Bulletin of the Museum of Comparative Zoology at Harvard College

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BULLETIN

OF THE

MUSEUM OF COMPARATIVE ZOOLOGY

AT

HARVARD COLLEGE, IN CAMBRIDGE.

VOL. XXI.

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

UNIVERSITY PRESS:

Joun Witson AND Son, CAMBRIDGE, U.S. A.

613334
a Se

CONTENTS.

No. 1.— Contributions from the Zodlogical Laboratory. XXIV. Contribu-
tions to the Morphology of the Turbellaria.—I. On the Structure of
Phagocata gracilis, Leidy. By W. M. Woopworrn. (4 Plates.) April,
Soe:

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

No. 38. —Contributions from the Zodlogical Laboratory. XXVI. On Some
Points in the Anatomy and Histology of Sipunculus nudus, L. By H. B.
Warp. (3 Plates.) May, 1891 .

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

No. 6. — Contributions from the Zodlogical Laboratory. XXVIII. The De-
velopment of the Pronephros and Segmental Duct in Amphibia. By
H. H. Fierp. (8 Plates.) June [August], 1891 .

PAGE

185

201

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

In the fall of 1887, Mr. HE. 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.
Mark, 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) situated 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, ? in., breadth, 5. 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, contractilis, mucosa, an-
tica auricularia. Aperture due, ventrales, ad os et ad generationem
pertinens. Proboscides multe.

1 Contributions from the Zodlogical Laboratory of the Museum of Comparative .
Zoology, under the 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. l. 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 fontis Pennsylvanie.

“ 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 Planariz (’51, p. 264) he
uses the name proposed by Haldeman, “ Planaria gracilis,” and says
that it is “common about Cambridge in pools and rivulets.” He
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%, p. 2), “ Die Plana-
rien und Nemertinen Nord-Amerikas,” the species is described under
the name given to it by Leidy, but no mention is made of the most

striking 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): “Das erwachsene Thier soll 23 Riissel haben, die es beim
Fressen alle hervorstreckt ; Ref. vermuthet, dass der Riissel eine trich-
terférmige ausgezackte Miindung besitzt, und dass die beweglichen Fort-
sitze des Riisselrandes fiir ebenso viele einzelne Riissel gehalten worden
sind.”

~

MUSEUM OF COMPARATIVE ZOOLOGY. 3

Diesing, like von Siebold, was incredulous ; in his ‘‘ System,” he says
(750, p. 207), “(Esophagus protractilis multi partitus (proboscides
multz Leidy).” Twelve years later, in the “ Revision der Turbellarien,”
he writes (’62, p. 506), “ esophago multipartito.”

Stimpson (758, p. 23) in his Prodromus apparently followed Diesing,
for he says “cesophago protractili multipartito.”

Recently, Professor Leidy (’85, p. 49) has figured Phagocata gracilis
ina popular account of “ Planarians.” These are the only descriptions
of Phagocata I have been able to find.

Phagocata gracilis, Lerpy.

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 44 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. 20a). 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 6and 20c¢). 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 the
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 auriculate 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. /actea, 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

6 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
first 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 Lumbriculide 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 LI. 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 500 nitric acid. I have used
with entire success a modification of his method, viz. a cold saturated solu-
tion of corrosive sublimate in 500 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, Grenacher’s alcoholic borax carmine
followed by differentiation with acid alcohol proved to be the most use-
ful and reliable method. I have stairied both zn toto and on the slide.
Good sections for topographical study were obtained by staming in
alcoholic borax carmine for 24 hours and cutting in the horizontal plane
sections 30 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 14% silver nitrate for the purpose of
demonstrating the epithelium, Depigmenting was accomplished by the
use of a 1% 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-
ricr 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 lijima (’84, p. 366), and are directly related to local modifi-
cations of the hypodermis.

I 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 the head region move in different directions, as Minot
(77, p. 407) has observed in the case of other fresh-water planarians.

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 Dendroccelum 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 trne 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 auf die Basalmembran aufsitzen,’ but are con-
nected with it by fine processes “‘ welche etwa kammférmig 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 Koérperinnern.” His figure (Taf. XX.
Fig. 4) is confusing, and in addition was drawn, as he admits, from a
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 zz 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 zntercellular 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 Fidchen ... 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 planarians] 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, ‘‘ The 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 planarians. 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 ») both abso-
lutely and in proportion to the length of his largest specimens (20 mm.),
whereas my sections were eat 5-10 » 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 aia the rhabditi
that lie between them. There are no unicellular glands in it. Lang
(784, 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 Rhabdocceles.

The dermal rods or rhabditi are defined by Graff (’82, p. 49) as “die
stark lichtbrechende glasartige homogenen Stibchen, welche weder
einen Faden noch einen Nadel einschliessen und durch ihre glatte Ober-
fliche, regelmassige Gestalt und ihren Glanz auffallen.”

12 BULLETIN OF THE

In Phagocata 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 orifice. 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 D, 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 » to 164 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 Rhabdoceeles 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 is by 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. Later, 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 comes 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 Ausliufer, deren Epithelzellen
reichlich damit gefiillt sind.” According to Moseley (774, 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-
hypodermal 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 eté temoin une seule fois de Ja rup-
ture d’une cellule productrice chez Mesostomum tetragonum ; il m’a été
impossible de retrouver dans cette cellule rompue la moindre trace du
noyau.” Tijima (84, p. 371) writes as follows: “Die Bildungszellen
sind rundlich und mit einem ausserordentlich feinkérnigen Inhalt ver-
sehen.” And again: ‘‘ Haben die Rhabditen ihre definitive Grésse er-
reicht, so durchbrechen sie die Zellenwand, welche 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 rhabditi 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 find
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 as
«‘junge kugelige Stabchen.” Iam inclined to regard them as residual
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 Rhabdoceels 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. 56)
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. 69), 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
mesenchymatése Gebilde, die eine factische Wanderung durch dass sie
umgebende Bindgewebe unternehmen, wihrend ihr Inhalt sich zu den
fadenférmigen Stiibchen ausbildet. Endlich treten sie durch die Basal-
membran (wovon spiter die Rede sein wird), driingen 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 hypodermis, 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: “ Lasst man sie
[planarians] aber in Uhrschalchen mit Wasser langere Zeit unbehelligt,
so dass sie sich festsetzen, und stért sie dann plitzlich, so ziehen sie sich
stark zusammen, machen heftige Bewegungen und suchen zu entfliehen.
An der betr. Stelle aber findet man bei schneller Untersuchung Massen
von Rhabditen in allen Stadien der Auflésung, und wenn man das Wasser
schnell ausgiesst, findet man dort ein Kliimpchen zahen Schleim, — die
Stiibchen ldsen 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
value of the dermal rods have been given by naturalists. The larger
number of observers consider them homologous with the nematocysts? of
Celenterates ; 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 dertmal 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,” ‘Schleimstibchen,” ‘ Schleimbléckchen,” and “ Korner-
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,.—wno 1. 2

18 BULLETIN OF THE

gonopore and at the edges of the body. Another proof consists in
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 ‘“ Stabchendriisen,” 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 ‘‘ Schleimprépfchen.”

