antero-ventral face
of the brain (Figs. 3, 20, 27), and includes a low rounded prominence
(o. ceb.) which I regard as the cerebral organ proper. Microscopically,
this appears to be continuous with the brain, but internally the con-
nective-tissue capsule separates it almost entirely from the ganglionic
mass. The histological character of this prominence, and its relation to
the brain, require more extended consideration.

When one examines a longitudinal section of this region (Plate III.
Fig. 27), perhaps the most striking feature is the extremely prominent
cuticula (4 fj. in thickness), which covers exactly the convex surface,
and only that portion, for at the margin of this convexity (f, Fig. 27)
it passes abruptly over into the very thin cuticula of the canal wall.
At each lateral edge of the cavity there is a considerable thickening of
the cuticula, which extends a short distance into the subjacent tissue
and has in cross section the outline of a small retort. The cuticula pre-
sents a sharp outer boundary, and there one finds no remnants of cilia
in the sections, yet I am inclined to think that cilia are present in the
living animal. For in preserved specimens the entire lower portion of
this canal is filled with a granular coagulum, which might easily enclose
and obliterate cilia, if indeed any were preserved in this deep and nar-
row canal, where fluids evidently could not readily penetrate. The lat-
eral cilia, which are perfectly distinct in the anterior half of the canal,
become gradually less so, until in the lower portion, which is filled with
this coagulum, they entirely disappear. In partly macerated specimens
this thick cuticula breaks up into small blocks along lines extending
perpendicularly to the surface, so that one may reasonably assume that
there is a ciliated condition of this surface in the living animal.

It is difficult to study the cells which underlie this cuticula, inasmuch
as the cell boundaries are very indistinct ; the most evident feature is
the regular row of nuclei which lies close under the cuticula. From
these a crowded mass of nuclei (cl. gn. 1) and fibres extend at right an-
gles to the surface into an irregular group of fibre's (transsected in Fig.
27, corns, a.), — the anterior commissure already described. If one ex-
amines the nuclei, their resemblance in size, shape, and optical proper-
ties to those of the central nervous system is evident. An actual
entrance of the fibres into this anterior commissure can also be easily
observed. The connection of these fibres and nuclei with the hypodermal

MUSEUM OF COMPARATIVE ZOOLOGY. 175

cells is very difficult to prove in sections ; but in a badly preserved and
hence partially macerated preparation there was in many places a defi-
nite continuity of these cells with the fibres and underlying nuclei.
The probability of a direct continuity of the hypodermal cells with the
central nervous system through the anterior commissure seems to me to
be strong evidence in favor of the special sensory nature of the organ.
An examination of its morphological relations also yields much that is
favorable to this view.

The existence of a glandular area, the direct connection of the organ
with the central nervous system, and its median position near the an-
terior extremity of the body, all point to its close relationship to such
sense organs as are cited by Dewoletzky ('87, p. 278), and as are com-
mon in the class Vermes. These have their origin, according to Dewo-
letzky, in " ein Paar flimmernder Hauteinstiilpungen." Whether the
same holds for this cerebral organ of Sipunculus can naturally be de-
cided only upon embryological evidence. Hatschek ('83, p. 115) says
that toward the close of the larval stage two " Wimpergruben " are
formed, one on either side of and near the median line. Further, he
says, " Es sind dies wohl Sinnesorgane die sich wahrscheinlich auch am
erwachsenen Thiere werden nachweisen lassen." These would by their
fusion produce an organ which, in position at least, would correspond
to that which I have described ; and from the absence of any other
structure to which these Wimpergruben can be traced, it is allowable
to assume their genetic connection with this cerebral organ until the
development shall furnish positive evidence on the question. That this
organ might be the apical area (Scheitelfeld) which, by the recession of
the brain from the surface, had come to be connected with the exterior
by means of a canal, is disproved by Hatschek's ('83, p. 108) observa-
tion that there is a complete separation of the ganglion from the body
wall at the time of its retreat ; according to the same author, the forma-
tion of the Wimpergruben was subsequent to this separation.

If, now, the other members of the group of Sipunculids be examined
for similar structures, two cases are found which require consideration.
Shipley ('90, p. 18) has described an infolding of the preoral lobe which
extends to the surface of the brain, and from which a pair of retort-
shaped tubes penetrate into the ganglionic mass, one at each dorsal
lateral angle of the bram. The cells of the inner limb of the tubes
socrete a black pigment. Andrews ('90, p. 418) finds in S. Gouldii two
similar tubes proceeding from the lateral edges of a transverse pit an-
terior to the ridges of the ciliated cushion. These tubes extend into

176 BULLETIN OF THE

the ganglionic mass, and contain a coagulum, but have no pigment.
Comparing these two accounts with each other and with that just given
of the cerebral organ in S. nudus, it will be seen that the tubes lack
pigment in S. Gouldii, and that both tubes and pigment are wanting in
S. nudus, unless the regions of thickening in the cuticula on the lateral
aspect of the cerebral organ noted above be the rudiments of such struc-
tures. The optic nature claimed by Shipley for the tubes in Phy-
mosoma agrees with their reduction or disappearance in the forms
inhabiting the sand. The position of the organs would seem to indicate
an homology between the ciliated cushion of S. Gouldii, the deep pit of
the preoral lobe in Phymosoma, and the cerebral organ in S. nudus.
As to the histological character of the organ in Phymosoma, nothing is
found in the account of Shipley. Andrews describes that of S. Gouldii
as ciliated and well supplied with nerves. The deep location of the or-
gan in S. nudus may be merely for protectibn, or perhaps due to the
development of the glandular ai'ea, or even necessitated by the recession
of the brain from the surface. The canal is much longer in S. tessela-
tus, where the brain also lies deeper in the body, than in S. nudus.
An analogous variation may be seen in the deep-seated lateral organs
of the Enopla as compared with those of the Anopla.

Finally, if it be asked why the whole structure may not be regarded
as a degenerate organ, of which the pigmented tubes were originally the
active portion, I can only say that the active glandular area and ciliated
canal cannot be explained on such an assumption, and still less can the
special nervous supply. 1 studied the structure a long time with this
idea in mind, but finally became convinced that it was untenable in
every respect. Although the evidence is far from complete, I regard it as
an actively functional organ, morphologically the equivalent of the cili-
ated cushion of Phascolosomes, and possibly with a more highly specialized
function, since it certainly has a more highly differentiated form.

Such organs are by no means rare. Dewoletzky ('87, p. 277) has
given a list of similar ones, and has considered at length their probable
function, which he regards as " some sort of general perception as to the
character of the surrounding medium."

IV. Conclusions.

If now the account I have given of certain points in the anatomy and
histology of S. nudus be compared with that given by Andrews ('00) for
S. Gouldii, it will be noticed that, while there is a general similarity, a

MUSEUM OF COMPARATIVE ZOOLOGY. 1 1 i

correspondence in details is wanting. The dermal glands are hardly
more than similar in type, and a direct correspondence between the
different kinds is not to be found ; for the bicellular are entirely want-
ing in S. Gouldii, and the multicellular of S. nudus agree with neither
group described fur S. Gouldii. Whether the non-glandular organs of
Andrews correspond to the small papillae described above cannot be
definitely determined, on account of the brevity of Andrews's description
and the laclj: of figures. On the other hand, Andrews lias emphasized
the fact that a close agreement exists between the dermal bodies of
S. Gouldii and those of various Phascolosomes. Agaiu, hi the arrange-
ment of the musculature, in the uniform unhanded circular layer, in
the absence of diagonal fibres, and in numerous other details, S. Gouldii
is unlike S. nudus, and in the same degree that the former resembles
Phascolosoma. In the light of these facts, a modification of the generic
characters given by Selenka ('S3) to Sipunculus, which include S. Gouldii
in the same genus with S. nudus, would seem advisable.

Striking as is the similarity between the anatomy of the nervous sys-
tem in the Annelids and in the Sipunculids, certain characteristic dif-
erences are worthy of note. The peripheral system of plexuses is
very highly developed in the latter, and consists almost entirely of
fibres, whereas the dermal plexus of Capitellids, Nemertines, and Poly-
chsets is composed largely of ganglionic cells. In the ventral nerve
cord of Sipunculids there is no metameric arrangement of the lateral
branches, nor any concentrations of the ganglionic elements in the cord
itself. On the other hand, there is present a splanchnic nerve and an
intestinal plexus in both Sipunculids and Aunulata, and the complicated
structure of the supracesophageal ganglion in Sipunculus agrees in gen-
eral with that of various Annelids and Nemertines.

As regards the histology of the central nervous system, it will be
noticed that the description given in this paper for S. nudus corresponds
closely with that given by Rohde ('87) for Chsetopods, and by Burger
('90) for Nemertines. It is of interest, however, to note more exactly
the points of likeness and difference. If further investigation should lead
to the discovery of a minimal cell body for the nervous nuclei (Nerven-
kerne) of Rohde, — and I think this probable on account of the extreme
difficulty experienced by Burger ('90, p. 106) and myself in finding this
cell substance, — then these nervous nuclei would correspond in general
character and occurrence with the first class of ganglionic cells described
by Burger in Nemertines, and with the first type in Sipunculus. The
first class of Rohde agrees in general with the second of Biirger ; but

von. xxi. — no. 3. 12

178 BULLETIN OF THE

both differ from the second type in Sipunculus in one important point,
namely, their arrangement. While they are (always?) found grouped
in clusters in the brains of Polychsets and Nemertines, such an arrange-
ment is never unquestionably present in Sipunculus, though indications
of a regular grouping were sometimes noticed. This may be regarded,
perhaps, as indicating a less highly specialized condition in the Sipuncu-
lid nervous system. According to Rohde and Burger, these cells have
nuclei slightly smaller and more deeply stained than those of the first
class. I did not find any such difference between the two groups in
Sipunculus. The third type of cells in the Sipunculid brain shows
also a general correspondence to Class III. of the Nemertiues and
Class II. of the Chretopods. In both Chaetopods and Nemertiues there
exists a fourth type, — the paired " giant cells " of the central nervous
system, with their accompanying "giant fibres." These are entirely
lacking in the Sipunculids. No one of the large cells has acquired any
uniform or considerable superiority of size over its fellows. Furthermore,
no giant fibres can be found in the ventral nerve cord, so that these
elements probably do not exist in the Sipunculid nervous system. This
may be regarded as further proof of the lower grade of specialization in
the Sipunculids.

The earlier investigators regarded these "giant cells " as " Bildungen
ganz verschiedener Art" (Spengel, '81, p. 40), but the more recent
writers incline toward the opinion that they are homologous throughout
(Eisig, '87, and Friedlander, '89). Now, either these "giant cells" are
neomorphic in both groups, and hence not at all homologous, or the
Sipunculids were separated from the primitive stem before the separa-
tion of Nemertiues and Annelids took place, and before the differentia-
tion of these elements had been effected. A complete disappearance of
giant cells and giant fibres in the Sipunculids is hardly probable, in the
light of the persistence of these and all other, nervous structures. This
would put the origin of the Sipunculids farther back than has usually
been maintained, and would make their relationship to the Annelids
somewhat distant. Of importance in this connection is the simple un-
differentiated condition of the ventral nerve cord, which shows no trace
of a metameric concentration of ganglionic cells, such as is found in the
Annelids. According to the researches of Andrews ('90), moreover, the
lateral branches lack that metameric character which has heretofore
been assigned to them, and I have been able to confirm this in part for
S. nudus. Lack of metamerism in the adult, as well as in the larva,
would serve to strengthen the view of only a remote relationship

MUSEUM OF COMPARATIVE ZOOLOGY. 179

between Annelids and Sipunculids, as has long been maintained by
Hatschek ('80 and '83) on embryological grounds.

The existence of at least giant fibres has been proved for Echiurus by
the researches of Greeff ('79) and Speugel ('80, p. 487), and more recently
for Thalassemia by Eietsch ('8G, p. 402), so that the presence of corre-
sponding ganglionic cells may be reasonably assumed. This is, then, a
further ground for separating the Sipunculids from the Echiurids, and
for assigning to the latter a closer relationship to the Annelids than the
former have. This position has been defended from an embryological
standpoint by Hatschek ('80, p. 71) and Conn ('86, p. 399).

In spite of the well known conservatism of the nervous system, I am
well aware of the dangers of such conclusions based upon the study of a
single system or a single form. The foregoing comparison is offered, then,
merely as a new side light on the unsettled question of the position of
the Sipunculids, and in the hope that the accumulation of evidence from
various sources may some day bring a clear and full solution of the
problem.

January 20, 1891.

Addendum.

During the correction of the proof-sheets there has appeared a second paper
D}' Shipley ('91) on Phyruosoma (P. Weldonii, n. s.). It is interesting to note
that the gland cells there described (p. 114) correspond very closely to the
multicellular glands of S. nudus, except that no connection with nerve fibres is
reported. Shipley affirms positively (p. 115) "the absence of those skeletal
cells which formed so interesting a feature" of P. varians (Shipley, '90, p. 9).
That such a tissue does not exist in S. nudus has already been emphasized.
This is then strong proof that it is an individual peculiarity of the one species,
rather than an ancestral relic. In general the claimed relationship of Sipuncu-
lids and Phoronis seems to me to have little in its favor beyond the external
similarity of the two forms.

It is a pleasure to see that Shipley and I have both arrived independently at
the same conclusions regarding the vascular system. He ('91, p. 116) does not
regard it as important in respiration, and explains the csecal diverticula of the
dorsal vessel, which might be looked upon as strengthening the view of its
respiratory nature, as merely reservoirs for the increased overflow from the
tentacles, which are exceptionally numerous in this species.

180 BULLETIN OF THE

BIBLIOGRAPHY.

Ambronn, H.

'90. Cellulose-Reaction bci Arthropoden und Mollusken. Mittli. aus d.
Zool. Station Neapel, Bd. IX. Heft 3, pp. 475-478. 15. Juni, 1890.
Andreae, J.

'SI. Beitrage zur Anatomie und Histologic des Sipunculus nudus, L.
Zeitschr. f. wiss. Zool., Bd. XXXVI. Heft 2, pp. 201-258, Taf. XII.,
XIII. 1. Nov., 1881.
Andrews, E. A.

'90. Notes on the Anatomy of Sipunculus Gouldii, Pourtales. Studies
Biol. Lab. Johns Hopkins Univ., Vol. IV. No. 7, pp. 389-430, PI.
XLIV.-XLVII. Oct., 1S90.
Brandt, A.

'70. Anatomisch-histologisclie Untersueliungen uber den Sipunculus nu-
. dus, L. Mem. de 1'Acad. Imp. d. Sci. d. St. Pefersbourg, VII e Ser.
Tom. XVI. No. 8, pp. 1-4G, Taf. I., II. 10 Fev., 1870.
Burger, O.

'90. Untersueliungen iiber die Anatomic und Histologic der Nenicrtincn
nebst Beitragen zur Systematik. Zeitschr. f. wiss. Zool., Bd. L. Hefte
1, 2, pp. 1-277, Taf. I.-X. 10. Juni, 1890.
Conn, H. W.

'86. Life History of Thalassema. Studies Biol. Lab. Johns Hopkins Univ.,
Vol. III. No. 7, pp. 351-401, PI. XX.-XXIII. June, 1886.

Dewoletzky, R.

'87. Das Seitenorgan der Nemertincn. Arb. aus d. zool. Inst. Wien,
Tom. VII. Heft 2, pp. 233-280, Taf. XII., XIII.

Eisig, H.

'87. Monographic der Capitelliden des Golfes von Ncapel. Fauna u. Flora
d. Golfes v. Neapel, No. XVI. xxvi + 905 pp., 37 Taf.
Friedlander, B.

'89. Ueber die markhaltigen Nervenfascrn und Neurochorde der Crustacecn
und Anneliden. Mittli. aus. d. Zool. Station Neapel, Bd. IX. Heft 2,
pp. 205-265, Taf. VIII. 24. Sept., 1889.
Greeff, R.

'79. Die Echiuren (Gephyrca armata). Nova Acta k. Leop.-Car. deutsch.
Acad. d. Naturforscher, Bd. XLI. Pt. 2, No. 1, pp. 1-172, Taf. XVI-
XXIV.

MUSEUM OF COMPARATIVE ZOOLOGY. 181

Hatschek, B.

'80. Ueber Entwickelungsgeschichte von Echiurus und die systematische

Stellung der Echiuridse. Arb. aus d. zool. lust. Wien, Tom. III. Heft 1,

pp. 45-78, Taf. IV.-VL
'83. Ueber Entwickelung von Sipunculus nudus. Arb. aus d. zool. Inst.

Wien, Tom. V. Heft 1, pp. 61-140, Taf. IV.-IX.

Hoyer, H.

'90. Ueber den Nachweis des Mucins in Geweben mittelst der Farbe-
methode. Arch. f. mik. Anat., Bd. XXXVI. Heft 2, pp. 310-374. 27.
Sept., 1890.

Keferstein, W.

'62. Untersuchungen iiber niedere Seethiere. Zeitschr. f. wiss. Zool., Bd.

XII. Heft 1, pp. 1-148, Taf. I.-XI. 16. Juni, 1862.
'65. Beitrage zur anatomisclien und systematischen Kenntuiss der Sipuncu-

liden. Zeitschr. f. wiss. Zool., Bd. XV. Heft 4, pp. 404-445, Taf. XXXI.-

XXXIII. 25. Oct., 1865.

Keferstein, W., und Ehlers, E.

'61. Zoologische Beitrage gesammelt im Winter 1859-60 in Neapel und
Messina. II. Untersuchungen iiber die Anatomie des Sipunculus nudus.
pp. 35-52, Taf. VI.-VIII. Leipzig, 1861.
Leydig, F. %

'61. Die Augen und neue Sinuesorgane der Egel. Arch. f. Anat. u. Physiol.,
1861, pp. 588-605.
Nansen, F.

'87. The Structure and Combination of the Histological Elements of the
Central Nervous System. Bergens Museums Aarsberetning for 18S6,
pp. 29-215, PI I.-XI.
Rietsch, M.

'86. Etude sur les Gephyriens armes ou Echiuriens. l re et 2 me Partie.
Recueil Zool. Suisse, Tom. III. pp. 313-515, PI. XVII.-XXII. Juin-
Juillet, 1886.
Rohde, E.

'87. Histologische Untersuchungen iiber das Nervensystem der Chaeto-
poden. Zool. Beitrage (Schneider's), Bd. II. Heft 1, pp. 1-81, Taf.
I.-VII.
Selenka, E.

'83. Die Sipunculiden, eine systematische Monographie. Semper's Reisen
in den Philippinen, II. Theih' Bd. IV., 131 pp., 14 Pis. Wiesbaden, 1883.
Shipley, A. E.

'90. On Phymosoma varians. Quart. Jour. Micr. Sci., Vol. XXXI. pp. 1-27,

PI. I.-IV. April, 1890.
'91. On a New Species of Phymosoma, with a Synopsis of the Genus and
some Account of its Geographical Distribution. Quart. Jour. Micr. Sci.,
Vol. XXXII. pp. 111-126, PI. XI. March, 1891.

182 BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY.

Spengel, J. W.

'77. Auatouiische Mittheiluugen iiber Gephyreen. Amtlicher Bericlit der
50. Versammlung deutscber Naturforscher und Aerzte in Miiuchen
vom 17. bis 22. Sept., 1877. p. 189.
'80. Beitrage zur Kenntnis der Gephyreen. II. Die Organisation des
Ecbiurus Pallasii. Zeitschr. f. wiss. Zool., Bd. XXXIV. Heft 3, pp.
460-538, Taf. XXIII.-XXVI. 30. Juli, 1880.
'81. Oligoguatbus Bonellise, erne schmarotzeude Eunicee. Mittb. aus d.
Zool. Station Neapel, Bd. III. Heft 1, pp. 15-52, Taf. II.-IV. 9. De-
cember, 1881.
Vogt, C, und Yung, £.

'88. Lebrbucb der praktiscbeu vergleicbenden Anatomic Bd. I., 906 pp ,
mit 425 Holzscbnitten. Viebweg und Sobn, Braunscbweig, 1888.

EXPLANATION OF FIGURES.

All figures were drawn with the aid of an Abbe' camera, unless otherwise
stated. They represent without exception preparations of Sipunculus nudus, L.
The method of staining and systems employed are indicated briefly for each
specimen.

ABBREVIATIONS.

can.o.ceb. Canal of cerebral organ. h'drm.

cl. gn. I., II, III. Ganglionic cell I., II., leu'cy.

or III. mb. ha.

cl.JU. Filamentous hypoderm cell. mit.

cl. pig. Pigment cell. mu. ax.

cl. sns. Sensory cell. n'gl.

corns, a. Anterior commissure of brain. n'gi.ul.

coins, d. Dorsal " " n. mu. ret.

corns, oe. Oesophageal connective. nl. sns.

corns, v. Ventral commissure of brain. n. spl.

c<m't. lis. Connective tissue. n. ta.

cps. enc. Capsule of brain. o. ceb.

emu. d. Dorsal horn of tentacular fold. or.

ct. Cutis. pa'mit.

eta. Cuticula. pap.

gl." Bicellular gland. pli. ta.

gl." dt. Duct of bicellular gland. plx. ><. drm

gl." env. Envelope " pr'c. dg.

gl." nl. Nucleus " " rm. gl.

gl." vl. Vacuole " rm mu.

gl.'" Multicellular gland. spht.

gl.'" dt. Duct of multicellular gland. va. sng. d

gl.'" env. Envelope " in. sng. v.

gl.'" n.fbr.' Nervous fibrilla to multicel- z. lev.

lular gland. z. pap. a.
gl.'" nl. Nucleus of the multicellular

gland. 2. pap. p.
gn. su'ee. Supracesophageal ganglion.

Hypodermis.
Leucocytes.
Basement membrane.
Mitome.

Circular muscles.
Neuroglia.
Neuroglia nucleus.
Nerve of retractors.
Nucleus of sensory cell.
Splanchnic nerve.
Tentacular nerve.
Cerebral organ.
Mouth.
Paramitome.
Papilla.

Tentacular fold.
. Dermal nerve plexus.
Digitate processes of brain.
Glandular branch of plexus.
Muscular "

Sphincter of cerebral canal.
Dorsal blood-vessel.
Ventral bloodvessel.
Smooth zone of introvert.
Anterior papillate zone of

introvert.
Posterior papillate zone of

introvert.

Ward. — Sipunculus.

PLATE I.

Fig. 1. Anterior half of the introvert. The base of the figure corresponds to the

middle of the posterior papillate zone. The slight contraction at the

centre of the zona levis is not usually found. Camera outline. Simple

microscope. X 3.
" 2. Anterior aspect of tentacular fold. The figure is diagrammatic only to

the extent that secondary folds are omitted. Camera outline. Simple

microscope. X 4.
" 3. Sagittal section of introvert. Diagrammatic in regard to details. Simple

microscope. X 8.
" 4. Longitudinal section of body wall of introvert in the anterior portion of

the posterior papillate zone. Muscles diagrammatic. Bohmer's hsema-

toxylin. Zeiss 3. A. X 98.
" 6. Transverse section of body wall at about the region indicated by the line

gl." in Fig. 4. Hamann's carmine. Zeiss 1. D. X 370.
" 6-8. Sections of bicellular glands in the three dimensions of space. Kleinen-

berg's haematoxylin. Zeiss apochr. 4 mm. Oc. 6. X 425.
" 6. Soon after the beginning of secretion. The membrane dividing the two

cells Is shown at *.
" 7. At the period of greatest activity in secretion.
" 8. At the close of secretive activity.
" 9. Transverse section of duct of bicellular gland immediately below the

cuticula. Zeiss apochr. 4 mm. Oc. 6. X 425.
" 10, 11. Longitudinal and transverse sections of bicellular glands to demon-
strate position of nuclei. Hamann's carmine. Zeiss apochr. 4 mm.
Oc 6. X 425.
" 12-14. Multicellular glands. Zeiss apochr. 4 mm. Oc. 6. X 400.
" 12. Longitudinal section. The duct is filled with a secreted material. Klei-

nenberg's haematoxylin.
" 13. Transverse section. At the left centre of the section a cell has fallen out.

Hamann's carmine.
" 14. Longitudinal section to demonstrate nuclei and connection of gland cells

with nerve fibres. Mayer's cochineal.

\\ AH ■ L"LL\S

/

■ > | •

Pi. i

m

n.

..

-

gV.'mv.

6.

nl'vl

d

mucrr

■

i

HI

%

prkdg

■"jr/

ranorrb

:

-

I 1

nl"vl

tj,- pap.

■

plx ndrm.

••■. mil

. .. ■. I rm.a!

. m

■ • i •■:.♦.;

• ••■»•-

....

.-•

■ .3!

*

nal

ng, l nl

PLATE II.

Fig. 15. Transverse section of epithelium of tentacular fold to show the leucocytes
in situ. Hamann's carmine. Zeiss 1. E. X 500.

" 16-18. Sense papillae from anterior papillate zone of introvert.

" 16. Tangential section through a single papilla. Hamann's carmine. Zeiss
1. D. X 300.

" 17. Transverse section. Apical area retracted. Hamann's carmine. Zeiss
1. D. X 330.

" 18. Transverse section. Papilla fully expanded. Orth's picro-litho-carmine.
Zeiss 1. D. X 220.

" 19. Lateral sagittal section of brain at point of departure of the anterior com-
missure from the central fibrous mass. Plane of section indicated on
Figure 22 by dotted line " 19." Mayer's cochineal. Zeiss 1. A. X 50.

" 20. Median sagittal section of brain. Plane of section indicated on Figure
22. Mayer's cochineal. Zeiss 1. A. X 50.

" 21. Cells of tentacular epithelium isolated by maceration ; a, c, and e, sensory,
b, d, and/, filamentous cells. Zeiss apochr. 4 mm. Oc. 8. X 725.

" 22. Central nervous system. Composite figure from maceration preparations
controlled by serial sections. The splanchnic nerve (n. spl.) should
project forward under the brain. For the sake of clearness it is repre-
sented as if turned posteriad ; * denotes inferior muscular branch (?).
The numbers denote the planes of sections represented in Figures 19,
20, 24, and 25. X 8 (about).

WARD SlPUN r ( L'LUS.

■ .in O.i • •

■ ■

■*>)

ft !■ I 4

until ret

._ /

20

.1

21

l '-t h . <S

if

-.

*

i».

r7 /;/

ZS

<7.<jm/p

PlJ

riqnll

/ r/f/ftj/

i In

n

It tin

rl

- _ 9 _

Ward — Sipunculus.

PLATE III.

Fig 23. Transverse section of hypodermis of tentacular fold with sensory cells.

Weigert's picro-carmine. Zeiss apochr. 4 mm. Oc. 8. X 725.
" 24. Transverse section of brain. Plane of section shown in Figure 22, Plate

II Grenadier's alcoholic borax carmine. Zeiss 1. A. X 50.
" 25. Transverse section of brain. Plane of section shown in Figure 22, Plate

II. The section was cut somewhat obliquely, and the right half lies
posteriad. Grenadier's alcoholic borax carmine. Zeiss 1. A X 50.

" 26. Transverse section of body wall passing through the canal of the cere-
bral organ at about the middle of its course. The left of the figure
is dorsal. Hamann's carmine Zeiss 1. A. X 50.

" 27. Transverse section of the cerebral organ : only one half is represented,
and but a small section is drawn in detail. The cerebral canal begins
at the angle near the number 27, and extends forward at right angles to
the surface marked eta. The transition from the cuticula of the cere-
bral organ to that of the canal is marked by a t- Czokor's cochineal
and picric acid. Zeiss apochr. 4 mm- Oc 8. X 510.

" 28-31. Ganglionic cells. Zeiss apochr. 4 mm. Oe 8. X 510. Fig. 28, Class

III. Fig. 29, Class II. Fig. 30, Nuclei of Class I. Fig. 31, Class I.
" 32. Oblique section through a ganglionic cell of Class II., showing the

regular arrangement of the paramitone. Hamann's carmine. Zeiss
apochr. 4 mm. Oc. 8. X 725

\\'.\UD SlPUNCULUS.

rlrjn.1

-

i

Pi. HI.

■

;

ffis en c.

— •j

r/n
clfil

•

fori tils

rati bt ■

cLgnM.

vajsngd. 30

r/.tn.i

>■

— ^

JV

#i

i jphi

No. 4. — Three Letters from Alexander Agassiz to the Hon.
Marshall McDonald, United States Commissioner of Fish and
Fisheries, on the Dredging Operations off the West Coast of Cen-
tral America to the Galapagos, to the West Coast of Mexico, and
in the Gulf of California, in charge of Alexander Agassiz,
carried on by the IT. S. Fish Commission Steamer "Albatross,"
Lieut. Commander Z. L. Tanner, U. S. N., Commanding.

Steamer Albatross, Panama, U. S. of Colombia,
March 14, 1891

My dear Colonel McDonald : —

We returned yesterday from our first trip. The route extended from
Panama to Point Mala, and next to Cocos Island ; from there we ran in
a southerly direction, then northwesterly to Malpelo Island, and back to
the hundred-fathom line off the Bay of Panama. We spent several days
trawling off the continental plateau of the Bay. This trip being rather
in the nature of a feeler, I cannot tell you just what I think it means.
But I believe I can to some extent conjecture probabilities from what
has been accomplished.

I have found, in the first place, a great many of my old West Indian
friends. In nearly all the groups of marine forms among the Fishes,
Crustacea, Worms, Mollusks, Echinoderms, and Polyps, we have found
familiar West Indian types or east coast forms, and have also found
epiite a number of forms whose wide geographical distribution was
already known, and is now extended to the Eastern Pacific. This was
naturally to be expected from the fact that the district we are exploring
is practically a new field, nothing having been done except what the
" Albatross " herself has accomplished along the west coast of North
and South America. The " Challenger," as you will remember, came
fvom Japan to the Sandwich Islands, and from there south across to
Juan Fernandez, leaving, as it were, a huge field of which we are
attacking the middle wedge. As far as we have gone, it seems verv

VOL. XXI — NO 4.

186 BULLETIN OF THE

evident that, even in deep water, there is on this west coast of Central
America a considerable fauna which finds its parallel in the West Indies,
and recalls the precretaeeous times when the Caribbean Sea was prac-
tically a bay of the Pacific. There are, indeed, a number of genera in
the deep water, and to some extent also in the shallower depths, which
show far greater affinity with the Pacific than with the Atlantic fauna.
Of course, further exploration may show that some of these genera are
simply genera of a wider geographical distribution ; but I think a suffi-
ciently large portion of the deep-sea fauna will still attest the former
connection of the Pacific and the Atlantic.

I am thus far somewhat disappointed in the richness of the deep sea
fauna in the Panamic district. It certainly does not compare with that
of the West Indian or Eastern United States side. I have little doubt
that this comparative poverty is due to the absence of a great oceanic
current like the Gulf Stream, bringing with it on its surface a large
amount of food which serves to supply the deep-sea fauna along its.
course. In the regions we have explored up to this time, currents from
the north and from the south meet, and then are diverted to a westerly
direction, forming a sort of current doldrums, turning west or east or
south or north according to the direction of the prevailing wind.
The amount of food which these currents carry is small compared with
that drifting along the course of the Gulf Stream. I was also greatly
surprised at the poverty of the surface fauna. Except on one occasion,
when during a calm we passed through a large field of floating surface
material, we usually encountered very little. It is composed mainly of
Salpa3, Doliolum, Sagittas, and a few Siphonophores, — a striking con-
trast to the wealth of the surface fauna to be met with in a calm day in
the Gulf of Mexico near the Tortugas, or in the main current of the Gulf
Stream as it sweeps by the Florida Reef or the Cuban coast near Havana.
We also found great difficulty in trawling, owing to the considerable irreg-
ularities of the bottom. When trawling from north to south, we seemed
to cut across submarine ridges, and it was only while trawling from east
to west that we generally maintained a fairly uniform depth. During the
first cruise we made nearly fifty hauls of the trawl, and in addition sev-
eral stations were occupied in trawling at intermediate depths. In my
dredgings in the Gulf of Mexico, off the West Indies, and in the Carib-
bean, my attention had already been called to the immense amount of
vegetable matter dredged up from a depth of over 1,500 fathoms, on the
lee side of the West India Islands. But in none of the dredgings we
made on the Atlantic side of the Isthmus did we come upon such masses

MUSEUM OF COMPARATIVE ZOOLOGY. 187

of decomposed vegetable matter as we found on this expedition. There
was hardly a haul taken which did not supply a large quantity of water
logged wood, and more or less fresh twigs, leaves, seeds, and fruits, in all
possible stages of decomposition. This was especially noteworthy in the
line from the mainland to Cocos Island, and certainly offers a very
practical object lesson regarding the manner in which that island must
have received its vegetable products. It is only about 275 miles from
the mainland, and its flora, so similar to that of the adjacent coast, tells
its own story. Malpelo, on the contrary, which is an inaccessible rock
with vertical sides, and destitute of any soil formed from the disinte-
gration of the rocks, has remained comparatively barren, in spite of its
closer proximity to the mainland.

The most interesting things we have found up to this time are repre-
sentatives of the Ceratias group of Fishes, which the naturalists of the
" Albati'oss " tell me they have not met befoi'e on the w r est coast of
North America. The Crustacea have supplied us with a most remark-
able type of the Willemoesia group. The paucity of Afollusks and
also of Echini is most striking, although we brought up in one of the
hauls numerous fragments of what must have been a gigantic species of
Cystechinus, which I hope I may reconstruct. We were also fortunate
enough to hud a single specimen of Calamocrinus off Morro Puercos, in
700 fathoms, a part of the stem with the base, showing its mode of at-
tachment to be similar to that of the fossil Apioerinidae. The number
of Ophiurans was remarkably small as compai-ed with the fauna of deep
waters on the Atlantic side, where it often seems as if Ophiurans had
been the first and only objects created. The absence of deep-sea corals
is also quite striking. They play so important a part in the fauna of
the deeper waters of the West Indies, that the contrast is most marked.
Gorgonise and other Halcyonoids are likewise uncommon. We have
found but few Siliceous Sponges, and all of well known types. Star-
fishes are abundant, and are as well represented in the variety of genera
and species as on the Atlantic side of the Isthmus. I may also mention
the large number of deep-sea Holothurians (Elasipoda) which we ob-
tained, as well as a most remarkable deep-sea Actinian, closely allied
to Cerianthus, but evidently belonging to a new family of that group.
We found the usual types of deep-sea West Indian Annelids, occasion-
ally sweeping over large tracts of mud tubes in the region of green mud.
Although we dredged frequently in most characteristic Globigerina ooze,
I was much struck with the absence of living CJlobigeiina- on the surface.
Only on two occasions during a calm did we come across any number

188 BULLETIN OF THE

of surface Globigerinse and Orbuliuse. On one occasion the trawl came
up literally tilled with masses of a species of Rhabdamina closely allied
to E. lineata. Thus far no pelagic Alga; have been met with.

It is interesting to note that at two localities we came across patches
of modern greensand similar in formation to the patches discovered off
the east coast of the United States by the earlier dredgings of the Coast
Survey, of Pourtales, and of the " Blake." Having always been more or
less interested in pelagic faume, and having paid considerable attention
to its vertical distribution during my earlier cruises in the " Blake," I
was naturally anxious to reconcile the conflicting statements and ex-
periences of the naturalists of the " Challenger" and " Gazelle " on one
side, and my own observations on the other. Both Murray and Studer
contended that, in addition to the deep sea and pelagic faunae, there was
what might be called an intermediate fauna with characteristic species,
having nothing in common with the other two; while I maintained,
on the other hand, from my experiments in the " Blake," that there
was no such intermediate fauna, but that the pelagic fauna might de-
scend to a considerable depth during the daytime to escape the effects
of light, heat, and the disturbing influence of surface winds, and that
this surface fauna on the Atlantic side — off shore in deep water — did
not descend much deeper than 150 to 200 fathoms. In order to test
this point, Dr. Chun, under the auspices of the Naples Station, made an
expedition to the Ponza Islands. Dr. Chun applied to a tow-net an
apparatus for closing it, similar to the propeller in use on our ther-
mometer and water cups. He towed to a depth of 1,400 meters, if I am
not mistaken, but never at any great distance from the mainland or
from the islands of the Gulf of Naples, and came to the conclusion that
the pelagic fauna existed all the way to the bottom. At the time, I
considered his experiments inconclusive, and was of course anxious to
repeat them in a strictly oceanic district, in great depths, and at a con-
siderable distance from shore. I had an apparatus constructed by
Ballauf of Washington, similar to that used by Dr. Chun. Unfortu-
nately, in testing it we found the pressure of the tow-net against the
propeller shaft so great as to make the machine useless, or at any rate,
most unreliable. Thanks to the ingenuity of Captain Tanner, we over-
came these obstacles. He devised a net which could be closed at any
depth by a messenger, and which worked to perfection at 200, 400, 300,
and 1,000 fathoms, and had the great advantage of bringing np anything
it might find on its way up above the level at which it was towed. The
lower part of the bag alone was closed by a double set of slings pulled

MUSEUM OF COMPARATIVE ZOOLOGY. 189

by two weights liberated from a bell crank by a messenger. We found
that, in towing the net at 200 fathoms for twenty minutes, we got every-
thing in any way characteristic of the surface fauna which we had fished
up with the tow-net at the surface. In addition to this, we brought up
five species of so called deep-sea Fishes, Scopelus, Gonostoma, Beryx, and
two others, which had thus far been bi'ought up in the trawl, and con-
sidered characteristic of deep water. Also a peculiar Amphipod, and the
young of the new species of Willemoesia mentioned above. We then
tried the same net at 300 and 400 fathoms, and in neiiher case did we
bring up anything in the closed part of the bag, while the upper open
part brought up just what we had found previously at a depth of 200
fathoms, plainly showing that in this district the surface fauna goes
down to a depth of 200 fathoms, and no farther. Next came our single
attempt to bring up what might be found, say within 100 fathoms of
the bottom, and Captain Tanner's net was towed at a depth of 1,000
fathoms where the soundings recorded 1,100. Unfortunately, we deep-
ened our water while towing only twenty minutes to over 1,400 fathoms,
so that we failed in our exact object. But we brought up in the closed
part of the bag two species of Crustacea, a Macruran and an Amphipod,
both entirely unlike anything we had obtained before. I hope in the
next cruise to follow this up, and determine also the upper limits of the
free-swimming deep-sea fauna. In the upper part of the bag (the open
part) we brought up a couple of so called deep-sea Medusa?, which must
have been collected at a comparatively moderate depth, judging from
their perfect state of preservation.

I can hardly express my satisfaction at having the opportunity to
carry on this deep-sea work on the " Albatross." While of course I
knew in a general way the great facilities the ship afforded, I did not
fully realize the capacity of the equipment until I came to make use of
it myself. I could not but contrast the luxurious and thoroughly con-
venient appointments of the " Albatross" with my previous experiences.
The laboratory, with its ingenious arrangements and its excellent accom-
modations for work by day and by night, was to me a revelation. The
assistance of Messrs. Townsend and Miller in the care of the specimens
was most welcome, giving me ample time to examine the specimens
during the process of assorting them, and to make such notes as I could
between successive hauls, while paying some attention also to the work
of the artist, Mr. Westergren. He has found his time fully occupied,
and we have in this trip brought together a considerable number of
colored drawings, giving an excellent general idea of the appearance

190 BULLETIN OF THE

of the inhabitants of the deep waters as they first come up. These
drawings can be used to great advantage with the specimens in making
the final illustrations to accompany the reports of the specialists who
may have charge of working up the different departments. . . .

We left Panama ou the 22d of February, and returned to Panama
after an absence of twenty days.

II.

Albatross, Acapulco, April 14, 1891.

We have reached the end of our second line of explorations. After
coaling we left Panama, and reached Galera Point, where we began our
line across the Humboldt Current, which was to give us a fair idea of
the fauna of that part of the coast as far as the southern face of the
Galapagos. With the exception of three good casts, the trawling on
that part of the sea bottom proved comparatively poor, nor did the sea
face of the southern slope of the Galapagos give us anything like the
rich fauna I had expected. Theoretically, it seemed certain that a sea
face like that of the Galapagos, bathed as it is by a great current coming
from the south and impinging upon its slope, and carrying upon its
surface a mass of animal food, could not fail to constitute a most favor-
able set of conditions for the subsistence and development of a rich deep-
sea fauna.

In the deeper parts of the channel between Galera Point and the
southern face of Chatham Island we found a great number of Elasi-
poda, among them several genera like Peniagone, Bathodytes, and Eu-
phrosine, represented by numerous species. The Starfishes of this
our second cruise did not differ materially from those collected during
our first trip, but we added some fine species of Freyella, Hymenaster,
Astrogonium, Aster in a, and Archasteridse to our collections. Among
the Sea-urchins on two occasions we brought up fine hauls of a species
of Cystechinus with a hard test, many specimens of which were in
admirable state of preservation. Among the Ophiurans nothing of
importance was added, unless I may except a lot of Ophiocreas attached
to a Primnoa, and a pretty species of Siirsbea attached to a species of
Allopora, from the south side of Chatham Island.

The Gorgonians were remarkably few in number, which is undoubtedly
due to the unfavorable nature of the bottom we worked upon. Nearly
everywhere except on the face of the Galapagos slope we trawled upon a

MUSEUM OF COMPARATIVE ZOOLOGY. 191

bottom either muddy or composed of Globigerina ooze, more or less con-
taminated with terrestrial deposits, and frequently covered with a great
amount of decayed vegetable matter. We scarcely made a single haul
of the trawl which did not bring up a considerable amount of decayed
vegetable matter, and frequently logs, branches, twigs, seeds, leaves,
fruits, much as during our first cruise.

Our Crustaceans, from the nature of the bottom, naturally consisted
of the same groups of deep-sea types which we obtained before. I may,
however, mention a haul containing a goodly number of Xephrops, a
genus we had not previously obtained.

Among the Worms the Maldaniee and limicolous types were unusually
abundant at some localities, the empty mud tubes often filling the bottom
of the trawl. Some very large specimens of Trophonia were collected,
and remarkably brilliantly colored (orange and carmine) Xemerteans
and Planarians.

The Mollusks were very scanty, and the absence of Comatuke or other
Crinoidswas equally disappointing, even when trawling on the extension
of the line started three years ago by the "Albatross," on the eastern
face of the Galapagos slope, when on her way from Chatham Island to
San Francisco. We took up this line off Indefatigable Island, hoping
to obtain from that quarter our best results, but our hauls were very
disappointing. The ground proved not only most difficult to dredge
upon, but also comparatively barren, and it was not till we got into the
oceanic basin again, between the Galapagos and Acapulco, that our
catches improved. But even then they were not to be compared with
the hauls at similar depths in the Atlantic off the West Indies, or along
the course of the Gulf Stream.

Among the Fishes, our most important catches were fine specimens
of Bathyonus, of Bathybrissa, of Bathypteroides, and a few specimens of
Ipnops in excellent condition.

From the nature of the bottom we naturally expected rich hauls of*
Siliceous Sponges, but we did not find many, and I do not think there
are many novelties among those we have collected. On two occasions,
a number of specimens of Ascidians were brought up ; among them was
a fine white translucent Corinascidia.

Among the Bryozoans, the most noteworthy haul was a number of
beautiful specimens of the delicate Naresia. in excellent condition, On
the line from the Galapagos to Acapulco we brought up a good many
Foraminifera from the mud bottoms. On several occasions the bottom
must have been covered with huge masses of a new type of an arena

192 BULLETIN OF THE

ceons Foraminifer, forming immense curling sheets attached by one
edge to stones or sunk into the mud. This Foraminifer seems to in-
crease in size by forming irregular more or less concentric crescent-
shaped rings. When it conies to the surface, it is of a dark olive-green
color.

During this second cruise we continued our experiments with the
Tanner closing tow-net, in order to determine the lower limits of the
surface pelagic fauna, and to determine also if there is any so called in-
termediate pelagic fauna at other depths, or within a short distance
from the bottom.

On the 25th of March, at a point not quite halfway between Cape
San Francisco and the Galapagos, in 1,8.;;.' fathoms of water, the Tanner
net was sent down to tow at a depth which varied from 1,739 to 1,773
fathoms. The net was towed within these limits for a period of some-
thing over twenty minutes. The messenger was then sent down to close
the net; time occupied seven minutes. The net was then drawn up to
the surface. The lower part of it was found to have closed perfectly,
and contained nothing beyond a few fragments of leaves. The lower
bag was carefully washed in water which had been strained, and the
water examined with all possible care, and sifted again. It contained
nothing. The upper part of the net, however, which had remained
open on its way up, was found to contain the identical surface things
which on former occasions we had found in the Tanner net down to a
depth of 200 fathoms. They were a small species of Sagitta, and species
of Doliolum, Appendicularia, a huge Sagitta, a large number of Leucifer
and Sergestes, and several species of Smizopods and Copepods ; two spe-
cies of Hyperia, probably parasitic on a Salpa, which was also quite abun-
dant ; several finely colored Calanus, some Isopods, and a number of
fragments of what must have been a very large Beroe, measuring from
five to six inches in diameter ; Leptocephalus, several specimens of Sto-
mias, of Scopelus, of Melamphses, and other species, many of which, like
some of the Schizopods, had been considered as typical deep-sea forms.
Among the so called deep-sea Medusae, several specimens of Atolla and
Periphylla weve also found in the open part of the net. I may mention
also as of special interest a huge Ostracod, allied to Crossophorus, with a
thin semi-transparent carapace, and measuring somewhat more than one
inch in length. The largest Ostracod previously known is not more
than one third of an inch long. On two other occasions this same
Ostracod was brought up in the tow-net from a depth of less than
200 fathoms.