If we are to consider the parent cells as glands, what part do the
rhabditi play in the economy of the worm? 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 this 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
Rhabdoceeles 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 and 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,
birnférmiger, sich hauptsachlich mit Picrocarmine intensiv farbender
Drisen.” Graff (’82, p. 375) says, concerning the same species, ‘‘ Ueber-
dies finden sich hier unter der Haut zahlreiche einzellige birnformige 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 stabchenformigen Korpern oder von irgend
einem Pigmente etwas wahrnehmen konnen.” (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 Kérpers in beson-
deren Driisen und farben sich dusserst intensiv in Carmine und Hama-
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 Anschauung ist auch heute noch die von
Schultze gegebene und von Stein auch fur die Stiibchen der Infusorien
adoptirte, wonach die Stabchen indem sie dem dussern Drucke einen
Widerstand entgegensetzen, in uhnlicher Weise befordernd auf der
feinere Tastgefiihl der Haut einwirken, wie der Nagel auf Tastver-
mégen 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.
The 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 slowly 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; 1p and 6.5m 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 (’84, p. 375), does not exist, nor is there any appear-
ance of the fibrous structure described by Lang (84, p. 63) 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 pigment 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,
’814, p. 193) and Planaria abscissa (Lijima, ’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 a 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 Planaria 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 1).
Branching ends were observed only 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 pseudoccele; 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, mw. 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 pseudoceele.
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 psendoccelar 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 ste//ate 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 Pseudoccelous 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 evidences from embryonic conditions. Graff (’82,
p- 72) was unable, from the evidence found in Rhabdoceeles, to estab-
lish “a distinction between muscle fibres and connective-tissue fibres ”’ ;
and Hamann (’85, p. 96) has shown that in Echinoderms 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 Rhabdoceeles.
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 Rhabdoccles, and most prominently in
the Mesostomide. 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 Rhabdoceles 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 Rhabdoceeles 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 IL. 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. 16).
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, eth. 7.). 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 through 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 been able to demonstrate
that the layer is a true epithelium. Isolated pharynges were killed
with hot 14% 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 IL. 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. 151), 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

ments.” Iijima (’84, p. 389) also saw “eine senkrechte Streifung.”
Lang (’81, p. 196, and ’84, p. 109) speaks of it as a ‘ cuticuladnliches
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 the outer to
the inner epithelium, to both of which they are attached by their finely
branched ends (Figs. 12 and 16). 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 16, 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 pseudoccele
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
lijima. 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 1V. 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 pressure, 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 diverticule 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, 2. 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 57 and 38 represent portions of
two successive horizontal sections close to the ventral surface. The
sections are 30 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 (0) 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 (a. /.’p.) together with transverse commis-
sures (com. t.) and lateral nerves (n./.). It may be seen from Figure 37
how the median branches from the peripheral nerves (2. p7i’ph.) break up
into a network or plexus, which is distributed to the muscles (plz. 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 ofa 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
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

parts,

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 Pl. polychroa and in D. lacteum, and
Loman (’87, p. 76) has found the same conditions in Bipalium suma-
trense and B. javanum. In Gunda ulvee and Pl. abscissa there exists,
according to Tijima (’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 Pl. 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 (?) condition shown in
Rhynchodesmus.

The brain is formed on the same plan as that of Gunda (Lang, ’81,
p- 67; ’81*, 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, n.). 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 4. 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 II]. Figs. 26-31,
Plate IV. Figs. 39 and 40), but I cannot say 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,
con’t. 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 a 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. 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 such 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 stiil 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 product of one of the parent 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, e’th.).

I have not succeeded in ascertaining the exact manner in which the
spermatozoa find their way into the vasa deferentia, 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. 482), 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 vesiculee seminales. In Polyclads and in Rhabdoceles 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. 460) 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 pends 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 meshwork 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. con’t. tis.). Tijima (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. 35

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, vt’m.). 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 rndimentary 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
which 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 appear 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 —wno. 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 resembling 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 Dotterstrange durch Vermehrung einzelner
Zellen, welche in dem Mesenchym sich befinden, ihren Ursprung nehmen.”
But his evidence that the “ Dotterstrange”’ arise i 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. 63) maintains that “ le vitellogéne n’est autre chose qu’une partie
différenciée de l’ovaire,” and according to Lang (’814, p. 228), “ Die
Keimstécke und Dotterstécke der Tricladen sind einander gleichwerthig.
Sie entstehen aus Zellen, die anfangs nicht von einander unterscheiden
lassen.” Among Rhabdoceeles 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 organs.. 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 Keimdotterstécke 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
Pl. tenuis (Iijima), inasmuch as the cells which form it still retain a more
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, wt.) 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 IL. Fig. 15, e’th.), 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
T 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 lijima (84, p. 419) asa
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 “un liquide

MUSEUM OF COMPARATIVE ZOOLOGY. ob

spécial” to support the vitality of the male elements, and possibly to
aid in fecundation. He states that in Pl. polychroa the cocoon is pro-
duced in the uterus, but as regards Dendrocelum lacteum he agrees
with lijima 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 “ muskulésen Driisenorgan” of Iijima
(84, p. 422), or the “ vésicule (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 more remote they are from it, the smaller they are.

The rhabditi or dermal rods lie between the cells of the hypodermis,
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 “Stibchenstrassen ” of Rhabdocceles are homologous with the
slime glands in Phagocata.

38 BULLETIN OF THE

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

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

The pseudoccelar 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 Rhynchodesmus.

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 vesicule
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 Rhabdocceles. 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.

atr.
cu.
cl. rhb.

cl. sp’2.
com. a.
com. p.

com. t.
con't. tis.
dt..sal.
eth.

eth. ex.
eth. i.
e’th. phy.
go’ po.
drm.

Wdrm!

in.

mb. ba.
ms’chy.
Mu. cre.
MU. Cre. ex.
MU. CFC. i.
mu. l.

mu. 1. ex.

(he

mu.

ABBREVIATIONS.

Genital atrium.

Cilia.

Parent cells of the rhabditi.

Parent cells of the sperma-
tozoa.

Anterior commissure of the
brain.

Posterior commissure of the
brain.

Transverse commissure.

Connective tissue.

Salivary duct.

Epithelium.

External epithelium.

Internal epithelium.

Epithelium of pharynx.

Gonopore.

Hypodermis.

Aborted cells of hypoder-
mis.

Intestine.

Basement membrane.

Mesenchyma.

Circular muscles.

External circular muscles.

Internal circular muscles.

Longitudinal muscles.

External longitudinal mus-
cles.

Internal longitudinal mus-
cles.

mu. T.
Mu. SAg.

n.

n I.

n, la.

nl. con’t. tis.
nl. e’th.

n. l.’p.

nl. rhb.

n. opt.
n. pi’ph.

Radial muscles.

Sagittal muscles.

Sensory nerve.

Lateral nerve.

Anterior longitudinal nerve.

Nucleus of connective tissue.

Nucleus of epithelium.

Posterior longitudinal nerve.

Nucleus of parent cells of
the rhabditi.

Optic nerve.

Peripheral (marginal) nerve.

Mouth opening.

Eye.

Oviduct.

Penis.

Median pharynx.

Lateral pharynx.

Nerves to muscular plexus.

Rhabditi.

Secretions which do not
form rhabditi.

Spermatozoa.

Anterior trunk of intestine.

Lateral trunk of intestine.

Uterus.

Vasa deferentia.

Vagina.

Parovarium (vitellarium).

Enlarged ends of vasa def-
erentia.

pati pains
Hfetyige?
ie

rt huey ;
pibslale

e? ip: i
SOS.
-atety Teuteatr

cave By ee (fred

WoopwortTu. — Phagocata,

Fig. 1.

x

10.

PLATE I.

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

Cross section near the lateral maggin of the dorsal side, in a region
where there were no rhabditi, showing the hypodermis in its primitive
condition. X 900.

In Figures 1 and 2 the basement membrane did not take the stain.

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 900.

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.

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

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.

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.

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

Showing the appearance of the rhabditi after having been acted upon by
pieric acid. X 900.

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
(n/. rhb.) in Figure 10 are not represented as being granular, as they
should be.

WooDWORTH-PHAGOCATA Py

B Meise] lith Boston.

Woopworts. — 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 yet differentiated. > 300.

15. 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. > about 20.

20a. Outline to show the appearance of the living worm while in progres-
sions |) <2:

20) 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. XX 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 300.

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WoopwortTH. — Phagocata.

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 (x. pi’ph.). The right-hand side of the section is a little
more dorsal than the left. X 52.

“ 96-31. Froma series of cross sections through the brain region. The sections
are taken at intervals of 604. Figure 26 is the most anterior. 52.

“ 99-36. From a series of horizontal sections through the brain region, cut
from the dorsal side. The sections are consecutive, and 30m in
thickness. X 52.

B Meisel lth. Boston

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5
a

a or ere

Wows

io

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- a

ee

s

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Woopworta. — Phagocata.
i=]

PLATE IV.

Figs. 57 and 88. ‘Two consecutive horizontal sections (80 in thickness) from the

ventral side passing through the floor of the pharyngeal chamber.
Figure 57 is the more ventral, and shows the marginal nerve; the re-
lation of the latter to the longitudinal trunks is evident upon comparing
Figures 387 and 38. X 27.

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

“41. From an isolation preparation, showing one of the sub-hypodermal glands
from the region of the gonopore. > 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.
x 450.

“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 260.

* 46. Sagittal section through the brain, showing the two commissures. 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. 380.

WooDWORTH-PHAGOCATA

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No. 2.— Zhe Compound Eyes in Crustaceans. By G. H. ParKer.!

TABLE OF CONTENTS.

PAGE PAGE

Ii Introduction . . . 1... & GolniCumacer: 1 221: cs = 99

BieGieBetinaky ii Jeih aj pet @.toiSehizopoda,). ....,.-, « .99

III. Arrangement of the Ommatidia 60 8. In Stomatopoda .. . . 104

IV. Structure of the Ommatidia. . 66) 9Sin Decapoda. ./. = ~« - 108

iinvAmphipoda <.., . = .) 68 V. Ommatidial Formule . . . 115

2. In Phyllopoda . . . . . 73} VI. Innervation of the Retina. . 116

8. In Copepoda. . . . . . 77| VII. Theoretic Conclusions . . . 118

4.InIsopoda .... . . 84| VIII. Bibliography . . . .. .« 181

5. In Leptostraca . . . . . 98| 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.
In order to familiarize myself with the subject, I determined to study
at first in detail the structure of the 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 Holl, 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,
Evadue, 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 Zodlogical Laboratory of the Museum of Comparative
Zodlogy, 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, 790°, 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 cells, —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
ommatidium consists of a cluster of cells regularly arranged around a

MUSEUM OF COMPARATIVE ZOOLOGY. 47

central axis. With very few exceptions, the different ommaitidia in the
retina of any given Crustacean agree with one another in the number
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*, 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.

Tue RetTINa.

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 Nebaliz, and the
Branchiopodide, and is represented by a simple thickening in the super-
ficial ectoderm.

Branchiopodide. —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 has 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.

Tsopoda. —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, when 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. ‘

_ Lhave 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.

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

Stomatopoda. — 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 Nebaliz 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 Nusbaum.

Decapoda, — Carriére (’85, p. 169), 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 Homarus (Parker, ’90*, p. 5), and I have since seen the same con-
dition in Gelasimus, Cardisoma, Cancer, Hippa, Palinurus, Pagurus,

VOL. XXI.—No 2, 4

50 BULLETIN OF THE

Cambarus, Crangon, and Palemonetes. There is, therefore, considera-
ble ground for the support of Carriére’s 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 Branchipodide, the
Nebaliz, 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 TYPE 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 Apuside, the Estheride,
and the Cladocera.

Estheride.—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.

Apuside, — In-Apus, according to Grobben (’79, p. 256), the plan of
the eye is essentially similar to that in the Estheride. 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 Estheride (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 hypodermis 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 retina 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 Apuside
and Estheride, or may become closed and partially obliterated, as in
the Cladocera. The right and left retinas either remain separated, as in
the Apuside, or become closely approximated, as in the Estheride, 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 Apuside, in which the retinas are sepa-
rate and the optic pocket permanently open, depart only slightly from
the primitive condition. In the Estheridz, 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 Apuside, the Estheridz, 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 Apuside and Estheride.

In the THIRD RETINAL TYPE, as in the more differentiated 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 Carriére (’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. crn’con.), 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. x. opt.). In
addition to the capsular membrane, the eye in Gammarus possesses
still another membrane (Fig. 1, mé.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 proximal ends of the retinular cells project. This membrane, there-
fore, has the form of a meshwork. According to Carriére (’85, 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. Not unfre-
quently the nuclei of certain accessory pigment cells (Fig. 2, nl. ’drm.)
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 diffivult to decide whether a given
nucleus was zz 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. 161,
Fig. 125), 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
Carriére (85, p. 161, 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 eyes, instead of
one, contrary to the condition in the more typical Amphipods. 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 Carriére, 785, 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 Claus, ’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 ;
proximally, 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 Gammarus, 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 Gammarus. In this type the retina is characterized by
two peculiarities: first, it is not continuous with the hypodermis, 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 790, p. 89) has demonstrated in Orchestia and Sunamphitoé,
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 tothem. 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-
nation ; 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 ocenpied 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 Grenacher (’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 may 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 retinular 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, ce/.). This peculiar condi-
tion was first clearly described by Leydig (’50, p. 331), although as early
as 1806 Jurine (’06, p. 447) remarked that the eye in this genus was
contained in a transparent membranous sac, which apparently contained
a fluid, and Miiller (’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 Claus (’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
Amphipods 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. ba.).

Grobben (’79, p. 258) has suggested that possibly the eye 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 Estheride, 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 modificdtion of the type presented by
the Amphipods. The satisfactory determination of this question can
be settled, however, only by embryological evidence. _

Eucopepoda. —In adult specimens of those true Copepods which
possess rndiments of the lateral eyes, — the Pontellide and Coryceide,
—the retina is apparently separated from the hypodermis. In the
Coryewide 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 Corycxide and Pontel-
lidee 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 (’80, 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 the 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 Cumacez, 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-
chiopodide, the Nebaliz, Stomatopods, Schizopods, and Decapods. In
the Isupods, 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 Apusidze and Estheride, or it may become closed, as in the
Cladocera. In the Apuside, so far as I am aware, the eyes are not

60 BULLETIN OF THE

capable of motion, and in the Estheride they are, if at all, only slightly
movable. In the Cladocera, where the second type probably reaches its
sreatest 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 retinule). This separation
is accomplished either by the formation of a corneo-conal membrane, as
in Amphipods, 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,! 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 made 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), Nebalia (Claus, ’89, Taf. X. Fig. 10), Gammarus
(Sars, ’67, p. 62), Orchestia (Frey und Leuckart, ’47, p- 204), Phronima (Claus, 779,
Taf. VI. Fig. 48), Cymothoa (Miiller, ’29, Tab. IIT. Figs. 5, 6, Bullar, ’79, p. 514),
Lygidium (Lereboullet, ’43, p. 107, Planche 4, Fig. 2°), Serolis (Owen, ’48, p. 174),
Arcturus (Beddard, 90, Plate XXXI. Fig. 4), Anceus (Hesse, ’58, pp. 100 and 103,
Dohrn, ’70, Taf. VIII. Figs. 33, 34), Sguilla (Milne-Edwards, ’34, p, 117, Will, 740,
p- 7, Frey und Leuckart, *47, p. 204, Leydig, ’55, p. 411), and Mysis (Sars, ’67,
Planche III. Fig. 7, Grenacher, ’79, Taf. X. Fig. 112). Ihave observed the hexag-
onal arrangement in the following genera: Apus. Branchipus, Estheria, Evadne,
Argulus, Gammarus, Caprella, Talorchestia, Idotea, Serolis, Porcellio, Spheroma,
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 VIII.). 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 ina 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,! as well as in three families of the
Macrura, the Hippide, Paguride, and Thalassinide,? the arrangement

1 The presence of hexagonal facets has been recorded in the following genera
of Brachyura: Portunns (Will, 40, p. 7); Zlia (Will, 40, p. 7, Leydig, 755, p. 411) ;
Cancer ; Maja; Carpilius (Frey und Leuckart, “47, p. 204); Herbstia, 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 Dromia 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 Guanhumi, Latr.,
Cancer irroratus, Say, and Gelasimus pugilator, Latr., are hexagonally grouped.