MUSEUM OF COMPARATIVE ZOOLOGY. 193

The surface at this point was also examined with the tow-net, and the
pelagic animals found to be the same as those brought up in the open
part of the tow-net on its way from the bottom. The number both of
species and specimens was, however, much less than in the Tanner net.
On the following day the Tanner tow-net was sent to be towed at a
depth of 214. fathoms. In twenty minutes the messenger was sent down
and the net hauled up. The bottom part of the net came up tightly
closed. Its contents were examined in the same manner as before in well
sifted water, and the water was found to be absolutely barren, while the
upper part of the net, which came up open, and was not more than eight
or nine minutes on the way, was well filled with surface life. The net
contained this time a number of Hyalceas and Criseis, in addition to
the things collected the day before. An examination of the surface fauna
at this same point with the tow-net showed the presence only in smaller
numbers of the same species which the open part of the same net con-
tained, except that there were a larger number of bells and fragments of
Diphyes and of Cristalloides than in the Tanner net. The point at
which this experiment was made was about 250 miles from the Galapa-
gos, and about the same distance from Cape San Francisco. There
were myriads of Nautilograpsus swarming on the surface of the water;
they literally filled the surface tow-net. On two other occasions, once
at a distance of 350 miles in a southeasterly direction from Aeapulco
(depth 2,232 fathoms), we tried the same experiment with the Tan-
ner net, and invariably with the same result. The net was towed at
a depth of 100, of 200, and of 300 fathoms, each time for twenty
minutes, the messenger sent down, and the bottom part closed. At the
depth of 100 fathoms, the closed part of the net contained practically the
same tilings as the open part of the net ; at 200 fathoms, the lower
part of the net contained but few specimens of the surface life ; and
at 300 fathoms, the closed bottom net came up empty.

On the following day the surface was carefully examined, and the tow-
net sent to 175 fathoms, where it was towed for twenty minutes, and the
messenger sent down to close it. The lower net came up well filled with
the surface pelagic species, which on this day were unusually varied,
it having been smooth and calm the previous night, and the morning
before the towing was made. This haul was made in the evening, at,
8 p.m. The previous hauls had been made at about 10 a. m., in a bril-
liant sunlight. Again on the 11th of April, about thirty miles southeast
of Aeapulco, in a depth of over 1,800 fathoms, the Tanner net was sent
to a depth of 300 fathoms, and the messenger sent down to close it.

VOL. XXI — NO 4. 13

194 BULLETIN OF THE

There was nothing in the lower part of the net which had been closed,
while the open part contained an unusually rich assortment of surface
species, and among them a large number of Scopelus, of iSchizopods,
and of Khizopods, mainly Collozoun and Aeanthometra.

These experiments seem to prove conclusively that in the open sea,
even when close to the land, the surface pelagic fauna does not descend
beyond a deptli of 200 fathoms, and that there is no intermediate pelagic
fauna living between that depth and the bottom, and that even the free-
swimming bottom species du not rise to any great distance, as we found
no trace of anything within GO fathoms from the bottom, where it had
been fairly populated.

The experiments of Chun regarding the distribution of the pelagic
fauna have all been made in the Mediterranean, within a compara-
tively short distance from the shore, and in a closed basin show-
ing, as is well known, special physical conditions, its temperature to its
greatest depths being considerably higher than the temperature of
oceanic basins at the limit of 200 fathoms, or thereabout, which we
assume now to be the limit of the bathvmetrical ran^e of the true
oceanic pelagic fauna. At 200 fathoms our temperature was from 49°
to 53°, while, as is well known, the temperature of the Mediterranean
soon falls at 100 fathoms even to about 56°, a temperature which is
continued to the bottom in this closed basin. Of course, if temperature
is one of the factors affecting bathymetrical distribution, there is no
reason except the absence of light which would prevent the surface
pelagic fauna from finding conditions of temperature at the greatest
depth similar to those which the surface fauna finds within the limit
of 200 fathoms in an open oceanic basin.

Arriving as we did at the Galapngos at the beginning of a remarkably
early rainy season, I could not help contrasting the greeu appearance of
the slopes of the islands, covered as they were by a comparatively thick
growth of bushes, shrubs, and trees, to the description given of them by
Darwin, who represents them in the height of the dry season as the
supreme expression of desolation and barrenness. Of course, here and
there were extensive tracts on the sea-shore where there was nothing to
be seen but blocks of volcanic ashes, with an occasional cactus standing in
bold relief, or a series of mud volcanoes, or a huge black field of volcanic
rocks, an ancient flow from some crater to the sea ; but as a rule the
larger islands presented wide areas of rich, fertile soil, suitable for cul-
tivation. The experiments at Charles Island, where there is a deserted
plantation, and at Chatham Island, where Mr. Cobos has under success-

MUSEUM OF COMPARATIVE ZOOLOGY. 195

ful cultivation a large plantation producing sugar, coffee, and all the
tropical fruits, as well :is extensive tracts on which his herds of cattle,
sheep, and donkeys roam towards the higher central parts of the island,
show the fertility of these islands. They are indeed as favorably situ-
ated for cultivation as the Sandwich Islands or Mauritius, and there is
no reason why, if properly managed, they should not in the near future
yield to their owners as large returns as do those islands.

I obtained from Mr. Cobos a piece of the so called sandstone said to
occur on Indefatigable Island, and which of course I was most anxious to
see, as the occurrence of true sandstone would have put quite a different
face on the geological history of the Galapagos from the one usually re-
ceived. This I found to be nothing but coral rock limestone, either a
breccia or slightly oolitic, identical with the formation found back of the
beach at "Wreck Bay on Chatham Island. 1 found there an old coral
rock beach, extending on the flat behind the present beach, composed
entirely of fragments of corals, of mollusks, and other invertebrates,
cemented together into a moderately compact oolitic limestone, wdiich
when discolored, as it often is and turned gray, would readily be mistaken
for sandstone. This coral rock is covered by just such a thin, ringing
coating of limestone as characterizes the modern reef rock of other local-
ities. On nearly all the islands there are a number of sandy beaches
made up of decomposed fragments of corals and other invertebrates, and
cemented together at or beyond high-water mark into the modern reef
rock I have described. The coral is mainly made up of fragments of
Pocillopora, which is found covering more or less extensive patches off
these coral sand beaches, but which, as is well known, never forms true
coral reef in the Panamic district. The only true coral reef belonging to
this district is that of Clipperton Island, (if we can trust the Admiralt}-
charts,) situated about 700 miles to the southwest of Acapulco. But
neither at Cocos Island, nor at the Galapagos, nor anywhere in the Pana-
mic district, do we rind true coral l'eefs, — nothing but isolated patches
of reef-building coral. The absence of coral reefs in this district has of
course already been noted by other naturalists, who have been struck by
this feature in an equatorial region. Dana has ascribed it to the lower
temperature of the water due to the action of the Humboldt Current com-
ing from the south, pouring into the Bay of Panama, and then flowing
westward with the colder northerly current coming down the west coast
of Mexico and Central America. From the investigations made this year
by the " Albatross," I am more inclined to assume that the true cause
of the absence of coral reefs on the west coast of Central America is due

196 BULLETIN OF THE

to the immense amount of silt which is brought down the hill and moun-
tain sides every rainy season, and which simply covers the floor of the
ocean to a very considerable distance from the land, the land deposits
being found by us even on the line from the Galapagos to Acapulco at
the most distant point from the shore to the side or extremities. The
mud in Panama Bay to the hundred-fathom line is something extraordi-
nary, and its influence on the growth of coral reefs is undoubtedly greatly
increased from the large amount of decomposed vegetable matter which
is mixed with the terrigenous deposits.

The course of the currents along the Mexican and the Central and
South American coasts clearly indicates to us the sources from which the
fauna and flora of the volcanic group of the Galapagos has derived its
origin. The distance from the coast of Ecuador (Galera Point and Cape
San Francisco) is in a direct line not much over 500 miles, and that
from the Costa Rica coast but a little over GOO miles, and the bottom
must be for its whole distance strewn thickly with vegetable matter.
The force of the currents is very great, sometimes as much as 75 miles
a day, so that seeds, fruits, masses of vegetation harboring small rep-
tiles, or even large ones, as well as other terrestrial animals, need
not be afloat long before they might safely be landed on the shores of
the Galapagos. Its flora, as is well known, is eminently American, while
its fauna at every point discloses its affinity to the Mexican, Central or
South American, and even AVest Indian types, from which it has proba-
bly originated ; the last indicating, as well as so many of the marine
types collected during this expedition, the close connection that once
existed between the Panamic region and the Caribbean and Gulf of
Mexico.

I have already referred to the physiognomy of the deep-sea fauna,
showing relationship on the one side to Atlantic and West Indian types,
and on the other to the extension of the Pacific types, which mix with
the strictly deep-sea Panamic ones. The western and eastern Pacific
fauna, while as a whole presenting very marked features in common, yet
also present striking differences. The vast extent of territory over which
some of the marine types extend, through all the tropical part of the
Pacific, may readily be explained from the course of the great western
equatorial current and the eastern counter current, which cannot fail
to act as general distributors in space for the extension of a vast number
of marine Vertebrates and Invertebrates.

Mr. Townsend made quite a large collection of Birds from Chatham
and Charles Islands, considering the short time we were there.

MUSEUM OF COMPARATIVE ZOOLOGY. 197

As soon as we have reached G nay mas, I shall be able to give you a
better resume of the character of the deep-sea fauna of the Panamic
region, and of its relationship on the one side to the Pacific fauna and
on the other to the West Indian region.

III.

Guatmas, April 25, 1891.

We left Acapulco on the 15th of April, for our third cruise, into the
Gulf of California, and steamed as far as Cape Corrientes without
attempting to do any trawling. The character of the bottom, as indi-
cated on the charts, promised nothing different from what we had dredged
off Acapulco, and on the line from there to the Galapagos Islands.
We made one haul off Cape Corrientes, bringing up nothing but mud
and decomposed vegetable matter. This induced us to keep up the Gulf
of California, till we were off the Tres Marias. We there made several
hauls, and obtained some Umbellulse, Pennatula?, Trochoptilum, An-
thoptilum, and a fine Antipathes, a few Comatulse, a large Astropec-
ten, some fine specimens of Urechinus and of Schizaster, a few Holo-
thurians, Lophothuria, Trochostoma, and two species of Elasipoda, besides
a few fragments of Gastei-opods, with an empty shell of Argonauta.

Among the Crustacea there came up the usual types found living upon
muddy bottom, such as Glyphocrangon, Heterocarpus, Notostoma, Penta-
cheles, Nematocarcinus, Nephrops, together with species of Lithodes
and of Munida. The usual types of limicolous Annelid also were found
here, Halinrecia, Terebella, Maldania, and the like, a few Ophiurans,
Ophiopholis and Ophiocantha, a few fragments of Farrea, and a huge
Hyalonema of the type of H. toxeres. Among the Fishes there were a
few Macrurans, Bathypteroides, Lycodes, and Malthe. The trawl was
usually well filled with mud, and with the mud came up the usual
supply of logs, branches, twigs, and decayed vegetable matter.

On going farther north into the Gulf of California, the nature of the
bottom did not change materially, and we found the trawling most diffi-
cult from the weight of the mud brought up in the trawl. But occa-
sionally a haul was made which more than repaid us for the time spent
on the less productive ones. Two of the hauls are specially worthy of
mention, as being characteristic of the deep-water fauna of the Gulf of
California, one made in 995 fathoms, and the other in 1,588 fathoms.
We obtained in these hauls a number of Ophiomusium and Ophiocreas,

198 BULLETIN OF THE

some fine specimens of Schizaster, a new genus allied to Paleopneustes,
and also the same species of Cystechinus, with a hard test, and of Phor-
mosoma, which we had obtained before on the line from the Galapagos
to Acapulco. Beside these, there came up a number of specimens of an
interesting species of Pourtalesia, most closely allied to Pourtalesia
miranda, the first type of the group dredged in the Florida Channel
by Count Pourtales.

The deeper haul was specially rich in Holothurians, among them a
fine large white Cucumaria, some specimens of Trochostoma, several
species of Bathodytes, some of them remarkable for their white color,
their huge size, and comparatively small number of ventral tentacles.
"With these were numerous specimens of an interesting species of Eu-
phronides. In this haul I was specially struck with the Elasipoda, and
the great variety in the consistency of the skin in individuals of one and
the same species ; it varied in different individuals from extreme tenuity
to a comparatively tough gelatine-like consistency. On carefully sifting
the mud, we found a number of interesting Foraminifera, and of deli-
cate and minute Gasterepods and Lamellibranchs, fragments of the shell
of an Argonaut a, and two species of a huge ribbed Dentalium. Among
the Starfishes were specially noticeable a large Brisinga, a long-armed
Cribrella, and several species of Astropecten. The usual types of Worms
were found in the mud at these greater depths. In addition to a num-
ber of Macruroids, we obtained a pink Amphionus, a large black Beryx-
like fish, a fine Nettastoma, and a couple of species of Lycodes. The
usual surface species of Stomias and of Scopelus also came up in the
trawl. Among the Crustaceans were a fine lot of Arcturus, of Colos-
sendeis, of Glyphocrangon, and of a Caridid with a deep blue patch on
the base of the carapace, making the strongest possible contrast to the
dark crimson coloring of the rest of the body. Blue is a very unusual
color in the deep-sea types, although the large eggs of some of the
deep-sea Macrurans are often of a light blue tint.

We brought up in the trawl at various times, and subsequently also
in the Tanner net, from depths of less than 200 fathoms, the same
gigantic Ostracod which I mentioned in one of my previous letters,
several specimens of Atolla, and fragments of a huge Periphvlla. winch
must have been at least fifteen inches in diameter. Also a most inter-
esting new type of Bougainvillia, remarkable for having eight clusters
of marginal tentacles, but only four chymiferous tubes.

We continued our experiments with the Tanner tow-net. On the
16th of April, about 120 miles from Acapulco, we sent the net to tow

MUSEUM OF COMPARATIVE ZOOLOGY. 199

at a depth of 175 fathoms, and after towing for about twenty minutes
sent the messenger to close it. On examining the bottom part of the
net, which came up tightly closed, we found it to contain practically the
same things as we obtained in the surface net at the same spot.

On two occasions we sent the net to be towed at deptbs of 800
fathoms and of 700 fathoms, the depths at these points being in one
case 905 fathoms and in the other 773 fathoms. At the greater depth,
the water shoaled somewhat while towing, as the closed part of the net
came up partly filled with fine silt; while during the second haul, the
twisting of the swivel wound the straps of the weights round the rope,
and the net came up open, but must have dragged very close to the
bottom, as it contained a fine specimen of Xettastoma, and some Pense-
ids, which we supposed to be deep-sea types. Otherwise the net con-
tained only the customary surface species of Sagitta, Pteropods, Copepods,
Schizopods, Tunicates, and Fishes. These two hauls were made about
the middle of the Gulf of California, at a distance of some fifty miles
in a southwesterly direction from Guaymas.

On the 23d of April, a few hours before reaching Guaymas, we made
one more attempt with the Tanner tow-net, at a depth of 620 fathoms,
sending the net to be towed at a depth of from 500 to 570 fathoms.
We found in this case in the bottom part of the net, which came up
tightly closed, a Scopelus, a Penseid, and a Hyalea, while the upper
open part of the net contained the same surface species we had obtained
before.

My experience in the Gulf of California with the Tanner self-closing
net would seem to indicate that in a comparatively closed sea, at a
small distance from the land, there may be a mixture of the surface
species with the deep-sea bottom species, a condition of things which
certainly does not exist at sea in an oceanic basin at a great distance
from shore, where the surface pelagic fauna only descends to a com-
paratively small depth, about 200 fathoms, the limits of the depth at
which light and heat produce any considerable variation in the physical
condition of the water. The marked diminution in the number of spe-
cies below 200 fathoms agrees fairly with the results of the " National "
Expedition.

The more I see of the "Albatross," the more I become convinced that
her true field is that of exploration. She is a remarkably fine sea boat,
and has ample accommodation for a staff of working specialists such as
would be needed on a distant expedition. The time will soon come
when the Fish Commission will hardly care to continue to run her,

200 BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY.

and I can conceive of no better use for so fine a vessel than to explore a
belt of 20° latitude north and south of the equator in the Pacific, from
the west coast of Central America to the East Indian Archipelago.

The success of the " Albatross " thus far has depended entirely upon
the zeal, energy, intelligence, forethought, and devotion of Captain Tan-
ner, if I may judge of the past by the present. He never spares himself,
and he is always ready to make the most of the time at his disposal for
the benefit of the special object he has in charge. He looks after every
haul of the trawl himself, and will not allow any one else to jeopard
in any way the material of the vessel, or the time it requires to make a
haul. That responsibility he assumes himself, and it constitutes his
daily work. In looking over the records of the " Albatross " during her
voyage from New York to San Francisco, I am struck with the amount
of work which has been accomplished. It would be but a just return
to Captain Tanner, if Congress would make the necessary appropria-
tions to work up and publish all that he has brought together, not
only on that cruise, but also what has been left untouched thus far of
the immense collections made by him in the Caribbean, and off the
east coast of the United States, to say nothing of his explorations in
the Gulf of California, on the coast of California, on the coast of Alaska,
and in the Pehring Sea, from which he has accumulated endless and
most interesting material, which no other ship could get together unless
she had another Tanner in command.

We reached Guaymas on the 23d of April, in the afternoon, and I
parted from the ship with great regret, but more than satisfied with the
results of this expedition.

Allow me, in concluding, to thank you most cordially for having given
me the opportunity to join the " Albatross " on this extended cruise, and
for your kindness in urging the President to allow the vessel to be
detailed for this work.

As soon as it may become practicable, I shall send you a full resume
of our work, accompanied with sketches of the Tanner tow-net and a
detailed chart of the route we followed.

Very respectfully yours,

ALEXANDER AGASSIZ.

Cambridge, May, 1891.

Xo. 5. — The Development of the Pronephros and Segmental Duct
in Amphibia. By Herbert H. Field. 1

I. Introduction

II. Descriptive Part 203

A. Rana 204

Stage 1 204

Stage II 209

Stage III 213

Stage IV 219

Stage V 227

Stage VI 237

Stage VII 241

B. Bufo 242

Stage 1 242

Stage II 243

Stage III 244

Stage IV 245

Stage V 246

Contents.

Page
201

Page

C. Amblystoma 247

Stage 1 248

Stage II 250

Stage III 250

Stage IV 252

Stage V 252

Stage VI 257

III. General Discussion .... 262

The Kidneys of Amphioxus 262
The Pronephros of the Cra-

niota . 266

The Segmental Duct ... 288

Organogenetic Conclusions . 295

Phylogenetic Conclusions . 307

IV. Bibliography 323

V. Explanation of Figures . . . 341

I. Introduction.

The studies upon which this paper is based were undertaken with the
purpose of determining the relation which the urogenital system bears
to the germinal layers in Amphibia. At the time when they were begun,
especial interest in this topic had been awakened by the appearance of
Flemming's paper ('86), in which the author entirely confirmed the state-
ment previously made by Graf Spee ('84), that the system was of ecto-
dermal origin. This view was gladly welcomed on many sides, for it
was felt that an origin from this source was more in harmony with gen-
eral conclusions already accepted than was the method previously advo-
cated. Moreover, a new light seemed now to be cast on the phylogeny
of Vertebrates. Under these circumstances, it appeared highly desirable
that the position which Graf Spee and Flemming had taken be subjected
to the test of renewed investigation on other groups of Vertebrates than

1 Contributions from the Zoological Laboratory of the Museum of Comparative
Zoology, under the direction of E. L. Mark, No. XXVII.
vol. xxi. — no. 5.

202 BULLETIN OF THE

those employed by them. The researches of these authors had been
conducted on Mammalian material only, and it was the hope of the writer
to find in Amphibia a similar mode of origin for the excretory duct.

The material employed in the present investigations consisted of em-
bryos of liana, Bufo, and Amblystoma. The study of the problem was
begun with liana pipiens Schreb. (halecina), embryos of which had been
prepared in the spring of 1884 by Prof. E. L. Mark, who kindly placed
his series at my disposal. In the spring of 1889, while in Baltimore,
Md., I secured an abundance of the eggs of liana sylvatica Le Conte. 1
These eggs are large, measuring at the blastula stage two millimeters or
more in diameter. I also found them far better for embryological study
than those of other species of frogs examined. An advantage which
they possess for my purpose is that the germ layers are very well sepa-
rated from one another. Moreover, the body cavity appears at an early
stage, making the boundary between the somatic and the splanchnic
mesoderm very pronounced, both in the region of the protovertebra: and
of the lateral plates.

The eggs of Bufo studied, B. americanus Le Conte, were collected dur-
ing the spring of 1887, in Cambridge and in Jamaica Plain, Mass. At
this time I also collected a small quantity of Amblystoma eggs from a
pond in Jamaica Plain ; but a careful search, carried on during several
subsequent trips to this locality, failed to yield any more eggs.

Prof. J. S. Kingsley at this time kindly sent me from Indiana some
Amblystoma material which he had preserved ; but for the determina-
tion of many points at issue I was obliged to wait till another season
offered opportunities for collection. In the spring of 1889, therefore, I
made a trip to Baltimore, where I was able to collect an abundant sup-
ply of the eggs of this Amphibian, most if not all of the material col-
lected belonging to the species A. punctatum Linn. In this work I was
accommodated at the Biological Laboratory of the Johns Hopkins Uni-
versity, — a privilege for which I am under obligation to that institu-
tion. My thanks are particularly due my friend Dr. T. H. Morgan for
his kind assistance during my stay in Baltimore, and for material of his
collection.

I may here also express my obligations to Dr. John S. Billings, Sur-

1 Inasmuch as the observations of European investigators have usually been
made on R. temporaria, it is of interest to note that R. sylvatica Le Conte has been
regarded by some systematists as a variety of R. temporaria (Giintlier, '58, p. 17).
In any event, the development of the two forms may be assumed to be very
similar.

MUSEUM OF COMPARATIVE ZOOLOGY. 203

geon U. S. Army, for the favor of sending me from the Surgeon General's
library in Washington a number of papers to which I should otherwise
have been unable to gain access. I am further indebted to Mr. Samuel
Garman and to Mr. G. H. Parker for the revision of my proof-sheets, and
for suggestions during the progress of my work. Mr. Parker also read
the earlier portions of my manuscript.

The material was prepared by ordinary histological methods ; but in-
asmuch as many of the hardening reagents and stains which I tried gave
thoroughly unsatisfactory results, I may state in brief the treatment
which proved most successful. The embryos of both Eana and Bufo
can be satisfactorily killed in Kleinenberg's picrosulphuric mixture ;
they can then be successfully stained in Orth's lithium-picrocarmin.
The object should be exposed to the action of the stain as long as possible,
care being taken to guard against maceration. In order to accomplish
this purpose, it has frequently proved advantageous to stain the object
twice, removing it after the first staining to strong alcohol. In passing
the stained object through grades of alcohol, it is important to keep a
little picric acid dissolved in the several fluids in order to prevent the
alcohol from extracting the yellow stain from the specimen. Embryos
treated in this way show a very effective double stain. The nuclei are
bright carmine, contrasting with the yellow color imparted by the picric
acid to the yolk spherules among which they are found. As a killing
reagent, Merkel's fluid also gives good results. It should be followed
by Kleinenberg's hematoxylin, and the decolorizing should be watched
with care.

With Amblystoma the best method of treatment is that with Fol's
chromic-osmic-acetic mixture, followed by Czokor's cochineal. The
picrosulphuric mixture followed by picrocarmin, as recommended for
liana and Bufo, is also of service.

It is usually best to stain on the slide ; and, in my experience, satis-
factory results with hematoxylin can very rarely be reached by staining
in toto.

II. Descriptive Part.

In the following account of the development of the pronephros and
segmental duct, I shall first treat these organs descriptively. For this
purpose, I shall take up in succession Rana, Bufo, and Amblystoma, and
shall describe selected stages in the development of each. This account
will be followed by a general discussion of nephridial organs, in which
the results of other investigators will be reviewed.

204 BULLETIN OF THE

A. liana.

Stage I.

Plate I. Figs. 1-3.

At the first stage which I shall describe the embryo has departed only
a little from the spheroidal form presented by the egg during segmenta-
tion. The medullary plate is widely open, its lateral margins being only
slightly elevated above the general surface. At the hinder end of the
medullary plate the blastopore is plainly visible. An idea of the exter-
nal form of the embryo can be gained by reference to Goette's figure of
Bombinator ('7.1, Taf. III. Fig. 41), or to van Bambeke's of the Axolo-
tyl ('80, PL XII. Fig. 9). In water of 15 to 18° C. eggs of R. sylvatica
reached this stage in about sixty hours after fertilization ; the eggs of
"R. halecina develop somewhat more slowly.

The general relations of the germinal layers at this stage are shown in
Figure 2. The ectoderm consists of two distinct layers (Figs. 3 and 7,
ec'drm.' and ec'drm."). Except in the region of the medullary thicken-
ing (la. med.), which is produced by a proliferation of the deeper of
tli^se two layers, the ectoderm is nearly uniform in thickness. The two
layers present slightly different histological characters. In the outer
layer (Figs. 3 and 7, ec'drm.') the cells are large and columnar, and their
external surfaces project as rounded eminences, giving a roughly granular
appearance to the surface of the embryo. Each cell contains scattered
pigment grannies, which are especially massed along its external face.
Small yolk spherules (spk. vt.) are present in considerable numbers.
The cells of the deep layer (ec'drm.") are smaller than those of the outer,
and are somewhat flattened. The pigment granules are scattered through-
out the cells of this layer, without showing special accumulations. The
yolk spherules present the same appearance as those of the superficial
layer.

The entoderm and yolk cells (Fig. 2, en'drm. and cl. vt.) form the great
mass of the interior of the embryo. The wide lumen of the gut trav-
erses the dorsal portion of this mass. The chorda (n'ed.) has the form
of a longitudinal ridge, imperfectly cut off from the entoderm below, and
in contact with the medullary plate above. A single cell layer (en'drm.)
on each side of the chorda forms the dorsal roof of the intestine. As
this layer passes out laterally, it increases in thickness, becomes several
cells deep, and finally merges in the mass of large yolk cells (cl. vt.) lying
ventral to the intestinal cavity. All the cells of the entoderm contain
large yolk spherules. Pigment is present in considerable quantity in the

MUSEUM OF COMPARATIVE ZOOLOGY. 205

cells bordering the cavity of the intestine ; elsewhere it occurs only as
scattered granules.

At this stage two plates of mesoderm (Fig. 2, la. msWrm.) extend out
laterally, one on each side of the chorda, and pass ventrally around the
mass of yolk cells to be united in the median line below. Each plate is
thickest (Figs. 1,3, at la. pr'vr.) next the notochord ; as it passes out-
ward, it becomes thinner. Before the ventral surface of the embryo is
reached, it is reduced to a layer two cells thick, representing the somato-
pleure and splanchnoplenre (so'plu. and spVplu.) of this region. The cells
of the mesoderm are in general smaller than those of the yolk-entoderm.
The yolk spherules which they contain are also somewhat smaller than
those in the entoderm. Pigment is rarely present except in the form of
scattered granules.

In the foregoing account of the relations of the germ layers the de-
scription refers in the main to She typical condition, realized in the
middle trunk region ; in this and in subsequent stages modifications
occur in the head and tail regions. These special conditions are of no
consecpience for the present purpose.

There are certain histological characters, to which allusion has already
been made, which may serve as criteria for distinguishing the germ
layers. The most satisfactory of these is the size of the yolk spherules.
As I have indicated, the spherules are largest in the entoderm and
smallest in the ectoderm ; in the mesoderm they are of an intermediate
size. Measurements of spherules from the three layers in the region of the
future pronephros gave the following results : entoderm, mean diameter
of spherules, 8 /j. ; mesoderm, mean diameter, 5 /x ; ectoderm, diameter
rarely exceeds 3 /a. Excluding the head and tail regions, these dimen-
sions represent, I believe, fair averages for the whole body. The dis-
tribution of pigment affords another criterion for distinguishing the layers.
In the superficial ectoderm, the pigment (Figs. 3, 7) is massed along the
external surface of each cell. In the deep ectoderm, it is present in con-
siderable quantity, but is scattered throughout the cell. Except in cer-
tain specialized regions, there is little pigment in either mesoderm or
entoderm. I have also noted the differences in the mean sizes of the
cells : the yolk cells are in general the largest, and those of the ecto-
derm the smallest, the mesodermal cells being of intermediate size.
The great variability of this character prevents its having much weight,
however, in determining to which of the three layers a given group of
cells belongs.

I shall now consider in greater detail some of the modifications which

206 BULLETIN OF THE

the mesoderm exhibits, particularly such as occur in the region where
the pronephros is subsequently developed. For this purpose I have
selected two embryos of Stage I. which show slightly different condi-
tions. The account will first relate to the specimen which is shown, by
the less differentiation of the medullary plate as well as by other features,
to be the younger. This embryo measures 2.31 mm. in length. In fol-
lowing a series of cross sections forwards, the three germ layers become
apparent at about 0.35 mm. from the posterior end, or a short distance
in front of the blastopore. Here the structure of the mesoderm is rather
obscure, since in a transverse section of the animal this layer is cut
obliquely. The condition, however, is here nearly the same as that
which I am about to describe for a more anterior section.

Figure 3 represents a section of this embryo 0.91 mm. from the pos-
terior end. On the ventral side of the embryo the mesoderm consists of
two layers, each of which is only a single cell in thickness. These two
layers, which represent somatopleure and splanchnopleure, are separated
by a narrow space, the ccelom (coeL). In the lower left-hand corner
of the figure, the beginning of this two-layered condition of the meso-
derm can be seen. On following the mesoderm towards the dorsum, it
becomes gradually thicker. In the mesoderm of this region there is
found an extensive cavity (coel.), which is usually irregular in outline,
and might be mistaken for a wholly artificial condition. That the two
layers were once in contact is shown by the correspondence of outline on
the two sides of the space. The separation along this line is so regular,
however, in successive sections, and recurs so frequently in other em-
bryos, that the cavity must be regarded as an artificial expansion of an
already existing split, rather than as an indifferent rupture of a solid
mass of cells. In many sections of this embryo it is easy to trace a line
of division reaching from the ventral cavity (ccelom) to the large lateral
cavity just described. This, then, represents a portion of the coelom
(normally, I believe, closed), and the layers of mesoderm on the two
sides of it are consequently somatopleure and splanchnopleure. The
mesoderm in this region, as I have stated, is several cells deep. Along
the inner and outer edges of the wedge-shaped plate of tissue constitut-
ing the mesoderm of either side, the cells, except where artificial rup-
tures occur, are in close contact, and form an epithelial lamella. The
central portion of the plate, where this is more than two cells in thick-
ness, contains cells of a more rounded shape, which do not form definite
rows, but which are closely applied to the outer layer, — a condition
which becomes quite evident when the coelom is artificially enlarged.

MUSEUM OF COMPARATIVE ZOOLOGY. 207

The somatopleure of this region, then, is a layer at least two cells in
thickness. The splanchnopleure, on the other hand, in this as in later
stages, consists of a layer one cell in depth, extending from the ventral
surface of the animal to the protovertebral plate. 1 Naturally no sharp
line of division can be drawn at this stage between the protovertebral
plates and the adjacent portions of the lateral plates. In the section
under consideration, the protovertebral plate is rather compact, and it is
difficult to indicate with certainty the boundary between the somatic
and splanchnic layers. A study of this portion of the mesoderm, how-
ever, has convinced me that the coelom (coel. 1 ) is already outlined, and
lies in such a position as to leave only a single layer of cells dorsal
to it, — a condition which is perfectly evident in later stages. It is
indicated by such a distribution of pigment as is seen to the right in
Figure 3.

On following the series of sections farther towards the head, a con-
striction of the mesoderm appears beneath the lateral margin of the
medullary plate, and the open ccelom is continued into the protover-
tebral plate. In a section 1.2 mm. from the posterior end the somatic
and splanchnic layers are each but one cell thick in the region of the
protovertebral plate. The cells of the somatic layer, which in the proto-
vertebral portion are of a high columnar form, become tile-like beneath
the pronounced lateral thickening (compare Fig. 1, eras, gn.) of the
medullary plate. The somatopleure immediately lateral to the medul-
lary plate is rather thick, and becomes thinner both towards the median
dorsal and median ventral lines. The regularity of the bounding walls
of the body cavity in this region, and the occurrence of a space where no
other signs of distortion are apparent, lead me to believe that the separa-
tion of the two layers of mesoderm is here perfectly normal, and not, as
in more posterior regions, an artificial separation of two closely applied
lamella?.

It is, in general, very difficult to observe karyokinetic conditions in
mesodermal or yolk cells, owing to the presence of the large and nu-
merous yolk spherules •, but I am reasonably certain that I have ob-
served cells in the somatopleural thickening, dividing in a plane parallel
to the surface of the layer -, i. e. the cells were dividing in such wise as
to increase the thickness of the layer.

In a section 1.32 mm. in front of the posterior end, the lateral portion

1 The differentiation of the protovertebrse has not yet begun in this region,
and I shall designate the thick masses of mesoderm on each side of the chorda as
protovertebral plates.

208 BULLETIN OF THE

of the medullary plate is greatly thickened, and the lateral plates are
thereby wholly cut off from the proto vertebral plate. The thickening of the
medullary plate is the hinder portion of a considerable ganglionic mass,
which is the basis for the subsequently differentiated ganglia Gasseri,
acusticum, and nodosum. 1 The somatopleural thickening may be traced
to a point about 80 fj. farther forward, where the body cavity is no longer
expanded. The relations of this thickening to the nephridial organs will
be discussed in connection with Stage II. (page 211).

In a slightly older embryo, measuring 2.34 mm. in length, the condi-
tion of the mesoderm is nearly the same as in the one last described.
The somatic layer shows a marked thickening (Plate I. Fig. 1, eras. su'jjIk.),
which is greatest immediately lateral to the protovertebral plate. An
anterior ccolomic chamber is also present. The anterior limit of the
thickening is situated, as before, about 0.1 mm. in front of the hinder
end of the enlargement which is destined to give rise to the cranial
ganglia. The thickening (Fig. 1, eras, so'plu.) of the somatopleure is
slightly more pronounced than in the younger embryo.

The results of this study may be summarized as follows. There
exists already at this stage a slight somatopleural thickening, which is
maximum along a line immediately lateral to the protovertebral plate.
This thickening is associated with a local expansion of the ccelom. It
is most pronounced in the region directly posterior to the cranial gan-
glionic mass. Posteriorly it is lost in a general lateral thickening of the
somatic layer. The location of the thickening corresponds closely with
the region in which the pronephros and segmental duct later arise.
Whether we have in this thickening the first rudiment of the excre-
tory system will be discussed in connection with Stage II.

1 I may here note that I have been able to make out for the series of spinal and
craniai ganglia in Rana, Bufo, and Amblystoma an origin not unlike that described
by Beard ('88. pp 166. 183) in Selachii and Aves. and by Schultze ('88, p. 349) 'in
Rana. The ganglia are developed from the ectoderm at the lateral margins of
the medullay plate (Fig. 3, fnd. gn. spi.). The differentiation of the ganglia is
already apparent before the neural tube is infolded A spinal ganglion does not
arise as an outgrowth from the neural tube, nor as a separate thickening of indiffer-
ent ectoderm, but is differentiated from a first rudiment (Anlage) common to it and
to the neural tube.

MUSEUM OF COMPARATIVE ZOOLOGY. 209

Stage II.

Plate I. Figs. 4, 5. Plate II. Figs. 13, 14.

This stage includes embryos with a distinct medullary groove, the
edges of which, however, have not yet fused to form a complete neural
tube. Several proto vertebrae can be distinguished.

In treating of the structure of the pronephros in this stage I shall first
consider two embryos, which, judging from external appearances, seem to
have reached the same stage of development. These embryos are about
as far advanced as the one figured by Hertwig ('83, Taf. V. Fig. 6). In
both the medullary groove is widely open. They are about 2.5 mm.
long, and have been sectioned, one transversely, the other frontally.

Following the series of cross sections forward from the tail end, and
comparing them with those of the preceding stage, the changes which
have occurred will be apparent. In the posterior region, the mesoderm,
as it passes outward and downward from the chorda, tapers much more
rapidly than in the earlier stage. Even as far posteriorly as a few sec-
tions in front of the blastopore, this condition can be observed ; and, in a
section 0.72 mm. from the posterior end, the thick central mass of meso-
derm, the protovertebral region (Fig. 4, la. pr'vr.), has a triangular out-
line in cross section, and is readily distinguishable from the lateral plate
(la. I.), with which it is continuous at its outer angle. The protover-
tebral plate consists of an outer epithelial layer and a central mass of
cells. It is the former which is prolonged into the lateral plates. Each
of these is here in general only one cell deep. Between somatopleure
and splanchnopleure a few scattered cells occur, which can be assigned
only with difficulty to either layer.

At 0.96 mm. from the posterior end the hindermost protovertebra
visible in cross section can be distinguished. Between this point and
the ganglion nodosum four protovertebrse are to be observed. Passing
farther forward, it is difficult to assign boundaries to the protovertebra?.
There is certainly 'one which is partially broken up into mesenchymatic
tissue. 1 Still farther forward the series of the protovertebrie is con-
tinued by mesenchyme of a yet looser structure. Inasmuch as I have

1 I use this expression merely as descriptive of tissue of a certain histological
character, quite independently of its origin. Indeed I am convinced, from observa
tions which appear in the sequel, that not merely the head mesenchyme, but also
much of that in the trunk, arises in relatively late stages from mesodermal tissue,
substantially in accordance with the account of Balfour (78, pp. 107 et seq.), which
lias recently found champions in Ziegler ('88) and others.

vol xxi — no 5 14

210 BULLETIN OF THE

reached no conclusions respecting the number and position of the head
somites, and since great diversity of opinion exists in the accounts to
be found in the literature, I shall make no attempt to number the
protovertebra? with which I shall have to do in any other way than
by beginning with the most anterior that is readily distinguishable.
Disregarding, then, the one which is wholly broken up into mesenchy-
matic tisue, somite I. lies in the same transverse plane as the fun-
dament 1 of the ganglion nodosum, and extends backward to the hinder
end of that structure. This protovertebra also shows signs of extensive
conversion into mesenchyme, although part of it at a later stage undergoes
muscular differentiation. Somite II. is the first of the series of well
developed trunk protovertebrse. In the specimen under consideration
somites I. to VI. are already differentiated.

As I have stated, the somatopleure in the middle of the trunk consists
of a layer one cell deep, to which a few loose cells lying between it and
the splanchnopleure may possibly also be assigned. In the region of
somite IV. the somatopleure becomes thickened. The thickening is
greatest at the level of the lower margins of the protovertebne (com-
pare Plate II. Figs. 15, 16), and tapers both dorsally and ventrally.
It is to be remarked in this connection that the protovertebra? are not
yet fully separated from the lateral plates ; but that in cross sections
through the middle of a somite, — i. e. midway between the anterior and
posterior faces of a protovertebra, — the coelom can be traced to the
dorsal margin of the protovertebra, and furthermore that the somato-
pleure and splanchnopleure are seen to be continuous with the somatic
and splanchnic layers of the protovertebne. The somatopleural prolif-
eration extends forward as far as the anterior face of somite II. The
cells in the thickening have a columnar shape, and are at least two deep.
In some sections I have observed, in addition, a third row of thin
cells next the body cavity. Near the ventral limit of the thickening
a nearly horizontal line of division in the substance of the thickening
can be observed. When seen in cross section, this line is slightly con-
cave above. It is here that ruptures produced by artificial causes are
likely to occur, and the line thus indicated marks, I believe, the lower
limit of the pronephros. The somatopleural thickening is the funda-
ment of the pronephros, and I shall call it in the following pages the

1 In the following pages I shall use the word fundament as an equivalent of the
German expression Anlage, the term fundamentum having been adopted as the
basis for the lettering of the figures of such structures in the " Contributions "
from this Laboratory.

MUSEUM OF COMPARATIVE ZOOLOGY. 211

pronephric thickening. The dorsal portion of the expanded body cavity
is the pronephric chamber.

The question whether the somatopleural thickening described in
Stage I. be an early condition of the pronephric thickening is only to be
answered by considering the fate of the former. Behind somite IV. this
early thickening wholly disappears, and the one which is seen at a later
stage is an independent formation. ' This conclusion is justified by a com-
parison of Figure 4 (Plate I.), showing the somatic layer to be only
one cell thick in the posterior region of an embryo of the present stage,
with Figure 3, which shows a two to three layered somatopleure (so'piu.) in
a somewhat more anterior region of an embryo of the next younger stage.
In the region of somites II., III., and IV., however, the somatopleure
never wholly thins out ; but the thickening is here moulded into a more
definite form, and becomes the fundament of the pronephros. To my
mind, it is as if the mesoderm, in the process of becoming thinner, was
overtaken by the necessity of affording material fur the formation of the
pronephros and duct, and, as a matter of physiological economy, used
for that purpose an accumulation of cells already present. Indeed,
from the form of the thickening in anterior portions of the embryo, I am
disposed to regard the differentiation of the pronephric thickening in this
sense as having begun already in Stage I.

The corresponding series of frontal sections shows five well developed
protovertebrse, representing somites I.-V. (Plate II. Figs. 13, 14). A
mass of mesenchymatic tissue in front of somite I. is doubtless the rem-
nant of the rudimentary anterior protovertebra observed in the series of
cross sections, and behind somite V. the differentiation of a sixth is
faintly indicated. Above the level of the lower border of the chorda the
protovertebrae are sharply marked off from one another, and the somatic
layer is relatively thin. Near their ventral margins, however, the suc-
cessive protovertebrse are in close contact, and the somatic layer shows a
pronounced lateral thickening (Fig. 13, eras. pr'?iph.).

On passing ventrally to the region of the lateral plates, the inter-
protovertebral constrictions vanish. Since frontal sections, however, do
not here cut the layer of mesoderm perpendicularly, certain sections in
the series show a distinctly segmented splanchnic layer, while the so-
matic thickening in the same frontal plane is unsegmented. Farther
ventral there are no traces of segmentation in either layer. Here the
splanchnopleure (spVplu.) uniformly consists of a single layer throughout
its entire extent. The somatopleure facing the ganglion nodosum, and
also that in the posterior region, is thin ; but in the anterior portion of

212 BULLETIN OF THE

the trunk, immediately behind the ganglion nodosum, there is a marked
thickening (eras, pr'nph.), which ends abruptly in front, but gradually
thins out into indifferent somatopleure behind. This thickening is
distinctly present through a length of 0.5 mm., which is slightly greater
than the extent of protovertebrse II., III., and IV. Still farther ven-
trally, the antero-posterior extent of the thickening is much diminished,
the reduction taking place from both ends, so that in passing ventrally
the region in which the structure is last visible is situated approxi-
mately beneath protovertebra III.

Another pair of embryos, one of which was 2.5, the other 2.G mm. in
length, presented a condition of the pronephros somewhat more advanced
than that just described (Plate I. Fig. 5). In these embryos the lips of
the medullary fold in the most advanced region were in contact, but had
not yet fused. The anterior limit of the pronephric thickening was the
same in position as in the younger pair of embryos, lying near the middle
of somite II. A study of the arrangement of the nuclei in this region
made it evident that there were at this stage in general three layers in the
thickening. The innermost of these is the thinnest, and is destined to be-
come the peritoneal covering of the pronephros ; the other two represent
the two walls of the pronephric pouch, soon to be described. The prone-
phric thickening ic the region of the anterior face of somite IV. is shown
in Figure 5. The section gives a somewhat false impression as to the
somatic layer of the protovertebra, unless the relation of the section to
the successive somites be borne in mind. The considerable thickening
which this layer apparently undergoes on passing into the protovertebra
is due to the circumstance that the section here passes obliquely through
a portion of the anterior wall of somite IV. Sections through the middle
of a protovertebra show a gradual thinning of the somatic layer as far a.;
the dorsal angle of the mesoderm (compare Plate II. Fig. 15, which is
a cross section of the following stage), where this layer is almost pave-
ment-like. The pronephric thickening extends rather farther posteriorly
than in the former pair of embryos, and while it is manifestly difficult
to set a limit to the structure, I am confident that the thickening ex-
tends into somite V. This posterior extension of the thickening is to
be regarded as the fundament of the pronephric, or, according to the
later nomenclature of Balfour, the segmental duct.

The corresponding series of frontal sections shows six well differen-
tiated protovertebra?, representing somites I. -VI. The same group of
cells which I interpreted before as the last remnant of a rudimentary

MUSEUM OF COMPARATIVE ZOOLOGY. 213

anterior somite is still present, and a few more posterior protovertebra;
are in process of formation. .Frontal sections just ventral to the chorda
are very instructive. By following through a series of these, an idea
can be had of the successive changes which take place in passing from
the protovertebrae to the lateral plates, — a region of prime importance
for problems respecting the development of the urogenital organs. In
sections approximately tangent to the chorda at its ventral border (com-
pare Fig. 5), the plane of the section passes through the ventral floor of
the protovertebra, and cuts the somatic mesoderm near the place where
the protovertebra passes into the lateral plate. The body cavity is ex-
panded in the anterior part of the trunk. The mass of tissue on the
median side of the body cavity appears very broad, owing to the circum-
stance that the plane of the section, as before noted, lies in the floor of the
proto vertebra. The somatic layer is several cells thick, and very com-
pact in structure, owing to the fact that the section passes through the
dorsal margin of the pronephric thickening. In following the series of
sections farther ventrally, the boundaries between the segmental con-
stituents of the pronephric thickening become indistinct ; and in a sec-
tion 90 fi farther ventral they have wholly vanished. This section,
however, still shows traces of segmentation in the splanchnic layer,
which is here reduced in thickness, the plane of this section having
passed ventral to the floor of the protovertebra. Still farther ventrally
the segmentation of the splanchnopleure likewise vanishes, and finally
the pronephric thickening gives place to undifferentiated somatopleure.
I have looked in vain for prolongations of the body cavity into the prone-
phric mass at this stage. I believe that the pronephric thickening is to
be regarded as a solid proliferation of the somatopleure, in which, how-
ever, the somatic layer of the protovertebra? takes some part.