2 The outline of the corneal facets is stated to be hexagonal in the following
genera: Pagurus (Swammerdam, ’52, p. 88, Cavolini, 92, p. 180, Milne-Edwards,

MUSEUM OF COMPARATIVE ZOOLOGY. 63

of the ommatidia is invariably hexagonal. In the remaining macrurous
Decapods? 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,? 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 (734, 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-

“04, p. 117, Will, ’40, p. 7, Frey und Leuckart, 47, p. 204, Chatin, ’78, p. 8); |
Callianassa ; and Gebbia (Milne-Edwards, 54, p. 117). In Pagurus longicarpus,
Say, and Appa talpoida, 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: Ga/athea (Will,
"40, Fig. III. c.); Astacus (Miiller, 26, Tab. VIL Fig. 13, Leydig, ’57, p. 252,
Fig. 134, Reichenbach, 786, Taf. XIV. Fig. 226, Huxley, ’57, p. 353); Homarus
(Newton, 73, Plate XVI. Fig. 3, Parker, 7902, p. 8); Palemon (Grenacher, ’79,
Taf. XI. Fig. 118 A, Patten, ’86, Plate 31, Fig. 115); Peneus (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 Palemonetes vulgaris, Say.

2 According to Chatin ('78, p. 13) the outline of the facet in Typton 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 (’86, Plate 31, Fig.
116), the ommatidia are hexagonally arranged, although it must be borne in mind
that Will's (740, Fig. III. c.) 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 (’40, 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
put 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 im Homarus, 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.
In Typton, 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 the 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. Possibly he may have examined some other
than G. strigosa, 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. IEf
this is not the true explanation, it is still possible that the specimens
studied by Patten were somewhat immature, in which case the hexagonal
arrangement might very naturally be present. From what has been said,
I think it must be evident that the apparent contradiction in Will’s and

i

MUSEUM OF COMPARATIVE ZOOLOGY. 65

Patten’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°, 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%, 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?, Plate IV. Fig. 55),
the space between the cones of adjoining ommatidia is considerable ;_ in
adults, it is proportionally very much less (compare Parker, ’90*, 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 preceding 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-
theids, Palinuride, Astacidee, and Carididz. 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.

Tur 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 greatest 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.

.
— a)

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
larvee 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

68 BULLETIN OF THE

the ommatidia 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 Carriére (’85, p. 156) in Gammarus, by Claus (’87, p. 15) in
the Platyscelidze, by Della Valle (88, p. 94) in the Ampeliscide, and
by Watase (’90, p. 295) in Talorchestia. I have also identified this struc-
ture in Gammarus, Caprella, and Talorchestia.

In Gammarus, as Carriére (’85, p. 156, 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. VI. 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 hypodermis of this same character also occurs in Talorchestia,
although in 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, ther@fore, 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. In 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. nw.) 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 Della Valle (’88, p. 94), how-
ever, in some of the Ampeliscidz 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%, p. 103) ; Cyamus (Miiller, ’29, p. 58, Frey und Leuckart, ’47, p. 205); Gam-
marus (Miiller, 29, p. 57, Frey und Leuckart, ’47, p. 205, Pagenstecher, ’61, p: ol,
Sars, 67, p. 61, Leydig, ’78, p. 235, Grenacher, ’79, p. 109); Hyperia (Gegenbaur,
* 788, p. 82, Grenacher, ’79, p. 111, Carriere, ’85, p. 160) ; Phronima (Pagenstecher,
61, p. 31, Schmidt, 78, p. 5, Claus, ’79, p. 151); Talitrus (Grenacher, ’79, p. 109);
and the Platyscelide (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 Blane’s (’83, p. 635) 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.?

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- 112), 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 Jn Caprella (Mayer, ’82, p. 122), in Gammarus (Grenacher, 779, p. 110, Car-
riere, 85, p. 156), in Hyperia (Grenacher, 74, p. 652), in Oxycephalus (Claus, ’71,
p. 151), in Phronima (Schmidt, ’78, p. 5, Grenacher, ’79, p. 112, Claus, ’79, p. 130),
in Talorchestia (Watase, ’90, p. 296), and in the Platyscclide (Claus, ’87, p. 15).

In Gammarus ornatus, 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, aud 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 cl. rti.',
Figs. 4-7).

In Talorchestia, according to Watase (90, p. 296), the retinula is
composed of only four cells. I have studied T. longicornis 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 Della Valle’s
(88, p. 94) observation, that in the Ampeliscide the retinule 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 Ampbipods, 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- 111) has shown that, in transverse section, the distal end of the
rhabdome is cross-shaped. In G. pulex, according to Carriere (’85,
p- 157), the distal end of the rhabdome in section shows four rays, the

ie BULLETIN OF THE

proximal five. In Carriere’s opinion, these rays indicate the five rhab-
domeres. In Gammarus ornatus, the species which I have studied, the-
rhabdome (Plate I. Fig. 6, rib.) 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 (785, 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’drm.). 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 (Carriére, 785, p. 161); a third kind is applied to the
retinule, 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 ommatidium.

These three kinds of accessory pigment cells, with the possible excep-
tion of those which surround the retinula, occur in the lateral eyes of
Phronima (Carriére, ’85, p. 164).

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 (Claus,
’79, p. 125, Carriere, ’85, p. 160), a uame 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 detinitely
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 Branchiopodide and Apuside,
the other in the Estheridze and Cladocera. On account of the greater
convenience, the eyes in the Apusidze and Branchiopodide will be con-
sidered first, then the eyes in the Estheride, and finally those in the
Cladocera.

Branchiopodide and Apuside. — The ommatidia in these two families,
and especially in the Branchiopodide, 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,
the 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 LV. Fig. 30, nd. h’drm.) is similar to that described by Claus
in B. torticornis. According to Patten (86, p. 645), 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 Miiller
(29, p. 56), Burmeister (35, p. 533), Zaddach (’41, p. 46), and Frey
und Leuckart (47, 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. 295). 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 Claus (86, p. 320), and in
B. Grubii according to Patten (86, p. 645), the corneal cuticula is
unfacetted.

The cone in Branchipus, as Spangenberg (’75, p. 30) first demon-
strated, is composed of four segments. This observation has since been
confirmed by Grenacher (79, p. 115), Claus (’86, p. 320), and Patten
(86, p. 645). 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 (’86, 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. 653) 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 LV. 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, rib.) it is circular, or at
times squarish, but never pentagonal, as might be expected from the
fact that it is surrounded by jive retinular cells.

The retina in B. vernalis contains no other cells than the three kinds
already described. According to Clans (86, 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 Branchiopodide and Apusidz 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.

Estheride.— 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. Woodworth.

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, x. ern.). 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 freqnently 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 Estheria
californica and E. tetracera described by Lenz (’77, p. 30). In Lim-
nadia Agassizii, however, cones composed of fowr 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. 360) statement that in Limnadia
the cone consists of four segments may be accurate, contrary to the
opinion of Lenz.

76 BULLETIN OF THE

The retinular cells in Limnadia cover the greater part of the sides of
the cones, and completely hide the rhabdome (Plate IV. Fig. 36). 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, ed. 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 Estheridee
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 Evadne 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. pr’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, Claus, ’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 (Carriére, ’84, p. 678).
Weismann’s assertion (’74, p. 364) that the cone in Leptodora is com-
posed of four segments is disproved by Carriere’s later observations, and

MUSEUM OF COMPARATIVE ZOOLOGY. Fa

Claus’s statement (76, p. 372) 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 the 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 jive
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 Claus, 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 Claus (’63, p. 47) has already stated,
is provided with a spherical Jens (Plate IT. Fig. 18, Zvs.), 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
(x. 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, one for each retina (compare Claus, *63, Taf. IIT.
Fig. 6), 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 lamin (Plate III.
Fig. 29, lvs.). A considerable portion of its distal surface is intimately
connected with thé superficial cuticula (Plate II. Fig. 18), 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-

2 he

MUSEUM OF COMPARATIVE ZOOLOGY. 79

mals only in that their concentric lamelle 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 number 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 not 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 size, 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
hypodermal thickenings which occur beneath the adjoining cuticula
(Plate II. Fig. 18, and Plate III. Fig. 29, h’drm.). 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 hy-
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
(758, p. 71) described such lenses in Sapphirina, and Leuckart (59,
p- 250) observed similar ones in the lateral eyes of Coryceus 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 Claus (’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 Coryceus the lens is implanted
in the superficial cuticula, and further describes it in Corycus as com-
posed of two parts, an outer and an inner. According to Grenacher
(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 Pontellidee and Coryceide 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, I believe that a similar origin will be demonstrated
for these parts in the Coryceide. The differences which Grenacher
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 Ponteila; 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 Grenacher.