Stage III.

Plate I. Fig. 6. Plate II. Figs. 11, 12, 15-17.

In embryos of this stage the medullary canal is wholly closed, the fun-
daments of two pairs of gills are present, and the auditory vesicle consists
of a shallow depression of the deep ectoderm.

The pronephric thickening has now begun to assume a more definite
form, and during this stage becomes converted into a tubular organ.
I shall first consider the structure as seen in a series of cross sections
from an embryo measuring about 2.7 mm. in length. Figures 15 to 17
are from this series. The anterior end of the pronephric thickening is

214 BULLETIN OF THE

located in somite II. The plane of the section from which Figure 15
was drawn passes somewhat behind the middle of this somite, so as to
show the location of the constriction between the protovertebne and the
lateral plates. In the middle of the somite, the arrangement of the cells
composing the pronephric thickening appears to be that of a fold in which
the layers are in close contact. The thickening is composed of three layers
of cells, and it is possible to trace the somatic layer of the protovertebra
into the outer layer of the thickening. The lateral indifferent somatopleure
is continuous at the ventral border of the thickening with the inner or
thin layer which lies next to the body cavity. Near the upper border of
the thickening this inner layer appears to be folded abruptly on itself to
form the middle layer of the thickening. The middle and outer layers
are continuous with each other distally, i. e. ventrally. 1 This anterior
knob of the pronephric thickening (Fig. 15, fnd. nph'st. 1 ) is the fundament
of the first nephrostome, a later stage in the development of which is
shown in Plate III. Fig. 18 (nptistm. 1 ). In Figure 15 the three lay-
ers are indicated by the arrangement of the nuclei. Of these the two
outer form the fundament of the first nephrostomal tubule. The inner-
most Liver represents the underlying peritoneum. In the region be-
tween somites II. and 111., it is impossible to distinguish definite layers
in the thickening.

On entering somite III., the pronephric thickening has a far greater
breadth, and it consists of three layers, the meaning of which is to be
understood by a comparison with the condition in the region of the first
n< [ 'hrostome, just described.

In somite IV. (Fig. 16) a division of the thickening into a dorsal and
ventral part is indicated, near the termination of the dotted line (eras.
pr'npk.). The dorsal part is the fundament of the third nephrostome,
and the ventral part represents the anterior portion of the segmental
duct (more properly, common trunk, see page 228). The ventral por-
tion of the thickening can be traced backwards from this point through
a distance of about 0.37 mm. Figure 17 is drawn from a section
through a region near the anterior boundary of somite VII., and shows

1 The correlative terms distal and proximal are so frequently employed by Ger-
man writers as synonymous respectively with posterior and anterior that it seems
advisable to allude to the fact that they are not used in the present paper in that
sense, but invariably with their primitive signification; thus, the distal portion of
a process is that part which is most remote from the point of attachment, whether
the structure project anterioriy or posteriorly, medially or laterally, dorsally or
ventrally.

MUSEUM OF COMPARATIVE ZOOLOGY. 215

the thickening (eras, pr'nph.) near its posterior termination. The mass
is evidently a thickening in situ of the somatopleure. On either side of
the fundament of the segmental duct the somatopleure is one cell thick,
whereas in the fundament itself it is two or three cells in thickness. If
the additional cells arose by a free backward growth from the anterior
pronephric mass, we should expect to find them lying on the external
face of a continuous somatopleural layer. But, as a matter of fact, no
such continuous inner layer exists ; on reaching the thickened region, the
somatopleure merely becomes several cells in thickness, the outer cells
presenting really a somewhat more compact condition and a more linear
arrangement than the inner ones.

The constrictions between the protovertebral and the lateral mesoderm
can be distinctly made out only in intersegmental regions. As is shown
in Figure 15, between somites II. and III. the level of the constriction
is immediately dorsal to the nephrostomal portion of the pronephric
mass.' In the region between somites III. and I\ T . the division occurs
at a corresponding position. This series of sections shows no sharp sepa-
ration between protovertebral and lateral mesoderm posterior to somite
IV., the protovertebral plate being here only partly broken up into suc-
cessive metameric blocks, which do not as yet possess sharp ventral
boundaries.

In frontal sections, the pronephi'ic thickening shows a similar condition
(compare Figs. 11-14) to that which obtains in the case of the embryo
described under Stage II. (page 213), the most noticeable difference
being an increase in the thickness of the pronephric mass. The longi-
tudinal extent of the thickening corresponds approximately to that of
five somites, though the posterior limit is of necessity somewhat un-
certain. The posterior portion has every appearance of having arisen in
the same way as the part lying beneath somites II., III., and IV. The
latter, however, represents, as we have seen, the future pronephros; the
former is the fundament of the segmental duct.

In an embryo slightly older than those last described, the evidences of
an incipient canalization of the pronephric system are more pronounced.
In the region of somites II.— IV., the two outer layers of the pronephric
thickening are separated from the peritoneal layer by a distinct line of
division, [n the intersegmental regions, the outline of these two layers is
that of an elongated ellipse, the nuclei being disposed, for the mosl part
alternately, on either side of its major axis. The significance of this
distribution becomes apparent on studying later stages, in which a lumen

216 BULLETIN OF THE

has appeared in the organ. It is then found that the lumen occupies
the position of the major axis of the ellipse, and that the nuclei of the
bounding cells lie close to the interior surface of the wall. If a tube so
constituted be compressed laterally, so that the lumen wholly disap-
pears, it is evident that the cells of the opposed walls would be likely to
accommodate themselves to one another so as to present an alternate
arrangement of their nuclei.

Opposite the middle of a somite, the relations are somewhat different.
Here the two layers of what I shall hereafter call the pronephric pouch
do not remain confluent at its dorsal extremity, but separate, the outer
becoming continuous with the somatic layer of the protovertebra. the
inner with the deepest layer of the thickening, and thus finally with the
lateral somatopleure. In this region the body cavity can be seen to
project for a short distance between the two layers of the pronephric
pouch, as shown in Plate 1. Fig. 6, ccel . This figure demonstrates very
clearly the relations of the pouch to the lateral mesoderm and t he over-
lying somites.

In the case of the } - ounger set of embryos which have been con-
sidered in this stage, it will be remembered that the boundary between
the lateral mesoderm and the protovertebra was evident only in inter-
segmental regions. In the somewhat older individual now under con-
sideration, the constrictions between these two portions of mesoderm have
advanced into segmental regions as well ; so that now, for the first time,
the precise relations between the fundaments of the nephrostomes and
the protovertebrse lying above them can be accurately determined. The
last remnant of the communication between the protovertebral cavity
and the body cavity I shall call the communicating canal, following in
this the nomenclature of Renson ('83). The section shown in Figure 6
passes through this canal (can. comn.). and it is to be especially noted
that the constriction between the somites and the lateral plates takes
place dorsal to the region of communication between the pronephric sys-
tem and the body cavity. Immediately dorsal to the pronephros, the
somite sends out a lateral fold of the somatic layer, which is destined to
form the capsule of the pronephros, to which I shall have occasion to
refer in later stages.

In somite IV., the division of the pronephric mass into a dorsal and
ventral part is faintly indicated, but the dorsal part shows no trace of
the lumen which is destined to become the third nephrostome. In this
embryo, the constrictions between the protovertebra; and lateral plate
have advanced into more posterior regions. In somite V. the constric-

MUSEUM OF COMPARATIVE ZOOLOGY. 217

tion occurs immediately dorsal to the fundament of the segmental duct,
which, as I have shown, is continuous anteriorly with the ventral half
of the thickening appearing in somite IV. A series of measurements
from the dorsal median line shows that the ventral portion of the pro-
nephric thickening remains at a nearly constant level, so that the pro-
tovertehiTe must reach a somewhat more ventral position in the posterior
region than in somites II. -IV.

Figure 11 (Plate II.) represents a frontal section through the dorsal
part of the pronephric pouch in one of the oldest embryos of this stage.
It shows the coarse of the earliest fundaments of the three tubules which
emerge from the somatopleure beneath proto vertebrae II., III., and IV.
The most anterior outgrowth, arising in somite II., inclines outward and
backward into the region of somite III. ; the second outgrowth proceeds
from its origin beneath protovertebra III. directly outward ; and the
third outgrowth inclines forward, so that its distal extremity also lies in
the region of somite III. As the review of the previous stages has
shown, these fundaments of the tubules have not arisen as separate out-
growths from the somatopleure, but have been differentiated from the
originally continuous pronephric thickening, the three fundaments being
confluent distally.

In this section the nuclei are abundant along a central band, but scarce
or wholly absent in peripheral parts. This peculiar arrangement be-
comes intelligible when we consider that the plane of the section passes
almost tangentially through the curved dorsal wall of the pouch. As we
have seen in transverse sections, the nuclei lie close to the inner lumen
of the pouch ; it is therefore only in the deeper central parts of the sec-
tion that they are encountered. In a section 0.03 mm. farther ventral
(Fig. 12), the lumen of the pouch can be made out, though it is not con-
spicuous. It is difficult to say whether at this stage the lumen is contin-
uous throughout the whole structure. In many embryos the evidence of
such continuity seems indubitable ; whereas in others, apparently quite
as far advanced in other respects, the lumen seems to consist of uncon-
nected portions. In some instances where no trace of a separation of the
walls could be seen, a line of pigment indicated the position of the lumen.
Occasionally I have met with a distinct prolongation of the body cavity
into the pronephric mass. This condition has been most frequently en-
countered in the case of the fundament of the first tubule. I am not,
however, inclined to place much weight on such observations as proving
the claim that the lumen of the pronephros forms as an ingrowth of the
cui'lom proceeding from the nephrostomes and advancing into the duct.

218 BULLETIN OF THE

On the contrary, the lumen is already potentially present, as shown by
the arrangement of the nuclei before any actual separation of the walls
occurs. 1 am of opinion that, in the cases referred to, the separation is
largely artificial, and that the ruptures take place most frequently at the
nephrostomies for the reason that the walls, which elsewhere form a closed
ring, here have in cross section the form of a sharp re-entrant angle bor-
dering on a large open space. It is evident that in the former region the
walls would be less liable to be torn apart in the preparation of the ma-
terial than in the latter. In general, however, it must be admitted that
the development of the lumen, like that of the system as a whole,
actually advances from anterior to posterior regions.

The fundaments of the three pronephric tubules shown in Figure 11
are not to be regarded as outgrowths from the somites. They are, it is
true, very closely related to the segments in their arrangement, but, as
transverse sections prove (Plate I. Fig. 6, and Plate II. Fig. 15), they
lie wholly ventral to the lower boundaries of the protovertebrae. The
frontal section figured (Fig. 12) was chosen for the reason that it was
the one which indicated most precisely the course of the fundaments of
the three tubules. The plane of the section is parallel to a tangent
to the dorsal margin of the structure, and passes only a little below that
margin, not through the nephrostomes. These begin in a more ventral
unsegmented region.

In the oldest embryos of this stage, the fundament of the duct has
developed very rapidly. Anteriorly, it has in cross section a distinctly
elliptical outline, and its cells have, with reference to the major axis of
the ellipse, the same arrangement that I have described for the inter-
segmental regions of the pronephric pouch. On following the structure
backwards, this distribution becomes less and less obvious, until the
cells seem to have no definite arrangement. In this region the funda-
ment of the duct is in far more intimate union with the somatopleure
than was the case in anterior somites. In the region of somite IX.
the last trace of the structure is to be seen as a simple thickening of
the somatopleure, similar in form to that described and figured in the
youngest embryos of this stage (Fig. 17), for a region just back of
somite VI. The region in which the duct is formed is throughout im-
mediately ventral to the constriction separating the protovertcbrse from
the lateral plates. 1

1 In «cctions from the posterior end of the embryo, it is necessary to guar;]
against the false appearances which arise from the obliquity of the plane of the

MUSEUM OF COMPARATIVE ZOOLOGY. 219

The mode of development which I have described in the foregoing
pages, taken in connection with frontal sections, which show that the
pronephric thickening tapers gradually backwards into indifferent soma-
topleure, seems to me to be very strong evidence concerning the precise
origin of the duct. I believe I am justified in concluding that the seg-
mental duct between somites V. and IX. arises in situ from a thickening
of the somatopleure serially equivalent to that from which in the anterior
region the pronephros is developed. Indirect evidence which can be
brought to bear on this question will be reserved for the fuller con-
sideration which can be accorded it, in connection with the following
stage (page 222).

Stage IV.

Plate I. Figs. 8, 9. Plate III. Figs. 18-26. Plate IV. Figs. 29, 39.

Plate V. Fig. 45.

I have placed in this stage embryos of frogs taken from five different
killings. They all belong to the fourth day after fertilization, and aside
from individual variation show an evident advance in organization on the
preceding stage. In all a distinct differentiation of muscular tissue has
begun, the auditory vesicle is wholly cut off from the epidermis, and the
ventral sucking (or more properly sticking) disks are well developed.
In the following description, I shall find it convenient to distinguish a
younger and an older set of embryos. In the younger set the embryos
are from "i\ to 3^ mm. long ■ they have about 14 protovertebrse and the
fundaments of 3 pairs of gills. The embryos of the older set are from
3 \ to 3| mm. long; they possess about 17 protovertebrse and the funda-
ments of 4 pairs of gills.

All the embryos of this stage have the pronephric pouch in its typical
form. A side view of this organ with the neighboring portion of the

section to the vertical axis of the protovertebra. Cross sections in this region fre-
quently encounter two contiguous protovertebra;. If the plane of the section
traverse the communicating canal of a protovertebra, it would also pass obliquely
through the dorsal portion of the next anterior protovertebra. The latter would
then appear in cross section as a distinct mass immediately lateral to the neural
tube and the chorda, and would resemble the condition which a protovertebra
presents when cut near its anterior or posterior wall. Immediately below this mass
there would be found on the same cross section the ventral portion of the more
posterior protovertebra, with the corresponding part of its cavity. The latter, how-
ever, being apparently a direct continuation of the body cavity, owing to the exist-
ence of the communicating canal, would appear to represent the dorsal part of the
body cavity, and the fundament of the duct would thus seem to be farther
removed from the dorsal angle of the body cavity than it really is.

220 BULLETIN OF THE

segmental duct is shown in Figure 39 (Plate IV.). In this drawing,
the outlines were obtained by reconstruction from a series of cross
sections. The pronephric pouch is suspended from the dorsal angle
of the body cavity by the nephrostomal funnels;. Elsewhere it is wholly
cut off from the mesoderm, and merely rests conformably on the outer
surface of the somatopleure. The precise relations of the parts can be
understood by referring to the series of cross sections shown in Figures
18 to 22 (Plate III.). Figure 18 represents a section through the left
pronephros in the region of the first nephrostome. The location of the
plane of thi*s section in the reconstruction is indicated by the dotted
line 18, in Figure 39. The Literal plates are here wholly cut off from
the protovertebne, splanchnopleure and somatopleure being continuous
with each other at the dorsal angle of the body cavity. Figure 19
shows the structure of the organ between the first and second nephro-
stomes. In this and the following sections it was found advisable to
depict the pronephric structures of the right side in order to exhibit
in each case the section which most clearly showed the structural con-
ditions. The next drawing (Fig. 20) in the series represents a section
through the second nephrostome. In the preceding section, — not fig-
ured, — the three portions into which the lumen is here divided are
continuous. The constriction between the middle and the ventral lumen
is artificial ; for the cells occasioning this local closure do not belong to
the proper wall of the pouch, but form a group within the cavity. In
several instances 1 have observed such groups of cells lying entirely free
in the lumen of the pouch (Plate V. Fig. 4">). In the present case,
however, the mass is very intimately connected with the adjoining w r alls.
This condition is preserved through a distance corresponding to the
thickness of two or three sections, but the mass terminates by becoming
free from both walls, so that in cross section it has the appearance of an
" island" of tissue occupying the lumen of the pocket. The occurrenre
of these islands within the cavity of the pouch is of significance in
determining precisely how the organ is developed. It is difficult to com-
prehend how they could be formed, provided the canals were produced
by a fold of the somatopleure. On the other hand, they are perfectly
intelligible on the assumption that the canals arise by the rearrange-
ment of a solid mass of cells into a peripheral layer with a central
lumen. According to the latter view, the islands would represent
residual portions of the pronephric thickening winch had not been trans-
formed into peripheral wall.

Returning now to the section last under consideration (Fig. 20), the

MUSEUM OF COMPARATIVE ZOOLOGY. 221

ventral union of the walls of the pronephric cavity is, as I have shown,
artificial ; the constriction between the middle portion of the lumen and
the dorsal, or nephrostomal, portion is more apparent than real, for it is
formed by the posterior wall of the nephrostomal tube, the plane of the
section not having cut exactly in the axis of the tubule. In the section
following that shown in Figure 20, the pouch is detached from the peri-
toneum, and presents an appearance similar to that shown in Figure 19.
Before the third nephrostome is reached, the canal is divided by a hori-
zontal constriction into two tubes. The dorsal portion forms the tubule
of the third nephrostome ; the ventral portion is the anterior end of the
segmental duct. Figure 21 shows these parts in the region of the third
nephrostome. The section corresponds in position with the dotted line
21 in Figure 39.

In the following sections the duct rapidly assumes a more dorsal posi-
tion (compare Fig. 39). It then proceeds directly backward, at the level
of the constriction between protovertebrse and lateral plate. Figure 22
shows the duct in the region of somite VI. It has not yet been formed,
however, throughout its entire length. On passing posteriorly, it grad-
ually loses its lumen ; then the circular arrangement of the nuclei indi-
cating the position of the lumen also vanishes ; the structure at length
terminates as a simple thickening of somatopleure in the region of the
tenth somite. In a few individuals, however, I found slight evidences
of a mode of ending different from that just described. In one case
the indications seemed so strong as to compel me to seek confirma-
tion of the view that the duct takes its origin in situ. I shall therefore
give the details of the evidence on this point, and discuss its probable
significance.

Figure 23 represents in cross section the fundament of the duct in this
specimen, as shown in the fifth section in front of its termination. The
section of the mass here contains about eight cells, which are in close
contact with the somatopleure. In the second section behind this one
there are shown parts of four or five cells (Fig. 24). The protoplasmic
patch in the centre (rd.) is wider than an average cell of the fundament,
and probably represents the anterior ends of two cells lying in the fol-
lowing section (Fi?. 25, c. and d.). Dorsal to this mass of protoplasm is
a nucleated cell (b.), and above this a small area of protoplasm with a
faint nucleus («.) which is doubtless a portion of a cell the principal part
of which was cut off by the preceding section. On the ventral side of
the centre of the fundament there is also a round nucleated cell (<?.). In
the next following section (Fig. 25), there are two nucleated cells in the

222 BULLETIN OF THE

centre of the mass (c. and c/.), which, as I have said, doubtless corre-
spond to the central protoplasmic area (cd.) seen in the preceding sec-
tion. The most prominent cells of that section are here represented by
two faint circles of protoplasm (b. and e.). In the next following section,
not figured, the duct terminates as a single non-nucleated mass, probably
corresponding to the dorsal cell in Figure 25. This remnant lies in a
distinct depression of the somatopleure (Fig. 26,/.). This depression
continues backwards through the space of three sections. Instead, then,
of terminating in a thickening of the somatopleure, the end of the duct
lies in a groove of unmodified somatopleure. There is no tissue directly
behind the duct for its further growth, and the inference is natural that
the somatopleure is mechanically depressed before the growing tip of the
duct. In fact, I believe this to be actually the case, and that in this
region the duct does grow by a simple cell proliferation within its own
mass.

The key to the situation is to be found in the location of the pos-
terior end of the duct in this specimen. An enumeration of the somites
shows that the sections figured lie at the hinder end of somite XL To
show the bearing of this fact, I shall anticipate some of the results
of a study of Stage V. In a series of frontal sections of the latter
stage, I have succeeded in locating with reference to the successive
somites the position at which the duct opens into the cloaca. The open-
ings are in the same vertical plane with the middle of somite XII. The
posterior end of the duct, then, in the specimen which I have just de-
scribed, is within the distance of half a somite from its final termination.
In order to empty into the cloaca, the duct has to grow inward from its
position at the lateral margins of the protovertebrce to a position much
nearer the median plane. It is difficult to comprehend how the duct
could make this extension, except by proliferation of its own cells. It is
just in this region that I find evidences of such a mode of growth. If the
inference I have drawn from the facts adduced be correct, it seems to
me to add strength to the conclusion I have reached in regard to the
general mode of formation of the duct, inasmuch as it has been shown to
be possible to detect free growth where it exists.

That the duct arises in the way I have described, and is not developed
from the ectoderm, is shown, moreover, by certain indirect evidence which
may be properly discussed at this point. As I have already stated, the
duct is developed in such intimate connection with the somatopleure
that I have been led to believe that it arises throughout its entire length
from a proliferation in situ of that layer. In almost all of my prepa-

MUSEUM OF COMPARATIVE ZOOLOGY. 223

rations the duct in its backward growth is separated by a considerable
space from the ectoderm, and I have observed no instance in which it
was impossible to distinguish a perfectly sharp line between the funda-
ment of the duct and the overlying ectoderm.

In describing the germ layers in Stage I., I referred to certain histo-
logical criteria which might be employed in determining to which germ
layer a given group of cells belonged. The most valuable of these is the
difference in the size and abundance of the yolk spherules, which even in
that early stage served to contrast sharply the mesoderm from the ecto-
derm. In later stages, this character is equally pronounced. "When
the duct appears, the cells which constitute it are not distinguishable in
histological features from those of the adjacent mesoderm, but are very
different from those of the neighboring ectoderm. It seems to me ex-
tremely improbable that the cells of the fundament of the duct, with
their numerous large yolk spherules, should have been recently derived
from those of the ectoderm, which are provided with only few spherules of
much smaller size. It would be entirely contrary to our conceptions 01
the physiological nature of yolk, if in the course of embryonic develop-
ment this material was increased instead of diminished in quantity.

A similar argument seems to me to afford evidence that the duct
arises in situ. If the duct had grown freely backward from an anterior
proliferation, such growth would in all probability have been associated
with the consumption of yolk in the cells of the fundament, and the
spherules would be smaller or less numerous than those of the adjacent
mesodermal cells. This, however, is not the case.

I conclude, therefore, that the segmental duct arises throughout its
entire length by a proliferation in situ of the somatopleure. Its posterior
end, however, grows across to the cloaca free from adjacent tissue.

Returning to the pronephric pouch, I purpose describing the relations
of that organ to the somites. The section represented in Figure 29
(Plate IV.) shows graphically these relations. The plane of section in
this case was very nearly tangential to the somatopleure at the points
where the nephrostomes emerge. In this section it is evident that
the three nephrostomes lie precisely under the first, second, and third
somites, behind the ganglion nodosum. These correspond to the so-
mites which I have numbered II., III., and IV.; so that the proneph-
ric pouch remains in the same position as the pronephric thickening
of earlier stages. In Figure 21 (Plate III.) the last remnant of the
canal connecting the body cavity with the cavity of the protovertebrse
is faintly indicated (above the letters coel") in the same transverse

224 BULLETIN OF THE

plane as the third nephrostome ; and Figure G, as we have seen, shows
more plainly the same condition in the case of the second nephro-
stome at an earlier stage.

The structure of the protovertebne in this stage (Plate V. Fig. 45)
merits especial consideration. Already in younger stages there is a
differentiation of a peripheral epithelial layer surrounding the dense cen-
tral mass, or kernel of the proto vertebra. Laterally this peripheral part
is represented by the entire somatic layer, which is separated from the
kernel by the protovertebral cavity (coel.). Along the median and ven-
tral boundaries of the somite, a layer having an epithelial character is
also to be seen. Thus the central mass which is to develop into the myo-
tome lies on the median side of the coelom, and is wholly surrounded by
an epithelial layer. Frontal sections show that this layer can be traced
inward for some distance between successive somites, both from their
median and lateral surfaces. Since the development of the protovertebrse
proceeds from before backwards, a single frontal section shows successive
stages in the changes which they undergo. From such a section it is
apparent that neither the median nor the lateral portion of the pe-
ripheral layer develops muscular fibres. That portion of this layer,
however, which is included between the kernels of successive proto-
vertebrse, is apparently differentiated into muscle, and becomes merged
in the myotomes. Very soon after the first development of muscle
fibres in the myotomes, the peripheral portions which have not been
converted into muscle separate from the central mass, and, while yet ad-
hering in a lamella, show evident signs of disassociation. It is to be
noted, that, in regions where traces of the communicating canal are still
distinguishable, the median peripheral kiyer, not the kernel, is seen to be
continuous with the splanchnopleure. The somatopleure, on the other
hand, may be traced, as before, into the outer layer of the protovertebra.
This peripheral layer I believe to be wholly converted, with the excep-
tion stated, into mesenchymatic tissue. In the stage before us we see
that it is distinctly breaking away from the myotome, and that the cells
are acquiring a flat tile-like form. In the following stage no layer that
could properly be called epithelial is present. In its stead there is a
considerable quantity of loose mesenchyme, and the lateral face of the
myotome is covered by a sheath consisting of very delicate fibrillar con-
nective tissue.

Not merely is mesenchyme produced by the thin peripheral layer of
the protovertebra?, but in anterior regions considerable portions of the
kernels of the proto vertebrae also undergo a metamorphosis in this direc-

MUSEUM OF COMPARATIVE ZOOLOGY. 225

tion. Thus, if I be not mistaken, a protovertebra immediately in front
of somite I. has been wholly converted into mesenchymatic tissue ; the
kernel of the succeeding protovertebra (somite I.) has given rise to a
considerable quantity of mesenchyme ; and the process has been mani-
fested, though to a less degree, even in succeeding somites. Further-
more, having established the continuity of splanchnopleure and somato-
pleure with the median and lateral peripheral layers respectively of the
protovertebrae, it seems to me the more probable that the former as well
as the latter may give rise to mesenchyme. I have, in fact, seen condi-
tions directly in front of the first nephrostome which indicated a very
extensive production of mesenchyme from the lateral plate in that
region.

My reason for dwelling at so great length on the derivatives of the
peripheral layer of the protovertebra is, that this layer plays an impor-
tant part in forming certain accessory portions of the pronephric system.
I refer to the capsule of the pronephros. Already in the preceding
stage I noted the occurrence of a lateral fold of the somatic layer im-
mediately dorsal to the constriction between protovertebra) and lateral
plates (Fig. G). In the younger individuals of Stage IV. the fold covers
the dorsal surface of the pronephric pouch, and extends a short distance
down on its lateral surface (Figs. 18-21, fnd. cps.). In the older set of
embryos it has reached the somatopleure ventral to the pronephros, and
thus forms a complete investing capsule.

In frontal sections the fundament of the capsule may be seen to
consist of a series of segmental outgrowths from the successive pro-
tovertebrge. Later, these segmentally arranged structures fuse into a
continuous enveloping sheet.

Lateral to the pronephros the capsule presents in general a two-layered
condition, the result of its having been formed as a fold ; but on ascend-
ing to the level of the lower boundary of the somite, these two layers
separate (Plate V. Fig. 45) ; one passes beneath the protovertebra, cover-
ing the pronephros on its dorsal aspect ; the other is continuous with the
somatic layer of the protovertebra, forming a lateral sheath to the myo-
tome. These layers are present in the region both of the pronephros and
of the duct, but are seen in their simplest condition in the region of the
second nephrostome (Fig. 20) ; not merely because this is the middle ol
the pronephros, but also because the process is somewhat modified in the
protovertebra next in front of it (somite II.). Somite II. is one of those
in which a considerable portion of the kernel of the protovertebra is con-
verted into mesenchyme. For this reason the inner layer of the capsular

15

226 BULLETIN OF THE

fold, after separating from the layer which forms the lateral sheath of the
myotome, passes inward, and is there lost in a loose mass of tissue (Fig.
18), resulting from the disassociation of certain cells of the somite in that
region. Intersegmental regions also present appearances which are con-
fused by the occurrence of cells belonging to the partition between two
successive somites. The points which I especially wish to emphasize in
this description are (1) the origin of the capsule from the somatic layer
of segmented mesoderm, and (2) the fact that the layer from which the
capsule is developed is also in other regions converted into mesenchy-
matic tissue.

In the younger specimens of this stage a horizontal fold of the splanch-
nopleure is to be noticed, forming a slight ridge directly across the body
cavity from the pronephros. It first appears in front of the second ne-
phrostoine, and develops from this point backwards. It is the fundament
of the glomus or pronephric glomerulus. 1 In the earliest trace of this
organ that I have been able to find (Plate I. Fig. 8) there were already
a few small meseuchymatic cells (ms'chy.) located in the angle of the
fold. The source of these cells I have been unable to determine with
certainty. The nuclei of all the cells in the fold itself lie very close to
the body cavity, and it does not seem probable that those small cells
could be produced by delamination from the splanchnopleure without an
actual migration of the nuclei of the somatopleural cells to the basal, or
entodermal, surface of that layer. I have never seen signs of such migra-
tion, and I therefore do not believe that it occurs. Furthermore, the
folded portion of the somatopleure does not at once become thinner than
the neighboring portions of that layer. In older stages, such a thinning
takes place, but it seems to be due to a superficial extension of the layer,
rather than to delamination. The position of the nuclei of the large
entodermal cells in this neighborhood is equally unfavorable for the
formation of these small cells by delamination. The only remaining
explanation is that the latter have migrated into their present position
from relatively remote parts. Other loose cells may be found between
entoderm and splanchnopleure, and the question here raised is only a
part of the larger problem as to the source of all such cells, including
those which bound the yolk veins. The fate of the cells which I have
found in the fundament of the glomus, I shall consider in treating of a
later stage. I may, however, here anticipate to the extent of stating
that they are connective-tissue elements.

1 Tlie former term seems to me preferable, and will be employed in the follow-
ing pages. The exact relations of the glomus to the mesonephric glomeruli will be
explained in the general discussion.

MUSEUM OF COMPARATIVE ZOOLOGY. 227

Figure 9 shows the fundament of the glomus in one of the older em-
bryos of this stage. Within the hollow of the fold may be seen two
cells (ms'cky.), which are to be regarded as the descendants of the first
small cells to which I referred in the younger embryo. Their differen-
tiation in the direction of connective tissue can be noticed throughout
the whole exteut of the fundament. The scattered rounded cells near
them probably represent embryonic blood cells in the region of the
aorta.

Stage V.

Plate I. Fig. 10. Plate IV. Figs. 31-34, 40.

The embryos belonging to this stage are on an average about thirty
hours older than those of Stage IV. At this period almost all of the
eggs are hatched ; and, the duct having opened into the cloaca, the pro-
nephros becomes functional. The larvse of this stage measure 5-7 mm.
in length, the rapid increase in size being largely due to the growth of
the tail.

The form which the pronephros presents in this stage has been studied
by means of reconstructions in the case of four pronephridia. The dia-
grams on Plate IV., Figures 31 to 38, represent in a rough way the num-
ber and distribution of the convolutions which the tubules present in this
and the following stage. Of these, Figures 31 to 34 relate to the present
stage. Figure 40 is a more accurate view of the pronephros which I
have diagrammatically represented in Figure 32. In Figure 40 the out-
lines were taken with but little modification from the original recon-
struction. I have not hesitated, however, even in this case, to remove
defects plainly due to artificial causes, such as distorted sections and
inaccurate superposition.

Comparing this drawing with Figure 39, it is easy to follow the
changes that have taken place. In the earlier stage the fundaments
of the three tubules are already present. The first modification which
may be noted is the deepening of the constrictions which are indicated
between the successive nephrostomes. In this way are formed three
transverse tubules, joining distally a longitudinal canal ; the former are
the nephrostomal tubules, the latter I shall call the collecting trunk.
In this case the continuation of the collecting trunk pursues a nearly
straight course to the posterior margin of the gland, where it emerges as
the segmental duct. 1 A. second change which is apparent in Figure 10

1 In Figure 40 the first nephrostomal tubule and the collecting trunk have a
pink color, the second tubule is yellow, and the third is orange, whereas the seg-
mental duct is uncolored.

228 BULLETIN OF THE

is the growth of the collecting trunk in the region between the second
and third nephrostomal tubules, and the consequent separation of the
latter. The further complication in this case is mainly due to a con-
volution of the second tubule; slighter contortions occur in other parts.
In the case of the pronephros diagrammatically represented in Figure 31,
however, a canal, which corresponds to what we should regard in Figure
32 as the anterior portion of the segmental duct, has been folded first
forwards, reaching nearly to the level of the first nephrostome, and then
backwards. The bends which are convex anteriorly may be called the
anterior bends ; those which nre convex posteriorly, the posterior bends.
The universal occurrence of this condition in all older embryos makes
it desirable to distinguish this bent portion of the tube and its deriva-
tives both from the original longitudinal canal of the pronephros, which
I have called the collecting trunk, and from the straight posterior por-
tion, or segmental duct proper. In the following pages I shall speak
of each nephrostomal tubule as extending from its origin in the nephro-
stome to its junction with the longitudinal canal, or collecting trunk.
In the case of the first nephrostomal tubule, the point of union with the
collectini;' trunk is usually marked by an abrupt change of direction;
where this does not occur, however, the distinction between the two
portions must lie somewhat arbitrary. The collecting trunk forms the
continuation of the first nephrostomal tubule, it receives in its backward
course the second tubule, and may lie regarded as terminating at the
point of entrance of the third tubule. The common trunk arises from
the point of junction of the third tubule with the collecting trunk, and,
after making various convolutions, leaves the gland at its posterior end
as the segmental duet. In the two pronephridia shown in Figures 31 and
33, we have before us examples respectively of the two principal forms
of convolution which are to be recognized in subsequent stages, viz. the
contortion of the second tubule and that of the common trunk. The
third tubule finally undergoes convolution to some extent; but the first
tubule and the collecting trunk take almost no part in the process.
Although complication has appeared both m the second tubule and in
the common trunk, it is to be noticed that these processes do not have a
fixed sequence. I have numbered the diagrams on Plate IV. with refer-
ence to the state of development shown by the larvae. In doing this, I
have not been guided by the age alone, for the large amount of indi-
vidual variation makes that method nearly valueless ; but I have en-
deavored, by passing in review a large number of characters, to gain a
notion of the relative degree of development shown by the larvae.

MUSEUM OF COMPARATIVE ZOOLOGY. 229

The first of the series of diagrams (Fig. 31) shows complication to
have taken place to a considerable extent in both the convoluted regions.
In the next diagram (Fig. 32) the second tubule alone takes part in the
complication. Figures 33 and 34 represent respectively the right and
left pronephridia of one individual. In the right pronephros (Fig. 33)
the typical condition of the common trunk is present, while the neph-
rostomal tubules have undergone no contortion. Likewise in the left
pronephros (Fig. 34) it is the common trunk to which the increasing
complication is due ; but in this case there are two additional bends
introduced by a slight folding backward of the middle of the anterior
bend. The convolutions of the common trunk lie principally in the
ventral portion of the gland. The tubes which in cross section are seen
in the dorsal part are mainly the several nephrostomal tubules, and the
collecting trunk. This condition is likewise retained in later stages.

The position of the pronephros with reference to the myotomes has
not changed since the preceding stage. The whole structure is slightly
longer, but the myotomes have also lengthened to the same extent.
The thi*ee nephrostomes are situated, as before, beneath the first, second,
and third myotomes posterior to the ganglion nodosum, and are seg-
mental in position.

In all the embryos of this stage the duct has opened into the cloaca.
It is to be remembered in this connection, that the morphological posi-
tion of the duct is outside the somatopleure ; so that the ccelom and
two layers of mesoderm intervene between it and the intestine. As
might be expected, the union does not take place until the segmented
and unsegmented portions of the mesoderm have become separated
from each other. The passage to the cloaca is then effected through
the split thus produced, and consequently around the dorsal angle of
the body cavity.

In the frog, there is a sharp histological contrast between ectoderm
and entoderm, and there is therefore no difficulty in assigning a limit
to the proctodreal invagination. The region into which the duct opens
is the hind gut, and the intestine at this point is unquestionably lined
with entodermal cells. The portion of the primitive gut posterior to the
openings of the segmental duct forms the Amphibian cloaca, and corre-
sponds precisely, I should say, with that part of the cloaca of Amniota
which Gadow ('88, p. 28) has recently designated by the name urodaeum.
The wall of the intestine is not wholly passive in the union occurring
between it and the duct. In front of the excretory openings, the lumen
of the intestine has an elliptical form, its major axis being vertical.

230 BULLETIN OF THE

On passing backwards, the dorsal half broadens and finally exhibits
two lateral processes, or cornua, the walls of which are composed of a
layer one cell deep. The ducts open into the distal ends of these cornua
(see Fig. 27, showing the condition in Stage VI.). Behind the outlets
of the segmental ducts, the lumen of the intestine has a nearly circular
outline, and descends rapidly to the anus, or, as it may now more cor-
rectly be called, the cloacal aperture. I was able to see in Stage IV. faint
traces of these intestinal cornua. The cells of the dorsal roof of the
intestine showed in this region a looser structure, and a line of pigment
indicated the region of the outfolding. The cells of the duct and those
of the cloaca are histologically very different from each other, so that
it is for a long time possible to draw a line sharply separating the two
constituents where they have come in contact.

The pronephric system of tubules presents in this stage quite uniform
histological characters. I shall therefore describe its typical condition,
and then consider the modifications that are to be found in certain of
its regions. The walls of the tubules are very thick, measuring on the
average about 25 //. in thickness. They accommodate themselves readily
to the structures with which they come in contact, becoming thinner
opposite elevations in neighboring surfaces, and thicker next to sinuses.
The size of the lumen varies greatly. In the segmental duct proper, the
diameter of the lumen is about 25 /x ; it is usually somewhat greater
in the region of the convoluted tubules. The walls of the tubules are
composed of an epithelium, consisting of a single layer of columnar cells.
The radial dimension of the cells in the case of thick walls is approxi-
mately three times their width. "Where the plane of the section cuts
the wall of a tube tangentially, the cells may be seen to have a polygo-
nal outline. The nuclei invariably occur close to the central lumen of
the tube ; each is large, and is usually provided with a single distinct
nucleolus. The eccentric position of the nuclei is attended with a
corresponding distribution of the cell protoplasm. By the picro-carmin
method which I have employed, the yolk spherules take a bright yellow
stain, and the nucleus a light red. The active protoplasm has a faint
pink coloration, which, however, is wholly invisible if too much picric
acid be left in the preparation. In young cells, where only a small
amount of yolk has been consumed, the delicate tint of the protoplasm
cannot be seen, since all the light passing through the section encounters
yellow yolk spherules. As the consumption of the yolk progresses, the
protoplasmic matrix comes into view. In the wall of the tubules, the
yolk is crowded to the outer surface of the cell, and a sheet of protoplasm

MUSEUM OF COMPARATIVE ZOOLOGY. 231

first becomes visible close to tbe lumen. It is here also that pigment
makes its appearance.

The histological character of special regions now claims our attention.
The pronephridia shown in Figures 31 to 33 are all histologically very
similar, but in the case of the gland represented in Figure 34 some
notable differences occur, which I shall consider later. The somato-
pleure covering the pronephros is at this stage very thin. Each of the
cells composing the membrane is thickest in its central portion, and
tapers rapidly towards its margins. In the more advanced larvae, the
cells have elongated to such an extent that the peripheral portion is
reduced to a thin protoplasmic plate, which is nearly devoid of yolk
spherules. The central mass, on the other hand, contains the nucleus,
and nearly all the yolk spherules. The peritoneum is continuous with
the columnar epithelium of the walls of the tubules at the outer rim
of the nephrostomes, which have the characteristic form of a funnel.
Before reaching the periphery of the funnel, however, the columnar layer
becomes slightly thinner, and at the rim it tapers rapidly, until it becomes
continuous with the peritoneum (compare Fig. 18 of a younger and Fig.
28 of an older stage). The nephrostomal funnels are always deeply pig-
mented. The pigment is most abundant along the incurved surface, but
is quite dense even up to the rim. It continues for a variable distance
into the tubules. In the case of the pronephros, represented in Figure
31, the whole system of tubes was pigmented from the nephrostomes to
the posterior bend near the beginning of the common trunk. The pig-
ment granules are always disposed in a layer along the free surface of the
cells. The nephrostomal tubules show in general the typical character,
which I have previously described. The collecting trunk appears to be
quite rigid, for I have never seen such a reduction of its lumen due to
pressure as other tubes exhibit. The calibre of this canal is usually larger
than that of the nephrostomal tubules. The portion of the duct which
lies behind the third nephrostome is nearly straight and of uniform calibre.
Generally the lumen is slightly wider, and the wall thinner, than in the
nephrostomal tubules. In the pronephridia, shown in Figures 31 and
33, the loop embracing the anterior bend of the common trunk seems
to have but little rigidity. It follows a tortuous course, and fre-
quently the walls are so closely pressed together that the lumen is
locally obliterated.

A peculiar modification in the pronephros, represented in Figure 34,
has been alluded to. In this case, the common trunk, after proceeding
from the level of the third nephrostome for a certain distance forward

232 BULLETIN OF THE

along tne ventral border of the gland, as in the other embryos, under-
goes a change of structure at about the level of the second nephrostome.
The lumen there begins to enlarge, and the wall to become thinner.
Farther forward, the cavity of the tube becomes greatly dilated, and
the bounding wall is reduced to a delicate pavement epithelium, having
the same appearance as the peritoneum covering the pronephros. The
tube again contracts shortly before attaining its most anterior bend. A
similar dilation also occurs in the following stage, in the description of
which I shall again refer to the chamber thus produced and suggest its
possible function.

The pronephros at this stage is completely invested in a loosely fitting
capsular membrane. The cells of which this envelope is composed have
become very thin, so that they form a delicate sheet not more than 6 fx
thick. The nuclei occur in slightly enlarged portions of the cells. They
are rather small, and show a tendency to be flattened in the plane of
the layer. At the lower outer angle of the myotome, the capsular
membrane is continuous with the myotomal sheath, as in the earlier
stage. The capsule covers the pronephros so loosely as to leave exten-
sive "spaces between the enveloping membrane and the tubules. These
spaces, together with those between the convoluted tubes, form an ex-
tensive and complicated system of sinuses, which bound the pronephric
tubules on every side (compare Fig. 28, belonging to the next older stage).
Behind the last nephrostome, a considerable space 'intervenes between the
wall of the capsule and that of the duct. There is thus formed a single
continuous but irregular channel, which accompanies the duct through-
out its entire course ; it is also prolonged into the region of the gland as
a large ventral sinus, which is triangular in cross section (compare the
lower of the spaces marked sn. sng. in Fig. 28). This channel is the
fundament of the posterior cardinal vein. The course of the vessel may
be traced at this stage for a short distance behind the point where the
ducts open into the cloaca. There are two veins connected with the an-
terior end of the pronephros. Of these the ventral one is the larger, and
is continuous posteriorly with the cardinal vein. The dorsal vessel of
the pronephros also unites with the cardinal vein by means of the spaces
between the tubules. After passing forward and leaving the pronephros,
the ventral vessel proceeds medianwards to empty into the sinus venosus,
this vessel constituting the ductus Cuvieri. The dorsal vessel of the
pronephros can be traced forward into the head. In the somite next in
front of the first nephrostome, it lies between the ganglion nodosum and
the myotome. It can be traced for some distance along the base of the

MUSEUM OF COMPARATIVE ZOOLOGY. 233

cranium, passing close to the median wall of the auditory vesicle and the
ball of the eye. 1

There are two kinds of cells found within the capsule of the pronephros
concerning which I have as yet said nothing. Those which are more
numerous are scattered, Of circular outline and of unform size. Each has
in general three or four large yolk spherules, and the nuclei are rather

1 In order to ascertain, if possible, what vein of the adult this vessel represents, it
vrill be necessary to describe here its condition in later stages. In the oldest embryos
I have examined, 8.5 mm. long, the vein runs forward from the pronephros parallel
to the aortic root and its continuation, the carotid artery. The vein is separated
from the arterial trunk by the ganglia nodosum and faciale. Recalling the earlier
position of the vein, it will be seen that it has been transferred from the median to
i lie external side of the vagus nerve. In an intermediate stage, I have been able to
see the nerve during its transit through the vein, thus confirming an observation of
Kastschenko ('87, pp. 275, 276).

Following the vein farther forward, it is found to pass immediately ventral to
the auditory vesicle, directly in front of which it sends a branch around the ganglion
faciale to the side of the cranium. Slightly farther forward the vein divides, its
branches passing around the more anterior of two ventral processes of the gan-
glion faciale. The two trunks thus formed separate. One enters the orbit, and can
be traced to the anterior end of the optic bulb. The other passes below the eye,
and pursues a nearly straight course to the anterior horn lip.

A description of the venous system of the adult frog has been given by Gruby
('42) and by Ecker ('64-81). The distribution of the veins which enter the dorsal
portion of the pronephros corresponds most closely with the internal jugular of these
authors. From the figures of Goette ('75), however, there can be no doubt that
the vein I have described corresponds to the one which he calls the external jugular.
I have been able to find a vein entering the sinus venosus directly which agrees ac-
curately with the inferior jugular of Goette, but I have found none corresponding
to the one he calls the internal jugular. It is stated by Goette (pp. 759, 760) that
the vein named by him external jugular receives large branches from the maxillary
and mandibular regions. This character would seem to connect it with the exter-
nal jugular of Gruby and Ecker. According to Goette (p. 765), however, the exter-
nal jugular of Gruby and Ecker is the same as his inferior jugular. I believe this
statement to be true, and it seems possible that Goette, who confesses that his studies
upon the veins did not extend beyond the first larval periods, may have erred in
his account of the distribution of these rudimentary vessels.