The retina and lens in Pontella are not separated by an intervening
space as in the Coryceide, 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 transverse sections this mass is
seen to consist of two bodies (cl. con. 1, and el. con. 2, Fig. 25), which
extend as far as to the lens (compare Figs. 25-27). Each body con-
tains a nucleus (v/. con., Figs. 25 and 27) and consequently represents
acell. 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 Coryczus,
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 always 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
(nl. 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. rtn.’).

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. h’'drm.). ‘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
scme 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 Ponvtella would be as follows: corneal hypodermis, undifferen-
tiated ; cone cells, two; retinular cells, five; undifferentiated pigment
cells (ectodermic?) 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 2, in Grenacher’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 z
by 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, ca/.), The cells in
this layer (Fig. 12, /’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, cta.), which, as
Miiller (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. pr’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,
con.), which lie with their distal ends usually somewhat below the
preconal membrane (Fig. 13). Each cone, as Claus (’75, p. 256) bas
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. pr’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’drm.),
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. h’drm.') 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 me
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 Sphzroma, were kindly fur-
nished me from the collections in the 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
types was first identified by Grenacher. 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. JBellonci (’81%, p. 98, Tav. II. Fig. 11 n.) figures similar cells
in the ommatidium of Spheroma, and Beddard (’90, p. 368) 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 n/. con. and nl. crn. in
Figs. 50 and 51, Plate V.).. Inan 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, 753,
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_
(67, 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 probable 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 Miller.
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 Bellonci (’81*, p. 98) in
Spheroma. 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
Serolis 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*, 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 Isopods, 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%,
p- 98) in Spheroma, by Sye (’87, p. 23) in Jeera, and by Beddard |
(790, p. 368) in Arcturus. In the three genera which I have examined,
Idotea, Asellus, and Spheroma, 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 (’67, 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 parts. 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 (’90, Plate XXXI.
Figs. 1 and 4), cones of three segments are occasionally met with.

The cellular composition of the retznula in Isopods was first made out
by Grenacher (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-
sisting of seven cells has also been demonstrated by Buller (’79, p. 513)
in Cymothoa, and by Beddard (’88, p. 443) in Aiga and Ligia. As
Beddard (’88, Plate XXX. Fig. 13) has shown, the seven cells in the
retinula of Auga pass through the basement membrane and become con-
tinuous with the nerve fibres. In Porcellio, as I have observed, the
fibrous ends of the seven retinular cells not only can be identified as nerve
fibres below the basement membrane, but each cell contains a well de-
veloped fibrillar axis (Plate V. Fig. 46, ax. .), 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
seven retinular cells (Plate V. Fig. 48). In positions either distal or
proximal to this, however, only szz 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. rtn!.). 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 about 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 funce-
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. 368), the retinula is also composed of six cells.

In Spheroma, Bellonci (81, p. 98, Tay. II. Fig. 12) has figured and
described a retinula consisting of jive 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 retinulz of Isopods may be as few as five. My own observations,
however, do not confirm Bellonci’s acgount. In the species of Spheroma
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 Grenacher (’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. Lach ray projects
into its retinular cell, not between two cells. My own observations on
Porcellio confirm Grenacher’s statements. A second representative of
this type of rhabdome has been described by Bellonci (’81, p. 98) for
Spheroma. Here, however, the rays, although they agree in number
with the retinular cells, project between the cells, not into them.

The second type of rhabdome is well represented in the eye of Arc-
turus furcatus. In this species, according to Beddard (’90, pp. 368,
369), 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 lost.

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 Grenacher (79, p. 107), and it is probable
that the pigment cells described by Bellonci (’81, p. 99) as intervening
between the retinular cells in Spheroma 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, nl. 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. Grenacher believed that those in Porcellio are undifferentiated
hypodermal cells ; this interpretation probably holds good for those in
Spheeroma and Idotea.

The hyaline cells, the second kind of accessory cells, have been iden-
tified by Beddard (’87, p. 235, ’88, Pl. XXX. Fig. 9, 2.) in Aga 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. Unditferentiated 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. 389-341) about seven years ago. Recently
Beddard’s observations have for the most part been confirmed by Watase
(90), and it must now be admitted withont 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, n/. ern.) 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 be confused with the nuclei of the cone
cells, which lie in a deeper plane. In making sections, the corneal
-euticula 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 60 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 i sit on the corneal cuticula.

The eyes in the aduuw, 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 an
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 (vl. crn. 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 1s that shown in
Figure 66. This includes only the most superficial layer of the retina,
and contains two nuclei (compare v/. crn. 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. crn. 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 surronnding 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 be identified in a posi-
tion somewhat proximal to that in which the smaller of the four nuclei
occur (compare ni. 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. Schythei
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 8. Schythei.

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 rualbtivine five struc-
tures can be observed (Fig. 61). Four of these (Fig. 61, rhb’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 I have described as the limits of the different pieces.

Evidently the squarish masses (72b’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 cells 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 8. 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
already 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
(788, 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 strue-
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 (’90, Plate XXIX. Fig. 1) has 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 satistac-
torily the retinule, 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 his 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 this genus there are two such
cells for each ommatidium. I believe that the number has been given
correctly, for 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. oa

vanced embryos (compare nl. dst., in Figs. 65 and 69). In transverse
sections at this stage (Fig. 69) 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 n/. 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. Not 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 ni. dst. in Fig. 60 with nl. rtn.! in Fig. 63).

Because of this resemblance, I believe that the pigment cells which
surround the cone can be fairly 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*, 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 ommatidinm 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
for the ommatidia of many Isopods.

The retinula in the species of Spheroma which I studied presents
an appearance which suggests the differentiation of simple retinular cells
into proximal and distal cells. In Sphzeroma 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 Sphe-
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. 68 with Figs. 71 and
72). The plane of separation in the cone of Spheeroma divides the retin-
ula by passing through the single small retinular cell shown in the lower
part of Figure 58 (Plate V.) and between 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 Sphe-
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 retinule. Beddard (’84%,
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. Watase (’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 retinule, certain small nuclei
which are usually surrounded with more or less black pigment. These,
I believe, represent the cells described by Beddard and Watase. In the
embryo certain scattered nuclei (ni. 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. Iam 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, fill 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°, p. 22) has
remarked, the number of these cells is subject to variation, there being
sometimes one, sometimes two, for each ommatidium. In 8. 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. 450) 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%, 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 (’90, 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 ameeboid 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. 64), 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 presence 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 Aiga; 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.— No. 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 ectodermic (‘) origin.

Leptostraca,

The histological structure of the ommatidia in the Nebaliz has been
investigated, so far as I am aware, only by Claus (’88, pp. 65-84). 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 (Claus, ’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 greater 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 two sets described by Claus. The proximal nuclei, which, accord-
ing to Clans, belong to the retinular cells, occupy positions not unfre-
qnently 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.

Cumacee.

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 Cumacez. 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 wheu examined
in’ sections, showed no evidence of eyes. My material consisted of
specimens of Diastylis quadrispinosa, G. O. 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. 8S. I. Smith.

Schizopoda.

The species of Schizopod the eyes of which I have studied is Mysis
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 Miiller’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 (/. crn.) lie in a plane somewhat nearer the external surface of
the eye than the nuclei of the cone cells (ni. con.). In transverse sec-
tions at the proper levels, each ommatidium will be seen to contain two
elongated nuclei (Fig. 75, nl. ern.) 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 76). 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 Semper’s nuclei mentioned by Claparéde (’60,
p. 194) in Mysis flexuosa, and described by Sars (67, p. 33) in M. ocu-
lata. Nusbaum (’87, p. 179) also observed four similar nuclei in the
developing eye of Mysis chameleo, and Grenacher (’79, p. 118) described
the same number in Mysis vulgaris. In the last named species, accord-
ing to Grenacher, 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
to 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,
Grenacher (’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, cta.). In this respect, therefore, different species probably vary.