Since the preceding description was written, a paper by Marshall and Bles
('90 b ) has appeared, which adds another to the divergent accounts I have re-
viewed. The inferior jugular of Marshall and Bles corresponds closely with the
vein I have alluded to under that name. The anterior cardinal vein is described by
these authors ('90 b , p. 236) as "formed by the union behind the ear of a jugular
vein returning blood from the brain and dorsal part of the head, and a facial vein
which lies superficially along the side of the head ventral to both eye and ear."
These vessels are described in tadpoles, measuring 9 mm. in length. My own
observations on larvae of nearly this size do not agree with this description.

234 BULLETIN OF THE

smaller than in the cells of the tubules. Very similar elements abound
in the fundaments of blood-vessels at this stage, and it is evident that
the cells are embryonic blood corpuscles. The spaces in which they
occur constitute a complicated system of communicating blood sinuses,
and are continuous with the lumens of the vessels entering and leaving
the pronephros.

The other class of cells to which I have referred are mesenchymatic.
I have carefully studied these cells in the endeavor to ascertain their
precise origin. A mode of reasoning similar to that employed in dis-
cussing the probable origin of the inner cells of the glomus leads to
the conclusion that the mesenchymatic cells of the pronephros cannot
have been given off from the walls of the tubules. As I have stated,
the cells in these walls are very thick, and their nuclei lie close to the
lumen of the tube. Under these circumstances, it is difficult to under-
stand how any cells of the tubule should divide so as to give off from
their basal surfaces cells as small as those in question. The usual pro-
cess of cell division, if it took place parallel to the surface of the layer,
would result in the production of a small cell on the side toward the
lumen and a large outer segment. Such a large cell might, it is true,
by repeated divisions, break up into numerous small cells, but for several
reasons I do not believe this to have been the case. If such a delamina-
tion and subsequent cell division took place, it would naturally be a
conspicuous process ; but I have never observed any evidences of it.
This method of origin would involve a considerable thinning of the tubes,
which does not take place.

There remain two other possible sources for the mesenchymatic cells
of the pronephros. They may have arisen from the tissue bounding
the pronephros, viz. the capsular membrane and the adjacent somato-
pleure, or they may have come from remote regions. In judging
between these possibilities, it is important to consider the sudden
appearance of the cells and the small amount of differentiation they
have undergone. It seems to me highly improbable that they should
have already accomplished any extensive migrations. Under these cir-
cumstances, such positive evidence as I am able to adduce is the more
convincing. In studying the youngest stages in which mesenchyme
was preseut in the pronephros, this tissue was usually found near the
somatopleure or the capsule, and frequently consisted of a row of cells
closely applied to one of these layers. Occasionally I have seen a layer
of mesenchymatic cells arranged along the somatopleure in a very definite
manner, so that the nuclei of the two layers lay directly opposite each

MUSEUM OF COMPARATIVE ZOOLOGY. 235

other and the intercellular regions precisely corresponded. In such
cases the evidence seems strongly in favor of delamination, but I have
never seen a nucleus dividing in that direction. This negative evi-
dence, however, should have little weight, since all cell divisions occupy
a comparatively short time, and are also obscured by the numerous yolk
spherules. The observations just recorded agree very well with the
rapid thinning of the somatopleure to which 1 alluded in discussing the
histology of the tubules. 1 conclude, then, that the mesenchymatic tissue
of the pronephros arises from the adjacent somatopleure, and probably
also to some extent from the capsular membrane.

The glomus (Plate 1. Fig, 10) has attained in this stage nearly its final
dimensions. The lateral plate having become wholly detached from the
protovertebrre, the glomus has the appearance of being attached to the wall
of the body cavity at its dorsal angle (compare Fig. 9, of a younger stage,
and Fig. 47, of Bufo). There is some individual variation, but in general
it may be stated that the ridge constituting the organ under consideration
extends continuously from the first nephrostome backward to a position
slightly behind the third. It appears in cross section (Fig. 10) obovate,
being attached by the narrower end. In structure it is very compact,
so that it is difficult to locate with precision cell boundaries in the dense
interior mass. The investing portion consists of a single layer of cells,
which is continuous with the peritoneum. These cells are large, and
have the form of spheres flattened on their inner surfaces (compare Plate
VI. Figs. 49 and 50, la. pVton., which represent this layer in Bufo). They
are slightly pigmented, and a distinct row of pigment granules can usually
be seen close to the inner surface of the layer. These outer cells are
evidently the representatives of the large cells of the splanchnopleure
which was folded, at a previous stage (see page 227), to form the earliest
fundament of the glomus (Fig. 9, jnd. glm.). In certain favorable re-
gions I have seen a thin structureless membrane lying directly within the
outer cellular layer (compare Figs. 49, 50, rnb.ba.). When any of the
cells of that layer become detached, which frequently happens, this base-
ment membrane usually remains in place, and gives a sharp outer con-
tour to the glomus in that region. Besides the compact mass of large
cells there occur within the organ one cr two cells in each section (com-
pare Figs. 49, 50, enUh.) which have an elongated form. They lie close
to the basement membrane, with which their long axes are parallel. In
sections each cell has a central swollen portion containing the nucleus,
from which it tapers in both directions. 1 have not been able to trace
the delicate lateral portions to their terminations, but I believe that

236 BULLETIN OF THE

these cells form a complete endothelial lining, which follows closely the
delicate basement membrane. They doubtless represent the loose cells
alluded to in Stage IV. as occurring within the fundament of the glomus.
Their origin 1 have already discussed in connection with that stage,
where I showed that they were probably not derived from the outer
layer of the glomus. Although the structure of the central mass of
cells is, it must be admitted, somewhat obscure, 1 have found no evi-
dences of the complication which Hoffmann ('86, p. 591) has recently
maintained' for it.

On the contrary, a comparison of many individual cells in this mass
with the loose cells in the cavity of the aorta has made me confident
that most ol the cells contained in the glomus are embryonic blood cor-
puscles. It is possible, however, that others are derived from infolded
portions of the outer layer of the glomus. They appear to have no
representatives in early stages of the organ. In my opinion, then, the
glomus is essentially a blood sinus, the wall of which projects into the
body cavity, carrying before it the peritoneal layer.

The junction of the two aortic roots takes place very nearly opposite
the first nephrostome (compare Plate IV. Fig. 28, rx. ao.). The aortic
trunk thus formed (Fig. 10, ao.), since it occupies the space between the
chorda and mesentery, passes close to the attachment of each glomus.

The precise relations of the aorta to the glomus are rather difficult to
observe", since the former is peculiarly liable to injury in sectioning.
The interior of the chorda at this stage consists of a frail vesicular tissue,
whereas its outer sheath is tough, and resistant to cutting instruments.
In sectioning, therefore, it collapses, and occasions serious distortion of
the adjacent parts.

In the younger individuals of this stage, the cavity of the aorta did
not seem to be sharply marked off from the root of the glomus; in sev-
eral instances, indeed, 1 was able to observe a continuity between the
endothelium of the aorta and that lining the glomus. In older indi-
viduals I have repeatedly noticed distinct branches from the aorta pass-
ing into the glomus (Plate I. Fig. 10, va. sng.). These observations
were made, however, only on the most favorable sections, and I have
been unable to ascertain the number or distribution of such branches.
In both of the two most obvious cases, however, the vessel entered the
hinder end of the glomus. Occasionally, the vessel to the glomus seems
to be only a lateral branch given off from a vessel which can be traced
between entoderm and splanchnopleure for some distance ventral to the
glomus (Fig. 10, va. sng., the lower of the two dotted lines).

MUSEUM OF COMPARATIVE ZOOLOGY. 237

I have spoken of the expanded dorsal portion of the body cavity, into
which the nephrostomes open, and which contains the glomus. This
portion of the body cavity constitutes the so-called pronephric chamber.
It is not to be regarded as a closed cavity. Elsewhere the somatopleure
and the splanchnopleure are closely applied to each other, but there is
absolutely no fusion of these layers ventral to the pronephros.

Stage VI.

Plate III. Fig. 37. Plate IV. Figs. 28, 30, 35-38, 41. Plate VI. Fig. 51.

The larvae included in this stage are in general two or three days
older than those of the preceding stage. They are about 8 mm. long
from the anterior end to the tip of the tail. In this stage, the body no
longer tapers gradually from the branchial region to the posterior end ;
but a definite line of separation is established between the trunk and
tail regions. In the tail a distinct membranous fin has appeared, both
along the dorsal and the ventral median lines. The horn lips can be
seen surrounding the mouth, and the external gills project prominently
on both sides of the body.

The pronephros of this stage has developed along lines foreshadowed
in the preceding stage. The general form of the organ can best be un-
derstood by reference to the series of diagrams (Figs. 35-37) and the
reconstruction (Fig. 41) given on Plate IV. As will be evident at once,
the gland has reached a high degree of complexity, produced, however,
by a continuation of the same process of complication which had begun
in Stage V. Thus the first nephrostomal tubule * and the collecting
trunk retain throughout a nearly unmodified condition ; the third ne-
phrostomal tubule usually becomes slightly complicated ; the second
exhibits the greatest number of convolutions. The common trunk,
however, is the part which has been principally concerned in producing
the increased complexity of the gland. It is to be noted that this con-
tortion is not of a wholly indefinite nature ; indeed, there is consider-
able uniformity in the pronephridia of different individuals of the same
stage of development. In Figure 31, representing a pronephros of a
larva in Stage V., it is to be seen that there are only two bends in the
common trunk, which extends forward to the anterior end of the gland.
From this simple condition the later complications may be derived by a
few simple steps. In order to follow the changes' it will be advisable to

1 The same colors have been employer! for corresponding parts in both Figures
40 and 41. Consult explanation of Figure 41.

238 BULLETIN OF THE

distinguish : (1) posterior bend adjacent to the collecting trunk, (2) as-
cending arm, (3) anterior bend, and (4) the descending arm, which is
continuous with the segmental duct. The simplest condition which I
have found in Stage VI. is represented in Figure 35. This diagram
relates to a larva of R. sylvatica Le C, and it is of interest to note its
close similarity to Figure 38, which represents the pronephros of a
larva of II. pipiens Schreb. (halecina) of this stage. These two pro-
nephridia will be considered together. In both, the ascending arm of
the common trunk makes either an S-shaped bend or a loop interpo-
lated near the middle of its course ; the transverse portion, or anterior
bend, is thrown into one or two slight fulds, and the descending arm
shows two loops, one in the middle of the gland, and the other near
its posterior end. The two remaining diagrams (Figs. 3G, 37), though
taken from different individuals, are alike in all essential particulars.
The principal changes from the condition shown in the simpler prone-
phridia just described consist in the development of an additional loop
in the course of the ascending limb, and of several slight folds in the
transverse portion ; the loops present in the younger individuals of this
stage have persisted and become more extensive. In the case of the
larva whose pronephros is represented in Figure 37, I made a compar-
ative study of the pronephridia found on the two sides of the body.
The comparison showed that a slight want of symmetry existed between
the two sides. Occasionally the direction in which equivalent tubes
were bent did not correspond. On the right side of the body (the one
figured), for example, the hindermost loop of the descending arm was
formed by an inward bend, while in the left pronephros the corresponding
tube is bent outward. In the descending arm of the left pronephros a
small loop occurs in addition to those present on the right, while one of
the two loops occurring in the ascending portion of the right side is
almost unrepresented on the left ; thus, the right pronephros approxi-
mates in this respect the simpler organ represented in Figure 35. A
more striking anomaly of the left pronephros consists in the occurrence
of a slight bend of the collecting trunk between the junctions of the sec-
ond and third nephrostomal tubules, so that the latter connects with an
ascending portion of the collecting trunk. Finally, the third nephro-
stomal tubule of the left side joins the collecting trunk farther posteri-
orly than does the one on the right side. In general, however, it seems
to me that the several pronephridia studied show a rather remarkable
uniformity even in the details of the arrangement of their tubules.

The position of the pronephros with reference to the muscle plates is

MUSEUM OF COMPARATIVE ZOOLOGY. 239

the same in this stage as in the foregoing. The lateral plate is now
wholly cut oft' from the myotomes ; but a study of serial sections shows
that each nephrostome lies beneath a myotome. These myotomes corre-
spond to somites II., III., and IV.

The course of the duct in this stage is the same as in Stage V. The
openings into the cloaca (Plate III. Fig. 27, dt. sg.) are now situated at
the bottom of a depression in the dorsal wall of the cloaca (clc). While
the excretory products enter the main cloacal chamber by a single aper-
ture, a glance at the histological characters of the short median unpaired
trunk shows that it is lined with entodermal cells, and is therefore really
a diverticulum of the roof of the cloaca. The ducts of the two sides,
therefore, are not to be regarded as uniting into a common trunk, bu'
as opening separately into a dorsal diverticulum of the cloaca.

The histological characters of the pronephric system have not under-
gone any great changes since the preceding stage. Figure 28 (Plate IV.)
shows a cross section of the left pronephros of the larva, whose right
pronephros is diagrammatically represented in Figure 37. The plane
of the section passes through the first nephrostome, and the transition
from the pavement cells of the peritoneum to the columnar epithelium
of the tubules is clearly shown. This section also shows, besides the
first nephrostomal tubule, the anterior ends of two loops which belong
to the transverse portion of the common trunk. The walls of all the
tubules are thinner than in the preceding stage, and since the nuclei re-
main of about the same size as heretofore, they now occupy a far larger
proportion of the cell, and in the case of the thinnest-walled tubules are
frequently almost in contact with both the outer and inner surfaces of
the cell. The amount of yolk in the cells is considerably lessened,
especially in those parts which exhibit the greatest number of convolu-
tions. In some cells, a single large spherule is the sole remnant of the
formerly abundant yolk. Pigment is present as scattered grains in the
walls of all the tubules ; it also shows a tendency, as in the previous
stage, to accumulate along the free surfaces of the cells. The nephro-
stomes, however, are densely pigmented on the surface that is directed
towards the body cavity and the lumen of the tubule. The duct pos-
terior to the pronephros (Fig. 30) offers no features worthy of special
mention. It is accompanied throughout by the cardinal vein (vn. crd.),
on the median side of which the earliest fundaments of the mesonephric
tubules are visible.

I have described a special enlarged region of the convoluted duct in
a larva of Stage V. A similar condition is apparent on both sides of the

240 BULLETIN OF THE

body in the case of the individual whose pronephros is represented in
Figure 35. The dilated chamber (Plate VI. Fig. 51) is here formed by
a great expansion of that portion of the ascending arm of the common
trunk (trn. com.) which is adjacent to the collecting trunk (trn. clg.).
A similar dilated chamber occurs in the pronephros represented in
Figures 36 and 41 ; but in the latter case neither the dilation of the
lumen nor the thinning of the wall is very pronounced. In both these
cases the expanded chamber is present in portions of the tubular system
which are exactly equivalent to each other. Under these circumstances,
the expansion of the descending limb of the duct occurring in the prone-
phros of Stage V. (Fig. 34) seems quite anomalous. The dilated
chamber is invariably, however, superficial in position, lying close to the
capsular membrane. I have been unable to reach an entirely satisfac-
tory opinion regarding its function. Since it is situated so near to the
nephrostomes, it does not seem very well adapted to serve as a reservoir
for the storage of fluids secreted by the gland, for by far the larger por-
tion of the secreting surface is situated between it and the duct. How-
ever, the chamber doubtless receives whatever fluids are gathered by the
nephrostomes or are secreted by the peritoneal tubules, and it is pos-
sible that the enlargement exists solely for this purpose. In following
the duct from the dilated region towards its outlet, a greatly contracted
portion is reached, and this may serve for the better retention of fluid
contained in the chamber.

The capsule in these larvao is not so well marked as in those of the pre-
ceding stage. Between the pronephric tubules and the ectoderm there
has arisen a considerable quantity of mesenchyme, and the capsule now
appears merely as the line along which this mesenchyme comes in con-
tact with the pronephric tubes and blood sinuses.

In discussing the blood supply for the preceding stage, it seemed
advisable to consider the vessels in older larvae as well, and I shall there-
fore merely refer here to the account given in that connection. 1

1 In all the larvae of this stage which I have examined, I have observed a peculiar
sac, of which I have been unable to find any mention in the literature. In the
oldest larva of this stage it consists of a capacious sinus lying in the triangular area
bounded by the myotomes, the somatic peritoneum, and the ectoderm. It extends
backwards from the niveau of the third nephrostome for a length of two or three
myotomes, and appears to be closed upon all sides. The sac lies in a ninss of loose
mesenchyme, but possesses firm walls, so that any opening would naturally be easily
recognizable. In the interior of the sac, cells which are undistinguisluible from
blood corpuscles are found in considerable numbers In a younger larva the sac
occurs in a corresponding position, is nearly filled with blood cells, and is in open

MUSEUM OF COMPARATIVE ZOOLOGY. 241

The glomus is somewhat larger and more compact than in the preced-
ing stage, and for that reason its structure is more obscure ; but I have

© © '

seen nothing which would lead me to believe that it differs materially
from the condition exhibited by the younger glomi of Stage V. The
organ is bounded by a definite peritoneal layer and contains blood cells
together with embryonic connective-tissue stroma. The blood cells are
usually contained in definite channels, and, being closely packed together,
frequently appear in cross sections to be disposed with considerable reg-
ularity around a central point. This arrangement is naturally suggest-
ive of a tubular or a rod-like structure ; but the histological characters
of the cells and the conditions exhibited by adjacent sections show that
this impression is illusory. In short, I have been unable to find within
the glomus any traces of the rods and thick-walled tubes which have
been described by Hoffmann ('86, p. 591).

No closed pronephric chamber exists at this stage. In the most
anterior sections in which the pronephric tubules appear, a blind anterior
diverticulum of the body cavity is to be seen ; but this unites with the
general body cavity surrounding the intestine even before the niveau
of the first nephrostome is passed. Throughout the remainder of the
pronephric region the lung bud (Plate IV. Fig. 28, fuel, pul.) forms a
ridge on the splanchnic side of the coelom. This ridge partially separates
the pronephric chamber from the general body cavity ; and in the region
of the third nephrostome a still more perfect closure is effected on the
right side of the body by means of the approximation of a portion of the
midgut to the peritoneum covering the pronephros.

Stage VII.

The age of the larvse of this stage, reckoned from the time of fertiliza-
tion, is about forty-seven days. A large gap therefore intervenes between
Stages VI. and VII., and the older larva are studied merely for the pur-
pose of observing the process of degeneration in the pronephros. In the
larvse of Stage VII. the mesonephros has already attained a degree of
complication comparable to that gained by the pronephros at Stage VI.,
i. e. the same average number of tubes appear in cross sections through
the two glands. The mass of contorted tubules in the case of the meso-
nephros, however, is formed wholly by the transverse tubules, while the

communication with the cardinal vein. In a larva of intermediate age, the sinus
communicates with the cardinal vein by means of a very narrow canal. Respect-
ins the fate and the significance of this singular structure. I have no suggestions to
offer.

VOL. XXI. — NO. 5. 16

242 BULLETIN OF THE

duct pursues a direct course through the glaud. The duct is situated
in the dorsal portion of the mesonephros adjacent to the lower borders
of the myotomes ; its relations are therefore different from those of the
longitudinal caual of the pronephros, since, as we have seen, the common
trunk in the pronephros is greatly convoluted, and its windings occupy
the ventral portions of the gland.

The marked signs of degeneration which the pronephros presents in
this stage prevented my reconstructing the gland, since it proved to be
impossible to follow any given tube throughout the entire series of sec-
tions. Indeed, I am convinced that the tubules are no longer strictly
continuous. I must therefore content myself with a brief description
of the histological features noticed.

The lumen of the tubules is greatly enlarged, and is frequently filled
with a dense coagulum which stains similarly to protoplasm. The cell
walls are very thin and show a tendency to become shredded or frayed
along the interior surface. The membranes between the cells in the
wall have become indistinct, and the number of nuclei in a given area
is far less than in a corresponding portion of the wall in Stage VI. The
nuclei are stained only feebly, but contain deeply staining granules, and
seem to be disappearing, since one can observe numerous gradations
between the typical nuclei and those which have become so pale as to
be neai'ly invisible. The ground substance of the walls is slightly vacu-
olated and contains numerous scattered dark granules. Between this
remnant of the cellular wall of the tube and the basement membrane,
I have frequently seen small cells with deeply stained nuclei. These
may possibly represent intrusive connective-tissue elements.

I regret that I have not been able to make an extended study of the
degeneration of the pronephros ; but the limit which I have set to my
work is perhaps the least arbitrary which I could easily make.

B. Bufo.

The development of the pronephros and the segmental duct in Bufo is
very similar to that which I have described for Rana. For this reason,
I can treat the development in Bufo much more briefly, and shall lay
principal stress upon those features which are unlike in the two genera.

Stage I.

In the case of Rana, this stage included embryos which showed an ill
defined somatopleural thickening lying immediately posterior ^o the
cranial ganglionic mass. This proliferation proved, on comparison with

MUSEUM OF COMPARATIVE ZOOLOGY. 24

<->

older specimens, to be the first indication of the pronephric thickening.
A similar condition of the somatopleure is presented by embryos of Bufo
about 2 mm. long, in which the medullary folds are widely open.

The general relations of the germinal layers at this stage are almost
identical with those in Rana, and the same histological criteria for distin-
guishing them can be employed. The ectoderm is very sharply marked
off from the mesoderm. The former is deeply pigmented, while the ad-
jacent mesoderm is almost destitute of pigment. The yolk spherules
of the ectoderm measure on the average abo,ut 2 /x ; those of the meso-
derm, about 4 p.

In embryos in which the medullary tube is still widely open, the
somatopleure and splanchnopleure are separated from each other by a
distinct space, the coelom, which can be traced with perfect distinct-
ness into the protovertebral plate, where it becomes slightly expanded.
In the anterior half of the embryo, both the somatic and the splanch-
nic layers are only one cell in thickness. Posteriorly, and in the middle
trunk region, however, certain loose cells bordering on the coelom become
associated with the somatic layer ; but this layer is never, except at the
extreme hinder end, more than two cells in thickness.

Stage II.

Embryos in which the medullary tube is just closed exhibit a con-
dition of the mesoderm slightly different from that of Stage I. In the
posterior portion of the embryo, the mesoderm is quite thick in the re-
gion of the protovertebral plate, and becomes gradually thinner as it
approaches the ventral portion of the body.

Anteriorly, the protovertebral plate shows traces of the differentiation
of four or five pi'otovertebrse. Of these, the most anterior lies in the same
transverse plane as the ganglion nodosum, and, following the method of
designation which was employed in the case of Rana, would properly
represent somite I. This protovertebra, as in Rana, shows signs of
transformation into mesenchyme, and is considerably compressed in the
region of the ganglion.

The thickening has the general form which I have described for the
corresponding stage of Rana, and its anterior margin is situated under
somite II.

In the region of its greatest thickness, which is somewhat lateral to the
boundary between the protovertebra and the lateral plate, it is two or
three cells deep. It thins out slowly on the ventral side, much more
rapidly on the side of the protovertebra, or dorsally. The thickening

244 BULLETIN OF THE

involves the ventral portion of the lateral wall of the protovertebra itself,
although the greater part of the thickening is in the region of the lateral
plate. I have not been able to find any sharp plane of division marking
the lower limit of the thickening. The latter extends posteriorly through
a distance of three or four somites, but it is difficult to make out its rela-
tions to the protovertebne, in consequence of the small amount of differ-
entiation which these exhibit at this stage. It seems to me, however,
that the thickening reaches backward into a region posterior to that
in which the prouephric tubules later develop, and therefore represents
already the first fundament of both the pronephros and the anterior end
of the segmental duct.

Frontal sections show the same relations between the pronephrio
thickening and the protovertebra) that I have described for Rana, but
in Bufo the coelom is entirely obliterated by the growth of the proue-
phric thickening, and consequently the prouephric chamber described in
a corresponding stage of Rana does not exist in Bufo. This circumstance
renders the determination of the precise boundaries between the two lay-
ers of mesoderm somewhat more difficult in the Toad than in the Frog,
but still there is usually an unmistakable line of division between soma-
topleurc and splanchnopleure even in the former. The pronephric thick-
ening at this stage is from two to three cells thick, and is a solid
mass.

Stage III.

In embryos of this stage, the fundament of a single pair of gill-folds
is present; the fundament of the auditory vesicle consists of a simple
thickening, which is just beginning to separate from the superficial ecto-
derm ; and five or six protovertebne have made their appearance. The
embryos measure from 2.25 to 2.50 mm. in length.

The pronephric thickening becomes sharply marked off in this stage
from the undifferentiated mesoderm lying ventral to it, and the canali-
zation of the structure is accomplished by the arrangement of the cells
ai'ound a lumen. Segmentally, the pronephric thickening has in general
the form of a close fold of somatopleure, whereas intersegmental^ it ap-
pears as a flattened tube. The points of continuity with the coelom are
situated each directly beneath the middle of a protovertebra, and the
somites in which they appear are II., III., and IV.

The duct arises as a backward continuation of the pronephric thicken-
ing, and contrasts very sharply in histological characters with the ecto-
derm, in consequence of the pigmentation and paucity of yolk spherules
in the latter.

MUSEUM OF COMPARATIVE ZOOLOGY. 245

Stage IV.

Plate V. Fig. 43.

Embryos of this stage measure from 2.8 to 3.1 mm. in total length.
Muscular fibres have begun to appear in the myotomes, the auditory
vesicles are entirely detached from the external ectoderm, and the pro-
tovertebrae have been differentiated as far back as the anus.

The pronephric pouch of Bufo is very similar to that of Rana. It
communicates with the coolom by means of three nephrostomes, and
from its ventral margin the duct takes its origin. The nephrostomes
are segmental in position, and are situated beneath protovertebrae II.,
III., and IV. 1

The duct can be followed for some distance posterior to the hinder-
most pronephric nephrostome as a distinct elliptical tube with a central
lumen. The lumen, however, disappears further posteriorly, and the
duct terminates either as a simple thickening of the somatopleure, or its
posterior end merely rests upon the mesoderm in the region of somite
XI. The hinder tip of the duct (Fig. 43, /W. dl. sg.) in both cases re-
sembles very closely the adjacent mesoderm both m the size and in the
abundance of yolk spherules, and it differs from the ectoderm both in
these features and in the scarcity of pigment. In Bufo I have never
been so fortunate as to find the growing end of the duct situated in a
groove of depressed mesoderm ; but I believe that the fundament ex-
tends itself from the region of its origin in the somatopleure to the pro-
jecting cornu of the cloaca by means of an independent growth on the
part of its own cells. The greater part of the duct, however, arises from
a local proliferation of somatopleure.

The pronephric capsule in Bufo arises as a downgrowth from the outer
peripheral layer of the protovertebrre. In this stage, however, it has not
reached the somatopleure ventral to the pronephros, but merely forms a
two-layered scale-like sheet of tissue covering the dorsal portion of the
gland.

The pronephric chamber is present at this stage. The general bodv
cavity, however, has not yet appeared, the somatopleure and splanchno-
pleure being in other regions in close contact.

1 I have preserved in the enumeration of the body somites of Bufo the same
designations that were employed in tlie case of Rana. In Bufo, however, the ker-
nel of the degenerate protovertebra in front of somite I. gives rise to a few muscle
fibres.

246 BULLETIN OF THE

Stage V.

Plate V. Figs. 42, 46. Plate VI. Figs. 47, 49, 52.

At this stage the larvae were hatched and swam about freely in the
aquaria. The larvae measured from 4 to G mm. in length, and each had
a distinct tail, which protruded for a distance of 1.5 to 2 mm. behind
the anus. The pronephros was probably already functional.

The character of the convolutions of the pronephric tubules was
studied in the case of four pronephridia. In this feature one of them
corresponded very closely with the condition in the pronephros of Rana
represented in Figure 33. The remaining pronephridia differed from
this type solely in the circumstance that the third nephrostomal tubule
joined the collecting trunk at t he extreme posterior portion of the bend,
which m Rana usually forms the first portion of the common trunk.

The position of the pronephros with reference to the somites remains
in general nearly the same as in the preceding stage. In individual
cases, however, the nephrostomes do not appear to lie precisely under the
middle of the myotome.

In embryos of this stage, the segmental ducts already open into the
cloaca. These openings are situated beneath myotome XII. It is
obvious from this fact that the duct in the older embrj'os of Stage IV.
had already very nearly reached the region of its final communication
with the cloaca. In Bufo the lumen of the gut is very narrow, and is sep-
arated from the lateral walls of the body by an extensive mass of yolk cells.
The cloacal cornua are therefore in this case very long, extending to the
outer surface of the entoderm. The ducts reach these cornua by passing
between the dorsal angle of the body cavity and the overlying myotomes.

The histology of the pronephros in Bufo does not present any note-
worthy features of difference from that in liana. The tubes are all
slightly smaller in Bufo, and their walls contain somewhat more pigment
than do those of Rana.

The capsule envelops the pronephros and duct in the way that I have
described for Rana, and it also encloses a series of blood sinuses which
are developed from the posterior cardinal vein. I was not able to obtain
in Bufo any additional evidence in regard to the origin of the mesenchyme
of the pronephros.

Two veins emerge from the anterior end of the pronephros. One of
these is the immediate continuation of the posterior cardinal vein, which,
in passing forward as the ductus Cuvieri (Plate V. Fig. 42, dt. Cuv.),
makes a rapid ventral descent to open into the sinus venosus. The

MUSEUM OF COMPARATIVE ZOOLOGY. 247

other vein (Fig. 42, vn.jgl.) passes forward between the myotome and
the vagus nerve. It evidently is one of the jugular veins, but I have
not been able to study its distribution in later stages, and am therefore
unable to state more precisely which vein of the adult it represents.

The structure of the glomus in Bufo is far more evident than in cor-
responding stages of Rana. In treating of the development of the glo-
mus in the latter, I reached the conclusion that it arises as a simple
fold of splanchnopleure, into which mesenchymatic cells migrate. In
later stages I was able to identify the original outer sheath with a
distinct basement membrane, and found within this membrane a large
number of embryonic blood corpuscles, and occasionally certain cells
which resembled in their histological characters those of the sheath or
peritoneal layer. In Bufo the vascular system is less developed than in
the corresponding stage of Rana ; and, owing to the small number of the
blood corpuscles, the remaining cellular elements come more plainly into
view. The usual form of the glomus is that of a hollow peritoneal sac
lined with endothelium (Plate VI. Figs. 47, 49, 50), and containing scat-
tered blood corpuscles (Fig. 46). At the entrance to the sac the endo-
thelium {en'th.) is continuous with the loose mesenchyme surrounding
the aorta, and, in certain regions, the lumen of the latter can be traced
into the interior of the glomus. This organ, then, exhibits markedly
the character of a blood sinus, the walls of which project into the body
cavity. Occasionally one encounters in Bufo certain minor pocketings of
the peritoneal layer of the glomus, — ! invaginations into the lumen of
the glomus at the place, e. g., occupied by the letters ccd." (Fig. 52).
If the cells at the apices of such invaginations were to become detached,
this condition woidd serve to indicate the source of the pigmented cells
found in the interior of the glomus in the case of Rana, although I have
as yet reached no final conclusion in regard to this matter.

In this stage the body cavity exists as a distinct lumen only in the
region from which the nephrostomes emerge, where it constitutes a pro-
nephric chamber.

My studies on the development of the excretory organs in Bufo have
not extended beyond the present stage.

C. Amblystoma.

Plate V. Fig. 44. Plate VI. Fig. 48. Plate VTI. Figs. 53-56.
Plate VIII. Figs. 57-65.

Amblystoma shows in the development of its excretory system many
features of similarity to the Anuran forms already described. The dif-

248 BULLETIN OF THE

ferences, however, are far greater than those which exist between Rana
and Bufo, and wdl require for their presentation a fuller treatment than
was given in the case of the latter genus ; but the development in all
three genera is sufficiently similar to allow the recognition of the same
successive stages, based upon the degree of complication exhibited by
the pronephros.

Stage I.

Plate VI. Fig. 48.

In embryos of this stage, the two lateral medullary folds have just
fused to form the neural tube. The embryos have a slightly elongated
form and measure about 3.7 mm. in length. They are slightly more
advanced than the embryo of Amblystoma represented by Bambeke
('80, Blanche XI. Fig. 35). The eggs from which I derived my series
of embryos had been deposited for a variable length of time before they
were collected, and I am unable to give the ages of the several stages. 1

The general arrangement of the germ layers (Blate VI. Fig. 48) is
similar to that which I have described for Bana and Bufo. The ecto-
derm (ee'drm.) consists in general of a single layer of cells, each of which
has the form of a cube slightly flattened. Scattered ectodermal cells
form an incomplete deep layer, which may gain in some regions, e. g. in
the head, a very considerable development. The outer face of each
ectodermal cell possesses a thin layer of pigment, but this is by no means
so dense as in Bana and Bufo. At this stage yolk spherules are abun-
dant in all the cells of the ectoderm.

The entoderm has nearly the same arrangement as in Bana, but the
yolk cells are relatively more abundant, and the lumen of the gut is
narrower. In the anterior region, the chorda consists of a simple fold in
the dorsal roof of the intestine ; but in the posterior portion of the body
it is represented by a single row of high columnar cells, which form a
layer convex from side to side towards the lumen of the intestine. This
layer is the one which 0. Hertwig ('83) has named the chorda-ento-
blast. The cells of the yolk entoderm are in general the largest in the

1 A quantity of the eggs of Amblystoma punctatum Linn, raised in the laboratory
during the present season reached the several stages as follows : Stage I., 5 days;
Stage II., 5 days, 12 hours; Stage III., 6 days, 15 hours; Stage IV., 7 days, 15
hours ; Stage V., 8-14 days ; Stage VI., 15-20 days. These figures are only approx-
imate, and between Stages II. and V. the individual variation is frequently more
than sufficient to cover the entire interval between two successive stages. The
temperature of the water varied somewhat during the period, but I believe that
10 or 11° C. would be a fair average.

MUSEUM OF COMPARATIVE ZOOLOGY. 249

body, and contain very large yolk spherules. The majority of the ento-
dermal cells contain no conspicuous accumulations of pigment ; but the
latter may occasionally be found in considerable quantity, particularly in
the cells bordering on the gut.

In the dorsal portion of the body, the mesoderm consists of two lateral
masses of tissue, each of which spreads outward and ventraJward from
the neural tube, and joins its fellow of the opposite side in the ventral
median line. Each of these masses of mesoderm is thickest next to
the medullary tube, and gradually becomes thinner in passing outward
around the mass of yolk cells. In the dorsal half of the body (Fig. 48)
each mass of cells consists of two distinct layers, which are continuous
with each other along the sides of the neural tube. They represent the
first division into somatic (la. so.) and splanchnic (la. spl.) mesoderm, and
the slight space which separates them is the coelom (cce/.). On passing
outward and ventrally, the two layers of mesoderm gradually approach,
and at length are continuous with, each other ; for a short distance
farther, it is still possible to trace two rows of nuclei, indicating approx-
imately the territory occupied by the layers ; but this arrangement
finally disappears, and before the ventral surface is reached the meso-
derm has the form of a layer only one cell in thickness (ms'drm.).

In both somatic and splanchnic layers, the cells are of a nearly cubical
form, but those of the parietal layer are rather thicker, and may be even
columnar. The mesoderm of the ventral side of the body, on the other
hand, is composed of more flattened elements. The cells of the meso-
derm are in general intermediate in size between those of the ectoderm
and of the entoderm. Their yolk spherules are much smaller than those
in the entoderm, but resemble those in the ectoderm too closely to af-
ford a thoroughly satisfactory criterion for distinguishing the two layers.
The mesodermal yolk spherules are, however, slightly larger than those of
the ectoderm ; and in doubtful cases they may be taken into account.

The pigment of the mesoderm is usually collected along that surface
of the cell which faces the coelom, and may in part serve as a guide for
following that cavity in cases where the bounding layers of mesoderm are
in close contact with each other.

I have spoken of the somatic mesoderm as a layer a single cell in
thickness; this is not, however, an adequate representation of the actual
condition. In many sections there may be observed, from place to place,
an additional cell associated with the otherwise single layer. The occur-
rence of an incomplete second layer of cells is most noticeable in the
anterior portion of the trunk, in a region directly lateral to the protover-

250 BULLETIN OF THE

tebral plate. It is probable that this slightly thickened somatic layer
is the first indication of the pronephric thickening.

Stage II.

Plate V. Fig. 44.

Embryos of this stage measure nearly 4 mm. in length ; the medul-
lary tube has become entirely separated from the superficial ectoderm,
and three protovertebrse can be distinguished in longitudinal sections.

The fundament of the pronephros forms in this stage (Plate V. Fig. 44)
an evident thickening of the somatic mesoderm lying immediately lateral
to the protovertebral plate. Throughout the greater part of the thick-
ening, the layer is obviously two cells thick, and occasionally three nuclei
may be seen in a line perpendicular to its surface. The cells constitut-
ing the thickening are closely compacted, and do not appear to form
definite layers. The fact that the thickening passes through a stage in
which it is only two cells in thickness precludes the possibility of its
being a disguised fold with closely applied walls, for in that event there
must be at least three layers of cells involved. Neither the anterior nor
the posterior limit of the thickening can be clearly determined at this
stage. I am also uuable to state definitely its relations to the protover-
tebra?, inasmuch as these cannot be adequately made out in transverse
sections, and the extent of the thickening cannot be satisfactorily ob-
served in such longitudinal sections as pass through both the protover-
tebrre and the pronephric thickening. The latter may be traced for a
distance of about 0.5 or 0.G mm. Each protovertebra at this stage
measures about 0.27 mm. in length, so that the thickening extends
through a length of about two protovertebra;.

In slightly older embryos the pronephric thickening becomes in gen-
eral three cells in thickness ; but it is still a solid proliferation, with no
indication of extensions of the ccelom between the layers.

Stage III.

Plate VII. Figs. 55, 56.

At this stage the young Amblystomas are about 4.3 mm. long and dis-
tinctly elongated in shape ; but they show as yet no trace of a tail.
They are further characterized by the possession of about eight well
marked protovertebra?.

In all the embiyos of this stage the pronephric thickening is at least
three cells in depth, and has a definite ventral boundary. The thickening
extends as far forward as the front face of somite III., and posteriorly

MUSEUM OF COMPARATIVE ZOOLOGY. 251

tapers gradually into undifferentiated somatopleure. The backward pro-
longation of the thickening is the first fundament of the segmental duct,
and may be traced at least as far back as somite VI. Both portions of
the thickening appear to arise in the same way ; namely, by cell prolifer-
ation in the somatopleure.

It is a matter of some difficulty to ascertain when the first trace of a
lumen appears. Before the two walls actually separate, the nuclei fre-
quently show an arrangement which is suggestive of an evagination ;
but one cannot always trust such appearances. Later, a line of pigment
can be traced from the body cavity for some distance into the interior of
the thickening, and finally the two walls separate, leaving a clearly defined
lumen. In all cases, the two regions of continuity with the ccelom are
opposite the middle of protovertebra3 III. and IV. respectively ; and
there is no indication whatever of a continuous fold.

Although the pronephric mass thus shows evident signs of segmen-
tation, yet, as is to be seen by a comparison of segmental and interseg-
mental regions (Plate VII. Figs. 55 and 56), the proliferation is not
interrupted in the latter regions. In frontal sections through prone-
phridia in which a definite lumen has begun to appear (compare Plate
VII. Fig. 55), there can be seen two narrow canals leading from the
cavities of protovertebrae III. and IV. and extending outward as ccelomic
diverticula into the pronephric mass. From this condition the hasty
conclusion misdit be drawn that the narrow canals are in fact outgrowths
from the protovertebral cavities. This however, in my opinion, is not the
case. If the relations of the mesoderm in such a transverse section as is
shown in Figure 55 be regarded, it will be seen that a frontal section
through the pronephric region (in the figure cited, a horizontal section a
little below the level of the letters coelJ) would cut through the proto-
vertebral cavity near its floor, and at the same time pass through the
lumen of the pronephric thickening. Since, moreover, these two spaces
are continuous by means of the communicating canal, it might at first
appear that the latter belonged to the pronephric tubule. The fate of
that portion of the tube, however, shows this interpretation to be incor-
rect, and that it was only by means of the communicating canal that the
lumen of the pronephros communicated with the protovertebral cavity ;
for when the separation of the protovertebra 1 from the lateral plate takes
place, the communicating canal, which is assumed to be the stalk of the
pronephric diverticulum of the protovertebra, becomes closed, and the
pronephros is thereby left in communication with the body cavity alone
(compare Mollier, '90, Taf. XII. Figs. 10 c, 10 (/., tr x and tr 2 ).

252 BULLETIN OF THE

Stage IV.

Larvae of Amblystoma do not possess a conspicuous wide!}- open
pronephric pouch, such as has been described in Anurau species; but
the proliferation becomes at once converted into a tubular organ. In-
deed, the condition of the pronephric thickening in Stage III. is the one
which is most similar to the Anuran pronephric pouch, since it is then
a continuous structure having connections with the ccelom in segmental

regions.

In slightly older embryos, the dorsal half of the pronephric thicken-
ing is no longer continuous through the region between protovertebrre
III. and IV.; and from this region backward to the hinder face of pro-
tovertebra IV. the mass is distinctly divided into two tubes. Of these
two tubes, the more median and dorsal is the second nephrostomal
tubule ; the more lateral and ventral is the common trunk. Finally,
it is to be observed in a number of cases that an anterior loop of the
common trunk occurs a short distance in front of the point of junction
with the nephrostomal canals. The pronephros thus has a form w T hich
approximates very closely to the condition which forms the starting
point for the next stage (Plate VIII. Fig. 58).

Stage V.

Plate VIII. Figs. 57-60.

This stage includes embryos which have attained a length of from
5 to 6 mm. Many of the older embryos of the stage have already
hatched ; they possess well developed tails and swim about freely.

The general form of the pronephros has been studied by means of a
number of rough reconstructions, some of which are represented by the
diagrams on Plate VIIT. In Figures 57 to 60 inclusive, which belong
to this stage, no windings have been reproduced which were not of suf-
ficient magnitude to form definite antero-posterior loops ; and, further,
in plotting these loops, no attempt has been made to preserve in the
diagram the natural direction in which the tube is actually bent. How-
ever, the relative positions of the bends in an antero-posterior direction
have been accurately reproduced.

In the younger individuals of this stage, the pronephros (Fig. 58)
resembles in many respects that of Rana represented in Figure 33 ; but
it differs from the latter, notably in the occurrence of two instead of
three nephrostomes and nephrostomal canals. For this reason, there is
no canal which corresponds to the collecting trunk of Anura, save that

MUSEUM OF COMPARATIVE ZOOLOGY. 253

portion of the latter which intervenes between nephrostomes I. and II. ;
and in discussing the topographical relations of the tubules it will be
needless to distinguish this remnant of the collecting trunk from the
first nephrostomal tubule. In this simplest condition of the pronephros,
the common trunk makes a single loop, the anterior curve of which is
situated nearly as far forward as the level of the first nephrostome. In
somewhat older pronephridia (Figs. 59, 60) the main bend of the common
trunk occupies a position even in front of the first nephrostome, and a
number of minor folds intervene between the junction of the nephrostomal
canals and this most anterior fold. In none of the pronephridia of this
stage is there any evidence of convolution in the nephrostomal canals.

One individual of this stage departed from the normal condition, in that
it possessed three instead of two nephrostomal canals. This abnormality
occurred on both sides of the body, and appears to be correlated with a
less highly developed first nephrostomal tube. It is to be noted that
the third tubule (Fig. 57) appears as an appendage attached to the
most posterior loop of the common trunk. This topographical relation
suggests that it is the most posterior of the three nephrostomal tubules
which has been added to those normally present in Amblystoma, and
this inference is shown to be correct by the relations which the several
tubules bear to the body somites. The question whether the most
posterior of the three tubules in this case represents the third nephro-
stomal tubule of the Ann ran pronephros can be answered only b} r a con-
sideration of the relations which the several nephrostomes in the two
groups bear to the overlying protovertebrse, and will be recurred to in
the general discussion which follows. I may here anticipate to the extent
of stating that the first and second tubules of Amblystoma probably
correspond respectively to the second and third of Rana and Bufo, the
abnormal third tubule belonging to a yet mure posterior metamere.

The position of the pronephric nephrostomes with reference to the
myotomes was determined at an early stage by the location of the first
metameric diverticula which are developed within the pronephric mass ;
and in the present stage these relations have not materially changed.
The two nephrostomes of the normal pronephros lie beneath the third
and fourth myotomes respectively. In the case of the pronephridia
with a supernumerary nephrostome, the first two nephrostomes occur
beneath myotomes III. and IV. respectively, while the third nephro-
stome is found beneath myotome V. 1

1 Myotome I. of this enumeration reaches forward to the root of the vagus nerve,
and is flanked on its outer face by a portion of the ganglion nodosum, exactly as
in the case of the Anura described.

254 BULLETIN OF THE

In this stage the segmental duct in the younger embryos shows some-
what different conditions from those found in the older embryos.
In the case of unhatched embryos possessing the simple pronephros
shown in Figure 58, the duct on passing backwards gradually dimin-
ishes in calibre, and finally loses all trace of a lumen. The funda-
ment of the duct is in this region composed of four or five cells
in each cross section, which are frequently arranged with some regu-
larity about the centre as an axis. On proceeding to more posterior
regions the fundament of the duct becomes intimately connected with
the mesoderm, and is finally lost in that layer. In Amblystoma
the histological characters of the mesoderm and the ectoderm are
nut sufficiently unlike to allow one to base on them a definite con-
clusion respecting the layer which has furnished the material for the
fundament of the duct. In all cases which I have observed, however,
the duct neither unites with the ectoderm nor terminates freely ; but
its posterior end invariably is closely applied to the mesoderm, and con-
sequently is most probably derived from that layer. In view of the fact
that the yolk spherules of the fundament of the duct are of the same
size as those present in the adjacent mesoderm, I am of opinion that
the duct has undergone no extensive independent growth, but has arisen
in situ as a proliferation of the somatopleure.

In the older embryos of this stage, the duct has extended backwards
to the region of the cloaca, and joins the latter near the posterior face
of myotome XX. A distinct post-anal gut is present at this stage. Its
anterior portion contains an evident lumen, and appears as a direct con-
tinuation of the pre-anal portion; its posterior tip is solid, and extends
backward into the tail region for the distance of about one millimeter.
From the ventral floor of this continuous intestinal tube, a median di-
verticulum leads backward and downward to the anus. The histological
characters of this diverticulum differ markedly from those of the rest of
the intestine, and by comparison with younger stages it becomes evi-
dent that the former has resulted from a proctodeal invagination.
Where the intestinal tube is joined by the proctodeum the ventral por-
tion, or cloaca, is T-shaped. The lateral arms receive the segmental
ducts, and the ventral stem may be followed to the anus. In Ambly-
stoma, then, the segmental ducts open into the intestine at the point
where the proctodseal ectoderm and the entoderm pass over into each
other. It is somewhat doubtful with which of the two germ layers the
wall of the ducts becomes continuous; but it is possible that — in con-
trast to the condition obtaining in the Anura studied — the ducts open
upon an ectodermal surface.

MUSEUM OF COMPARATIVE ZOOLOGY. 255

In the younger embryos of this stage, the walls of the pronephric
tubules are all very thick ; they gradually diminish in thickness as the
embryo grows older. The lumen, on the other hand, is at first narrow,
but afterwards becomes much wider. Its size varies greatly in different
portions of the pronephros. For example, the lumen of the long" arm of
the common trunk, which forms the direct continuation of the segmental
duct, is usually much narrower than the average lumen of the other
pronephric tubules. The nephrostomal canals near their junction and
the adjacent portion of the common trunk usually have a wide lumen.
In the abnormal pronephros represented in Figure 57, however, the
lumen of the first nephrostomal tubule was very narrow, a circumstance
which, as I have already suggested, may possibly be correlated with the
presence of a third nephrostome.

The lining epithelium of the tubes is composed of polygonal cells,
which in the younger embryos have a high columnar form, but become
gradually thinner as development proceeds. The nuclei when stained
with Czokor's cochineal show a coarsely granular or reticulate structure,
and are located close to the lumen of the tubule. The protoplasm takes
a uniform delicate tint, which is masked, however, by the deeply staining
yolk spherules. These are most abundant near the basal surface of the
cell ; they decrease in number and in size with the growth of the larva.

In the younger embryos of this stage, the somatopleure is composed
of somewhat flattened cells, whose superficial dimension is approximately
double the thickness of the cell. The walls of the pronephric tubules
in these embryos have a thickness of about 37.5 /x, while the parietal
peritoneum has an average thickness of only about 15 fx. These two
epithelial layers are confluent at the nephrostomes, the wall of the tubule
diminishing rapidly in thickness to that of the peritoneum. The
nephrostomes, as well as many of the pronephric tubules, are slightly
pigmented on their internal surfaces ; but the pigmentation is by no
means so conspicuous as in Rana and Bufo. In the older larva) of this
stage, the peritoneum is much thinner; but since the walls of the tu-
bules have also diminished in thickness, nearly the same relations are to
be observed at the nephrostome as in the younger embryos.

As in Piana and Bufo, the pronephric capsule in Amblystoma develops
in the form of a downgrowth from the somatic layer of the protover-
tebrre. In Amblystoma the two-layered condition of the capsule and
its connection with the overlying protovertebrae are maintained in the
oldest larva? of this stage. It seems probable, moreover, that the down-
growth from the protovertebra) is met by a more or less pronounced

256 BULLETIN OF THE

upgrowth from the somatopleure immediately ventral to the pronephros.
The thickness of the capsular sheath gradually diminishes in the course
of the development of the larvae, but it is in general approximately equal
to that of the peritoneum in the same individual. In the older larvae,
moreover, the pronephros, and especially the segmental duct, become
partially covered by a downward extension of the myotome. In such
larvae the anterior limb bud is prominently developed at this stage, and
its cells cover in part the posterior ventral portion of the pronephros.

The sinuses within the capsule are bounded by mesenchymatic cells
and contain scattered blood corpuscles ; they are continuous posteriorly
with the posterior cardinal veins, so that the venous blood in passing
forward from the hinder portions of the body bathes the pronephric
tubules on every side.

The vessel emerging from the anterior end of the pronephros receives
a large vessel from the head, and from the point of union the ductus Cu-
vieri leads to the sinus venosus. The former vessel is one of the jugu-
lar veins. The distribution of this vein and its probable representative
in the adult will be considered in connection with the following stage.

The first trace of the glomus appears in embryos of this stage. It
consists, as in Rana and Bufo (compare Plate I. Figs. 8, ( J, and Plate VI.
Fig. 47), of a horizontal fold of splanchnopleure lying close to the dorsal
angle of the body cavity. This fold extends, when fully formed, from
the first nephrostome backwards to the second. The outer layer of the
organ consists, as shown by its development, of splanchnic peritoneum,
which is usually bounded within by a sharp contour. I am of opinion
that the latter is in reality a thin structureless basement membrane.
The interior mass of the glomus consists of several different elements.
In the young stages embryonic blood cells form a prominent constituent.
Other cells are present, which have an elongated form and arc evidently
connective-tissue elements, and there appear to be still other cells which
are of a less modified character and in which nuclear mitoses occur.
Many of the latter may well represent young stages in the development
of blood corpuscles, for I have observed mitotic division of blood cells
even in certain older larvae of Stage VI. In addition to the classes of
cells just mentioned, there are a few large cells whose nature is to
me quite obscure. These cells measure 60 /x or more in diameter, and
contain large yolk spherules, which are closely packed together and
make up almost the entire substance of the cell. The histological
characters of these cells ally them most closely with those of the ento-
derm, and in the youngest stages in which I have been able to identify

MUSEUM OF COMPARATIVE ZOOLOGY. 257

them they were closely associated with the yolk entoderm, which lies
medio-ventral to the regiuu of the glomus. It is probable that they
arise from the eutoderm and migrate into the interior of the splanchno-
pleural fold. I have been unable to lind in either liana or Bufo any
cells similar to these large cells in the glomus of Amblystoma, and I
have at present no suggestion to offer respecting their significance. The
glomus, as I have already indicated, is a highly vascular organ, and even
in the younger stages it is possible to find vessels which connect it with
the aorta. These vessels usually follow the splanchnic layer quite
closely, and appear to lie external to the large cells to which reference
has been made.

In the younger larvae of this stage the body cavity in the pronephric
region has the form of separate chambers, from each of which a single
nephrostomal tubule arises ; but elsewhere the cavity is wanting on ac-
count of the contact of the peritoneal surfaces. In the older individuals
it is expanded over a much larger area, but by the development of the
lung bud a dorsal portion of the cavity is partially separated from the
rest as a pronephric chamber.

Stage VI.

Plate VII. Figs. 53, 54. Plate VIII. Figs. 61 65.

The larvae included in this stage were taken from several different
killings made in the course of three or four days. They measure about
9 mm. from the anterior end to the tip of the tail. An anterior limb
bud is plainly visible upon surface view, and the tail is provided with a
distinct membranous fin.

The great complication in the structure of the pronephros which is
attained in this stage is accomplished by a continuation of the same pro-
cess of forming convolutions that has been described for the preceding
stage. Indeed, the separation of the two stages is at best quite arbi-
trary. Figures 61-65 represent various pronephridia of the present
stage. It is to be noticed that the portion of the common trunk of
which the segmental duct is the direct continuation can be traced from
the anterior limit of the pronephros backwards without convolution, or
after having formed a few insignificant loops. The common trunk from
its junction with the nephrostomal tubules to this most anterior bend is
thrown into a series of complicated convolutions, which may be so arranged
as to present a gradation of considerable regularity (Fig. 62), or may
be quite irregular (Fig. 65). In most cases, however, it is to be noticed
that the arrangement of the loops is in general favorable for a compact
vol. xxi. — no. 5. 17

258 BULLETIN OF THE

disposition of the tubes (Fig. G2). The convolution in this stage is
no longer confined to the common trunk, the nephrostomal tubules un-
dergoing slight contortion (Figs. G3-C5).

I have determined the positions of the pronephric structures to the
somites in these later stages by their relations to the spinal ganglia.
The first and second nephrostomes lie very nearly in the same transverse
plane as the first and second spinal ganglia respectively. In the young-
est larvae of this stage the boundaries between the myotomes may still
be made out in transverse sections, and the nephrostomes are then found
to lie beneath myotomes III. and IV. It is probable that in later
stages as well two myotomes occur in front of the first spinal ganglion.

The duct after leaving the pronephros pursues a nearly straight course
backwards to the cloaca. In the larva; of this stage, the post-anal gut
has atrophied, and the ducts open into the intestinal tract just at the
point where it bends downward toward the anus or cloacal aperture.
The outlets of the two sides of the body are quite widely separated,
never opening into an unpaired median depression in the dorsal roof of
the cloaca, as is the case in the corresponding stage of liana. The out-
lets of the segmental ducts are situated between the eighteenth and the
nineteenth spinal ganglion, which would correspond to somite XX. or
XXI. Their position is, then, the same as in the preceding stage.
(Compare page 254.)

In the series of embryos included under Stage V., it was shown that
the walls of the pronephric tubules became gradually thinner as the ani-
mal developed. In the pronephridia of the present stage the same pro-
cess has been continued, and the cells are frequently so reduced in
thickness that the nucleus appears to be in contact with the basal as
well as the superficial, or inner, surface of the cell. Occasionally tubes
occur whose walls are so thin that each nucleus causes a protuberance
into the lumen of the tube. But wherever the thickness of the epithe-
lium exceeds the diameter of the nucleus, it is to be noticed that the
latter lies close to the inner surface of the tube, whereas the yolk
sphei'ules are accumulated in the basal portions of the cells. The yolk
spherules are much less numerous than in the preceding stage. In
many cells they are wholly wanting, and in all they now form a much
less prominent constituent than the cell protoplasm.

The nephrostomes present no new features of interest in this stage.
Most of the pronephric tubules contain more or less pigment, which is
usually accumulated in irregularly distributed dark patches. In one or
two instances I have had a fair degree of success in dissecting out the

MUSEUM OF COMPARATIVE ZOOLOGY. 259

pronephros of a fresh specimen. In such an isolated pronephros the
course of the tubes can be followed with tolerable accuracy in conse-
quence of the pigmented areas occurring in their walls. The loss of yolk
spherules, to which the pronephric tubes have been subjected on reach-
ing the present stage, is shown in a striking manner by the transparency
of the gland as contrasted with tbe snow-white yolk-entoderm.

The histological characters of the duct (Plate VII. Figs. 53, 54) re-
semble closely those of the pronephric tubules. Its calibre is greatest in
the region immediately posterior to the pronephros (Plate VII. Fig. 54),
becoming less as the duct passes posteriorly (Fig. 53). Throughout its
course it is accompanied by the posterior cardinal vein (vti. crd.) . In
the older larvae of this stage, the segmental duct in its passage backwards
to the cloaca receives a large number of mesonephric tubules, which will
be described in the sequel.

The pronephros of the present stage is covered on its dorsal surface by
the main body of the myotomes. From the outer angle of each myotome,
moreover, a distinct fibrillar sheet envelops the entire lateral surface of
the gland. This layer is the capsule, whose origin has been discussed in
connection with Stage V. In the present stage, it frequently becomes
deeply pigmented.

The anterior portion of the pronephros is also overlaid by a stratum
of smooth muscle fibres, which arises from the dorsal fascia. This mus-
cular sheet is continuous in front with a muscle layer which is inserted
upon the ventral surface of the mandible, and probably represents the
depressor maxillae of the adult.

The pronephros is also covered in part by the shoulder girdle, which
in this stage is wholly composed of cartilage.

The vascular sinuses enclosed within the capsule are the direct con-
tinuations of the posterior cardinal vein. They also receive — usually
about midway between the first and second nephrostomes — a blood-
vessel, which may be traced nearly as far back as the cloaca, and which
accompanies in its course the ramus lateralis of the vagus nerve (see
Fig. 53, just median to n. I.). I am not aware of any prior mention of
a vessel having this course, and am unable to state whether this vein
has any representative in the adult.

The vessel emerging from the anterior end of the pronephros receives
a vessel from the head, and the two form the ductus Cuvieri, which pro-
ceeds downward and inward to join the sinus venosus. The anterior
branch may be traced forward into the head in the same direction as the
original trunk ; it accompanies in its course the ramus lateralis vagi.

260 BULLETIN OF THE

In consequence of the uncertainty as to what vein of the adult this
vessel represents, I shall here digress to describe its distribution at this
stage. For purposes of description, I shall follow it from its point of junc-
tion with the cardinal vein forward towards its finer branches. Before
reaching the ganglion nodosum, it sends a branch dorsalward, which can
be traced for a short distance between the lateral wall of the cranium and
the ganglion. The main trunk continues forward external to the ganglion,
and gives off a branch which passes around the posterior end of the audi-
tory capsule and enters the cranium. The original vessel now passes for-
ward through a narrow channel left between the auditory capsule and the
articulating portion of the mandibular cartilage. Near the anterior end of
the auditory capsule it divides into two branches, one of which passes dor-
sal to the eyeball, accompanying in its course the ophthalmic branch of
the trigeminal nerve ; the other branch passes ventral to the eyeball, and
continues into the anterior maxillary region, following the course of the
canalis nasalis. The main trunk runs nearly parallel to the aortic root
and its prolongation, the carotid artery, the efferent branchial trunks
joining the aortic root by passing immediately ventral to the vessel whose
course I have been following. The vein evidently corresponds to the
one described under Stage V. of Rana (page 233, foot-note), and appears
to me to represent in all probability the internal jugular of Gruby ('42)
and of Ecker ('64-'82).

The glomus is considerably broader and thicker than in Stage V. ;
but its longitudinal extent is about the same. In the middle of its
course its distal edge reaches across the body cavity and fuses with the
somatic peritoneum which covers the pronephros. The structure of the
organ appears to be nearly the same as in the preceding stage, but the in-
terior mass is so compact that one can distinguish little more than the
nuclei, which present quite uniform characters. Cells which are unques-
tionably endothelial are frequently evident along the basal surface of the
peritoneal layer ; they also traverse the interior of the glomus dividing
this space into compartments. Pigment is present both in the peritoneal
wall and in the interior mass. It has a scattered distribution, appearing
in the form of perfectly black patches. The large cells to which allusion
was made in Stage V. are present also in this stage. They have about
the same size and histological features that formerly characterized them.
The pronephric chamber has not changed materially from the condition
exhibited in Stage V. The most anterior pronephric tubules are situated
immediately latei*al to a diverticulum of the body cavity, which in sec-
tions through this region appears wholly isolated. On following the

MUSEUM OF COMPARATIVE ZOOLOGY. 261

series of sections backward, however, the chamber enlarges greatly, even
before the nephrostomes are reached, and is separated from the ventral
portion of the body cavity only by the lung bud. Between the first and
second nephrostomes, the pronephric chamber is divided into two parts by
the fusion of the distal edge of the glomus with the somatic peritoneum
covering the pronephros. Still farther posteriorly, an open communication
is established, not merely between these two portions of the pronephric
chamber, but also between the latter and the general body cavity.

In almost all the larvae of this stage, the mesonephric tubules have
appeared, and in many individuals they have already opened into the
duct. There is always a space intervening between the pronephros and
the mesonephros, in which no tubules are developed. This interval ap-
pears to be subject to some variation, but in the majority of cases it
comprises four somites.

In the most anterior region of the mesonephros the tubules show traces
of a metameric arrangement, but this is wholly lost in more posterior
regions. These relations can perhaps be best illustrated by the accom-
panying table, which shows the positions of the right mesonephric tubules
in the larva, whose pronephros is represented in Figure 64. The somites
have been reckoned by reference to the spinal ganglia, but the results are
here expressed in terms of the original metamerism of the myotomes.

Somite III. — Pronephric nephrostome I.
IV.— " " II.

" V. — Tubules absent.

VI.— "
VII.— "
VIII.— «
IX. — 1 mesonephric tubule.

tubules.

k

a.

X.

— 1

XL

— 1

XII.

— 2

XIII.

— 3

XIV.

— 3

XV.

— 4

XVI.

— 5

Each tubule of the mesonephros (Plate VII. Fig. 53) has the ordinary
form, which has induced several authors to call it "sickle-shaped," and
consists of cells which are wholly devoid of yolk spherules, in which the
nucleus occupies almost the entire body of the cell. Along the region
which corresponds to the cutting edge of the sickle, a few loose cells (fnd.

262 BULLETIN OF THE

glm.') occur, which constitute the earliest fundament of the glomerulus.
The nephrostomes, howevei", have not opened at this stage.

In the region between pronephros and mesonephros (Plate VII. Fig.
54) certain masses of cells are found on the median side of the duct in
the same position as that occupied in the posterior region by the meso-
uephric tubules. These cells do not form a continuous mass, but are
interrupted at intervals. The cords of cells thus formed do not, how-
ever, appear to correspond in their arrangement to the metamerism of
the body. It is possible that they represent rudimentary nephridial
tubules, but the evidence in favor of this interpretation must be regarded
as far from satisfactory.

1 have been unable to ascertain the precise mode of origin of the
mesonephric tubules, having sought in vain for nuclear mitoses which
"should throw light upon this question. There are in younger stages
many retroperitoneal (subperitoneal) cells which might be collected and
rearranged so as to produce the tubules ; or, again, the fundaments of
the tubules might be formed by proliferation from the peritoneum. The
cells of the tubule have evidently undergone very rapid division, as is
indicated by the complete consumption of the yolk ; and this circum-
stance seems to me to favor the second view. Furthermore, I have found
nuclear mitoses (Fig. 51) in the region immediately in front of the meso-
nephros which indicate that the cords of cells in this region arise from
the peritoneum. Although I am unable to assert that the mesonephric
tubules arise from the peritoneum, I am inclined to regard it as probable
that they do. There is no evidence, however, of a definite invagination
of the wall of the body cavity.

This is the oldest stage of Amblystoma which I have examined, and
with it I close the descriptive part of this paper.

III. General Discussion.

Having presented in a purely descriptive manner the facts of develop-
ment as yielded by my own studies, I shall now endeavor to use these
observations as a basis for the criticism of the results of other investiga-
tors, and in closing shall point out certain general conclusions which seem
to me warranted by such a review.

Eecent researches have extended greatly the number of animals in
which a homologue of the pronephros is known, so that it may now be
fairly assumed that the organ appears in the ontogeny of all Vertebrates.

In view of much recent evidence (Hatschek, -88 b , Rabl, '88, Ayers, '90)

.MUSEUM OF COMPARATIVE ZOOLOGY. 26

o

which clearly supports the view that Amphioxus is closely related to
Craniotes and occupies a position near the base of the Vertebrate phylum,
the kidneys of this animal are of prime interest in the present connection.
Notwithstanding the extreme importance of the subject, however, the
relation of the excretory system of Amphioxus to other Chordates must
still be regarded as a matter of considerable doubt.

At least seven different views have been advanced respecting the excre-
tory organs of this animal. According to the earliest of these views,
which originated with Joh. Muller ('42, p. 101, see also Langerhans, '76,
p. 322, and Rolph, 70, p. 140), certain modified groups of cells lying in
the posterior portion of the atrium are claimed to possess an excretory
function. I presume that no morphologist would endeavor to homol-
ogize these excretory patches with the kidneys of A^ertebrates. The
same is true of the glandular structures described by Owen ('66, p. 533,
Fig. 1G9, k), and the epithelial bands of Wilh. Muller ('75, p. 109).
Nor- can I see in the "pigmented canals," atrio-ccelomic funnels, of
Lankester ('75, pp. 260, 261, and '89, pp. 394-397) any features which
would definitely link them to Vertebrate nephridia.

The account given by Hatschek ('84) of his discovery of a single
nephridium, which he believes to open into the pharyngeal cavity, is too
brief to permit one to form a final judgment upon his interpretation.
The observation has not been confirmed by any subsequent investigator
save perhaps Lankester and Willey ('90, p. 459), who do not however
regard this organ, which they call the sub-chordal tube, as a nephridium.
There is nothing in its structure as described by either author which in
my opinion justifies its comparison to a Vertebrate excreton r tubule.

The most recent paper on this topic, which is by Weiss ('90), is of
considerable interest from the physiological researches which it records :
these show that a large portion of the atrial epithelium, as well as the
excretory patches of Muller, have a well marked excretory function. Of
greater morphological value is the description given by Weiss of certain
small tubules in which the excretory function is peculiarly active. These
tubules empty into the atrium at the upper margin of that cavity in the
region of each secondary gill bar. They seem to project into the ccelom,
but Weiss was unable to detect a continuity between their lumen and
the ccelom. Since the relations of these tubules to the ccelom are not
ascertained, I am of opinion that the observations of Weiss do not afford
satisfactoiy reasons for regarding them as homologues of either the Ver-
tebrate or the Annelidan nephridia. Weiss's account, however, is at
least very suggestive. An important feature is the metamerism of the

264 BULLETIN OF THE

tubules ; for while the metamerism of the gill bars does not correspond
in the adult to that of the myotomes, yet we should not lose sight of the
fact that according to Kowalewsky ('67, see his Figs. 3G and 39) such a
correspondence exists in the embryo. At such a stage, then, there would
be present a single excretory tubule for each myotome.

In a recent lecture before the Gesellschaft fur Morphologie und Physi-
ologie in Munchen, Boveri ('90) has endeavored to show the existence in
Amphioxus of homologues of the pronephros, the mesonephros, and the
segmental duct. The tubules which Boveri regards as pronephric are
probably the same structures as the excretory tubules of Weiss ; and I
infer that the same have been seen by Spengel ('90, p. 282), though this
writer makes no suggestion as to their significance. Both Weiss and
Boveri claim to have proved by feeding the animals with carmine that the
tubes are actually excretory. According to Boveri, also, they open into
the atrium at the upper margin c f each secondary gill bar ; but their
course is somewhat differently described by the two authors. Boveri
maintains that each tube communicates by means of several openings
with the dorso-pharyngeal coclom. As confirmatory of his position that
these canals represent the pronephric tubules of Craniota, he describes
the relations they bear to the gill vessels, which he identifies with the seg-
mental vessels described by Paul Mayer ('87, p. 343) in Selachii. Accord-
ing to Riickert ('88, pp. 239-242), the glomus of Elasmobranchs consists
of a rete mirabile in connection with these segmental vessels. Adjacent
to the excretory tubules, Boveri finds that the gills display an increase in
vascularity, and that anastomoses are formed between the branchial ves-
sels. This condition does not seem to have been noticed by Weiss.
Spengel, who made a special study of the gill vessels, describes a longi-
tudinal vessel at a corresponding level (longitudinal trunk of the liga-
mentum denticulatum), but does not discuss its significance. It seems
to me that Boveri's observations, provided they be confirmed, afford fairly
satisfactory evidence of the existence of true nephridia in Amphioxus ;
and, as I shall endeavor to show in the sequel, that these are constructed
on a type which may be assumed to represent a primitive condition of the
Vertebrate kidney.

The starting point of Boveri's researches was the hypothesis that the
atrial cavity and gonad ial pouches of Amphioxus correspond to the seg-
mental duct and mesonephros respectively of Craniota. The attempts
of Haeckel ('74», p. 37, and '74 b , p. 305) and of Huxley ('76, pp. 221,
222) to discover a homologue of the segmental duct in Amphioxus must,
in my opinion, be held to have at present merely an historical interest;

MUSEUM OF COMPARATIVE ZOOLOGY. 265

it remains for me to consider whether the theory of Boveri be better
grounded.

The arguments which are adduced in favor of the homology of the
gonadial pouches and the mesonephros may be reduced to the following
points of similarity. The gonadial pouches of Amphioxus are metameric
diverticula of the dorso-pharyngeal ccelom, in accordance with the estab-
lished views of Kowalewsky and Rolph, as confirmed by Boveri, who finds
in the adult a continuity of the epithelia belonging to the two tracts ; the
mesonephric tubules likewise are primitively metameric diverticula from
the dorsal portion of the body cavity (see Sedgwick, '80 a , et al.), The
generative cells develop in the walls of the gonadial diverticula; the
early occurrence of germinal cells at the proximal ends of the forming
mesonephric tubes has also been described by Riickert ('88, p. 257) for
Selachii. Finally, the canal by which the gonadial pouches primitively
communicated with the ccelom arches over the dorsal angle of the atrial
cavity in a way that is very similar to that in which the mesonephric
tubules curve outward to join the duct. The only reason — save those
that require the prior assumption that the gonadia represent mesonephric
tubules — which I can see for identifying the atrium with the segmental
duct is the fact that nephridial (pronephric 1) tubes open into it. This
argument seems to me of very little weight. Boveri himself believes that
the pronephros primitively opened directly to the exterior. Unless other
evidence can be adduced, I see no adequate reason for regarding the
formation of the atrial cavity as a step in the development of the seg-
mental duct. On the other hand, that interpretation seems to me quite
opposed to all that is known of the development of the segmental duct.
As I have shown in the preceding pages, there can be no doubt that,
in Amphibia at least, the duct develops solely from the mesoderm.
According to the opposed view — the ectodermal origin of the duct — the
development always proceeds from a pair of narrow rod-like thickenings
of ectoderm, one on each side of the body, which are very different from
the unpaired ventral groove from which, according to the most recent
account (Lankester and Willey, '90) the atrium develops. If, now, we
deny the homology of the atrium with the segmental duct, the outward
arching of the gonadia becomes a most insignificant topographical resem-
blance. It seems to me that it would be manifestly unfair to base so
far reaching a homology on the remaining points of resemblance, viz. the
early occurrence of germinal cells in the mesonephric tubules, and the
circumstance that the gonadia are metameric diverticula of the dorso-
pharyngeal ccelom.

266 BULLETIN OF THE

Turning now to Craniota, the pronephros in Amniota and Selachii
is a wholly degenerate structure ; in many Anamnia, however, it serves
for a longer or shorter time as a functional excretory organ.

The" pronephros of Dipnoi alone is wholly unknown. Beard ('90,
p. 157) speaks of the transformation of a part of the pronephros into the
Miillerian duct as "a well known fact"; but the only authority he cites
in this connection (Parker, '89) does not make such a statement, nor
have I succeeded in finding anywhere in the literature any account of the
pronephros of Dipnoi. Unless Beard has personal observations on this
matter, I believe that in Dipnoi absolutely nothing is known of the
pronephros or its transformation, save such inferences as may be drawn
from the adult anatomy. I shall therefore merely repeat the statement
of Ayers ('S5, p. 506), that the development probably proceeds as in
Amphibia, since the adult urogenital system in this group presents the
closest analogy with that of the Dipnoi.

The excretory system of Cyclostomes is similar to that of Amphibia.
In Petromyzon a pronephros develops in the Ammocoetes larva, but
aborts in the adult. The number of nephrostomes and of tubules is
small (4, according to Willi. Muller; 4 to 5, Shipley; 3, Kupffer;
according to Semon, an inner and an outer row of nephrostomes are
to be distinguished); and they communicate with an anterior expanded
portion of the body cavity. According to Fiirbringer ('78 a , p. 42), the
pronephros extends over about four somites. Opposite the nephro-
stomes, a vascular organ projects from the root of the mesentery into
the body cavity. This is the so-called glomerulus; as figured by Scott
('81, Taf. IX. Fig. 24), it strikingly resembles the glomus of Am-
phibia. According to Scott, the pronephric tubules develop secondarily
as outgrowths from the segmental duct. On the other hand, Shipley has
confirmed the statements of Muller and Fiirbringer, according to which
the nephrostomes and tubules are formed by the incomplete closure
of a longitudinal groove of somatopleure. Finally, Kupffer maintains
that the tubules arise as three separate evaginations of the somatopleure,
a result which is in harmony with my own observations on Amphibia. 1

In Myxine nothing is known of the early development ; but in late
stages an organ has been made known by the studies of "Willi. Muller

1 In Goette's ('88, p. 163) preliminary account of the development of Petromyzon
he states that a pronephros develops in the same manner as in Amphibia. This
would indeed be a conclusion acceptable to me, but until the accounts are more at
one in regard to the latter group the statement is somewhat vague. I await with
interest the publication of that portion of Goette's final paper which relates to the
excretory sj'stem.

.MUSEUM OF COMPARATIVE ZOOLOGY. 2G7

('75) and of FUrbringer ( ? 78 a , pp. 38, 39), which plainly represents the
Amphibian pronephros. Whether it ever persists in the adult is still a
matter of doubt (see Weldon, '84) ; but in young individuals, at least,
the segmental duct (ureter) is prolonged anteriorly to the heart region.
Here it gives off numerous coiled tubes, which branch and open by funnel-
shaped nephrostomies into the pericardial cavity. On its dorsal side,
the duct gives off a few tubules which terminate in glomeruli resembling
those of the mesonephros. This condition and the large number of
tubules constitute the main points of difference between the Amphibian
pronephros and that of Myxine.

The pronephros of Teleosts and Ganoids appears to me to be reduci-
ble to a single type of structure, which can be easily derived from the
condition present in Amphibia and Cyclostomes (and Dipnoi ?). The
so-called head-kidney of Teleosts described by Hyrtl ('51, p. 29) is prob-
ably derived from the embryonic pronephros, though mesonephric ele-
ments may also be found in the adult head-kidney (see Emery, '82,
p. MS).

According to Rosenberg ('67, pp. 42 et seq.) and Oellacher ('73, pp. 97-
100), the excretory organs arise as a pair of grooves of the somato-
pleure directly beneath the protovertebrse. A process of constriction,
which proceeds from a middle region forwards and backwards, leads to
the conversion of each groove into a tube, the segmental duct. The
anterior portion becomes wholly cut off from the body cavity, and is
thrown into numerous coils. The tip becomes considerably swollen, and
is invaginated by an outgrowth from the aorta forming a single glomer-
ulus on each side.

Goette's ('75, pp. 826, 827) account of the development of the pro-
nephric glomerulus in Teleosts is somewhat different, and affords abetter
basis for homologizing the pronephros of Teleosts with that of Amphibia.
Goette maintains that the somatopleural groove is imperfectly closed
in front, leaving a single nephrostome, opposite which a glomerulus
(glomus) is developed. Subsequently, the pronephric chamber becomes
separated from the rest of the body cavity, and comes to resemble a
Malpighian capsule with its contained glomerulus. While Furbringer
('78 a ) confirms Goette's view, Hoffmann ('86, p. 621 et seq.) has quite re-
cently reasserted that this Malpighian capsule is the blind infolded end
of the segmental duct, and the homology with the Amphibian glomus
and pronephric chamber, which appears to me probable, he denies.
Hoffmann's position does not seem to me-tenable in the light of com-
parative studies. Even though it should be shown that the ducts

268 BULLETIN OF THE

have absolutely no connection with the body cavity at the time when
the glomerulus is formed, I could nevertheless defend my position by
the assumption that the blind anterior end of the duct is a compound
structure, representing both nephrostomal canal and pronephric cham-
ber. It seems to me that, were it necessary to make this assumption,
an extensive comparative study would justify such an interpretation.

The pronephros of Teleosts was long supposed to remain functional
in the adult ; but recent investigations seem to favor the conclusion that
it never persists in fully mature individuals, with the possible exception
of a few degenerate animals like Fierasfer (cf. Balfour, 81 b , '82 ; Grosglik,
'85 and '86; Emery, '80, '81, and '85 ; Calderwood, '91).

The account given by Balfour and Parker ('82, pp. 415-424) of the
development of the pronephros in Lepidosteus is in very close agree-
ment with the development in Teleosts as described by Goette and by
Fiirbringer. The only conspicuous point of difference is, that, while in
Teleosts the pronephric chamber becomes wholly detached from the body
cavity, in Lepidosteus a remnant of the original communication probably
persists as a so-called peritoneal tubule. As among Teleosts, the pro-
nephros atrophies in adult Lepidostei.

Beard's ('89, pp. 114, 115) account of the early development differs
greatly from that just given. According to this author, the pronephros
is formed as a solid proliferation from the intermediate cell layer
(Balfour) in the region from the 4th to the 8th or 9th somite inclusive.
Externally, the proliferation fuses with the ectoderm. As a rule, there
are formed three pairs of pronephric nephrostomes, of which the most
posterior pair abort. The pronephric chamber is formed by the narrow-
ing of the ciliated opening and the widening of the part opposite the
glomerulus. Since Beard does not describe the development of the
glomerulus, the account seems to me decidedly vague; but I believe
I am right in accrediting to the author the view held by Hoffmann for
Teleosts, that the glomerulus is not developed in the body cavity. As
I understand him, it is developed in the course cf the pronephric
tubes.

All the studies on Ganoids thus far enumerated have been made upon
Lepidosteus. In Acipenser, Salensky ('78, '80) maintains, in opposition to
Kowalewsky, Owsjannikoff and Wagner ('70), that the excretory organs
first appear as a differentiation in the form of a solid cord of cells. There
is at that stage no trace of the coelom, nor of a division into proto vertebral
and lateral plate. Indeed, this cord of cells first marks the region where
the' latter separation will later occur. In its further development the

MUSEUM OF COMPARATIVE ZOOLOGY. 269

cord of cells acquires a lumen, either by a rearrangement of the cells, or
by destruction of the axial ones. Anteriorly the structure now opens
into the body cavity. The anterior portion elongates and becomes more
and more convoluted up to the time of "post-embryonic" development.
Opposite each of the peritoneal funnels are formed glomeruli [glomi] as
processes from the radix mesenteric They are covered by a pigmented
layer of peritoneum. Salensky does not seem to me to have been very
clear upon the earliest development, which was studied mainly by surface
views, and I am of opinion that these stages would show very different
conditions if more recent technical methods were employed. The most
interesting feature of the development, as described by Salensky, is the
occurrence of a glomus in the position which is typical for Amphibia and
Petromyzon.

The excretory system has probably been studied more carefully in
Selachii than in any other group. The independent researches of Bal-
four ('75 and '78) and Semper ('74 and '75) are in substantial accord,
and have formed the basis for all subsequent investigations. For our
purpose, the most prominent feature of the development as described
by these authors is the absence of any structure which demonstrably
represents the pronephros. According to Balfour, the first trace of the
excretory system appears as a solid knob springing from the " interme-
diate cell mass " near the level of the hind end of the heart. From this
anterior proliferation a solid cord of cells grows backward between ecto-
derm and mesoderm. The posterior portion is the fundament of the
segmental duct; the anterior knob persists in adult females as the
ostium abdominale of the oviduct. According to Balfour, this solid
knob represents a rudimentary pronephros.

Very recently the early development of the excretory organs has been
placed in a new light by the researches of Ruckert ('88) and van Wijhe
('89). According to Ruckert, the development begins with the forma-
tion of a pronephros as an outgrowth towards the ectoderm from the
ventral portions of several protovertebrce, extending from the third or
fourth trunk somite backwards for a distance of four to six somites.
The thickening extends ventrally in each somite to the region where the
segmented mesoderm passes into the unsegmented lateral plates. The
proliferation, in the formation of which the somatic layer is alone con-
cerned, shows on careful study a metameric character. From the pos-
terior end of each protovertebra a narrow canal can be traced outwards
and backwards, where it unites with a similar canal emerging from the
next following somite. The pronephric mass fuses for a time with the

270 BULLETIN OF THE

ectoderm and probably receives a contribution of cells from tbat layer.
The duct grows backwards as far as the cloaca at the expense of the
ectoderm. Having reached this stage of development, the pronephros
rapidly degenerates. This process takes place in a slightly different way
in the anterior and posterior regions. A variable number of the most
anterior evaginations flatten out into a simple longitudinal groove of per-
itoneum, the ostium abdominale ; the remaining ones become closed and
detached from the peritoneum ; thus there remains a longitudinal canal
communicating with the body cavity by the slit-like ostium. In inter-
preting the structure as a rudimentary pronephros, it is important to
note the discovery by Ruckert (pp. 239-242) of a structure which he re-
gards as a pronephric glomerulus, or glomus. This structure is developed
in connection with segmental blood-vessels which pass from the aorta to
the right subintestinal vein, and which have been described by Paul
Mayer ('87, p. 343). In Torpedo the vessels are present on the right side
in the same number as the segments of the pronephros, and as they pass
ventrally between the entoderm and the splanchnopleure it is to be noticed,
in regard to the middle vessels at least, that they send out buds, which
form projections from the median peritoneal wall opposite the pronephric
tubules.

It will be at once seen that the development of the pronephros as de-
scribed by Ruckert is in striking agreement with the account I have
given of the early stages in the development of the Amphibian pro-
nephros, and I have no hesitation in homologizing the two organs. The
earliest stage which has been observed in both groups is that which I
have termed the pronephric thickening. This is followed in both by the
stage of canalization ; but the Selachian pronephros never goes beyond
an early condition of the pronephric pouch, in which, however, the homo-
logues of the nephrostomal tubules and the collecting trunk appear.
The points of difference between the account I have given and that
given by Ruckert for corresponding stages of the Selachian pronephros
seem to me, with a single exception, to be either unreal or insignificant.
The exception to which I refer pertains to the participation of the ecto-
derm in the formation of the pronephric thickening. This condition I
am confident does not occur in Amphibia. Moreover, the evidence upon
which Ruckert bases his statement seems to me far from conclusive, nor
has his observation been . confirmed by any subsequent investigator.
Ruckert described the pronephric thickening as a product of the proto-
vertebra?. I cannot admit that this is true for Amphibia ; but I believe
that our differences of opinion are really due to the fact that we use dif-

MUSEUM OF COMPARATIVE ZOOLOGY. 271

fereut criteria for determining the boundaries of the protovertebrse.
There can be no doubt that the earlier pronephric thickening is made
up of metameric constituents ; but I should be unwilling to regard
all segmented mesoderm as belonging to the protovertebrai. On the
contrary, I am of opinion that the ventral extent of the protovertebrse
is for the first time defined when the longitudinal constriction appears
which divides the primitive ccelom into protovertebral cavity and pleuro-
peritoneal or (secondary) body cavity. When such a definite line of
demarcation has been established, the remnant of the pronephros in
Selachii, as well as the functional pronephros in Amphibia, remains con-
nected with the latter space. The remaining points of difference relate
to the number of tubules involved, — which, as we have seen, varies
even within the class of Amphibia, — and to their position with refer-
ence to the somites. The latter feature seems to me to be at once
difficult to determine and of minor importance.

Before the conclusion of this paper I shall endeavor to indicate how
the glomus of Amphibia may possibly have been derived from the
type of structure which is described by Riickert for Selachians and by
Boveri ('90) for Amphioxus.

The results gained by van Wijhe ('89) do not seem to me to differ
from those of Riickert in many respects which are of importance for a
comparative study. The great divergence of their descriptions in the
case of. many details seems to me to be occasioned mainly by the peculiar
conception which Riickert holds of the relations between the protover-
tebral and the lateral mesoderm. For these details and for the hotly
contested questions of priority, I must refer to the original papers (van
Wijhe, '86, '87, '88\ '88 b , '89, Riickert, '88, '89), and consider here those
features only which merit special attention because of their bearing on
the general questions of homology. Van Wijhe denies positively the
participation of the ectoderm in the formation of the pronephric thicken-
ing ; and he claims that the ostium abdominale is formed from the pro-
nephros by the fusion of the nephi-ostomes. Finally, structures which are
supposed by him (pp. 480-482) to represent the pronephric glomeruli
of Riickert are described as occurring on both sides of the body, not,
as affirmed by Riickert, on the right side alone, and van Wijhe inclines
to the view that they are actually equivalent to the glomi of Amphibia.
The body described by van Wijhe consists of a vascular rod, which passes
obliquely from the dorsal to the ventral lip of the pronephric pouch, and
represents the last trace of the partition between two peritoneal open-
ings, which have not yet fused. Riickert's description is not entirely

272 BULLETIN OF THE

clear, and also suffers from misleading typographical and grammatical
errors ; but it is certain that the structure he describes lies within the
splanchnic peritoneum, and is not to be confounded, as "was done by van
Wijhe, with the partition between two pronephric tubes. Riickert says
('88, p. 239), " Es [ein Paul Mayer'sches Quergefass] zieht dicht an der
medialen Grenze der Vornierenanlage vorbei und gelangt, indem es die
Leibeshbhle durchbricht, d. h. ihre Wandung vor sich herstiilpt, an die
Aussenfliiche des Darmes, wo es zwischen Ectoderm [soil wohl Entoderm
heissen] und Splanchnopleura gelegen, mit der rechten Subintestinalvene
confluirt." I cannot admit that the structure described by van Wijhe
is the homologue of the Amphibian glomus, nor do I believe that it
corresponds to the structure observed by Riickert.

The mode of development of the excretory system is much alike in the
three groups of Amniotes. It seems, however, best in the present in-
stance to deal with the Reptiles separately from Birds and Mammals.
The most important of the works on the Reptilian excretory system is
perhaps the monograph of Braun ('77), which, however, is of little ser-
vice in elucidating the earliest stages. Weldon ('83) first gave a satis-
factory account of the early development. According to this author, the
first» trace of the excretory system in Lacerta is found in the region of
the intermediate cell mass, and consists of a series of vesicles (Segmen-
talblaschen of Braun), which have a strictly metameric arrangement.
Throughout a region of five protovertebra; (from the 8th to the 12th),
there appears on the external wall of these segmental vesicles a rod of
cells at first composed of discontinuous parts. This rod is the fundament
of the segmental duct ; in the region between two successive protover-
tebrse, it is budded off from the unmodified " middle plate" (Waldeyer),
or intermediate cell mass. Behind the twelfth protovertebra, the duct
grows backward, free from adjacent tissue. The rod of cells soon ac-
quires a lumen, continuous anteriorly with the cavities of the segmental
vesicles.

The observations of Mihalkovics ('85) upon Lacerta agilis differ from
those of Weldon mainly in two particulars. In the first place, according
to Mihalkovics (pp. 42, 43), the most anterior three or four pairs of seg-
mental vesicles at the time of their origin communicate both with the
body cavity and with the protovertebral cavity. In other words, they
are formed as expansions of what I have termed the communicating
canal, or Mittelplattenspalten of the German authors. Some somites
in the series, however, may be without vesicles. Secondly, Mihalko-
vics (p. 48) maintains that the segmental duct buds off from the middle

MUSEUM OF COMPARATIVE ZOOLOGY. 273

plate as a continuous cord of cells at a time when only the first trace
of the segmental vesicles has appeared. Before the (3 or 4) anterior
segmental vesicles have entirely lost their connection with the body
cavity, they communicate distally with the lumen of the segmental duct,
and may therefore be regarded as typical nephrostomal canals. This
condition is never encountered in the posterior vesicles, which develop
independently of the ccelom in the solid Wolffian blastema, or middle
plate. In consequence of this difference in the mode of development
of the anterior and posterior portions, Mihalkovics is of opinion that the
first three or four segmental vesicles represent a rudimentary pronephros.

According to Strahl ('86), the segmental vesicles are budded oft" from
the ventral portions of the protovertebrae, and gain secondarily a con-
nection with the body cavity ; the duct does not appear until the vesicles
are evident.

Ostroumoff ('88 b , p. 81) confirms for Phrynocephalus the observations
of Mihalkovics regarding the anterior segmental vesicles, although he is
unable to ascertain the precise number that communicate with the body
cavity. He also interprets these anterior vesicles as a pronephros. The
duct, however, first appears in disjointed fragments lying between
successive vesicles.

According to Hoffmann ('89), there develops in Reptiles a pronephros
similar to that described by Riickert ('88) for Selachii. It appears as a
series of evaginations of the somatopleure. These are formed in the re-
gion where the protovertebrae pass over into the lateral plates. The orgar
extends over a variable number of somites (6-7 in Lacerta and 5-6 in Tro-
pidonotus). As protovertebrae separate from the lateral plate, the pro-
nephric evaginations remain in connection with the former, except in the
case of the first outgrowth (L. agilis, in L. muralis the first two), which
forms for a time a single pronephric ostium. The most posterior out-
growth extends backwards, and forms the fundament of the segmental
duct. The fate of the several evaginations is different. The most ante-
rior and possibly the next following outgrowth abort at an early stage ;
the remaining evaginations become detached from the protovertebrae nnd
fuse with one another, thus forming a tube closed in front, but continu-
ous posteriorly with the segmental duct. Hoffmann identifies these
evaginations with the segmental vesicles of Mihalkovics and Weldon,
but asserts that these authors mistook for a separate fundament of the
segmental duct a blind backward prolongation'bf the evagination belong-
ing to the immediately preceding somite. These backward processes are
described by Riickert for Selachii. Ostroumoff's ('88 b , pp. 78, 79) state-

VOL. XXI. — KO. 3. 18

274 BULLETIN OF THE

ment, apparently unknown to Hoffmann, that the duct first appears in
short fragments, each of which lies posterior to a segmental vesicle,
could be readily brought into accord with these observations.

In regard to the correctness of Hoffmann's conclusions that these evagi-
nations represent a pronephros, I am of opinion that there is considerable
room for doubt. The organ described by Hoffmann differs in two im-
portant respects from that of Selachii, and from the young stages of the
Amphibian pronephros as presented in the first part of this paper. In
the latter groups, while the metameric evaginations are yet continuous
with the coelom, they have also fused distally to form a longitudinal
canal (collecting trunk) ; this condition I wholly miss in Hoffmann's
account, according to which all the evaginations remain distinct from
each other till they have entirely separated from the coelom, and only
the more posterior outgrowths ever fuse together. Secondly, no struc-
ture comparable to the Amphibian glomus is described. The latter
objection would apply equally to the account given by Mihalkovics. 1
None of the previous investigators were more successful in finding
glomeruli of the pronephric type.