The cones in Mysis vulgaris, according to Grenacher (’79, p. 118), are
composed of two segments. The same number is also present in the
cones of M. stenolepis (compare Figs. 76-78, 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, mJ. pa.).

In Mysis the number of cells comprising the retinula is at least seven
(Figs. 85-87). Possibly, as I have elsewhere suggested (Parker, ’90*,
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 retinulz 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 to6. 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 IJ., 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 rudimeutary retinular cell in Homarus (compare Parker,
790"; pp. 20,21).

The riabdome in Mysis stenolepis lies in 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-
cher (’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 Grenacher’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, Grenacher 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 86). 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, how-
ever, is one which is more 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 Grenacher, and to regard it as belonging to the cone cells
rather than the rhabdome.

The distal retinular celis 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. Hach 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, ni. 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 hama-
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 Iam 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 ameeboid
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 ?) 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. W. 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 only 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. 94-96, nd. ern.) 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, n/. 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 (’68, p. 17), the cone is also
composed of four segments.

The retenular 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. px.) are
located near their distal ends.

The number of cells in the retinula of Squilla, as described by Grena-
cher (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 [ have already
shown in Mysis, may suggest the presence of eight in all, one of them
being rudimentary. This condition 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 6 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 has
essentially the same structure as ommatidium 1. In ommatidium 3 the
nucleus corresponding to the one seen in ommatidium | 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 6. 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 6 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 are 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 6 (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 6 (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 proximally, 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 Grenacher (79,
Taf. XI. 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 Grenacher’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 Grenacher in Squilla is prob-
ably artificial, as Grenacher himself suggests. In Squilla, according to
both Steinlin (’68, p. 17) and Grenacher (’79, p. 125), the rhabdome
in transverse sections is subdivided into four equal parts, somewhat as
in Mysis. I have not observed this condition in Gonodactylus.

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, x/. 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 Mysis,
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 hypodermis, two ; cone cells, four; proxi-
mal retinular cells, eight, one of which is rudimentary ; distal retinular
cells, two ; accessory cells (mesodermic ?) present.

Decapoda.

I have studied the eyes of the following species of Decapods: Gelasi-
mus pugilator, Latr.; Cardisoma Guanhumi, Laty.; Cancer irroratus,
Say ; Hippa talpoida, Say ; Palinurus Argus, Latr. ; Pagurus longicarpus,
Say ; Homarus americanus, Edw. ; Cambarus Bartonii, Fabr ; Crangon
vulgaris, Fabr.; and Palemonetes 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.
Richards for specimens of Palemonetes collected by him at Newport,
R. I. A number of eyes of two Crustaceans, Cardisoma and Palinurus,
were kindly obtained for me by Mr. Isaac Holden; they were collected
on the coast of Florida by Mr. Ralph 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
(86, pp. 626 and 642), who observed it in Penzeus, Palemon, 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%, p. 6).
More recently I have observed it also in Palzemonetes (Plate IX. Fig.
103, cl. ern.), 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 Carriére’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 correctly
interpreting them. In his account of the ommatidium in Palemon,
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. XI. Fig. 117, 2.) represent, in his opinion, the nuclei of the cone
cells; the more proximal (Fig. 117, A x’.) he considers as differentiated
parts of the cone itself. The positions occupied by these bodies in
Palzemon, and by certain bodies which I have observed in Palemonetes
(Plate IX. Fig. 103), are so similar that I believe the structures in the
two genera to be homologous. In Palemonetes the distal bodies lie in the
cells of the corneal hypodermis (Fig. 103 cl. ern.), and are the nuclei of
these cells. They represent what Grenacher considered the nuclei of the
cone cells in Palemon. The proximal bodies in Palemonetes (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
ofthe 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
Paleemon according to Grenacher (79, p. 123), and in Palemonetes
(Plate IX. Fig. 103, crn.) 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°, Fig. 2), or transversely, as in Palemonetes (Plate IX.
Fig. 105). In the hexagonal facets it either bisects opposite sides, as in
Cancer (Plate X. Fig. 126), 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 subdivided by 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 Paleemon, 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 seen 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 Pal-
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 (’68,
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 (’66, p. 93) in Palamon, and by Schultze
(67 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 rvetinula 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 which
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. Aaa

in this genus, as in Herbstia, it was maintained that there were only
four cells. Subsequent investigators have not confirmed this conclusion.
In transverse sections of the retinula of Palemon, 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 retinule of Astacus and Portunus. Since the publication of
Grenacher’s observations, a retinula containing seven cells has been
seen in Astacus by Carriére (’85, p. 169), in Penzus, Palemon, Gala-
thea, and Pagurus by Patten (86, pp. 630 and 643), and in Cambarus
by Watase (’90, p. 299).

In Homarus, as I (’90%, 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 Palemonetes, Palinurus,
Cambarus, Crangon, and Cancer, I have succeeded in ascertaining the
number both of the functional cells and of the nuclei in the retinule.

In Palemonetes each rhabdome is surrounded by at least seven re-
tinular cells (Plate IX. Fig. 114, cl. pzx.). 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 Paleemonetes, 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

In 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.! 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 #, 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 z). Imagine the
arrangement in Figure 118 extended over a large surface. The groups of four
cone cells could then be 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 any 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 one belongs to the rudi-
mentary cell.

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

The retinule in Decapods, according to all recent observers, contain
seven functional cells. In Homarus, Palinurus, Cambarus, Crangon,
Paleemonetes, and Cancer, the retinule 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 Palemon, 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 Palzmonetes (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 dista/ retinular cells in Decapods were collectively rec-
ognized by Miiller (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 Carriére (’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 Paleemonetes
(Plate IX. Fig. 108, cl. dst.) and in Cancer (Plate X. Fig. 127, el. 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 Palemonetes (Plate IX. Figs. 103 and 109, nl. dst.) the
nuclei are grouped distinctly in pairs, one pair for each ommatidium,

Each cone in Penseus, according to Patten (’86, p. 634), 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
Penzus. I am therefore inclined to doubt the accuracy of these two
observations.

The interommatidial space in the basal part of the retina in Pale-
monetes contains a light pigment similar to that described in the retina
of Mysis. Like this the pigment in Palemonetes 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
Penzeus, and by myself (90%, p. 25) in Homarus. I have also recently
observed them in Crangon, Cambarus, Cardisoma, Pagurus, and Pali-
nurus, as well as. in Palzmonetes.

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

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 (?)
in origin, often present.

TABLE OF OMMATIDIAL FoRMUL2.

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 formule 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 different
ommatidia in the same individual.

TABLE SHOWING THE CELLULAR COMPOSITION OF THE OMMATIDIAN
CRUSTACEANS.

Retinular Cells.
Cells of 3 er
Corneal Differentiated. Accessory
Hypo- Cells.
dermis. Proxi-
mal.

Groups of Crustaceans.
Distal.
Amphipoda,

Branchiopodide and Apuside,

Estheride,

Cladocera, i pr. (ect. %)

Copepoda: Pontella, , pr. (ect. ?)
Sapphirina, g
Argulus, i a

Isopoda: Idotea, pr. (ect. 2)
Porcellio, pr. (ect. 2)
Serolis, d pr. (ect. #)

pr. (ect. ?)
pr. (mes. 2)

pr. (mes. %)

pr. (mes. ?)