In regard to the former feature, however, the account of Mihalkovics
is more satisfactory, since the most anterior three pairs of vesicles stand
in precisely this relation to the body cavity and to the collecting trunk
(segmental duct). In reviewing Mihalkovics's interpretation, Hoffmann
says ('89, p. 272), since " die Vorniere als eine Ausstiilpung, die Urniere
nicht als solche entsteht, kommt es mir hochst wahrscheinlich vor ; dass
die Vermuthung von Mihalkovics, nach welcher die proximalen Urnieren-
kanklchen der Eidechsen der Vorniere der Amphibien entsprechen, eine
andere Deutung zulasse." I judge from this passage that Hoffmann is
inclined to regard as mesonephric tubules the anterior three or four seg-
mental vesicles described by Mihalkovics. I am quite unable to har-
monize this view with Hoffmann's prior identification ('89, pp. 267, 2G8)
of the pronephric evaginations described by him with the segmental
vesicles of Mihalkovics and Weldon. The mode in which the meso-
nephric tubules develop in Lacerta is asserted to be very similar to that
described by Riickert and van "Wijhe for Selachii. If I properly under-
stand Hoffmann's description, the space lettered c. in Tafel XVII. Figs. 3
and 4, is the lumen of a mesonephric tubule. From these figures it is
evident that the mesonephric tubule develops from a portion of meso-
derm ventral to the pronephros ; but according to both Riickert and van

1 Figures 18 and 19, referred to by Wiedersheim ('90 b , p. 413) in this connection,
do not relate to Reptiles at all. They represent sections of Duck embryos.

MUSEUM OF COMPARATIVE ZOOLOGY. 275

Wijhe, the mesoderm which produces the mesonephric tubules in Selachii
belongs to a regiou dorsal to that which gave rise to the pronephros
(see the diagrams appended to van Wijhe, '89, Taf. XXXII.).

In view of the difficulties to which I have alluded, it seems to me that
Hoffmann's position cannot be regarded as satisfactory. Furthermore, it
Hoffmann's observations 1 on the origin of the posterior mesonephric
tubules be accurate, the contrast which Mihalkovics endeavored to es-
tablish between the anterior and posterior tubules does not exist. If,
finally, these anterior three or four pairs of tubules develop in their
course typical Malpighian capsules remote from the peritoneum, — Mihal-
kovics is not clear on this point, — I can see no reason for regarding them
as pronephric. I am therefore of opinion that there is at present no
evidence which proves a pronephros to exist either in Lacertilia or in
Ophidia.

It remains for me to consider two recent papers by Wiedersheim
('90 a , '90 b ), which describe a very interesting condition of the excretory
system in Crocodilia and Chelonia. The anterior portion of the em-
bryonic excretory organs in these groups consists of a number of
tubules which take their origin in ciliated nephrostomes, and, after un-
dergoing contortion, join a longitudinal canal continuous with the seg-
mental duct. From the root of the mesentery a large glomus protrudes
into the body cavity. It lies in a distinct fold of the peritoneum, and
consists of a mass of highly vascular tissue receiving distinct vessels
from the aorta. It extends continuously opposite a number of nephro-
stomes, and is evidently equivalent to the Amphibian glomus. In some-
what more posterior regions the conditions are essentially the same ;
but the nephrostomes and the glomus having approached each other,
they are cut off from the main portion of the body cavity by a longitu-
dinal fold of peritoneum. In this manner, there is formed a pronephric
chamber comparable to that of Amphibia. In yet more posterior regions,
the pronephric chamber w 7 ith its contained glomus breaks up into a series
of capsules containing glomeruli, each of which then appears to form the
blind termination of a tubule. This is the region of the mesonephros with
typical Malpighian capsules. In the subsequent development of the em-
bryo, the anterior portion of this excretory system early atrophies, and
the hinder part alone constitutes the well known Wolffian body, or
mesonephros. In my opinion, the account given by Wiedersheim affords
a satisfactory basis for the view that the most anterior portion of this
excretory system is truly pronephric. It seems, however, quite impos-

1 Similar observations are recorded by Orr ('87, pp. 325-327).

276 BULLETIN OF THE

sible to draw a rigid line between pronephros and mesonephros. Indeed,
such is a part of the conclusion which I think we shall finally be able to
draw from the entire review.

The numerous accounts which have been recently given of the pro-
nephros in the higher Amniota may be conveniently treated under three
heads : —

(1.) According to Balfour and Sedgwick ('78, '79), the Miillerian duct
in the Chick first appears in a region somewhat behind the front end of
the Wolffian duct as three slender invaginations of the peritoneum which
covers the Wolffian body. These invaginations later fuse at their distal
extremities, and the most posterior involution grows backwards in con-
nection with the Miillerian duct. There is thus formed a longitudinal
canal with three peritoneal funnels, the whole structure being comparable
to the pronephros of Amphibia. Slightly in front of the nephrostomes
there is attached to the radix mesenterii avascular body which resembles
the Amphibian glomus. It receives blood-vessels from the aorta, and
projects into the body cavity enclosed in a distinct sac of peritoneum.
Gasser ('74, pp. 58, 59) had previously observed somewhat similar condi-
tions in the anterior end of the Miillerian duct ; and, by renewed inves-
tigation, Gasser and Siemerling were able to confirm the occasional
occurrence of the phenomenon, though a single invagination appeared
to be the rule. Multiple invaginations have also been mentioned by
Kollmann ('82 b , p. 20), Siemerling ('82, p. 29), Janosik ('85, p. 43), and
Mihalkovics ('85, p. 295) ; but Braun (79) and Renson ('83, p. 37) were
unable to find any evidence of such a condition. Braun also opposed
Balfour and Sedgwick in their view respecting the nature of the vascular
body, and Sedgwick ('80 b ) later came to the conclusion that this struc-
ture was really a series of greatly modified mesonephric glomeruli.
This interpretation was adopted by Balfour ('81 a , p. 590).

(2.) The second view is set forth in the recent account of Felix ('90),
who describes in a chick embryo with eight protovertebrae a series of
outgrowths, which, emerging from the lower hinder portions of protover-
tebrae IV.-VIIL, extend backward and outward toward the ectoderm.
The latter layer occasionally presents local thickenings in this region, and
in some cases a connection between the mesodermal outgrowths and the
ectodermal thickenings can be observed. In older embryos no trace of
the structures can be found. As was the case with the evaginations
found by Hoffmann ('89) in Reptiles, no fusion of their distal extremities
is recorded. This condition makes them at once unlike the Selachian
pronephros described by Riickert, and the early stages of the Amphibian

MUSEUM OF COMPARATIVE ZOOLOGY. 277

pronephros as detailed in the preceding pages. Moreover, Felix pro-
duces uo evidence to show that they stand in any genetic relation what-
ever to the Wolffian duct, or to the pronephric structures described by
other authors. In the present state of knowledge his interpretation
seems to me untenable.

(3.) The remaining views all have the common feature that they regard
certain rudimentary canals in connection with the anterior end of the
Wolffian duct as pronephric. The views are somewhat divergent, but I
have been able to compile from them a general statement which will in
a measure explain their conflicts. In bringing the observations of each
author under this general scheme, I shall frequently be driven to regard
his results as incomplete, but I shall as far as possible avoid questioning
his statements from an a priori standpoint.

In general three regions of the embryonic excretory organ may be
distinguished : the pronephros, an intermediate region, and the meso-
nephros. For criteria of these regions, I shall use in the main glomeru-
lar structures : those of the pronephros are glomi wholly external to the
tubules; those of the intermediate region are transitional glomeruli,
which develop in peritoneal canals, but project through the nephrostomes
into the body cavity ; those of the mesonephros are typical glomeruli,
which have only a mediate connection with the body cavity through the
tubule.

It now remains to consider the results of the observers whom I have
placed in my third group. The work of Gasser and Siemerling ('78, '79),
subsequently carried on by Siemerling ('82), relates to Birds alone. These
authors recognize two distinct portions of the Wolffian duct : a portion
lying in front of the fifth somite, and a posterior portion. The former
shows many irregularities, is broken up into discontinuous fragments,
and early atrophies ; the latter develops more slowly, but more regularly,
and persists as the duct of the Wolffian body. The first indications of
tubules consist of the so-called primary cords, which are continuous with
the coelomic epithelium by means of funnel-shaped ostia, while they are
distallv in contact with the duct. Gasser and Siemerling maintain that
they belong to the most anterior part of the mesonephros, a portion which
early atrophies. They are quite similar to the S-shaped canals of Kolli-
ker ('79). In front of the region of the " primary cords" similar evagi-
nations occur, but these never reach the duct. A typical glomus, which
may be single or may be divided into parts, projects from the radix
mesenterii opposite the openings of these evaginations. In embryos
of this stnge the space between the most anterior Wolffian tubule and

27S BULLETIN OF THE

the pronephric structures is traversed by a series of glomeruli which re-
semble most closely those of the mesonephros. Siemerling calls them
transitional glomeruli. The pronephros of our scheme would be repre-
sented in this account by the region in front of the fifth protovertebra ;
the intermediate region would correspond to the space occupied by the
transitional glomeruli, and also, as I believe, to that previously occupied
by the primary cords ; the mesonephros would form the rest of the
organ.

According to Sedgwick's ('81) account of the development in the
chick, the Wolffian duct, in separating from the proliferation in which
it arises (region between the 7th and 11th protovertebra 1 ), remains
connected with the peritoneal epithelium by short cords of cells. Be-
tween the 8th and 15th protovertebrae, the duct, as it grows freely back-
wards, comes secondarily into contact with such a cord of cells in each
somite. Behind the 15th somite, the fundaments of the tubules (inter-
mediate cell mass) do not join the duct until their differentiation is
somewhat advanced. The cords of cells in the region between the 7th
and 11th protovertebrae acquire lumens which maybe continued even
into the duct ; but both cords and duct soon entirely disappear. Al-
though no glomus is described, this region probably represents the
pronephros. Between the 12th and 15th protovertebrae typical nephro-
stomal funnels are formed, in which transitional glomeruli develop. This
portion of the organ would then correspond to the intermediate region of
the general scheme ; behind this region comes the typical mesonephros.
Sedgwick regarded the first mentioned region as pronephric ; but ho
hoped to be able to harmonize such a view with the position (cf. page 27G)
formerly taken by himself and Balfour (Balfour and Sedgwick, '79). 1

In the foregoing description 1 have assumed that the most anterior
portion of the Wolffian duct and the accompanying transverse canals
observed by Sedgwick corresponded to the pronephric region as de-
scribed by Siemerling. This interpretation seems to me in all probabil-
ity correct ; yet it should be recalled that the pronephros described by
Siemerling lies in front of the 5th somite, and is anterior to the region
in which the early proliferation to form the duct took place ; whereas,

1 Mihalkovics's statement, that Sedgwick abandoned his former view, is incor-
rect, as will be seen by referring to the closing paragraph of his article (Sedgwick,
'81, p. 468).

In the second edition of Foster and Balfour's ('83, p. 218) Elements of Embrj'-
ology, revised by Sedgwick and Heape, the anterior end of the Mullerian duct is the
only homologue of the Amphibian pronephros suggested.

MUSEUM OF COMPARATIVE ZOOLOGY. 279

the pronephros, according to Sedgwick, lies between the 7th and 11th
protovertebrae and arises in the same region in which the duct first
appears.

Lock wood ('87, pp. 657-GG3) describes three regions in the embryonic
excretory organ of the Rabbit. In the most anterior region (pronephros),
the duct consists of isolated fragments, which are connected with the body
cavity by 2—3 nephrostomes. Then follows a region of typical nephrosto-
mal canals with glomeruli, and finally typical blind mesonephric tubules.
Possibly the last two regions belong to the mesonephros ; but in none
of the accounts of Mammalian development have I been able to recog-
nize with certainty the intermediate region.

According to Renson ('83, p. 29), glomeruli develop in the Chick in
the i*egion of the pronephros, which is otherwise described in agreement
with Sedgwick's account. The pronephric tubules atrophy with the
exception of their nephrostomes, and in the hollow of each funnel there
appears a glomerulus which soon comes to project freely into the body
cavity. In a region directly posterior to that in which the free glomeruli
occur, there are found the so-called mixed glomeruli, which are situated
in the base of an infundibular depression, and are partially covered by
a fold of peritoneum. This, as well as the more anterior portion of the
system, Renson regards as belonging to the pronephros. lie also de-
scribes in the Rabbit a series of peritoneal involutions in connection
with a discontinuous duct. In this region he likewise observed a vascu-
lar structure, which he regarded as a very rudimentary external glomer-
ulus. A similar observation has been recorded for human embryos by
Lockwood ('87, pp. 6G2, GG3), and for Arvicola by Spoof ('83, p. 86, foot-
note). It is difficult to arrive at a satisfactory estimate of Rcnson's posi-
tion. There would lie no difficulty in classing him with Seilgwick, were it
not for the circumstance that he describes for the pronephric region (G or
7th to 11 or 12th somites) glomerular structures which, according to his
own comparison, develop in the same way as the transitional glomeruli
observed by Sedgwick in the "intermediate" region only (11th to 1-lth
somite). If, however, it should prove to be true that only the "mixed"
glomeruli develop in this way, the conflict would at once be removed, and
Renson's account would show the three primary regions in their typical
condition.

According to Mihalkovics the most anterior two or three tubules (4—7
somites) in the Chick and Duck are derivatives of the communicating
canals, and gain a connection with the duct while yet opening into the
body cavity by a distinct ostium. The posterior canals, on the contrary.

280 BULLETIN OF THE

are all differentiated from the solid " Wolffian blastema," and never have
any connection with the body cavity. Posterior to the last pronephric
canal, 5-6 free glomeruli are to be found. The anterior canals form
much earlier than the posterior, indeed they wholly abort before the
mesonephros attains its final development; and they together with the
free glomeruli are, in his opinion, to be regarded as equivalent to
the pronephros and glomus of Amphibia. Mihalkovics also mentions
the occurrence of transitional glomeruli ; these are typical glomeruli
which lie near the peritoneal covering of the Wolffian body. It seems
to me probable that these glomeruli really belong to the mesonephros,
and that at least a portion of the "external glomeruli" belong in reality
to the class which 1 have designated transitional glomeruli. This inter-
pretation would not merely be in agreement with the described position
of the glomeruli with reference to the somites, but it would also accord
well with the figures Mihalkovics gives of the two sets of glomeruli.
Thus, in his representation of a transitional glomerulus (Taf. I. Fig. 17,
g. a).), there is little reason to regard the structure as in any way different
from a mesonephric Malpighian body. I may here further remark, that
nearly all other modern investigators agree in deriving a part, if not all,
of the mesonephros from a layer of cells which primitively bounded the
coelom, rather than from a strictly indifferent blastema. In this light,
the validity of the principal contrast Mihalkovics sought to establish be-
tween the pronephros and mesonephros becomes at least very uncertain.
The account of Janosik ('85) affords the best basis for the general
scheme I have proposed. The most anterior region, or pronephros,
develops somewhat later [!] than the first tubules of the mesonephros
(primary cords ?). The duct in the region of the pronephros is broken
up into fragments, which receive rudimentary peritoneal canals. Three
typical glomi are developed on the radix mesenterii. In the next follow-
ing region (intermediate), from two to five peritoneal canals communicate
with the Wolffian duct. Xear the nephrostomal ends of these canals
transitional glomeruli develop. Both the pronephros and the intermedi-
ate region rapidly atrophy. The remaining portion of the embryonic
excretory organ is the true mesonephros. The mesonephric tubules are
either developed as sepai-ate buds from the peritoneum, or are differen-
tiated from a blastema which is directly derived from the peritoneum.
Janosik was able to confirm Kenson's discovery of rudimentary pro-
nephric tubules in the Rabbit, but was unable to find in this form any
trace of external glomeruli. Later, however, he ('87, p. 582) described in
a young human embryo, 3 mm. in length, a peculiar projection into the

MUSEUM OF COMPARATIVE ZOOLOGY. 281

body cavity. The structure resembled the external glomerulus of Birds,
and he was inclined to interpret it as such in this case.

In the preceding pages T have endeavored to present a comprehensive
resume of the development of the pronephros as described in groups of
Vertebrates other than Amphibia. In this review it has been shown
that an equivalent of the Amphibian pronephros has been claimed to
exist in all Craniota; and that a mode of development similar to that
described in the early part of the present paper has been found in
Selachii by Ruckert ('iS8) and van Wijhe ('89), in Petromyzon by
Knpfler ('88), and in Lepidosteus by Beard ('89). The Reptilian
pronephros as described by Hoffmann ('89), and that of the Chick
according to the account of Felix ('90), do not seem to me to be in per-
fect accord with this mode of development.

It now remains for me to compare the results of my studies, as detailed
in the descriptive part of the present paper, with those which have been
recorded by other writers on the development of the Amphibian proneph-
ros. According to the account which at present receives most general
acceptance, the pronephros first appears as an outfolding of the somato-
pleure in the form of a longitudinal groove. The anterior end of this
groove is destined to become the pronephros ; the remaining portion is
constricted off to form the segmental duct. Since the process of con-
striction advances from before backwards, stages may be found in which
a completed tube is continuous posteriorly with a mere groove of the
somatopleure. In the anterior region, the groove remains in communica-
tion with the body cavity, and grows down towards the ventral surface
of the embryo in the form of a broad pocket. The slit-like peritoneal
opening of this pouch closes throughout the greater part of its length,
leaving, however, two or three regions of incomplete closure, the funda-
ments of the nephrostomes. The nephrostomal tubules are formed by the
fusion of the walls of the pouch between two nephrostomes. The regions of
fusion extend in vertical lines from the nephrostomal margin of the pouch
nearly to its ventral border, where there is left an unfused and therefore
continuous longitudinal tract constituting the canal which I have called
the collecting trunk. This view of the development of the pronephros,
although suggested by Willi. Midler (75), was first described in detail
by Goette ('75) for Bombinator, and was later extended to other Am-
phibia by the researches of Furbringer ('77). It has been entirely con-
firmed by Wichmann ('84), by Hoffmann ('86), and still more recently
by Marshall and Bles ('90 a ).

282 BULLETIN OF THE

In opposition to this view, I would maintain : (1) that the first trace
of the excretory system consists of a solid proliferation of somatopleure,
the pronephric thickening ; (2) that the lumen of the system arises
secondarily ; and (3) that the pronephric tubules do not appear in con-
sequence of the local fusion of the walls of a widely open pouch, but
that they are differentiated at an early stage from the hitherto indifferent
pronephric thickening. 1

The development of the pronephros and duct from a solid mass of
mesoderm was a common feature in the accounts of those who wrote
prior to Willi. Midler and Goette, but since then this mode of origin,
though repeatedly maintained by single observers, has failed to gain
general acceptance. Clarke ('81) described a solid pronephric thickening,
and asserted that the lumen arose secondarily in this mass; the details
of the process are, however, not accurately given. Duval ('82) also
described the pronephros as first appearing in the form of a solid thick-
ening. He however states that it later acquires a slitdike opening into
the body cavity, and that by the imperfect closure of this opening the
successive nephrostomies are formed, as described by Goette and Fiir-
bringer. This latter statement I am unable to confirm. Gasser's ('82,
pp. 89-97) short note gives, on the other hand, an account of the early
development in Alytes, which is in substantial agreement with my own
observations. His account of the first differentiation of the nephrostomal
canals is not very full, but it is not improbable that he conceived it to
take place in a manner altogether similar to that which 1 have described.
His statements seem to me in general correct, 2 but incomplete.

Janosik ('85, p. 19) states, on the basis of personal observations, that
the first trace of the segmental duct in Bufo and Triton is a solid mass
of cells, which he is, however, incliued to regard as a disguised fold of
somatopleure.

According to a recent account by Kellogg ('90), a lumen does not
appear anywhere in the organ (except in the region of the nephrostomes)
until it has been separated from the peritoneum. Finally, Mollier ('90,
Ruckert and Mollier, '89) has published an account of the early develop-
ment, which is for the most part in close accord with the results of my
own studies. Since these results were gained entirely independently of

1 The large cavity which the pronephric pouch presents in Stage IV. of Rana
and Bufo is a secondary condition produced by the expansion of the lumens of the
several diverticula.

- 1 must here except his statement that the second tubule is differentiated before
the rest; this I believe to be an error.

MUSEUM OF COMPARATIVE ZOOLOGY. 2S3

Mollier's researches and were written out before his paper came into my
hands, it seems to me that my confirmation of his position affords ex-
cellent evidence of the correctness of the view advocated. In one feature
alone our accounts of the earliest condition of the pronephros would seem
to differ widely, but I am confident that the difference is apparent rather
than real. Mollier states that each of the diverticula which form the
first indications of the nephrostomal canals emerges from a protovertehral
cavity. This statement, as I have already shown, does not in my opinion
accurately represent the actual conditions. In the stage under consid-
eration, the dorsal portion of the mesoderm is in the anterior region
divided by transverse planes into a series of metameric blocks ; the pro-
nephric thickening also is made up of metameric constituents, and is
continuous dorsally in Amblystoma with two, in Rana and Bufo with
three, of the blocks of mesoderm. As yet no definite line can be drawn
between the protovertebree and the lateral plates ; in a slightly older
embryo, however, the protovertebra? begin to be constricted off from the
lateral plates, and it is at once evident that the pronephric tubules have
to do with the ventral segment of the mesoderm. This difference in our
accounts seems to me then very trivial, and my only excuse for dwelling
upon it is the circumstance that Rlickert and Mollier seem to attach great
morphological significance to this feature of their-- account. This relation
to the protovertebra; seems to me quite untenable.

Previous authors have been singularly reticent respecting the exact
position of the pronephros with reference to the body somites. Fiir-
bringer ('78 a , p. 5) states that the pronephros of Anura extends over
three, that of Urodela over two somites ; but I have looked in vain for a
statement which should show whether the nephrostomes are segmental or
intersegmental in position. Kellogg states that each nephrostome occurs
opposite the middle of a proto vertebrae. Marshall and Bles confirm this
statement, and contend that, in the case of liana, the nephrostomes lie
in the 2d, 3d, and 4th somites behind the auditory vesicle. According
to Mollier ('90, p. 213) the pronephros appears in Triton in the region
of the 1st and 2d trunk protovertebra? ; but since the most anterior two
protovertebra; are reckoned to the posterior region of the head, these
represent the 3d and 4th protovertebroe of the series. The enumeration
which I have given for Rana and Bufo is in precise agreement with that
of Marshall and Bles. For Amblystoma my account is in agreement
with that of Mollier for Triton.

I am not aware that any definite attempt has thus far been made to
ascertain which of the three nephrostomes of Anura is unrepresented in

284 BULLETIN OF THE

Urodela. At first sight it would seem probable, — and by implication I
accredit this opinion to Mollier, — that the rudimentary third tubule
occasionally present in Urodeles corresponds to the normal third tubule
of Anura. This view, however, is not in precise harmony with the rela-
tions of the nephrostomes to the myotomes. As I have already shown,
the first nephrostome in Amblystoma is situated beneath myotome III.,
whereas in liana and Bulb it occurs under myotome II. If now the
enumeration of the somites in the two cases correspond, it follows that
the first and second nephrostomal tubules of Amblystoma are equivalent
to the second ami third tubules respectively of Rana and Bufo, not to
their first and second tubules, and that the occasional rudimentary third
tubule of Urodeles belongs to a more posterior somite, and is unrepre-
sented in Anura. In Amblystoma the root of the vagus nerve arises
immediately in front of the somite which I have denominated I. j 1 the
same is true in the case of Rana and Bufo, and I am inclined to regard
these as equivalent somites. It is possible that somite II. of Ambly-
stoma is not represented in liana and Bulb ; but this is hardly probable,
since it belongs to the head region, which is hardly likely to vary in such
closely related groups, and since it is evident that the greater number of
protovertebne present in Urodeles as compared with Anura is largely
accounted for by additional protovertebra in posterior regions, particularly
in the region of the mesonephros, as I believe. In general, it seems
to me that we should be more ready to admit the aboition of the most
anterior tubule in Urodela than to assert the existence of an additional
protovertebra in Anura.

All the more recent writers are agreed that in Anura three pairs of
pronephric nephrostomes occur, Giles ('88, p. 135) alone claiming that a
degenerating pronephros may have four. In Urodela the typical num-
ber is two; but .Mollier ("DO, p. 224) has recorded the occasional occur-
rence of three pairs in Triton, and I have made similar observations in
Amblystoma. Spengel ('7*3, p. 19, Taf. II. Fig. 21) maintained, on the
evidence of a specimen in which the pronephros was largely degenerated,
that four pairs occur in Ccecilia ; the recent observations of Semon ('90,
p. 4G2), on the other hand, have shown that there exist in Ichthyophis
on each side of the body ten pairs of nephrostomes, therefore forty in all.

1 Somite I. of this enumeration probably corresponds to the one which has
been called somite XI by Honssay f'91). Houssay believes that he can identify
in Amphibia the somites which have been observed in the head region of Selachii.
If his conclusions are accurate, they are evidence in favor of the view that this
region of the body is very permanent.

MUSEUM OF COMPARATIVE ZOOLOGY. 285

Of the two nephrostomies belonging to any pair, one opens freelv into the
body cavity, the other communicates with a pronephric chamber, which
contains the glomus and is completely shut off from the body cavity. The
meaning of this condition I shall consider in the subsequent discussion.
The pairs of nephrostomes on each side are slightly more numerous than
the overlying protovertebne.

The origin of the so-called ventral part (common trunk) of the pro-
nephros has recently become the subject of controversy. According to
Goette the duct at first communicates with the posterior end of the widely
open pronephric pouch. At the same time that the nephrostomal canals
are formed by local fusions of the walls of the pouch, a similar process
constricts off the posterior ventral portion of the pouch ; this has the
effect of lengthening the duct, so that the point of its attachment is
carried forward to the place where the converging nephrostomal tubes
unite. The portion of the longitudinal canal in front of the most poste-
rior nephrostome represents the " ventral part " of the pronephros.

According to Fiirbringer, the longitudinal groove which forms the
earliest fundament of the pronephros and duct becomes entirely con-
stricted off from the somatopleure as far forward as the opening which
leads into the pronephric pouch ; this slit-like opening then elongates
posteriorly, so as to extend into the region formerly occupied by the
longitudinal canal alone ; the latter thus comes to lie ventral to the last
nephrostomal canal, and forms the ventral part of the pronephros.

Kellogg ('90) opposes the accounts of previous observers, and claims
that the ventral part " is formed from the ventral side of the dorsal part
of the pronephros, and anterior to the last nephrostome." Marshall and
Bles, alluding to Kellogg's description, declare that it is in exact ac-
cordance with the accounts of Goette and Fiirbringer. I have not been
able to satisfy myself as to the precipe manner by which Kellogg con-
ceives the formation of the ventral part to have taken place ; but I think
he has said enough to contrast his position strongly with that of Fiir-
bringer, according to whom the ventral part of the pronephros first ap-
pears as a portion of the somatopleural fold immediately posterior to the
part which gives rise to the nephrostomal canals. Kellogg argues, how-
ever, that, were the views of previous authors correct, some portion of
the pronephros would appear behind the last nephrostome ; but this is
actually never the case. The force of this argument I am wholly unable
to appreciate, and I must in consequence feel some doubt as to whether
I have properly interpreted Kellogg's previous statements.

According to Mollier, the " ventral part " is differentiated in the

286 BULLETIN OF THE

ventral portion of the broad pronephric thickening. Mollier's descrip-
tion is substantially in accord with my own observations, and it seems
to me probable that Kellogg's statements are to be understood in the
same way.

The structure of the functional pronephros was early the occasion of
much controversy. The discoverer of the organ, Joh. Muller ('29 and
'30), describes and figures it as a cluster of blind tubules, which radiate
in the form of a rosette from the anterior tip of the segmental duct.
This view was shared by the larger number of the early investigators.
According to von Wittich ('52), the gland is typically formed by the
convolutions of a single tube ; in the more complicated pronephridia,
however, this canal may give off branches. It is to Goette and Furbringer
that we owe the first accurate account of the process of convolution.

According to these authors, the gland is composed of two portions : a
" dorsal part " (collecting trunk and nephrostomal canals), which alone
receives the nephrostomes, 1 and a "ventral part" (common trunk), which
serves as the efferent canal, and is in communication with the anterior
end of the segmental duct. Both ventral and dorsal parts undergo ex-
tensive convolutions, and give rise to blind diverticula. Subsequent
authors have in general confirmed Fiirbringer's account, but have added
no new matter to the description. Selenka ('82) describes and figures
an interesting condition of the pronephros in Hylodes. The glands of
the two sides are unsymmetrical, and depart widely from the typical
structure known in Amphibia. Following the nomenclature which I
have proposed in the descriptive part of this paper, it is evident that the
nephrostomal canals and the collecting trunk are present, but do not
show the convolutions customary in these parts. The "ventral part" of
the gland, however, is not formed by the windings of the common trunk,
but is composed of great irregular blind pouches which communicate
with the collecting trunk, while the latter opens directly into the an-
terior end of the segmental duct. This condition of the pronephros evi-
dently represents the degeneration of the gland, and Selenka is inclined
to correlate the premature appearance of this complication in Hylodes
with the absence of gills in the larvae of this form.

Kellogg has studied the structure of the pronephros in Amblystoma
and Rana by means of reconstruction from cross sections. His pre-

1 Duval ('82, Fig. 7), figures the second pronephric nephrostome in Rana as open-
ing directly into the ventral part of the gland. I have never seen such a condition
in my preparations, nor do I know of similar observations being elsewhere recorded.
It seems likely that Duval has here fallen into error.

MUSEUM OF COMPARATIVE ZOOLOGY. 287

liminary notice, however, does not describe the process of convolution in
detail. An interesting feature is the statement that blind diverticula do
not appear until the tubes of the gland have become very much convo-
luted. In the pronephridia which I have studied, I have never seen a
blind diverticulum. My observations do not extend to sufficiently old
stages to allow me to deny that such diverticula appear anywhere in the
developmental history of the gland, but the organ can reach at least the
high degree of complexity shown in Figures 41 and 65, and yet be com-
posed of the windings of the nephrostomal canals, the collecting trunk,
and the common trunk without possessing any blind diverticula.

It is needless for me to discuss in this place the histology of the tubu-
lar portion of the pronephros. These details have little general interest,
and they have furthermore been accurately given by Fiirbringer and
Hoffmann ('86).

The dilated chamber which I have described (page 240) was also ob-
served by Hoffmann, but he was unable to determine what portion of the
system was concerned in its formation. Similar dilated chambers are
likewise described by Marshall and Bles, who regard them as steps in
the degeneration of the tubules. The early appearance of these dilated
regions in Rana (see page 232) seems to me to render this interpretation
improbable.

According to the usual account, the capsule arises as a differentiation
of the connective-tissue stroma, which lies between the pronephros and
the ectoderm. Duval ('82, pp. 25, 27) alone has claimed an origin from
the overlying protovertebrse ; but, singularly, his statement has been
wholly neglected by subsequent writers. His observations on this point
agree in all essential features with my own.

The glomus was discovered by Job. Muller ('30, p. 12), but the sig-
nificance of the structure was wholly problematical until Bidder ('46, p.
58) suggested its glomerular nature, which has since received general
acceptance. This view has, however, been opposed by Semper ('75, pp.
439 et seq.), and more recently by Hoffmann ('86, pp. 572, 573). Accord-
ing to Goette and Fiirbringer, the glomus arises as an outfolding of the
splanchnopleure opposite the pronephric nephrostomes. The interior of
the fold becomes occupied by mesenchymatic cells and with blood tracts,
which communicate with the aorta. According to Hoffmann, the inte-
rior is largely occupied by " columns " of large cells, which would seem
foreign to the nature of a glomerular structure. These ' columns of cells,"
he says, may be seen to arise, in Bufo at least, by the invagination of the
superficial covering of the glomus. I have myself seen continuous cylin-

288 BULLETIN OF THE

drical cords of cells in the glomus ; but in most cases I have been readily
able to satisfy myself that this appearance had to do with densely packed
blood cells lying in a definite vascular tract. I have also occasionally
met with invaginations of the superficial (peritoneal) epithelium of the
glomus (page 247) ; but it seems to me, even should it be shown that
they give rise in the interior to columns of cells, that this would not be
a very serious objection to the view which ascribes to the organ a glo-
merular function. In favor of that view, many arguments may be ad-
duced : (1) the highly vascular nature of the glomus; (2) its position
in an open chamber of the body cavity directly opposite the proncphric
nephrostomes ; (3) its serial relations with the mesonephric glomeruli ;
(4) its appearance and degeneration synchronously with the pronephros ;
and (5) the circumstance that its homologue, wherever found in other
classes of Vertebrates, is always in equally close relation with excretory
tubules. The last argument seems to me the most weighty, and I am of
opinion that a comprehensive comparative study proves beyond question
the glomerular nature of the structure.

In the descriptive part of this paper I have stated that, in satisfactory
sections through the blood-vessel which leads from the aorta to the glomus,
one could frequently observe that the ramifications within the glomus
did not appear to be terminal, but that the vessel seemed to give off
a lateral branch to the glomus, while the main trunk continued on to-
ward the ventral side of the body. An explanation of this condition has
occurred to me, which, if confirmed, will be of considei'able morphological
significance, though at present I can merely offer it as a suggestion. As
we have already seen, the glomus of Selachii, according to Riickert ('88,
pp. 239-242), does not receive a separate blood-vessel directly from the
aorta, but a rete mirabile is developed in connection with the segmental
vessels described by Paul Mayer. I have not succeeded in tracing
the main aortic branch to the ventral side of the larva ; but, as far as it
could be followed, the course of the vessel between splanchnopleure and
entoderm corresponds perfectly with that of one of the segmental ves-
sels described by him. It seems to me quite possible that, in Amphibia,
the dorsal portion, which is in communication with the glomus, is the
only part of these rudimentary vessels which is retained, and that the
remaining portion, having ceased to be of functional importance, fails to
develop.

Having completed my survey of our knowledge of the development of
the pronephros in the several classes of Vertebrates, I now turn to a

MUSEUM OF COMPARATIVE ZOOLOGY. 289

consideration of the development of the segmental duct. As is well
known, observers up to a very recent date have been almost unanimous
in ascribing a mesodermal origin to this structure. In regard to the
details of the process, however, they have been less at one ; and I shall
accordingly treat of their accounts under three heads, which seem to me
to represent fairly well marked phases of opinion.

According to one view, the duct arises as an evagination of somatopleure,
its lumen being therefore a detached portion of the bod// cavity. Such a
mode of origin was advocated by Rosenberg ('07, pp. 42 et seq.) for
Teleosts ; and this feature of his account has gained almost universal
acceptance both for Teleosts and for Amphibia, having been recently
entirely confirmed by Hoffmann ('8G) and Henneguy ('88, '80). Ac-
cording to Wilh. Miiller (T5) and Furbringer, the duct arises in this way
also in Petromyzon, and a similar claim has been made for Ganoids by
Kowalewsky, Owsjaunikow, and Wagner (TO), and by Balfour and Parker
('82). In Selachians, however, the weight of the evidence is distinctly
opposed to this view, and I am not aware of its having been advocated
by any one besides Schultz (To).

In Amniotes also such an account of the early development has not
received general acceptance ; it was first claimed in this class by Romiti
(T4), and was adopted, with some modification it is true, by R, Kowa-
lewsky (To), and by Dansky und Kostenitsch ('80, p. 24). Very re-
cently such a mode of origin has been reasserted by Fleischmann ('87)
for Carnivores and the Duck.

My own observations on Amphibia indicate that in this group the
duct does not arise as a fold ; and I am of opinion that, in both Cyclo-
stomes and Ganoids, the evidence that the duct arises by evagination is
at present unsatisfactory. It seems to me probable, on the contrary,
that the method of origin which is usually recognized as characteristic of
all the Anamnia with the exception of Selachii exists, if at all, only in
Teleosts. In view of the peculiar obstacles which Teleostean material
presents for embryological study, one should be cautious in affirming for
this group a mode of development which, in my opinion, is not proved to
exist in any other class of Vertebrates.

A second view of the origin of the duct is, that it arises from a solid
proliferation of somatopleure. According to Furbringer (T8 a ), Spoof ('83,
p. 84), and the earlier writers (Remak, '55, Kolliker, '61, Bornhaupt,
'67, Waldeyer, TO, and Foster and Balfour, T4), the duct arises in the
chick by a proliferation in situ of the subjacent mesoderm, and a similar
origin is maintained for Petromyzon by Scott ('82). The more recent

VOL. XXI. — NO. 3. 19

290 BULLETIN OF THE

view, however, affirms that the posterior end of the duct grows back-
ward free from adjacent tissue, the cellular material being wholly de-
rived from an anterior proliferation. For Selachii this method of origin
has been maintained by Balfour ('78), and for Amniotes by a large num-
ber of observers ; e. g. Weldon ('83) and Mihalkovics ('85) in Reptiles ;
Gasser ('77), Sedgwick ('81), Schmiegelow('81 and '82), and Janosik ('85),
in Birds ; Benson ('83) and Martin ('88), in Mammals. Gasser ('82)
believes that the segmental duct in Alytes has no direct connection with
the mesoderm, posterior to the pronephros; but he was unable to ex-
clude with certainty the possibility that the somatopleure immediately
behind the pronephros might take some part in the formation of the
duct. Mollier ('90, p. 22G) moreover asserts that such a participation
actually takes place in the two somites following those in which the pro-
nephros is formed, but that the posterior portion of the duct probably
grows back from this point independently of the mesoderm.

In so far as these authors maintain that the duct arises from a solid
proliferation of mesoderm and acquires its lumen secondarily, I entirely
agree with them ; but my observations on this point lead me to conclude
further that the duct arises throughout its entire length from a continu-
ous thickening of somatopleure, and that the only free growth which
occurs in the Amphibia studied by me is for the purpose of effecting a
union with the cloaca. In assuming this position, I am aware of being
in conflict with prior observations on Amphibia, and with the more recent
accounts of the development in other groups ; it seems to me, however,
that satisfactory evidence in favor of this mode of origin has been ad-
duced in the descriptive part of this paper.

Finally it remains for me to consider the third view, that of the ecto-
dermal origin of the duct, which is to-day advocated on so many sides.
As early as 1855 Bemak expressed himself as dissatisfied with the deriva-
tion of the excretory system from the mesoderm, although this mode of
origin was confirmed by his own observations. A decennium later His
C65 h , pp. 160-1G2) maintained that, in the Chick, the Wolffian and
Miilleriau ducts both arise as folds of the ectoderm ; but he abandoned
this position later ('08, p. 119), when it had been shown by Boruhaupt
('67) and Dursy ('G7) to be untenable. He then endeavored to interpret
the facts in harmony with his theoretical conceptions by maintaining that
the cells from which the Wolffian and Mullerian ducts arose were pri-
marily derived from the ectoderm, a view which was likewise adopted by
Waldeyer ('70). Meantime Hensen ('66) had indorsed the view of a
direct origin from the ectoderm. He states ('66, p. 81, foot-note) that

MUSEUM OF COMPARATIVE ZOOLOGY. 291

in the Rabbit the Wolffian duct arises from a solid rod-like thickening of
the ectoderm near the middle protovertebne. In a second short commu-
nication ('67, p. 502), Hensen merely reaffirmed his confirmation of His;
but finally he (75-76, pp. 369-372) published a fuller account of his
observations, accompanied with figures. These, however, are far from
conclusive, and it does not seem surprising that this single observation
was distrusted by subsequent writers.

In 1884 Graf Spee published an account of his very careful investiga-
tion of the subject, and reasserted the ectodermal origin of the Wolffian
duct. 1 Following this publication have appeared a large number of con-
firmatory papers, which have moreover extended the observations of
Graf Spee; so that at present the ectodermal origin of the duct has
been asserted for every class of Vertebrates, with the single exception of
the little known Dipnoi.

As stated in the Introduction to the present paper, it was my hope
in undertaking these studies to find in Amphibia results confirmatory of
Graf Spee's position. If, then, a contrary result has been reached, it
has been because I have been driven to that conclusion by evidence
brought out in the course of the investigation. In my opinion, the
entire excretory system of the forms I have studied unquestionably
develops without any participation of the ectoderm in its formation.
The duct develops from mesoderm throughout its entire length, and
at its posterior end, in Rana and Bufo at least, comes in contact with
one of the entodermal cornua of the hind gut ; so that nowhere in its
development does it come into organic union with the outer germ
layer.

I must in this case distinctly disavow the suggestion of Hertwig ('88,
p. 280), who endeavors to harmonize the accounts by assuming that
only the posterior end of the duct is formed from the ectoderm. This
explanation would by no means be admissible, unless it be granted that
the ectodermal constituent might in this case be reduced to nothing
at all. On the other hand, it must be confessed that a fundamental
opposition in the mode of development of an organ in two closely related
groups is at present hardly reconcilable with our general conceptions
of embryological processes.

1 Graf Spee, and subsequently Flamming ('86), did not clearly recognize the fact
that the Wolffian duet and the mesonophros develop in different ways, and were
led to defend an ectodermal origin for the excretory system. This interpretation is in
evident opposition to the accounts of others, and, in my opinion, is not justified by
their own observation, even should these prove to be accurate in every particular.

292 BULLETIN OF THE

It will therefore be of interest to review critically the most recent
accounts in the several groups, for the purpose of ascertaining whether
the ectodermal origin of the segmental duct be in any case actually
demonstrated. For this purpose, only those papers which have ap-
pealed since Graf Spee's researches need concern us. Of these, the
larger number are brief notices, which, in view of the extreme difficulty
of the investigation, cannot be regarded as conclusive.

In regard to Gyclostomes, the only papers that have appeared during
this period have been preliminary notices ; that of Kuplfer ('88) main-
tains an ectodermal, those of Goette ('88) and (Jwsjannikow ('89) a
mesodermal, origin for the duct.

In Teleosts, the duct has been claimed to be ectodermal by Brook
('87) and Ryder ('87) ; but on the basis of my own observations, which
are as yet incomplete, I am led to doubt the correctness of this claim,
which has already been opposed by the observations of Henneguy ('88),
of H. V. Wilson ('90), and of Mcintosh and Prince ('88). In the
account by Brook, it seems to me probable that the ectodermal thick-
ening observed has in reality a very different significance (lateral line
proliferation) from that attributed to it, an opinion which is shared by
Wilson ('90, p. 58). The only recent paper dealing with the develop-
ment of the Ganoidean excretory system is the preliminary notice of Beard
('89) on Lepidosteus. According to Beard, the duct is ectodermal.

In Amphibia, also, an ectodermal origin of the segmental duct has
been asserted by Perenyi ('87) and by Brook ('87). Their communi-
cations, however, are both short notices, and in the absence of the final
papers cannot be regarded as satisfactory evidence. Moreover, the meso-
dermal origin of the duct has been reaffirmed by Mollier ('90), Kellogg
('90), and Marshall and Bles ('90).

It is rather remarkable, that, in all the preceding classes, nothing but
preliminary notices have ever appealed in favor of the ectodermal view.
The same is true of Birds, where this mode of origin has been claimed
as probable by Beard ('87) and by Brook ('87). On the other hand,
a number of observers have carefully investigated the chick with this
special purpose in view, aud have been unable to find any evidence of a
participation of the ectoderm in the formation of the Wolffian duct.
Among these may be mentioned JanoSik ('85), Mihalkovics ('85), and
Hoffmann ('89). Peculiarly significant, however, is the fact that Graf
Spec (!<('>) was unable with the use of the most various reagents to see
any direct evidence of a genetic connection between the ectoderm and
the Wolffian duct in the Chick.

MUSEUM OF COMPARATIVE ZOOLOGY. 293

In Reptiles, a number of writers have asserted that the Wolffian duct
arises from the ectoderm. According to Perenyi ('87, '88, '89), irregu-
lar groups of cells are at an early stage budded off from the ectoderm
covering the middle plate, and on the first formation of the segmental
vesicles they form the cord of cells which has been recognized by prior
writers as the fundament of the duct. In my opinion, no conclusive
evidence is adduced to prove that the cells figured in the latter position
('89, Fig. 5, ceW.) are descendants of those which at an early stage
form part of the ectodermal thickening. Mitsukuri ('88) and Orr ('87)
have published short notes claiming an ectodermal origin for the duct ;
and, finally, Ost roundoff ('88 a , '88 b ) asserts that it is derived from the
ectoderm in Phrynocephalus. It seems to me, however, that Ostroumoff's
observations are incomplete at a critical point, and that no satisfac-
tory evidence is brought forward to show that the ectodermal thick-
enings which he describes and figures ('88 b , Tab. III. Fig. 5G) with all
desirable clearness, are unquestionably the fundament of the Wolffian duct.
They may be merely chance thickenings over the intersegmental depres-
sions in the underlying mesoderm. On the other hand, Mihalkovics
('85), Strahl ('8G), and Hoffmann ('89) have all sought in vain to find
satisfactory evidence of a participation of the ectoderm in the formation
of the Wolffian duct.

With all the preceding classes of Vertebrates, I am of opinion that
the weight of evidence is at present in favor of the view that the excre-
tory system is wholly derived from the mesoderm. For the remaining
groups, Mammals and Selachians, however, no such claim can be sus-
tained. The researches of Graf Spee on Cavia showed conclusively
that a cord of cells representing the fundament of the Wolffian duct is
continuous posteriorly with a ridge of tissue which is still in intimate
union with the superficial ectoderm, and that, in the further develop-
ment, a continuous cord of cells separates off from this ridge by the pro-
gressive formation of a split between the deep portion of the ridge and
the superficial ectoderm. At first, a distinct membrana prima is reflected
from the unmodified ectoderm over the ridge, and the partially separated
fundament of the duct may still be in connection with the superficial
ectoderm by means of such a membrane. Tins latter feature is also
dwelt upon bv Flemming ('86), who furthermore emphasizes the cir-
cumstance that in the ridge which forms the first rudiment of the
Wolffian duct mitoses are especially abundant, and that the nuclear
spindles are frequently perpendicular to the surface, i. e. are so situated
that the ensuing cell divisions would tend to thicken the layer. The

294 BULLETIN OF THE

general results of these two investigators have been confirmed by Bonnet
('87 and '88) in the Dog and Sheep, and a number of former advocates
of a mesodermal origin have satisfied themselves of the correctness of the
opposed view by a study of the preparations of these authors ; e. g. His
(see Spee, '84, p. 93),Waldeyer (see Janosik, '85, p. 13), and Mihalkovics
(see van Wijhe, '89, p. 501).