A few features in the table require explanation. Among the number
of cells recorded for the Estheridz, 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 for 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 fibrillee, 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, I 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,
Sphzroma, Serolis, Gonodactylus, Mysis, Palemonetes, Crangon, Cam-

MUSEUM OF COMPARATIVE ZOOLOGY. 1 lig

barus, Palinurus, Pagurus, Cancer, and Cardisoma. In the majority of
these, a fibrillar axis could be distinguished. In Cambarus, as in Homa-
rus, the nerve fibrillze 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, az. 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. 6-8), Porcellio (Plate V. Fig.
46), Idotea (Plate V. Figs. 53 and 55-57), Mysis (Plate VII. Figs. 87-89), Gono-
dactylus (Plate VIII. Figs. 101, 102), Paleemonetes (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 fibrillee of the optic nerve in the rhabdome
supports Miiller’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 fibrille
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 Stomatopods, 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", 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 reserable
the nuclei of the proximal retinular cells (x/. px.), and differ consider-
ably from those of the accessory pigment cells (nl. h’'drm.). 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 retinule 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 Spheroma, 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. 94), and Palemonetes (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*, 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 VIL. Figs. 77, 78, and 87). Consequently,
the axis in the distal cells either represents a rudimentary nervous 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 celis, 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 retinul, 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 retinulz, 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 retinule
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

1 ep 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 ommatidinm 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 Estheride
can be easily derived, and the retina in the Estheride represents a con-
dition from which the retina of the Cladocera may have arisen. That
this series of retinas, from Apus through the Estheride 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 Estheride 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. 123

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 four 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 formule
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 hypodermis 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, Nebalise, 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, Nebalize, 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 ommatidium 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. 86 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 retinule with five, six, or seven cells. These retinule 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 Branchipus, 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
arhabdomere. 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 retinule 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
Branchipus and Argulus, and with one of two cells in Amphipods. The
relation of the two kinds of retinule 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 retinule 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 Nebaliw. I believe, therefore, that all the
evidence at present deducible from the condition of the simpler retinule
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 Coryceide, 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 in
Pontella the 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, Coryczeus, and Copilia each retina is provided with a single lens,
but in Irenzus, 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 retinula 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 retinule 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 as 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 Chetopod 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 origin 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 mor-
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 5 and if they can occnr 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
layer 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 retinular 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 Grenacher’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 Grenacher sug-
gests; but possibly the two kinds of eyes may have had totally different
origins.

MUSEUM OF COMPARATIVE ZOOLOGY. 131

°

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ie

EXPLANATION OF FIGURES.

All the drawings were made with the aid of an Abbé 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 }%.

a.
ax. n.

brs. oc.

cl. con.

cl. ern.

el. dst.

cl. hyl.

cl. ms’drm.
cl. px.

cl. rtn.’

oy

cl. rud.
ench.
cal.
con.

cp. sng.
crn.
cta.

d,

dsc.

dx.

gn. opt.
h’drm.
hp.

in.

Ins.

mb. ba.
mb. ern.
mb. ern’con.

ABBREVIATIONS.

Anterior.

Axis of nerve fibrille.
Optic pocket.

Cone cell.

Cell of corneal hy podermis.

Distal retinular cell.
Hyaline cell.
Mesodermice cell.
Proximal retinular cell.
Retinular cell.

Rudimentary retinular cell.

Shell.

Body cavity.

Cone.

Blood corpuscle.
Corneal cuticula.
Cuticula.

Dorsal.

Sucking disk.

Right.

Optic ganglion.
Hypodermis.

Liver.

Intestine.

Lens.

Basement membrane.
Corneal membrane.
Corneo-conal membrane.

mb. vel.
mb. n. opt.
mb. pv’ ph.
mb. pr’con.
mu.

n. for.

nl. con.

nl. crn.

nl. dst.

ni. h’drm.
nl. hyl.

nl. ms’drm.
nl. px.

nl. rtn.’
n. opt.
oc.

v.
va. sng.

Intercellular membrane.

Membrane of optic nerve.

Peripheral membrane.

Preconal membrane.

Muscle.

Nerve fibre.

Nucleus of cone cell.

Nucleus of cell in corneal hy-
podermis.

Nucleus of distal retinular cell.

Nucleus of hypodermal cell.

Nucleus of hyaline cell.

Nucleus of mesodermic cell.

Nucleus of proximal retinular
cell.

Nucleus of retinular cell.

Optic nerve.

Eye.

Ommateum.

Posterior.

Pore of optic pocket.

Retina.

Rhabdome.

Rhabdomere.

Retinula.

Left.

Ventral.

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.

Fig.

“cs

bo

ge

PLATE I.

Gammarus.

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 116.

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.

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

A transverse section through the distal ends of the retinular cells and
cone. X 476.

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

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

A transverse section through the retinular cells somewhat proximal to
the basement membrane. > 475.

A transverse section through a single retinular celi in the region of its
nucleus. XX 475.

The proximal portion of a retinular cell viewed from the side. (Compare
Fig. 2.) Isolated in Miiller’s fluid. Not stained. X 478.

A cone isolated in Miiller’s fluid and viewed from the side. Not stained.
x 476.

PARKER — CRUSTACEAN EYE PL. I.

Zz:

cla. mbcrncon

_....nl.con

3....nbrin

GHP del. B Masel ih Becton
GAMMARUS 5 Meisel Jith Boston

PARKER. — Compound Eyes in Crustaceans.

14.

15.

5 alte}

19.

PLATE II.
Argulus.
(Figs. 11-17.)

A section in a plane transverse to the chief axis of the body and through
‘the right eye. Depigmented. x 140.

A longitudinal section of an ommatidium. X 475.

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.

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

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.

A transverse section through the rhabdome. Depigmented X 475.

A transverse section through the retinula somewhat proximal to the
rhabdome. X 4765.

Pontella.

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.

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. > 400.

BMeigel Jith Boston. —
COPEPODA.

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.in 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. 8) have been numbered in reference to the par-
ticular rhabdomeres with which they are associated. X 400.

PARKER — CRUSTACEAN EYE. PLL.

E Meisel Jith Boston.
PONTELLA.

gltit'a4

PARKER — Compound Eyes in Crustaceans.

Fig.

38.

39.

. 40.

41.
42.
43.

44.
45.

PLATE IV.

Branchipus.
(Figs. 30-32.) .

A longitudinal section of an ommatidium. x 400
A transverse section through the distal end of four cones. > 400.
A transverse section through the middle portion of a retinula. > 400.

Limnadia.
(Figs. 38-59.)

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

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.

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

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.

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

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

A transverse section through the central portion of four retinule. x 475.

Evadne.
(Figs. 40-45.)

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. > 140.

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

A transverse section through the proximal end of a cone. X 475.

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 478.

A transverse section through the central part of four rhabdomes. X 475.

A transverse section through a retinula. Depigmented. IMKleinenberg’s
alum-hematoxylin. X 476.

PARKER — CRUSTACEAN EYE.

nlhilrm
_.gi.opt.

45.

Gy \ @)

Ao@

6 FI clr tr.
4 eal G Z)

WY
~el.con

a

B Meisel ith Boston. —

PHYLLOP ODA.

PARKER. — Compound Eyes in Crustaceans.

Fig.

Fig.

Fig.

46.

47.

48.

49,

50.

PLATE V.
Porcellio.
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 476.

Idotea robusta, Kroyer.
(Figs. 47, 48.)

A transverse section through the distal end of a retinula. The bodies,
one of which is marked z, 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.

A transverse section through three ommatidia in the region of their

rhabdomes. X 475.

Idotea wrrorata, M. Edws.
(Figs. 49-57.)

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.

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.

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

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

A transverse section through the middle of a retinula. Near the centre
of each cell can be seen a small axis of nerve fibrilla. > 475.

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. 475.

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.

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

A transverse section of four retinular cells at the level in which their
nuclei occur. The axis of nerve fibrille 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).

Spheroma.
(Figs. 58, 59.)

A transverse section ofa retinula at a level slightly distal to the base
ment membrane. X 475.

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 fibrille is the
more transparent condition of the central part of the cells, due to the
partial absence of pigment granules. X 476.

PARKER — CRUSTACEAN EYE.

1SOPODA.

B Meisel lth Boston.

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

“

4

61.

62.

63.

64.

section includes a portion of a cone and the tissue lying between it
and two adjoining cones.

A transverse section of a retinula in the region of its rhabdome. The

' arrangement of the pigment granules and nerve fibrillz is indicated
in only one of the four cells. Of the two lines which appear to
separate the cone cells (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).

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.

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

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 occuffied 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.

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.

A transverse section at the level of the corneal hypodermis.

A transverse section through the distal end of a cone.

A transverse section made ina plane enly slightly proximal to that
shown in Figure 67.