The most recent paper on this subject is that of H. Meyer ('90), who
claims an ectodermal origin for the Wolffian duct in man. The embryo
upon which these observations were made was obtained by artificial
abortion, and was at once preserved by histological methods ; so that,
in the opinion of the author, it would be unfair to ascribe his results to
imperfect preservation, which so frequently renders observations on
human material untrustworthy. On the other hand, the mode in which
t'he duct is here claimed to originate, viz. as a conspicuous fold of ecto-
derm, is so different from the method of origin described in oilier Mam-
mals that one cannot regard this observation based on a single specimen
as conclusive evidence. 1

A few recent writers have reasserted the mesodermal origin of the
Wolffian duct even in the case of Mammals. Lockwood ('87, p. 642) crit-
icises the evidence adduced by Graf Spee and Flemming, and compares
their ectodermal ridge to a number of insignificant ectodermal thicken-
ings which may be observed over depressions in the underlying tissue in
diverse regions of the body. Lockwood entirely ignores the very defi-
nite relations which Graf Spee showed to exist at a certain stage between
the fundament of the duct present in anterior regions and the continuous
posterior ridge ; his entire criticism therefore seems to me quite unwar-
ranted. Fleischmann ('87) also reasserts in a preliminary note the meso-
dermal origin of the duct in Carnivora ; but his description of the mode
of origin is so entirely at variance with the accounts of recent authorities
that his statements can hardly be regarded satisfactory before the evidence
on which they are based is produced.

On the other hand, Martin (Stahl und Martin, '86, Martin, '88) accepts
tie main features of the development as described by Graf Spee and

1 During the correction of these proof-sheets another paper has appeared which
asserts a participation of the ectoderm in the formation of the duct in Man (Kollmann,
'91). In the region of the middle plate there is found, according to this author, a
close fold of ectoderm (Taf. III. Figs. 3, 4, Anlmje d. Urniere, Fig. 8 a ) which he
believes to he concerned in the formation of the duct, thus confirming Meyer's ('90)
account. The later stages studied by Kollmann, however, are too far advanced
to afford convincing evidence that his interpretation of the fate of this fold is
accurate.

MUSEUM OF COMPARATIVE ZOOLOGY. 295

Flemming, but interprets their observations in a fundamentally different
way. In the course of a painstaking investigation, in which more than
forty series of sections were used, Martin never encountered conditions
which in his opinion demonstrated a genetic connection of the duct with
the ectoderm. He believes that the duct arises from a proliferation of
mesoderm in the region between the 9th and 11th protovertebrse, and
grows backward by cell division within its own mass. The posterior
portion of the duct, however, fuses with the ectoderm so intimately that
in certain regions it is quite impossible to recognize a boundary between
them ; but Martin believes that the fusion is wholly secondary, and that
the ectoderm contributes no material to the duct. Keibcd's ('88 a , p. G35)
studies on Erinaceus led him at first to accept Martin's attempt at har-
monizing the two views ; but in Cavia ('88 b , pp. 424-428) his observa-
tions inclined him towards the original view of Graf Spee.

In my opinion, Martin is right in denying that an ectodermal origin of
the Wolffian duct has been demonstrated in Mammals. It is undoubtedly
true, that there is considerable evidence in favor of such a mode of origin ;
but it is not of a nature that would warrant one in concluding that the
duct arises in this way throughout all Vertebrates, or in asserting that
it develops in fundamentally different ways in Mammals on the one hand,
and in other Vertebrates on the other. AH that can be claimed, how-
ever, in accordance with Martin's view, is that it is possible to interpret
the conditions in Mammals in agreement with observations in other Ver-
tebrates, should these be shown to be less ambiguous.

In Selachians, the evidence in favor of an ectodermal origin of the
duct is perhaps even stronger than in Mammals. In the former group,
besides the preliminary communications of van "Wijhe ('86, '88*) and
Beard ('87), there have appeared two extensive papers by Ruckert ('88)
and van Wijhe ('89), which seem to place the ectodermal origin of the
segmental duct almost beyond question ; and, so far as I am aware, no
recent observer has expressed doubts upon this point. It nevertheless
seems to me that, before accepting this result as final, we have yet to
inquire whether Martin's interpretation of the condition in Mammals
cannot be applied also in Selachii.

It might be objected, that the latter view offers no explanation for the
intimate fusion which must be granted to exist between the posterior end
of the segmental duct and the ectoderm ; yet this argument cannot in-
validate the general conclusion, since a number of cases of such a union
of two epithelial structures in their growth have been recorded, where
no genetic connection is believed to exist. Such a conception is in-

296 BULLETIN OF THE

volved, for example, in the account — contested, it is true — given by
Carius ('88) of the anterior growth of the chorda in the " Kopffortsatz "
of Cavia, the chorda being in intimate fusion with the underlying ento-
derm ; or, again, the backward growth of the Amniotic Miillerian duct in
close connection both with the Wolffian duct and with the adjaceut peri-
toneum, as described by a number of recent writers.

In conclusion, then, I am of opinion that the more primitive condition,
and that shown by most Vertebrates, is the development of the segmen-
tal duct independent of connection with the ectoderm ; but that in cer-
tain groups the duct enters into a secondary union with the ectoderm.
The question whether the ectoderm here contributes material to the
fundament of the duct can at present receive no more deiinite answer
than that contained in the foregoing discussion.

It has frequently been asserted that the mesodermal origin of the kid-
neys is not in harmony with our conceptions of the derivatives of this
germ layer. As early as 1855 Remak saw a fundamental opposition in
the mode of development which he described for the excretory organs,
and that familiar in the case of other glands. According to his view,
which received very general acceptance, the kidney is a unique example
of a gland whose secreting surfaces are not derived from one or other of
the bounding germ layers, ectoderm and entoderm.

In my opinion, this view must now be considered inaccurate. It is
doubtless true that glands are usually developed either from the ecto-
derm or from the entoderm ; this circumstance may merely be due to
their apparently being seldom needed on mesodermal surfaces. Certain
special regions, however, seem to require glands. Such regions are the
sexual conduits in which, besides those glands which have special func-
tions, such as the deposition of the secondary egg membranes 1 (Ludwig),
we should expect to find glands similar to those which are found in the
course of other canals leading to the exterior, such, e. g., as the trachea. I
shall disregard the glands which develop in the ampullae of the vasa defer-
entia, since these are derived from the Wolffian duct and consequently map
be of ectodermal origin in Mammals, and shall take as a specific example
the genital tract of the human female. It seems very certain that in Am-
niotes the Miillerian duct develops entirely independently of the Wolffian
duct, as an evagination of the peritoneal covering of the Wolffian bod}'.
Moreover, whether we accept the view of van Ackeren ('89), that the
hymen marks the region of fusion between the fused Miillerian ducts and

1 The albumen secretion of the Hen's oviduct is a familiar example. According
to Giacosa ('72) the oviducal secretion in Rana is largely composed of mucin.

MUSEUM OF COMPARATIVE ZOOLOGY. 297

the sinus urogenitals, or that of Xagel ('89), who claims that the vagina
is a product of the sinus urogenitalis, the boundary between the two
constituents being marked by the os externum uteri, it must in either
case be granted that the entire genital tract from the ostia abdominales
of the oviducts to the os externum is of mesodermal origin. This entire
system is lined with a continuous columnar epithelium, which is con-
tinuous below with the stratified epithelium of the portio vaginalis. In
its histological characters this membrane closely resembles a typical
mucous membrane, and is subject to the characteristic disorders of this
form of tissue, cancer and catarrh. The Fallopian tubes are believed to
be without glands ; 1 in the region of the fundus and corpus, however, are
numerous lung tubular caeca? which have been called uterine glands. It
has not been demonstrated, however, that these structures exercise a
secretory function ; and they may merely serve to regenerate the mucosa
cast in menstruation. In the cervical region occur glands (glandulaB
Nabothi, Sehleimkrypten) which are much shorter than those in the
body of the uterus. These cervical glands secrete a viscous fluid of the
characteristic ropy consistency of mucus, which at periods mingles with
the catamenial flow, 2 and, in certain stages of pregnancy, forms a com-
plete plug in the cervical canal. This secretion forms a dense mass on
addition of alcohol; it swells conspicuously when placed in water; it
stains blue with hematoxylin, and pink with picro-carmin ; and, finally,
according to Overlach ('85) its formation is attended with the same
fundamental changes in the protoplasm at the distal end of the secreting
cell which are familiar in the case of ordinary mucous secretion. 3 It
is almost certain that the cervical glands produce true mucus. Not
merely, then, does the mesoderm give rise to glands, but it produces
glands of the same nature as those found in mucous 2 :>assa 9 es °f ec t°-
dermal origin.

A second view was that formulated by His ('65 a ), according to which

1 The vagina also is stated by Veith ('89) to be normally glandless.

2 Of interest in this connection are the observations of Artemjeff ('89), who
describes mucous corpuscles as a constituent element of normal lochia.

3 Through the kindness of Dr. C. S. Minot, I have been able to try in addition
a few simple chemical tests on the cervical secretions. The cervical plug from a
uterus of three months' pregnancy examined by me, proved to be soluble in po-
tassic, sodic, and calcic hydrates, and in sodic carbonate ; it is precipitated by
nitric acid, but redissolves in excess ; in strong acetic acid, on the contrary, it
appears not to redissolve. The substance gives the proteid reaction with nitric
acid, but not that with cupric sulphate. It also gave the specific mucin stain with
methylen blue recommended by Hoyer ("'90).

298 BULLETIN OF THE

the ectoderm and entoderm alone are capable of giving rise to epithelial
tissues. This view, which was associated with the derivation of the
urogenital tract from the ectoderm, was naturally revived by Graf Spee
('84). More recent evidence, however, shows that it is only the Wolffian
duct in regard to which the question of an ectodermal origin remains
open ; the Wolffian tubules, on the other hand, as well as the epithelia
of the female sexual tract, are distinctly mesodermal. The statement
that epithelia do not arise from the mesoderm is, in my opinion, either
insignificant or untrue. If, avoiding genetic characters, we define epi-
thelium so narrowly as to exclude endothelium, we must confess that,
except in certain specialized regions, epithelia do not develop from the
mesoderm ; but the conclusion is obviously of little morphological impor-
tance. On the other hand, if we employ broad morphological characters
in our definition, such a conclusion is manifestly inaccurate.

The ectodermal origin of the Wolffian duct has been supposed to
account for certain pathological new formations which frequently have
their seat in the urogenital organs. Tims His ('G5 b ) saw in the mode of
development which he described for the Wolffian and Mullerian ducts an
explanation for the occurrence of dermoid cysts in the ovary. It must
be confessed that the structure of many of these cysts suggests that they
have an ectodermal origin ; but their occurrence in very diverse parts of
the body shows that they do not require a normal ingrowth of ectodermal
cells into the region in which they arise. Thus in the dermoid cysts
which are occasionally found back of the optic bulb, the translocation
must be regarded as purely adventitious. 1

The suggestion has recently been made by Sutton ('8G, p. 344), that
testicular and ovarian carcinomata are to be explained by the occurrence
of degenerating ducts in the neighborhood of the genital ridge, and he is
inclined to regard the Wolffian duct as the means of transporting ecto-
dermal cells to this region. The weight of evidence seems to favor the
view that carcinomata cannot develop without an epithelial basis (Klebs,
'89, p. 771) ; but. this fact does not compel us to seek an ectodermal
source for these growths. In the case of adenomata, which also require
an epithelial basis, one can see more readily the source of the prolifera-
tion ; and these abound in the ovary. The germinal epithelium, in
consequence of its retention of embryonic characters, seems to be well
adapted to the formation of carcinomata, and, according to Birch-Hirsch-
feld's ('89, p. 202) enumeration, they are somewhat more frequent in the

1 Many dermoids may be explained as eases of foetus in fcetu, and those in the
ovary may often be due to extra-uterine gestation.

MUSEUM OF COMPARATIVE ZOOLOGY. 299

ovary than in the testis, even though the latter organ is in such intimate
relations with degenerating Wolffian canals.

The remaining portion of the present paper will be concerned with
those inferences of a general nature which can be drawn from the develop-
ment of the pi-onephros and segmental duct as traced in the preceding
pages. These general conclusions naturally fall into two groups : (1 ) such
as are of principal interest in elucidating the development of the excre-
tory system, and (2) such as tend to throw light on the development of
the Vertebrate type. Following this division, then, in our discussion, I
shall consider in the present section the organogenetic conclusions ; and,
in concluding, deal with the phylogenetic conclusions which seem to me
warrranted by our present knowledge.

In the historical review of the development of the pronephros, it proved
in several groups very difficult to draw a sharp line between the pro-
nephros and the mesonephros, and it was suggested at that point in the
discussion that this difficulty is in reality a fundamental one, and one
which is indicative of the true relations between these parts. The
question of the serial homology of the pronephros and mesonephros, as
it presents itself to the modern student, is to my mind simply this :
Are we to regard these two glands as derivatives of one continuous
ancestral organ, which at one time extended over all the somites now
occupied by each? The answer to this question naturally must come, if
at all, by a comparison of the two organs for the purpose of bringing to
light their features of similarity and those of contrast. Manifestly they
differ in the time of their appearance ; indeed, from this circumstance
the two glands were distinguished and named ; it remains to consider
whether they are constructed on the same or on different types.

In endeavoring to furnish an answer to this question, I shall proceed
to an anatomical comparison of the glands, taking into consideration both
of the principal portions involved, the glomerular and the tubular parts.
The glomerulus of the mesonephros resembles the glomus of the pro-
nephros in the following particulars : both are highly vascular structures
composed of ramifying blood-vessels and mesenchyme ; they project into
spaces which are in communication with the exterior by means of excre-
tory conduits; they originate outside of this space, and gain position
within it by pushing before them in their growth-its epithelial wall, which
then persists as an outer covering to the vascular process ; they receive
branches directly from the aorta; and, finally, they are developed in re-
gions of the body which at least nearly correspond to each other serially,

oOO BULLETIN OF THE

as is shown by the relations of the glomus and the glomeruli respectively
to the aorta, and by the existence of transitional glomeruli (Birds, Croc-
odilia, Chelonia). On the other hand, the features in which the glomus
differs from the glomerulus may be summarized as follows : the glomus
lies in the body cavity, instead of projecting into the lumen of a spe-
cialized excretory tubule, and it is a continuous structure, instead of
consisting of a number of separate parts.

Turning now to the tubular portions of the two glands, one can recog-
nize a number of common characters. In both can be distinguished a
longitudinal conduit and transverse canals, the latter communicating with
the body cavity by means of ciliated nephrostomes. The longitudinal
canal of the two glands is in reality a continuous structure, the segmental
duct. Since the proncphric and the mesonephric tubules are similarly
related to this continuous duct, it is evident that they must themselves
lie in approximately equivalent regions of the bod} 7 . The metamerism
of both glands primitively corresponds to that of the body somites; this
feature is apparent from my account of the Amphibian pronephros, and
has been proved for the most anterior mesonephric tubules in Ambly-
stoma (see page 2G1), as well as for the entire series in Selachii and cer-
tain other groups. Finally, the cardinal veins give rise to a meshwork of
vascular spaces which bathe in a like manner the tubules of the proneph-
ros and mesonephros. In addition to the different Ways in which their
tubules are related to glomerular structures, the pronephros and meso-
nephros are unlike, in that the tubules of the former develop in continuity
with the duct, while those of the latter join the duct secondarily. The
character of the convolution also is different in the two glands. As is
evident from the reconstructions (Plates IV. and VIII.) of the pronephros
in Rana and Amblystoma, the complication is here mainly due to the
convolution of the longitudinal canal (common trunk) ; whereas in the
case of the mesonephros the longitudinal canal (segmental duct) traverses
the gland as an almost straight duct, the transverse tubules alone being
highly convoluted.

The pronephros and mesonephros, then, present many striking ana-
tomical features of resemblance, but also differ in several respects. I
am however of opinion, that the similarities of structure are sufficiently
great to make it probable that pronephros and mesonephros have de-
veloped from a common beginning. I do not think, however, that such
tabulation of the resemblances and differences gives an adequate insight
into the true relationships of the structures. In the search for ances-
tral characters, it is a matter of indifference whether the organ in ques-

MUSEUM OF COMPARATIVE ZOOLOGY. 301

tion actually realizes a given character, or merely shows a tendency to
assume it, provided in the latter case it can be satisfactorily shown that
the realization of the tendency was prevented by intelligible causes.
Thus, in the gastrulation of meroblastic eggs, if it be recognized that
the great accumulation of yolk renders eniboly impossible, the substi-
tution of epiboly in these cases must be regarded as morphologically
insignificant.

The question now naturally arises, Are any of the contrasts between
pronephros and mesonephros of such a nature that they can be explained
as the result of a single modifying influence ? As I have already stated,
the most marked point of disagreement between the two glands is the
difference in time at which they appear. What influences may that
factor exert in modifying their development] At the time when the
Amphibian mesonephros appears, the myotomes are widely separated
from the peritoneum, and the continuous strip of ccelom immediately
ventral to the lower boundaries of the protovertebrse in the region of the
pronephros does not exist in the region of the body in which the meso-
nephros develops. In its place is a mass of cells which extends from the
dorsal angle of the body cavity upward towards the overlying myotomes.
This mass of cells has been regarded as the first rudiment of the meso-
nephros. The most natural explanation of the condition is that this mass
of cells is morphologically not a secondary proliferation from the perito-
neum, but is really the last remnant of the mesoderm which formerly
connected the dorsal angle of the permanent body cavity with the over-
lying protovertebrse. The correctness of this interpretation is shown by
comparison with the conditions in Selachians and in Amniotes, where,
according to the mutually confirmatory accounts of Sedgwick ('80 a ), Van
Wijhe ('8S a , '89), Ruckert ('88), and Hoffmann ('89), the mesonephric
tubules develop from the communicating canal. The first rudiment of
each mesonephric tubule is in reality that portion of the primitive meso-
dermal plate which lies immediately ventral to the protovertebrse, and,
corresponds to that portion of the ccelom into which, as shown in Figure 6,
the glomus projects, and from which the pronephric tubules emerge.
Each mesonephric fundament, then, presents on its outer side somatic, on its
inner, splanchnic mesoderm. When the fundaments of the mesonephros
have been converted into a series of blind tubules, they grow outward and
join the segmental duct. This process appears to me to be precisely
equivalent to the somatopleural evagination, which at an early period gave
rise in the anterior region to the nephrostomal tubules of the pronephros.
That portion of the differentiated mesonephric tubule into which the

302 BULLETIN OF THE

glomerulus projects is of different origin ; it is merely a portion of the
coelom, the walls of which are to be understood to be formed as I have
just stated in part by somatic, in part by splanchnic mesoderm.

Returning now to the two features in which the glomus was shown
to differ from the glomeruli, — viz. situation within the body cavity, and
continuity throughout successive somites, — it will be seen that it is im-
possible to maintain the former as a ground of distinction, since the
glomerulus also lies in a detached portion of the coelom, and that the
latter ground is equally untenable because it simply results from the fact
that, before the glomeruli appear, the space iuto which they would other-
wise project as a continuous organ has already been broken up into a
series of distinct tubes ; the glomerular organ is consequently broken
up iuto a corresponding number of separate vascular processes, each of
which becomes converted into a Malpighian capsule.

It seems probable, therefore, (1) that the pronephros and mesonephros
were primitively alike, and were portions of a single continuous gland ;
(2) that in Vertebrates which came to lead an independent existence
early in life, an anterior portion of the gland and the whole of the duct
are differentiated before the posterior part for the immediate purposes of
the larva; and (3) that the difference in structure between the two
glands is mainly due to their arising at different times relatively to the
differentiation of the body cavity and protovertebrse. Applying this
conclusion to the tubular portion of the glands, it becomes at once intel-
ligible why the tubules of the mesonephros must of necessity join the
duct secondarily. From this standpoint, the existence of convolutions
in the common trunk points to a less differentiated condition of the
pronephros, in that, for temporary purposes, the longitudinal canal,
including the common trunk, subserves at the same time the functions
of an efferent duct and of a secreting tubule.

The foregoing explanation of the nature of the pronephros is based
upon the assumption that it is developed as a larval excretory organ.
In order to justify this position, it will be necessary to consider whether
the pronephros is functional in those Vertebrates which, viewed from this
standpoint, would seem to require this organ, and in such alone. For
the present purpose, two methods of sexual reproduction may be dis-
tinguished : (1) that in which the mother spends her energy in pnxhicing
a large number of offspring, which are early forced to care for themselves ;
and (2) that in which the mother produces a small number of eggs, and,
either by giving to each a large quantity of reserve food yolk, or by
nourishing the young embryo within her own body, secures the existence

MUSEUM OF COMPARATIVE ZOOLOGY. 303

of her offspring without calling into play their individual activities. In
the former class may be reckoned Cyclostomes, Teleosts, Ganoids, Dip-
noi, and Amphibia. 1 Omitting from consideration the little known
Dipnoi, a functional pronephros appears in all the members of this group
without exception, and is most highly developed in those forms (Petro-
myzon, Amphibia) which pass through a protracted larval stage. The
other class includes Selachians, Reptiles, Birds, and Mammals. In every
member of this group the pronephros is rudimentary.

/ conclude, therefore, that pronephros and mesonephros are parts of one
ancestral organ; that the glomeruli are strictly homodynamous ivith the
glomus ; that the entire tubular portion of the pronephros is represented in
the mesonephros ; that the cavity of a Malpighian capsule and the nephro-
stomal canal connecting it ivith t/ie body cavity are detached portions of the
coelom, the equivalents of which are not thus differentiated in the pronephros ;
that the pronephros is developed as a larval excretory organ ; and that the
period at which it appears largely accounts for its peculiarities of structure.
This general conclusion, which is mainly based upon a study of the con-
ditions in Amphibia, is, in my opinion, in perfect harmony with the
recorded observations on other groups.

It must be remembered in this connection, however, that the pro-
nephros may possibly have been developed from the primitive excretory
organ independently in two or more groups, in response to similar physio-
logical necessities. While I have not been able to preclude this possibility,
I am nevertheless inclined to the opinion that in general a closer relation
exists, and that consequently the pronephros is homologous throughout
all Vertebrates. An interesting condition manifests itself in those forms
(Teleosts and Ganoids) in which the pronephros remains functional until
the individual is nearly adult. In these the pronephric chamber becomes
partially (Lepidosteus) or entirely (Teleosts) cut off from the body cavity
and comes to resemble an enormous Malpighian capsule. The region in
Crocodilus intermediate between pronephros and mesonephros shows a

1 In the one group, the eggs are holoblastic, or if meroblastic contain little yolk
(Teleosts); in the other, they contain much yolk, or the young are nourished by
means of a placenta (.Mammals). Mr. Samuel Garman has kindly called my atten-
tion to a number of cases in Amphibia where the period of larval life is greatly
reduced. The occurrence of holoblastic segmentation in this group appears to
me to afford adequate evidence that such conditions are secondary. Moreover,
there actually appears to be a reduction of the pronephros in such species as
abandon in part their larval life. In the case of Hylodes martinicensis, mentioned
by Mr. Garman in this connection, Selenka ('82) has shown the pronephros to
be very degenerate.

304 BULLETIN OF THE

similar differentiation of a part of the ccelom into a distinct excretory
chamber. The condition in this region differs from that of the meso-
nephros of this genus solely in the circumstance that the excretory cham-
ber is not broken up into metameric portions ; this process takes place
in the posterior region, and produces a typical mesonephros.

It now remains for me to review the opinions of previous writers in
respect to the nature of the pronephros. The existence of a larval excre-
tory system different from and earlier than the mesonephros appears to
have been first suggested by Marcussen ('-31 ) ; but this view received no
recognition until it had been reasserted by Willi. Mtiller ('75), who gave
to the pronephros a distinctive name, Vorniere. Semper ('75), on the
other hand, denied utterly the nephridial nature of the pronephros, and
regarded the glomus as equivalent to the suprarenals (Nebennieren) of
I'htLCiostomcs. Fiirbringer ('78") vigorously opposed this view, and main-
tained that the pronephros and its duct represent a primitive excretory
system which conspicuously differs from both mesonephros and meta-
ncphros. According to Balfour's ('75) earlier view the segmental duct is
formed by the backward growth of a single anterior evagination, which
may be regarded as the representative of a mesonephric tubule. He
('81) later interpreted the pronephros similarly to Fiirbringer, but was
still inclined to believe that each mesonephric tubule was "in a sort of
way serially homologous with the primitive pronephros." It is very
difficult for me to reconcile the latter opinion with his view that the
pronephros is a primitive excretory system derived from Plathelminthes,
while the mesonephros is a secondary (new) development which does not
appear until the trunk becomes segmented. Moreover, this view mani-
festly ignores the metamerism which is exhibited by the pronephros. It
appears to me therefore entirely unsatisfactory.

Sedgwick ('81) lirst distinctly stated the conclusion that the pro-
nephros and mesonephros are differentiations of a single ancestral organ.
This view, which was adopted by Renson ('83), does not seem to have
been generally accepted, although several authors, by describing what
they denominate a transitional region, seem to me implicitly to assume
an intimate connection between the two glands. Mihalkovics ('85, pp.
65, 6G) denied that they are wholly homologous, on the ground that the
pronephric tubules are peritoneal evaluations, whereas those of the meso-
nephros are differentiated in the solid Wolffian blastema. Mihalkovics
does not explain his use of the term complete homology, and I have
been unable to satisfy myself in regard to the precise relations which
he supposed to exist between the two glands.

MUSEUM OF COMPARATIVE ZOOLOGY. 305

Van Wijhe ('8S a , '89), Ruckert ('88), Hoffmann ('89), and Wieder-
sheim ('90), have distinctly denied the serial homology of the pronephros
and the mesonephros. The objections of these authors to the view which
I have adopted have been most clearly formulated by van Wijhe ('89,
pp. 509, 510), whose account I shall follow in my criticism of their
position. First, "the pronephros arises before the appearance of the
duct or the mesonephros, and is indeed the first part of the excretory
system that appears." This point of difference is, as I have stated, the
most conspicuous feature in which the two glands are unlike. It is, how-
ever, not a weighty argument against their serial homology. Secondly,
" the pronephros arises as an (in Selachii segmented) evagination of the
somatopleure ; its cavity, which may be temporarily obliterated by the
proliferation of the walls, is formed as an evagination of the body cavity
(Metacolom). The mesonephros, on the other hand, is not formed as an
evagination, and it is constituted of somatopleure as well as of splanchno-
pleure." This analysis seems at first sight to establish a fundamental
contrast between the pronephros and the mesonephros, and I admit fully
the cogency of the argument in disproving a comparison of the nephro-
stornal and glomerular portions of a mesonephric tubule with the
nephrostomal canal of the pronephros. On the other hand, however, I
would insist that a hitherto unnoticed homologue of the pronephric evagi-
nations is to be found in the outward growth of the primitive mesonephric
canal to join the duct. 1 (See page 301.) It is in precisely this way that
a tendency to a somatopleural evagination would of necessity manifest
itself. Thirdly, "the duct always appears in continuity with the pro-
nephros, but always discontinuous with the mesonephros, which only
secondarily fuses with it and empties into it." This circumstance, as I
have already shown, is a direct consequence of the condition explained
under the first head. Fourthly, "the mesonephros possesses Malpighian
corpuscles ; while the pronephros has none, the glomus of the latter not
being homodynamous with the glomeruli of the mesonephros because it
is a vascular tuft invaginated into the secondary body cavity (Meta-
colom)." This contrast appears to me morphologically inaccurate, as I
believe I have adequately shown in the preceding discussion.

A further objection, which van Wijhe does not mention in his enumera-
tion, is the occurrence of rudimentary mesonephric tubules in the somites
which formerly gave rise to the pronephros. To prove this assertion, it

1 This is the only portion of the mesonephric tubule which can properly be called
an evagination; the entire tubule comprises the evagination plus the communicat-
ing canal.

vol. xxi. — NO. 5. 20

306 BULLETIN OF THE

is usually regarded adequate to show the existence in the pronephric
region of metameric diverticula proceeding from the body cavity towards
the overlying protovertebrse. These diverticula are the communicating
canals, and it is undoubtedly true that from similar canals in the pos-
terior region mesonephric tubules are actually developed ; but, to my
mind, the occurrence of these diverticula in the pronephric region cannot
be brought forward a.s evidence of the existence of two sets of nephridial
tubules in these somites, until it can be shown that these remnants of
the canal-like communication between protovertebrse and lateral plates
exhibit some indication of the characteristic nephridial differentiation,
i. e. grow outward and join the duct. This, I believe, has never been
demonstrated. The existence of such a growth has, however, been as-
serted by several observers ; but it seems to me compatible with the
view I have expressed of the relations between pronephros and mesoneph-
ros. Since the time of the investigations of Balfour and of Semper on
Selachians, it has been a familiar fact, that, although at first only one
mesonephric tubule occurs in each somite, the further complication of
the gland is largely produced by the formation of new tubules which
proceed from the region of the primary Malpighian capsule. If the
development of more than one tubule in a somite became normal in the
ancestors of the Craniotes before the separation of pronephros and meso-
nephros took place, the development of such secondary tubules in the
pronephric region would at once be intelligible.

A more fundamental objection is contained in an ignored observation
of Gasser ('82, p. 9G) on Alytes, according to which a typical glomus is
developed in the body cavity of the mesonephric region, in addition to the
universally present glomeruli. Gasser's account is contained in a rather
short note unaccompanied by figures ; it has not been confirmed by any
subsequent observer ; nor have I been able to find such a structure in
either Rana or Amblystoma. I am therefore inclined to the opinion
that Gasser may have mistaken for the glomus either the germinal ridge
or the fat-body, both of which are developed in this region, although this
explanation would contradict the statement of Gasser that the mesoneph-
ric glomus is a transitory organ. Be that as it may, I cannot without
further evidence accept his account as final.

Semon ("JO) has recently asserted that the pronephros and mesoneph-
ros are built upon the same structural type. He was led to this con-
clusion by a study of the excretory system in Ichthyophis. I have
already alluded to the condition of the pronephros in this form. It is
characterized by the possession of a completely closed pronephric chain-

MUSEUM OF COMPARATIVE ZOOLOGY. 307

ber, from which a portion of the nephrostomies ("inner" nephrostomies)
emerge. Each nephrostomal canal, however near the nephrostomal end,
is joined by a branch which communicates with the permanent body
cavity by means of an "outer" nephrostomy. According to Semon, the
pronephric chamber, as well as the cavity of a mesouephric Malpighian
capsule, is a diverticulum of the ccelom ; and the nephrostomal canal
which joins the glomerular portion of a mesonephric tubule with the
body cavity is represented by those canals of the pronephros which
emerge from the open body cavity. The mesonephros is to be regarded
as a " generation " of excretory tubules younger than the pronephros,
and the latter may be conceived to have primitively extended throughout
the entire trunk. In many features Semon's view is similar to that ex-
pressed in the preceding pages. The point of difference which I would
here emphasize is the different way in which the nephrostomal canal of the
mesonephros is explained. According to my opinion, this canal is a rem-
nant of the communication between the protovertebral cavity and the secondary
body cavity, and is not represented in the tubular portion of the proneph-
ros. Semon, on the other hand, claims that it is the homologue of the
outer series of nephrostomal canals in the pronephros of Ichthyophis.
Considering the relations of the glands in that form alone, this view
seems well justified; but it neglects the significant relation which has
recently been shown to exist between the mesonephros and the com-
municating canal ; and I am of opinion that the view as applied to other
Vertebrates is untenable, unless it cau be shown that the -outer nephro-
stomal tubule of the Gymnophionian pronephros also develops from that
canal. The latter interpretation is, I must admit, at least possible;
but we must await further researches on the development of these
Amphibia before accepting such a conclusion.

The closing section of this discussion will be devoted to a consider-
ation of the evidence which the development of the excretory system as
a whole throws on the origin of Vertebrates.

Two methods of investigation, which are mutually dependent, yet
quite unlike in their application, may be employed in attempting to
draw phylogenetic conclusions. One of these methods is peculiar to
embryological research ; it is dependent upon the principle that on-
togeny is in part an abbreviated recapitulation of phylogeny ; its
method is to eliminate coenogenetic characters ; it accomplishes this
largely by the aid of a physiological estimate of the influences of lar-
val and embryonic environment, and it is comparative only throughout

308 BULLETIN OF THE

the extent of the group whose origin is sought. The other method is
common both to comparative embryology and to comparative anatomy ;
it is dependent upon the inherent improbability of the same physio-
logical requirements, being met by the same structural device in two
groups which are not genetically related ; it can employ equally well,
though with a somewhat different significance, both ccenogenetic and
palingenetic characters ; it is purely anatomical in its method, and it is
in the broadest sense comparative. The first I may designate as the
method of elimination, or the intensive method ; the latter as the
comparative, or extensive method.

1 have been led to make the preceding analysis in order to employ the
division thus indicated in the subsequent discussion, and also because it
is a contrast which does not appear to be generally recognized. Thus,
a recent text-book of zoology (Hatschek, '88, pp. 25, 2G) identifies the
methods of embryology with those of comparative anatomy, and declares
that palingenetic and ccenogenetic characters are equally valuable for
phylogenetic inferences. According to the preceding analysis, these two
statements are partial, relating only to the comparative method in em-
bryology, and ignore the higher use which renders embryological facts
of peculiar value.

Observing then this two-fold division in the following discussion, an
attempt will first be made to reconstruct from the ontogeny of Verte-
brates the ancestral history of their excretory organs.

The most general character which appears to be common to the ontogeny
of all Vertebrates is the intimate relation which exists between the excre-
tory tubules and the coelom. This relation is peculiarby well illustrated
by the pronephros, but it is true also of all the urogenital organs, and is
a fact which in my opinion throws considerable light on their evolution.
The ccelom appears to be an internal cavity developed to meet a num-
ber of physiological needs. It is likely that in the lower Invertebrates
the ccelom served largely a nutritive function (see, e. g., Chun, '80, pp.
248-253) ; but I am of opinion. that in the higher Invertebrates and in
Vertebrates the ccelom early became in large measure an excretory
space. This function of the ccelom, inferred from its relations with ne-
phridia, is in accord with its situation in the body. Evidently the
organs which would be most in need of a near place of discharge for
nitrogenous waste products are those which are in the highest degree
metabolic. Such are, par excellence, the muscle masses of the body,
and it is a familiar circumstance that in all Chordates the primitive
muscle plates develop from the lining wall of the dorsal segmented por-

MUSEUM OF COMPARATIVE ZOOLOGY. 309

tion of the ccelom. It is very probable that this arrangement represents
the earliest differentiation of a special excretory surface of which evi-
dence is preserved in the ontogeny of Vertebrates.

The next step in the specialization of the urinary organs is the es-
tablishment of definite conduits for the purpose of conveying the excreted
products to the outside. It is possible that simple apertures, such as
the abdominal pores, at first served this end ; or, if the enterocoelous
condition represent a phylogenetic stage, communications with the
intestinal tube may have afforded an outlet to the excreted fluids. Be
this as it may, it is evident that the ancestors of our present Vertebrates
early acquired specialized tubes subserving this purpose.

In the account of the development of the Amphibian pronephros and
duct given in the first section of the present paper, emphasis was laid
upon the fact that these structures are differentiated from a solid so-
matopleural thickening, and do not arise as a fold of the peritoneum.
Manifestly the former condition is coenogenetic ; such a solid thickening
could in no wise function as an excretory conduit. On the other hand,
it must not be rashly assumed that the somatopleural thickening is a
disguised fold of that layer. On the contrary, the pronephros, on canal-
ization, shows itself to be already composed of a series of metameric
evaginations of the ccelom, and it is perfectly conceivable that the pro-
nephric thickening is a modification from a condition where the separate
evaginations had their independent means of communication with the
exterior, the several diverticula being fused into a solid mass. Either
interpretation would be physiologically intelligible. In the first case, a
certain region of the peritoneum would first sink as a groove into the
parietes of the coelom. This channel might, like the nephrostomes, be
provided with vibratile cilia, and might thus serve to carry the fluids
lodged in it back to a single pair of orifices situated near the posterior
end of the coelom. As a further differentiation, it is to be conceived
that this groove became at intervals constricted off from the coelom,
forming a retroperitoneal duct with a series of nephrostomal tubules.

According to the second alternative, it is necessary to suppose that
the several evaginations communicated distally either directly with the
exterior or with an independent longitudinal duct. The nephridia of
Heteromastus and Capitella (Eisig, '88, pp. 242, 272), in which no ex-
ternal opening is present, show us that the gradational steps in the
formation of such outgrowths may be conceived to be functional.

In judging between the two views to which allusion has just been
made, it is important to consider whether the ontogeny of other groups

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ever presents either of these processes in an unambiguous manner. I
have already expressed my doubts in regard to the development of the
pronephros and duct by the incomplete closure of a groove of somato-
pleure. The best attested claim that has been made for such a mode
of origin was that made by Goette, Fiirbringer, Hoffmann, and Marshall
and Bles, for Amphibia ; but this position is distinctly contradicted by
my own observations. Indeed, this mode of origin has been recently
denied in the case of every class except Teleosts, a group in which it is
very difficult to obtain accurate evidence respecting the early history of
the mesoderm.

On the other hand, numerous recent investigators have described the
first rudiment of the pronephros as a scries of distinct evaginations.
Such observations have been recorded in Cyclostomes by Kupffer ('88),
in Ganoids by Beard ('89), and in Amniotes by almost all writers on
their early development. It seems to me, therefore, that the mode of
formation by means of serial evaginations has a far wider distribution,
and is more clearly attested, than that by means of an incompletely
closed fold. I am of opinion that the condition in Amphibia and Sela-
chia is to be regarded as derived from such evaginations by means of
coenogenetic modification ; and that the weight of internal evidence is in
favor of the view that the tubules were primitively distinct.

Typically the nephridial tubes ore strictly metameric, one pair of
tubules being developed in each metamere. The occurrence of several
nephridia in a somite occurs, as we have seen, in the case of the meso-
nephros of certain Amphibia. This condition seems to me to be a char-
acter secondarily acquired. The following reasons confirm this opinion :
(1.) In other forms, the strict metamerism of the nephridia is the ear-
liest ontogenetic condition, the duplication of the tubules appearing much
later. (2.) The dysmetameric arrangement seems to be correlated with
the limited number of somites which are, in such cases, involved in the
formation of the mesonephros ; thus, in the Anura, a group in which the
number of trunk somites is extremely small, the mesonephros departs
most widely from the metameric condition ; in Urodeles, the number of
somites is larger, and there is an indication of metamerism in the anterior
tubules ; and again in Ccecilia, where the number of somites is still lar-
ger, the mesonephros has the typical metameric arrangement. (3.) The
pronephros, which in general represents the least modified portion of the
excretory system, retains a metameric condition in those forms in which
this arrangement is absent in the mesonephros.

In order to ascertain the probable mode in which the metameric diver-

MUSEUM OF COMPARATIVE ZOOLOGY. 311

ticula primitively terminated, whether they opened on the surface or
joined a longitudinal duct, it will be necessary to consider the pronephros
alone, since the segmental duct is already present before the mesonephros
is formed, and we cannot expect to find an adequate criterion for deter-
mining whether the union of the mesonephric tubes with the duct be
primitive or secondary. In the pronephros there is in most cases no
evidence of a mode of termination more primitive than that of com-
municating with a duct. Two arguments, however, occur to me, which
seem to indicate that a series of direct outlets to the exterior may have
been early present. In the first place, the pronephric diverticula have
frequently been observed to enter into intimate union with the ectoderm.
Thus Kiickert ('88, p. 217) was led to believe that the pronephric thick-
ening of Selachians even received a contribution of cells from the outer
germ layer. The most natural explanation of this condition seems to me
to be, that the fusion of the diverticula with the ectoderm is the re-
capitulation in the ontogeny of a phylogenetic stage, which possessed
nephridia provided with direct openings to the exterior. Secondly,
Amphioxus, according to the most recent investigations, is provided with
a series of nephridia opening into the atrial chamber, which latter we
are, in my opinion, justified in regarding as a simple infolded portion of
the exterior. Accepting the homology of the nephridia of Amphioxus
and those of Craniotes, it seems to me probable that the ancestors of
Vertebrates possessed nephridia which resembled those of Amphioxus in
opening directly to the exterior.

If separate diverticula leading from the ccelom to the exterior be the
primitive condition of the Vertebrate excretory organs, we have still to
seek the origin of the segmental duct. On this point, the pronephros
alone can afford evidence. The participation of the ectoderm maintained
by many authors for the posterior end of the duct affords the suggestion
that it may have first been formed as a groove of that layer, or that a
primitive anterior opening was gradually shifted back to the cloaca. It
may be objected to this view, (1) that in many Vertebrates no participa-
tion of the ectoderm occurs, while in none has it been shown that the
mesoderm does not play a part in the formation of both anterior and
posterior portions of the duct ; and (2) that the longitudinal canal of the
pronephros, which forms the anterior prolongation of the duct, in no case
arises in this way. In the pronephros the longitudinal canal arises, as
testified by a large number of recent investigators for divers groups, and
as confirmed by my own observations on Amphibia, by the fusion of the
distal ends of the pronephric diverticula. This mode of development

312 BULLETIN OF THE

seems to me entirely in harmony with physiological requirements ; and
in this earliest fragment of the excretory system we have, in my opinion,
a remnant of the primitive mode of formation of the segmental duct.

The question at once arises whether there is any indication of this
mode of origin preserved in the development of the posterior portion of
the duct. A free backward growth, such as is maintained for many
Vertebrates, is evidently far removed from the primitive mode of forma-
tion, and is to be regarded as an adaptation to the needs of the proneph-
ros. The origin of the duct in situ from a somatopleural proliferation
is without doubt a modified condition ; yet it suggests a mode of origin
which is in agreement with that observed in the anterior region. I have
already emphasized the circumstance that in Amphibia the duct arises
from a mass of cells which is perfectly continuous with that from which
the pronephric tubules are differentiated ; and it is possible that both
regions represent disguised nephridial evaginations of which those in the
posterior region are never differentiated as actual canals except in such
portions as are converted into the duct. Further evidence in favor of
this view is afforded by the occasional occurrence of supernumerary pro-
nephric tubules such as have been observed by Mollier ('90, p. 224) and
myself (page 253). The acceptance of this interpretation would necessi-
tate a modification of our conception of the relations between pronephros
and mesonephros, since we should be obliged to regard the mesonephric
tubules as a second generation of tubules, the first generation having
been employed in giving rise to the duct. On the other hand, it is quite
possible that the entire backward growth of the duct is a wholly secon-
dary process to meet the needs of a prematurely developed portion of the
primitive excretory organ. This is the only interpretation which seems
admissible in those cases where the duct has been found to grow back-
ward free from adjacent tissue.

The conception of the phylogeny of the duct which I have just pre-
sented offers a partial explanation of the contradictory evidence which
has been advanced respecting the germ layer from which the duct arises.
With a narrower conception of the phylogeny of the duct, it is difficult
to understand why the ectoderm should participate in the formation of
the excretory system in one group, but not in another, and why the
posterior end of the duct should in some cases be formed at the expense
of a germinal layer different from that which gives rise to its anterior
portion and to the nephrostomal canals wherever they appear. If, how-
ever, we assume the existence of a phylogenetic stage in which a series
of nephridia open directly to the exterior, it is at once evident that a

MUSEUM OF COMPARATIVE ZOOLOGY. 313

very trifling difference of location would determine whether the longi-
tudinal canal, by means of which the duct arises, should develop from
the mesoderm or from the ectoderm. It is to be remarked, however,
that such an explanation is not wholly satisfactory, since one would
expect on this hypothesis tha«t those forms in which the ectodermal
origin of the duct seems well attested would show evidence of close
genetic relationship, while those classes in which the duct arises from
the mesoderm ought to form an equally well defined group. This
condition, however, is by no means realized. On the other hand, the
force of this objection is materially weakened if we regard the duct as a
recent acquisition, which its absence in Amphioxus gives some justifica-
tion for assuming. The explanation seems to me, nevertheless, in a
measure unsatisfactory, and I have adduced it merely as a possible solu-
tion of the problem to which the apparently diverse relations of the duct
to the germ layers gives rise.

An intimate relation is always very early established between the
excretory tubules and the cardinal veins. Such an arrangement is so
favorable for the process of secretion that there can be but little doubt
that this condition prevailed in the ancestors of all Vertebrates. There
does not appear to be any evidence which would indicate whether the
cardinal veins or the excretory tubules are the more primitive structures.

In addition to the means of excretion afforded by the epithelial walls
of the tubules, the Vertebrate kidney-organs possess peculiar glomerular
structures. These, as I have already shown, are all formed on the type
of the pronephric glomus. In their primitive condition, they consist of
vascular tufts, which receive blood from the aorta and project into the
body cavity from the root of the mesentery. 1 The origin of such a prim-
itive glomerular structure is not far to seek. It is readily conceivable
that fluid may at first have simply exuded from the aorta, and, travers-
ing the small amount of tissue intervening between it and the body
cavity, may have reached the orifices of the excretory tubes prior to
the development of any specialized organ subserving a glomerular func-
tion. This process being once established, any modification of structure
which should allow a portion of the aortic current to be brought into
closer relations with the excretory tubules would be of obvious utility,
and would be preserved.

The excretory system thus constituted would represent the proneph-

1 The view of the excretory system here presented explains the double blood
supply of the kidneys of lower Vertebrates, and also the circumstance that the
Malpighian bodies always receive their blood by a direct branch from the aorta.

314 BULLETIN OF THE

ric type of structure. I have already sketched the manner in which
the rnesonephros may be derived from the pronephros by supposing
the metameric segmentation of the body to extend to that portion of the
ccelom from which the nephrostomes emerge. The account given in the
preceding section of this paper regarded the tubules as passive in such a
metamorphosis. It is possible, however, that the transference of the
tubules to a segmented portion of the coelom may have been in part
effected by a dorsalward shifting of the nephrostomes. In either case,
I am of opinion that the mode of development which I have now sug-
gested is applicable alike to the pronephros and the rnesonephros, and I
may also add to the metanephros (see Sedgwick, '80).

I have now presented, in a suggestive manner rather than as a com-
plete argument, certain indications of the phylogeny of the excretory
system which may be obtained from internal evidence. It still remains
for us to consider what conclusions are justified by a comparative study
of the excretory system, and whether the phylogenetic stages suggested
in the foregoiug account are to be found in any/group of living animals.

The sole purpose of this discussion is to ascertain the most probable
phylogenetic line of development for the excretory system of Vertebrates.
For this reason, I shall avoid any discussion, which would necessarily be
lengthy, respecting the interrelationships of the diverse excretory organs
found in Invertebrates, simply endeavoring to seek out those classes
which possess nephridia similar to those of Vertebrates, and shall ignore
the further consequences which would follow from the assumption of a
homology in any single case.

In the preceding account, I have provisionally accepted the view
that Amphioxus belongs to the Vertebrate phylum, and have endeav-
<jred to interpret its kidneys in accordance with that view. With
Tunicates it is quite different ; not merely do they afford no assistance
in the solution of the problem in hand, but it lias hitherto proved im-
possible to find any organs in this group which can be regarded as
homologous to Vertebrate nephridia. 1 In my opinion, it cannot be
objected that the absence of such organs in Tunicates proves that the
Vertebrate nephridia arose within the Vertebrate phylum. A rigid ad-
herence to such a system of restriction in the case of other organs would
quickly lead to absurd conclusions.

The only classes of animals in which we need seek for a homology of

1 Hatschek ('84, p. 519) regarded the single nephridium described by him in
Amphioxus homologous with the neural gland of Tunicates ; but I have already
pointed out the probable inaccuracy of this observation.

MUSEUM OF COMPARATIVE ZOOLOGY. 315

the Vertebrate renal organs are those belonging to the bilateral cladus ;
and among these I shall consider only those forms which are usually-
included in the rather heterogeneous class Vermes. This restriction is
justified by the circumstance that the only similarities of structure
which are to be found between the excretory system of Vertebrates and
those of Atollusks and Arthropods recur with greater force in the case of
several groups of Vermes.

In comparing the kidneys of Vertebrates and those of Worms, I shall
distinguish three types of structure in the latter group: (1) the water-
vascular system of Plathelminthes, (2) the excretory system of Xemer-
tines, and (3) the nephridia of Annelids. The various organs which
serve as excretory and genital passages in Rotifers, Nematodes, Echi-
urids, and Sipunculids are either referable to one of these types, or are
valueless for the purpose in hand.

In endeavoring to find what points of similarity exist between the
excretory system of Plathelminthes and that of Vertebrates, I have been
unable to formulate any more definite statement than that both consist
of longitudinal internal canals, which bear numerous lateral branches,
and which open directly or indirectly to the exterior. On the contrary,
the two sets of organs appear to me to perform the function of excretion
by anatomical devices which are diametrically opposed. In Vertebrates,
the excretions are either (primitively) poured into the ccelom and con-
veyed thence by a simple series of conduits, or excreted from the blood
in the course of the tubuli uriniferi of the kidneys. In Plathelminthes,
oil the other hand, the excretory tubes ramify throughout the entire
body parenchyme, and, so to speak, seek out the waste products of
metabolism at the seat of their formation. It is a contrast such as
exists between lungs and tracheae, and appears to me of fundamental
importance. According to Fraipont ('80) and Francotte ('81 and '83,
pp. 734, 735), it is true, there is a communication between the excretory
tubules of Plathelminthes and certain interior canalicular spaces, which
they interpret as a rudimentary coelom. The evidence in favor of the
latter interpretation is certainly far from complete, but could not, if
true, overthrow the fundamental contrast which I have just emphasized.
Furthermore, I am not aware that any subsequent writers have con-
firmed this account of the termination of the excretory capillaries ; while
Pintner ('80, p. 302), von Graff ('S2 a , pp. 106 et seq., '82 b , p. 80), Lang
('81, p. 208, '84, p. 167), Iijima ('84, p. 400), Zschokke ('87, p. 1G5),
and Bohmig ('90, p. 243) have all asserted that the terminal sacs are
entirely closed. The conclusion seems warranted that no direct evi-

316 BULLETIN OF THE

dence in support of an intimate relation between Vertebrates and Plat-
helminthes is afforded by a comparison of their excretory organs.

In Nemertines the excretory system is in peculiar relations with the
blood vascular system. According to Oudemans ('85). the excretory
system of the Schizonemertini and the Hoplomertini consists of a longi-
tudinal tube, which is closely applied to the lateral longitudinal blood-
vessel of the oesophageal region, and this tube communicates with the
exterior by means of a single excretory pore or by a number of such
openings. In Carinella and Carinoma, however, the connection is much
more intimate, and the glandular portion of the excretory organ lies
embedded in the oesophageal blood lacuna? and communicates with the
latter by means of two or three evident openings. Oudemans asserts,
indeed, that the excretory system is in reality a detached portion of the
blood vascular system. Burger ('90, p. 92), however, has recently
thrown some doubt upon the existence of open communications between
the nephridia and the blood-vessels, but reaffirms the close dependence
of the excretory system upon the vascular trunks.

Comparing the nephridia of Nemertines with those of Vertebrates, it
seems to me that one cannot fail to recognize a pronounced difference in
type. In Vertebrates the nephridia are canals in close relation with the
ccelom ; they develop from its epithelium, and even in the adult arise
from chambers which must be regarded as detached portions of the
ccelom. The excretory organs of Nemertines lie between the vascular
trunks and the exterior, and show no such relations with the ccelom.

Among the Annelids, on the other hand, the Chsetopods possess an
excretory system, which seems to present several features of strong
resemblance with the nephridia of Vertebrates, and it remains to be
considered whether an actual homology can be postulated in this case.

The points of similarity may for the present purpose be classed under
seven heads. (1.) The Vertebrate and the Chaetopod excretory systems
agree in the fact that both primitively serve, at least in part, to convey to
the exterior such fluids as accumulate in the ccelom, this cavity being used
as a capacious excretory reservoir. (2.) In both groups certain portions
of the epithelial lining of the coelom become differentiated into specialized
excretory glands. In Vertebrates the only structures of this character are
the glomi and glomeruli ; but in Annelids there is evidence that con-
siderable areas of the peritoneum may become modified in this way. It
was suggested by Claparede ('69, p. 615), that the chloragogen cells
secrete certain elements from the blood and transfer them to the peri-
visceral fluid. This view of the function of the chloragogen cells has

MUSEUM OF COMPARATIVE ZOOLOGY. 317

been con firmed by a large number of observers, 1 and it has been further
shown that individual cells, having become charged with excreted con-
crements, loosen from the layer to which they belong, and float freely in
the ccelom, whence they are discharged through the nephridia. The
chloragogen layer covering the blood-vessels appears moreover from its
anatomical relations to be a portion of the visceral mesoderm, and it
has been shown to arise ontogenetically from that layer (Houle, 'SO,
pp. 201, 252, 200). The chloragogen cells are frequently distributed
upon special vascular processes, thus forming distinct glandular organs.
Similar in function is probably the glandular envelope of the ventral
vessel in Polyophthalamus (E. Meyer, '82, p. 81 G), and cases may be
found among Polyclnetes in which definite peritoneal glands are present
(Grobben, '88, pp. 255 et seq., Eisig, '87, pp. 227, 245, 681). I should not
wish to assert a strict homology between the glomus and the masses of
chloragogen cells ; yet it seems to me likely that the latter represent an
early differentiation of the splanchnic mesoderm of which we have more
specialized developments both in Annelids and in Vertebrates. 2 (3.) The
efferent conduits take their origin from the ccelom by means ot a series
of ciliated funnel-shaped openings, the nephrostomes. (4-.) The nephro-
stomes lead into transverse convoluted canals, along the course of which a
large part of the excretion takes place. (5.) The nephridial tubes arise
from the parietal peritoneum. (6.) They are typically strictly meta-
meric, one pair of tubules being developed in each metamere. The
deviation from this typical metamerism to which I have already referred
in the case of Vertebrates is paralleled by similar conditions in Capitella
(Eisig, '87, p. 50-4). (7.) The development of the Chtetopod nephridia
resembles in general that shown by those of Vertebrates. Both the
pronephric and mesonephric tubules arise as a series of metameric
outgrowths from the somatic mesoderm. In Polychretes this is evidently
the mode of origin of the nephridia. In Oligochastes the development is
more doubtful, but the method of origin described by Bergh ('00) is in
essence the same as that known in Polychsetes, and the mode of devel-
opment maintained by Wilson ('87, pp. 185, 186. and '80, pp. 410 et seq.)
may be interpreted so as not to be in fundamental opposition with such
a method.

There is one feature in regard to which the nephridia of most Anne-

i E. g. Timm ('83, pp. 122, 120): Kiikenthal ('85, p. 330); Meyer ('87, p. 648).

2 A further analogy is possibly to be found in the fact that in Amphibia certain
cells early loosen from the wall of glomus and fall into the ccelom, leaving inter-
vals between the remaining cells of the epithelium.

318 BULLETIN OF THE

lids and those of Vertebrates differ conspicuously, viz. in the mode in
which the nephridia terminate. It is a familiar tact, that in Annelids
each nephridium opens separately on the surface of the body ; while in
Vertebrates the nephrostomal tubules all connect with a pair of longi-
tudinal ducts opening into the cloaca. In commenting upon this fea-
ture of difference, it is important to note in the first place that the
contrast is not of universal application. It lias been recently shown by
K. Meyer ('87, pp. 618-625) and Cunningham ('87 a , 87 b , pp. 248-253)
that in Lanice conchilega, a terebelloid Annelid, certain nephridia open
into a longitudinal trunk, and only secondarily communicate w r ith the
exterior. On the other hand, it is probable that Amphioxus possesses
nephridia which open to the exterior (atria cavity) without the inter-
vention of a longitudinal duct. If such differences can occur among the
members of either group, it seems to me that it would be unjust to deny
the homology of the other portions of the system in consequence of
the fact that Vertebrates in general possess a longitudinal duct, while
Annelids in general do not. It appears to me, moreover, that the con-
dition of the nephridia in Lanice conchilega and the ontogeny of Verte-
brates both serve to indicate the manner in which the duct may have
secondarily arisen. In Lanice conchilega there is no doubt that the
nephridia] duct is a secondary growth, and it is highly probable that the
channel is formed by outgrowths extending from each of the nephridial
tubes backward and communicating with the next following nephrid-
ium. Two groups of nephridia can be distinguished in Lanice con-
chilega. The more anterior of these consists of a short longitudinal duct
which bears three nephrostomal tubules, and terminates at its posterior
end by a single pore. In the posterior set, the longitudinal duct is
merely a canal which connects the several nephridia, while these con-
tinue to retain their external orifices. I have already pointed out
that the ontogeny of Vertebrates presents a similar process in the devel-
opment of the longitudinal canal of the pronephros, and have shown
that such changes may likewise have taken place in the region of the
mesonephros. I by no means wish to imply by this comparison a belief
that the ordinary mode of development in Vertebrates is to be directly
derived from that presented by Lanice conchilega, nor to assume a close
genetic relation between Vertebrates and genera presenting this condi-
tion. I merely wished to emphasize the fact, that in Lanice conchilega
we have an instance of a species which, primitively possessing discrete
nephridia, such as may have been present in the ancestor of Vertebrates,
has acquired a longitudinal excretory canal by a process of transforma-

MUSEUM OF COMPARATIVE ZOOLOGY. 319

tion which resembles that by which Vertebrates acquire in their ontogeny
the segmental duct.

From the facts thus far brought forward, I conclude, (1) that the
group of animals whicn presents nephridia most closely resembling those
of Vertebrates is unquestionably that of the CliBetopod Annelids ; and
(2) that the Vertebrate excretory system can be readily derived from
that of Annelids by a series of steps which are in accord with the evi-
dence afforded by the ontogeny of Vertebrates.

In conclusion, I shall briefly allude to the opinions of previous writers
respecting the origin of the Vertebrate excretory organs. These opin-
ions fall, in the first place, into two classes, according to one of which the
excretory system is derived from Invertebrate ancestors ; according to
the other, it has arisen wholly within the Vertebrate phylum.

The most recent exponent of the latter view is van "YVijhe ('89, pp.
50G et seq.). The arguments offered by this author in support of his
position are in part dependent upon his denial of the serial homology of
pronephros and mesonephros. Van W\jhe also employs two arguments
which are independent of his position in regard to this point: (!) ne-
phridia are absent in Amphioxus, and therefore the common ancestral
form cannot have possessed them ; (2) the renal organs do not appear
until after the so-called " Acrania stage," and therefore could not have
appeared phylogenetically until this stage had been passed. Granting
both premises, it seems to me that neither conclusion follows. With
reference to the absence of kidneys in Amphioxus, the possibility — or
should I not say probability 1 — of extensive degenerative modification is
entirely neglected. Moreover, it has now been rendered probable that
true nephridia do exist in Amphioxus, an observation which removes at
a blow the whole basis of the argument. I do not believe many embry-
ologists would unite with van Wijhe in holding that characters which
appear simultaneously in the ontogeny of a form must necessarily have
arisen contemporaneously in its ontogeny. For my part, I am unable to
diagnose with accuracy the "Acrania stage" of Amphibia; but Ostrou-
moff ('88, pp. 77, 78) appears to have had the necessary insight, and
denies emphatically that the pronephros in the case of Eeptiles ai'ises
after the " Acrania stage."

Turning now to the hypotheses that have been advanced involving
the derivation of the Vertebrate excretory system from Invertebrate an-
cestors, Haeckel ('74, p. 37), Gegenbaur ('78, p. G28), and Fiirbringer
('78 a , pp. 95 et seq.) endeavored to show that the Vertebrate nephridia
were derived from those of Plathelminthes. Semper and Balfour, on the

320 BULLETIN OF THE

other hand, claimed that the transverse tubules of the mesonephros are
homologous with the segmental tubes of Annelids, an opinion which is
shared by Beard, Haddon, Kollmann ('82 a ) and others.

Riickert ('88, p. 2G2) has recently denied this homology, and asserted
that the Annelidan nephridia are represented in the pronephros alone,
since the latter is the only portion of the system of metameric tubes
which comes in contact with the ectoderm. With Riickert, I would
admit that the evidences of an Annelidan origin of the excretory system
are to be sought mainly in the pronephros; but, in view of the intimate
relations which exist between pronephros and mesonephros, it seems to
me more probable that both have had the same phylogenetic origin.
OstroumofF ('88, pp. 80 et seq.) also has asserted that the development of
the pronephros indicated that this system had been inherited from
Annelidan ancestors.

On the other hand, a number of authors maintain that, although the
mesonephros may be derived from the segmental organs of Annelids,
the pronephric system is represented by other Invertebrate nephridia.
Thus, Semper ('7G, pp. 387, 388) suggests the possibility that the duct
(pronephric system so far as present in Selachians J ) may represent the
unsegmented excretory tubules developed in the larva of Xephelis, or
may even represent an inheritance from the Plathelminthan water-vas-
cular system ; and Balfour ('81, p. GOT) was led to accept this view.

In addition to his hypothesis of an origin of the excretory system of
Vertebrates from that of Plathelminthes, Furbringer suggests the possi-
bility that the entire system may have arisen from Gephyreans, in which
unsegmented and segmented excretory organs are stated to coexist. This
view was adopted by Kollmann ('82 a ), and is offered as a suggestion in
Wiedersheim's ('86, p. 731) text-book.

Xone of the views which claim a double origin for the excretory system
seem to me to be tenable, in consequence of the fact that the pronephros,
an undoubtedly segmented element, develops in strict continuity with
the duct, the so-called unsegmented element. The view according to
which the excretory system is derived from that of Gephyreans, more-
over, is liable to special criticism. This claim that Gephyreans present
an excretory system of a double nature, segmented and unsegmented,
doubtless refers to the coexistence of two or three pairs of nephridia,
together with the so-called Analschlauche of Echiurids. I am imable to
see the slightest evidence that the Vertebrate excretory system is made

1 By this suggestion, Semper allows some justification to W. Mailer's conception
of a pronephros, although he had earlier contested it.

MUSEUM OF COMPARATIVE ZOOLOGY. 321

up of such remote parts; and the hypothesis must fall to the ground as
soon as it is proved that the Analschlauche are in reality only modified
nephridia opening into a proctodeum. In addition to the anatomical
evidence favoring such an interpretation, Hatschek ('80, pp. 60-62) has
recently advanced strong evidence from an embryological standpoint for
believing that they are nephridia which primitively opened directly to
the exterior.

Among the attempts to find a homologue of the segmental duct already
present in the Worms may be mentioned the longitudinal canal described
by Hatschek ('78, p. 117 seq.) in the larva of Polygordius. Were Hat-
schek's account accurate, it would doubtless warrant great changes in
our conceptions of the interrelationships of the Vermian nephridia, and of
the origin of the Vertebrate kidneys ; but his statements have not been
confirmed by any subsequent observations, though several investigators
have concerned themselves with this interesting form (Fraipont, '87,
p. 83 ; Eisig, '87, p. 662 ; E. Meyer, '87, p. 594 j Bergh, '85, p. 27, foot-
note).

The remaining views in respect to the nature of the duct agree in re-
garding it as a secondary growth. 1 Beard ('87, p. 651) and Haddon
('87) accept the ectodermal origin of the segmental duct, and endeavor
to give it phylogenetic significance by assuming that the ontogenetic
connection of the duct with the ectoderm indicates that it was repre-
sented in the phylogeny at first by a groove into which the nephridia
opened, and that this groove gradually became closed and was cut off
as a tube extending from the most anterior tubule backwards to the
cloaca. Ruckert ('88) and van Wijhe ('89, pp. 507, 508), on the
other hand, assert a gradual backward growth of a primitively anterior
opening.

None of these views are compatible with the fact that in many Verte-
brates the duct is demonstrably of mesodermal origin. They also seem
to me to give insufficient significance to the mode in which the pronephric
diverticula unite to form a longitudinal canal : and the first two authors
neglect it wholly.

Finally, the abandoned view of Balfour ('76, pp. 25, 26), according to
which the duct arises by the fusion of the distal ends of the several nephridia,
has been revived by Eisig ('87, p. 649), and was accepted by Ruckert
('88, p. 264) for the pronephric portion of the duct. This view seems to
me much the most probable, and I am inclined to accept it.

1 Here belongs also tlie view of Boveri ('90), which has already been discussed
(page 265).
vol. xxi. — no. 5 '21

322 BULLETIN OF THE

In the foregoing discussion, I have endeavored to show that there is
considerable evidence in favor of the view that the excretory system of
Vertebrates has developed from a system of metameric nephridia, such
as are present in Annelids. None of the evidence seems to me, how-
ever, final. The excretory systems of the two groups are very similar,
but we have no means of limiting definitely the part that has been
played by physiologically similar needs in moulding the structure of the
organs. Nor am I committed to the theory of the Annelidan extraction
of Vertebrates. I fully realize that such a theory can only be estab-
lished by investigations which shall include in their scope the entire or-
ganization of the two groups. So far as this larger theory has been
dealt with in this discussion, it has been with the view of contributing
such evidence as the excretory system offers, and I have naturally left
untouched the mass of evidence which proceeds from other organs. To
this in addition we must appeal for the justification of the broader
theory.

CamiiUdge, April 25, 1891.

MUSEUM OF COMPARATIVE ZOOLOGY. 323

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'82. Zur Entwicklung von Alytes obstetricans. Sitz.-Ber. d. naturf.

Gesellsch. Marburg, Jahrg. 1882, Nro. 5, pp. 73-97- Oct., 1882.

Gasser, E., und Siemerling.

'78. Das obere Eude des Wolff'schen Ganges im Huhnerei. Sitz.-Ber. d.

naturf. Gesellsch. Marburg, Jahrg. 1878, Nro. 3, pp. 62-65. Nov., 1878.

'79. Beitrage zur Entwickelung des Urogenitalsystems der Huhnerembry-

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Gegenbaur, Carl.

'78. Grundriss der vergleichenden Anatomic 2te verb. Aufl., viii + 655
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'82. Ktudes sur la composition chimique de l'oeuf et de ses enveloppes cbez
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{^Abstract given in Arch. Ital. Biol., Tom. II. pp. 226-230.]

Giles, Arthur E.

'88. Development of the Fat-bodies in Rana temporaria. A Contribution
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Goette, Alexander.

'75. Entwickelungsgeschichte der Unke (Bombinator igneus). viii -f- 964 pp.,

m. Atlas v. 22 lith. Taf. Leipzig: Voss. 1875.
'88. Ueber die Entwickelung von Petromyzon fluviatilis (vorl. Mitth.). Zool.
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Graff, Ludwig von.

'82 a . Monographic der Turbellarien. I. Rhabdocoelida. xii -f- 441 pp., 12

Holzschn., Atlas mit 20 Taf. Leipzig : Engelmann. 1882.
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'88. Die Pericardialdriise der chaetopoden Anneliden, nebst Bemerkungen
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'85. Zur Morphologie der Kopfaiere der Fische. Zool. Anzeig., Bd. VIII.

pp. 605-611. 12 Oct., 1885.
'86. Zur Frage iiber die Persistenz der Kopfnii're der Teleostier. Zool.
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'42. Rechercbes anatomiques sur le systeme veineux de la Grenouille. Ann.
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Gunther, Albert.

'58. Catalogue of tbe Batracbia Salientia in tbe Collection of tbe British
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Haddon, Alfred C.

'87. Suggestion respecting tbe Epiblastic Origin of tbe Segmental Duct.
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Haeckel, Ernst.

'74 a Die Gastraea-Tbeorie, die pbylogenetiscbe Classification des Tbierreicbs
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'78. Studien iiber Entwicklungsgescbicbte der Anneliden. Arb. zool. Inst.

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'84. Mittbeilungen iiber Amphioxus. Zool. Anzeig., Bd. VII pp. 517-520.

29 Sept., 1884.
'88 a . Lebrbucli der Zoologie. lte Lieferung, iv + 144 pp., 155 Abbild. im

Text. Jena : Fischer, 18S8.
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'66. Benierkuugeu iiber die Lymphe. Arch. f. pathol. Auat., Bd. XXXVII.

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Hertwig, Oscar.

'83. Die Eutwicklung des mittleren Keimblattes der Wirbeltbiere. vi -4-
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'88. Lehrbuch der Entwicklungsgeschichte des Menschen mid der Wirbel-
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His, Wilhelm.

'65 a . Die Haute uud Hohlen des Korpers Acad. Progr. Basel, 34 pp.

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'86. Zur Entwickelungsgeschichte der Urogenitalorgane bci den Auamnien.

Zeitschr. f. wiss. Zool., Bd. XLIV. pp. 570-643, Taf. XXXIII.-XXXV.,

4 Holzschn. 14 Dec, 1886.
'89. Zur Entwickelungsgeschichte der Urogenitalorgane bei den Beptilien.

Zeitschr. f. wiss. Zool., Bd. XLVIIL, pp. 260-300, Taf. XVIL, XVIII.,

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Houssay, Frederic

'91. Etudes d'embryologie sur les Vertebres. IV. Les Fentes Branchiales.
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Hoyer, Heinrich

'90. Ueber den Nachweis des Mucins in Geweben mittelst der Farbe-
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'76. On the Classification of the Animal Kingdom. Jour. Linn. Soc, Vol.
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'51. Das uropoetische System der Knochenfische. Deukschr. d. k. Akad.

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'84. Untersuchungeu iiber deu Bau uiid die Entwicklungsgeschichte der Siiss-

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'85. Histclogisch-enibryologische Untersuchungeu iiber das Urogenital-

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CI., Abt. 3, Febr.-Hefr, pp 97-199, 4 Taf. 1SS5.
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'87. Das Scldundspaltengebiet des Hiihnchens. Arch. f. Anat. u. Physiol.,

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'88 a . Zur Entwicklungsgeschichte des Igels (Eriuaceus europaeus). Auat.

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'90. Notes ou the Pronephros of Amblystoma punctatum. Johns Hopkins

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Klebs, Edwin.

'89. Die allgemeine Pathologie oder die Lehre vou den Ursacheu und dem

"Wesen der Kraukheitsprocesse. 2te Th. Storungen des Banes und der

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'61. Entwickelungsgeschichte des Menschen und der hoheren Thiere.

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'82 a . Die Doppelnatur des excretorischen Apparates bei den Cranioten.

Zool. Anzeig., Bd. V. pp. 522-524. 9 Oct., lsv\
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'91. Die Rumpfsegmente menschlicher Embryoncn von 13 bis 35 Urwirbeln.

Arch. f. Auat. u. Physiol., Jahrg. 1891, Auat, Abt., Heft 1, pp. 66-SS,

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Kowalewsky, A.

'67. Eutwickelungsgeschichte des Amphioxus lanceolatus. Mem. Acad.
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'70. Die Eutwickelungsgeschichte der Store. Vorlaufige Mittheilung. Bull.
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Kowalewsky, R.

'75. Oopa3oBaHie Haia.Tt MoiejionojioBuxi opraHOBi y KypnHux'B 3apoAfciineH.
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Kukenthal, Willy.

'85. Ueber die lymphoidcn Zellcn der Anneliden. Jena. Zeitschr. f. Natur-
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'88. Ueber die Entwicklung von Petromyzon Plancri. Sitz.-Ber. d. k. b.
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Lang, Arnold.

'81. Der Bau von Gunda segmentata und die Verwandtschaft der Platyhel-
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Langerhaus, Paul.

'76. Zur Anatomie des Amphioxus lanceolatus. Arch. f. mikr. Anat., Bd.
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Lankester, E. Ray.

'75. On some New Points in the Structure of Amphioxus and their Bearing

on the Morphology of Vertebrata. Quart. Jour. Micr. Sci., Vol. XV.

pp. 257-267, 4 woodcuts. July, 1875.
'89. Contributions to the Knowledge cf Amphioxus lanceolatus Yarrell.

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Lankester, E. Ray, and Arthur Willey.

'90. The Development, of the Atrial Chamber of Amphioxus. Quart. Jour.
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Lockwood, C. B.

'87. The Development and Transition of the Testis, Normal and Ab-
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Mcintosh, Wm. Carmichael, and E. E. Prince.

'88. On the Development and Life-Histories of the Teleostean Food- and
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Marcussen, Jean.

'51. Sur le developpement des parties geuitales et uropoietiques chez les Ba-
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Marshall, A. Milnes, and Edward J. Bles.

'90'. The Development of the Kidneys and Fat-Bodies in the Frog. Stud.

Biol. Lab. Owens College., Vol. II. pp. 133-158, PL X. 1890.
'90 b . The Development of the Blood-vessels in the Frog. Stud. Biol. Lab.
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Martin, E.

'88. Ueber die Anlage der Urniere beim Kaninchen. Inaug. Diss. Marburg.
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Mayer, Paul.

'87. Ueber die Entwickelung des Herzens und der grossen Gefassstamme
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'82. Zur Anatomie und Histologic von Polyophthalmus pictus Clap. Arch,
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'87. Studien iiber den Korperbau der Anneliden. Mitth. zool. Stat. Neapel,
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'90. Die Entwickelung der Urniere beim Menschen. Arch. f. mikr. Anat,
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'84. Untersuchungen iiber die Entwickelung der Harn- und Geschlechtsorgane
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Mitsukuri, K.

'88 The Ectoblastic Origin of the Wolffian Duct in Chelonia. (Prelim.
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Mollier, S.

'90. Ucber die Entstehung des Vornierensystems bei Ampliibien. Arch. f.
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Muller, Johannes.

'29. Ucber die Wolff'scheu Korper bei den Embryonen der Frosche and

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Muller, Wilhelm.
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Nagel, W.

'89. Ueber die Entwickelung des Urogenitalsystems des Menschen. Arch,
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'73. Beitrage zur Entwicklungsgeschichte der Knochenfische nach Beobacht-
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Orr, Henry.

'87. Contribution to the Embryology of the Lizard. Jour, of Morph., Vol. I.
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Ostroumoff, A.

'88 a . Zur Entwicklungsgeschichte der Eidechsen. Zool. Anzeig, Bd. XL

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Oudemans. A. C.

'85. The Circulatory and Kephridial Apparatus of the Nemertea. Quart.
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Overlach, M.

'85. Die pseudo-menstruirende Mucosa uteri nach acuter rhosphorvergiftung.
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Owen, Richard.

'66. Ou the Anatomy of Vertebrates. Vol. I. Fishes and Reptiles, xlii +
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Owsjanniskow, Ph.

'89. Zur Entwickelungsgeschichte des Flussneunaugens (Vorl. Mitth.).
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Parker, W. Newton.

'88. Zur Anatomie und Physiologie von Protopterus aunectans. Ber.
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Perenyi, Josef von.

'87. Die ektoblastiscbe Anlage des Urogenitalsysteins bei Bana esculenta
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Pintner, Theodor.

'80. Lntersuchnngen iiber den Bau des Baudwurmkorpers mit besonderen
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Rabl, Carl.

'88. Leber die Bildung des Mesoderms. Vortrag in der 3ten Sitz. d. Anat.
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Remak, Robert.

'55. Untersuchungen iiber die Entvrickelung der VTirbelthiere. vi -f- xxxviii
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Renson, George.

'83. Contributions a l'embryologie des organes d'excretion des oiscaux et
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Rolph, W.

'76. Untersuchungen iiber don Bau des Amphioxus lanceolatus. Morph.
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Romiti.

'74. Leber den Bau und die Entwickelung des Eicrstockcs und des "Wolff'-
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Rosenberg, Alexander.

'67. Untersuchungen iiber die Entwicklung der Teleostier-Niere. Inaug.
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Roule, Louis.

'89. Etudes sur le developpemeut des Annelides et un particulier d'un Oli-
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'88. Ueber die Entstehung der Excretionsorgane bci Selachiern. Arch. f.

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'89. Zur Ent vvicklung des Excretionssystem der Selachier. Eine Erwidcrung

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Ruckert, Jon., und W. Mollier.

'89. Ueber die Entstehung des Vornierensystems bci Triton, llaua, und
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'87. [Resume of Haddon ('87).] Amcr. Naturalist, Vol. XXI. pp. 587-
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'78, '80. HcTopia pa3BHTia CTep.mjtn (Acipenser ruthenus). TpyiH
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[History of the I >evelopment of the Sterlet (Acipenser ruthenus). Mem.
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'81. Studier over Testis og Epididymis Udviklingshistorie. 9G Sid., 3 Taf.

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MUSEUM OF COMPARATIVE ZOOLOGY. 337

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[Also in Stud. Morph. Lab. Univ. Cambridge, Vol. I.]
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338 BULLETIN OF THE

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EXPLANATION OF FIGURES.

All the Figures, unless otherwise stated, were drawn with the aid of an Abbe
camera lucida, and represent the appearance of the anterior faces of the sections.
Plates I.-IV. were made from preparations of Rana sylvatica Le Conte ; Plates
V.-VIII. from Rana sylvatica Le Conte, Bufo americanus Le Conte, and Ambly-

stoma punctatum Linn.

ABBREVIATIONS.

(For the meaning of letters a, b, c, d, e,f, in Figures 24-26, see the explanation

of those Figures.)

Ganglion nodosum.
Spinal ganglion.
Liver.
Intestine.
Lateral plate.
Medullary plate.
Mesodermal plate.
Peritoneal layer.
Protovertebral plate.
Somatic layer.
Splanchnic layer.
Median.

Basement membrane.
Mesoderm.
Mesenchyme.
Myotome.
Nervus lateralis.
Chorda dorsalis.
Nephrostome.

111 1st, L'd, and 3d pronephric
nephrostome, respectively.
Peritoneum.
Protovertebra.
Aortic root.

Root of the vagus nerve.
Sub-notochordal rod.
Blood sinus.
Somites I., II., etc.
Somatopleure.
Yolk spherules.
Splanchnopleure.
.n.,in. lst> 2d, and 3d nephro-
stomal tubules respectively.
Pronephric tubule.
Collecting trunk.
Common trunk.
Blood-vessel.
Posterior cardinal vein.
Jugular vein.

ao.

Aorta.

gn. nd.

can. comn.

Communicating canal.

(pi. spi.

cd. spi.

Spinal cord.

hp.

cl. ms'drm.

Mesodermal cells.

in.

cl. vt.

Yolk cells.

la. 1.

dt.

Cloaca.

la. med.

ccd.

Coelom.

la. ms'drm.

ccd.'

Protovertebral cavity.

la. pi'ton.

ccd."

Body cavity.

In. pr'vr.

cp. sng.

Blood cells.

la. so.

cps. pr'nph.

Pronephric capsule.

In. spi.

eras. gn.

Ganglionic thickening.

in.

eras, pr'nph.

Pronephric thickening.

nib. ba.

ci-as. so'plu.

Somatopleural thickening.

ms'drm.

d.

Dorsal aspect.

ms'ch if.

dt. sg.

Segmental duct.

mi/tin.

dt. CuV.

Ductus Cuvieri.

n.l.

dx.

Right side.

n'ed.

ee'drm.

Ectoderm.

nph'stm.

ee'drm.'

Superficial layer of same.

nph'stm J-' n -

ec 'drm."

Deep layer of ectoderm.

en'th.

Endothelium.

pi'ton.

fnd. arc. vr.

Deep layer of a vertebral

pr'vr.

arch.

rx. ao.

fad. cps.

Fundament of the pro-

rx. vni!.

nephric; capsule.

sh.-n'cd.

fnd. dt sg.

Fundament of the segmen-

sn. sin/.

tal duct.

so. 1 - u - etc.

fnd. gJm.

Fundament of the glomus.

so'plu.

fnd. glm.'

Fundament of glomerulus.

sph. vt.

fnd. gn. spi.

Fundament of a spinal gan-

spl'plu.

glion.

thl. nph'stm. 1 -

fial. mbm.

Limb bud.

fnd. ms'nph.

Fundaments of mesoneph-

t'J. pr'nph.

ric tubules.

trn. clg.

fnd. nph'stJ

Fundament of first pro-

trn. com.

nephric nephrostome.

va. sng.

fnd. pul.

Lung bud.

vn. crd.

glm.

Glomus.

vn.jgl.

FiuLD. — Prouepbroj iu Amphibia.

PLATE I.

Fig. 1. A portion of a cross section through the anterior trunk region of one

of the older embryos included under Stage I. X 110.
" 2. A cross section through the same embryo in the middle trunk region.

X 25.
" 3. A portion of a cross section through the middle trunk region of one

of the younger embryos in Stage I. X 02.
" 4. A portion of a cross section through the hinder trunk region of one of

the younger embryos belonging to Stage II. X 02.
" 5. A portion of a cross section through the anterior trunk region of one of

the older embryos from Stage II. The section passes through an

interprotovertebral septum. X 110.
" 6. A portion of a cross section from one of the older embryos in Stage III.

The plane of the section passes through the middle of Somite III.

X no.

" 7. A small segment of a cross section through the embryo shown in Figures
15-17 of Plate II. The Figure represents a portion of the ventro-
lateral ectoderm with three subjacent mesodermal cells. X G15.

" 8. A portion of a cross section through the embryo shown in Figures 18-22,
Plate III. It shows the fundament of the glomus. X 110.

" 0. A portion of a cross section through a slightly older embryo, showing
the glomus in a more advanced stage of development. X 110.

" 10. A portion of a cross section through an embryo of Stage V., showing
a branch of the aorta which gives off a small vessel to the glomus.
X 150.

■

. ■ ■

• -

' u .

■ ■ .

■■

4*

■

(

(cupl

...

spfp/u

■

■

■ ■ ■

■■

Field. — Pronephros in Amphibia.

PLATE II.

All the Figures on this plate are magnified 110 diameters.

Figs 11 and 12. Portions of two frontal sections through the pronephric thicken-
ing of one of the older embryos belonging to Stage III.

Fig. 11 shows the dorsal margin of the thickening.

Fig. 12 shows a section through the nephrostomal region.

Figs. 13 and 14. Portions of two frontal sections from one of the younger embryos
of Stage II.

Fig. 13 shows the ventral ends of the anterior protovertebrae.

Fig. 14 shows a section through the dorsal portion of the pronephric thickening.

Figs. 15-17. Portions of three cross sections through one of the younger embryos
of Stage III.

Fig. 15 shows the anterior end of the pronephric thickening. The plane of the
section passes a little behind the middle of Somite II.

Fig. 16 shows the pronephric thickening in the region of Somite V.

Fig. 17 shows the thickening near its posterior termination.

■

•o'plu

■-"'■</"</•>■

■

II,

ir>

■

Field. — Pronephros iu Amphibia.

PLATE III.

Figures 18-22 and 27 are magnified 110 diameters; Figures 23-26, 260

diameters.

Figs. 18-22 Portions of a series of cross sections through an embryo of Stage IV.
In Figure 18, the pronephros of the right side is shown ; in the
remaining Figures, the pronephric organs of the left side. The
location of the several sections on the reconstruction (Fig. 30) is
shown by the series of lines bearing corresponding numbers.

Fig. 18 shows the first nephrostome.

Fig. l'J is from a region between the first and the second nephrostomes.

Fig. 20 shows the second nephrostome.

Fig. 21 shows the third nephrostome and the anterior portion of the segmental
duct.

Fig. 22 shows the segmental duct in the middle trunk region.

Figs. 23-26. Cross sections of the fundament of the segmental duct near its pos-
terior termination, from an embryo of Stage IV.

Fig. 23 shows the duct five sections in front of its termination.

Fig. 24, three sections before its termination ; a., b., and c, cells in the fundament
of the duct ; <d., portions of the two cells c. and </., which are to be
seen in the following section.

Fig. 25 shows the duct one section in front of its termination ; c. and (/., cells in the
rudiment of the duct ; b. and c, portions of two cells bearing the same
lettering in Figure 24.

Fig. 26 shows the depression of the somatopleure (/. ) directly behind the tip of the
fundament of the duct.

Fig. 27. A cross section from a larva whose pronephros is shown in Figure 41.
It shows the opening of the segmental duct into the cloaca.

■ I , .

ta.

myim ■■>« (m ,

1

•sbticd
Jhdqps -

'iiiT/n

■

' I

llpi

h'stm '

■

,<>,:

i

'■,': ;..; | ffd ;';/

fndcp, ; '•*'■'

■so pi l J

5° .■•■'''. jp/>/u

spl'plu.

■•■pi,;

ytptti

i-nrf

cor!

m.) tin

tbl.prnph

rulmbni ,

,„■■■

c/r.

rn

Jm,

/)/ Ktf

o«/,

fnddLtq ec'drm f> ecflrt

apl'pln

.sop/u
soplu. M P lu

Flew). — Pronephros hi Amphibia.

PLATE IV.

Fig. 28. Part of a cross section through the anterior trunk region of a larva
belonging to Stage VI. The Figure shows the pronephros in the
region of the first nephrostome. X 110.

Fig. 29. Part of an oblique longitudinal section through a larva of Stage IV. The
plane of the section was directed so as to cut the somatopleure tan-
gentially along the line of the three nephrostomes. Its direction is
represented by the line 40 in Figure 20. X 110.

Fig. 30. Part of a cross section through the middle trunk region of a larva,
from which Figure 28 was also drawn. (Stage VI.) X 110.

Fig. 31-38. A series of diagrams illustrating the convolution of the pronephric
tubules. These diagrams, which are based upon reconstructions
from cross sections, merely serve to show the number and approxi-
mate location of the loops in a longitudinal direction. No attempt
has been made to indicate in the diagrams the changes in position
undergone by the tubules in a transverse direction. The gray tint
represents the common trunk and the anterior portion of the seg-
mental duct. The first nephrostomal tubule and the collecting
trunk are colored pink. The second and third tubules are repre-
sented in yellow and orange respectively.

Figs. 31-34 are from various larvae of Stage V.

Fig. 32 is a diagram of the reconstruction shown in Figure 40.

Figs 33 and 34 represent the right and the left pronephros respectively of the same
individual.

Figs. 35-37 are from various larvae of Stage VI.

Fig. 36 is a diagram of the reconstruction shown in Figure 41.

Fig. 38 is from a larva of Rana halesina.

Figs. 39-41. A series of reconstructions from cross sections of larvae in different
stages of development. In Figures 40 and 41, the common trunk
and the anterior portion of the segmental duct have been shaded
without color ; the collecting trunk and the first nephrostomal
tubule have been colored pink ; and the second and third nephro-
stomal tubules are respectively yellow and orange. X 65.

Fig. 39. Right pronephric pouch of a larva belonging to Stage IV., viewed from
the median side. The X's represent the position of the nephro-
stomes. The lines 18-21 show the various levels at which the sec-
tions represented in Figures 18-21 were made.

Fig. 40. Right pronephros of a larva belonging to Stage V., viewed from the
median side.

Fig. 41. Right pronephros of a larva belonging to Stage VI., viewed from the
ventral side, the external face being uppermost.

I

r

'i >»/

■

>■

K

■

Wi,

" 1 . 4

■

mi

Field. — Pronephros in Amphibia.

PLATE V.

Fig. 42. A portion of a cross section through the anterior trunk region of a larva
of Bufo, belonging to Stage V. The Figure shows the pronephros
in the region of the first nephrostomy X 110.

Fig. 43. A section through the rudiment of the duct near its hinder tip, from an
embryo of Bufo belonging to Stage IV. X 500. Zeiss apochr.
4 mm. Oc. B.

Fig. 44. A portion of a cross section through the anterior trunk region of an
embryo of Amblystoma belonging to Stage III. The section shows
the pronephric thickening in the region of its greatest development.
X 65.

Fig. 45. A portion of a cross section through the anterior trunk region of an em-
bryo of Rana belonging to Stage IV. The section shows the pro-
nephric pouch in the region of the second nephrostome. X 110.

Fig. 46. An embryonic blood corpuscle occurring in the glomus of a larva of
Bufo belonging to Stage V. X 955.

'

44

In pr'vr

%

%

%

m •

i

/ >

i i

■

//»/./ •

Field. — Pronephros in Amphibia.

PLATE VI.

Fig. 47. A portion of a cross section through the anterior trunk region of a larva
of Bufo belonging to Stage V. The Figure shows the pronephric
structures in a region between the first and second nephrostomes.
X 158.

Fig. 48. A portion of a cross section through the middle trunk region of an em-
bryo of Amblystoma belonging to Stage I. X 65.

Fig. 49. Anterior face of a cross section through the glomus of a larva of Bufo
belonging to Stage V. X 470. Zeiss apocr. 4 mm. Oc. 6.

Fig. 50. Anterior face of a portion of a cross section through the right glomus of
the same larva, including also the opposite peritoneal wall. X 710.
Zeiss apochr. 4 mm. . Oc. 12.

Fig. 51. A cross section (right side, anterior face) through the pronephros repre-
sented in Figure 37. The section passes directly in front of the
third nephrostome, and shows the expanded region of the common
trunk at the level of its union with the collecting trunk. X 90.

Fig. 52. A portion of a cross section through the glomus of a larva of Bufo be-
longing to Stage V. The Figure shows an infolding (opposite the
letters azL") of the outer peritoneal layer of the glomus. X 500.

.

0~'

jo.

■

Ty

•

_

>' ...

■

rit.su.
dri.

■

48. . \ i

-

« « • *,

1 * .

'"

lap

/n.s iln>i

Field. — Pronephros in Amphibia.

PLATE VII.

Fig. 53. A portion of a cross section through the posterior trunk region of a larva
of Amblystoma belonging to Sta«e VI. The section shows the fun-
dament of the second primary mesonephric tubule (<ii. sp.). X 90.

Fig. 54. A portion of a cross section through the middle trunk region of a larva
of Amblystoma belonging to Stage VI. The section shows one of
the "cords of cells" which occur between the mesonephros and the
pronephros ; it exhibits a case of nuclear mitosis in the peritoneum,
which suggests the origin of these cells. X 500. Zeiss apochr.
4 mm. Oc. 8.

Fig. 55. A portion of a cross section through the anterior trunk region of an
embryo of Amblystoma belonj,'in<,' to Stage III. The pronephric
thickening is shown in the region of the middle of Somite II.
X 00.

Fig. 56. A portion of a cross section from the same series. The pronephric
thickening is shown in the region of the posterior face of Somite II.
X 90.

/ill/ ..

PL.V1I.

vnrrtf.

nl
ec'dmi i$i

nirlin

ms'rJn

i/.y//

ignspi.

t'al

00 nil,

!>i tw

ind ijlm'

,n,l"

yri.crd.

■

lltsq ,

I

rnvtiH

ii In

cms/

.>. >

prvr.

riispi.

prw

cmsprnph

Field. — Pronephros in Amphibia.

PLATE VIII.

Figs. 57-60. Reconstructions of several pronephridia of Amblystoma larvae be-
longing to Stage V.

Fig. 57. Reconstruction of a pronephros showing three nephrostomal tubules.

Figs. 61-65. Reconstructions of several pronephridia of Amblystoma larvae be-
longing to Stage VI.

"

.

o8

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