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

A transverse section through the proximal ends of the cones.

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

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

7%

PARKER — CRUSTACEAN EYE.

PLVL

B Meisel Jeh Boston.
SEROLIS. see tact

are

sae
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Bias ¢ ig?
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PaRKER, — Compound Eyes in Crustaceans.

Fig. 73.

PLATE VII.

Mysis.

A longitudinal section of an ommatidium. The numbers at the left of
the figure indicate the levels at which the sections for Figures 75-89
were taken. X 475.

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

A transverse section of three ommatidia in the plane of the corneal hy-
podermis. X 476.

A transverse section through the distal end of acone. X 475.

A transverse section through the proximal end of a cone and the adjoin-

. ing distal retinular cells. X 475.

A transverse section similar to that shown in the preceding figure,
except that it is depigmented and stained in Kleinenberg’s alum-
hematoxylin. 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.

igang:
ste0n83:
STS.
Se CO:
telat
LOU
telah
F189:
yO)
ce nol;
coz:

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

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-hem-

_atoxylin. X 615.

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-hematoxylin. X 615.

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-hematoxylin. X 615.

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-hematoxylin. X 615,

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

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.

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.

A longitudinal section through the basal portion of one and parts of two
adjoining ommatidia. Depigmented, and stained in Kleinenberg’s
alum-hematoxylin. XX 615.

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.

A cone viewed from the side. Isolated in Miiller’s fluid and studied in
water. X 475.

te

PARKER — CRUSTACEAN EYE. PLVIL

h.Corny, = ESS) STE.
| fEN> aA

80.
sé ea _ xz
Le I
a.
Boo 50

B Meisel, lith Boston.
MysIs.

PARKER. — Compound Eyes in Crustaceans

Fig. 93.

aa

94.

101.

102.

PLATE VIII.

Gonodactylus.

Part of a tangential section through a superficial portion of the retina.
The extreme edges of the 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. > 275.

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.

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

A transverse section through the cells of the corneal hypodermis and the
distal end of the cone in a large ommatidium. X 275.

A transverse section through the distal part of a cone in a large omma-
tidium. X 276.

A transverse section through the middle of a cone from a large omma-
tidium. X 276.

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

A transverse section through six retinule 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. 475.

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

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.

. =
PARKER — CRUSTACEAN EYE. Pr, VIL
WE MLC: ntlcon. nloon. nlern
: d rare 5 ene
| ; s : Ss oes po 2 | : \ ¢ f oe P my \ =

B Meisel Ith Boston.
GONODACTYLUS.

[=
a

cS et ik ‘ te
a ak ¥ We i
i is ye af

4

PARKER. — Compound Eyes in Crustaceans.

PLATE IX.

Palemonetes.

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 z 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 ommatidia in the region of
the distal retinular nuclei.

Figures 110-112 represent three successive transverse sections, each through

five ommatidia, 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 ommatidia, 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.115. A transverse section of the retinula near the distal end of the rhabdome.

Depigmented.

114. A transverse section of four retinule at the level of the eighth retinular
nucleus.

115. A transverse section through four retinul in the region of the accessory
pigment cells; viewed by reflected light. The retinule 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.

PL.IX.

PARKER — CRUSTACEAN EYE

De® O<
©
g a}
@ ge” @O%,

q 6

B Meisel lth Boston

PALA.MONETES.

GHP del.

PARKER. — Compound Eyes in Crustaceans.

PLATE X.

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

Cambarus.

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
their distal retinular nuclei.

Palinurus.

Fig. 124. A transverse section through a retinula in its middle region. The out-
lines of the retinular cells cannot be 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 retinule. 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.

“ 151. A slightly oblique section through the basement membrane. The upper
part of the figure represents the retinule as seen im transverse sec-
tion distal to the basement membrane; the part marked mb. ba.
represents the region in which the membrane itself appears 1 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.

a

PARKER — CRUSTACEAN EYE Pu.X

ee

"o

; nips I) Si Pape
C5

GHP del. BMeisel lith. Boston
DECAPODA B Meisel, lith. Boston

No. 3.— On some Points in the Anatomy and Histology of
Stipunculus nudus, L. By Henry B. Warp.!

CoNTENTS.

PaGE PaGE
EL Jntroduction .... .°... . 148 2. Tentacular Fold . . . . 159
Methods) an soo: -s) ss, , 144 ao Oral Wall.) < ya. <. a »-, 159
II. External Anatomy ... . 145 b. Migratory Corpuscles . 160
ieintroverti: « . =. : . « 145 é¢. Musculature: . . . . 161
2. Tentacular Fold . . . . 147 d. Vascular System . . . 162
MS Eistology. « . . . . >. . 149 é. Aboral Wall. . .. . 164
ib body Walle. 3. os, 149 3. Nervous System . . . . 165
a. Cuticula and Hypodermis 149 Ge io as) wa, LGB
Bra WHAM cy Ss st oa), LOO . a. Ganglionic Cells . . 166
c. Pigment Cells . . . . 150 B. Internal Structure . . 169
d. Dermal Bodies. . . . 152 b. Cerebral Nerves . . . 170

a. Bicellular Glands . . 152 c. Ventral Nerve Cord and
B. Multicellular Glands . 155 le}tesat 5. a eres ee peen ae 4 |
y- Sense Papille . . . 157 4. Cerebral Organ... . 172
é. Museular Layers . . . 159!IV. Conclusions. . ..... 176
Bibliography . . . . . . . . 180 | Explanation of Figures . . . . 183

I. Introduction.

SomE two years ago, while working on Sipunculus nudus in the zoé-
logical laboratory at Gottingen under Prof. E. Ehlers, my attention was
attracted by a peculiar organ in the region of the dorsal ganglion ; and
Ithough 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 Naples Zovlogical 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 study
of the form seemed likely to yield results, and, on the advice of Prof.

1 Contributions from the Zodlogical Laboratory of the Museum of Comparative
Zoology, under the direction of E. L. Mark, No. XXVIL
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 reeent 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 impermeahle
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 cesophagus of adhering sand, the animals were brought into
a shallow dish of sea-water, and 50 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

a es 8

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 ef 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? (z. pap. p.), (2) a smooth zone (z. lev.),
(3) an anterior papillate zone (z pap. a.), and (4) the tentacular
crown (pli. éa.).

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 papillee are somewhat scattered. The lighter,
almost translucent appearance of the anterior portion of this zone, which
permits the central mass of the csophagus 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 wall of the first large composite nerve
given off from the ventral nerve cord (¢f. infra).

The papillz 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.’ Passing forward,
this general form is preserved until the point of transition from the dark
to the light portion of this posterior papillate zone isreached. Here the
papille 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 papille,
and leave irregular patches of skin entirely free. In breadth such a
papilla measures .25 mm.; in length, 37 mm. Iam unable to confirm
the statement of Andreae (81, p. 205), that they are arranged “in
gleichen Abstinden” ; for the relative distances are extremely variable,
being from 70 to 300 w 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 papille 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 papillz. 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 papille ; this measured 3 mm. in
breadth in the same specimen. The papille 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 papille
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. TrentTacuLaR Fo.p.

The tentacles (Tentakelmembran) originate in the larva as two folds
of the oral margin, —“‘lippenartige 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 Hatsvhek ('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, crnu. 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 papillee 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. 0. 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.- Bopy Watt.

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 (Hautkérper), 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

amination 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? 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 ameeboid 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 COMPARATIVE 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 ameeboid () cells, distinguishable from the surrounding mass by their
refractive power, and containing from five to many deeply stained nuclei
3m 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 hematoxylin, 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. Seme-
what similar cells were found by Biirger (’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 amceboid character, I was
unable to determine. It is to the presence of large numbers of these
cells that the papillz 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- 386) 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 Ehlers (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 haema-
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-
pille 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 p, 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 hematoxylin.
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 adjoiming cell. The space thus formed measures 12 X15 x 25 p, and
communicates with the exterior by means of a narrow canal opening
simply on the surface of the cuticula. The duct measures 6-8 p 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 penetrate
between the cells, which in consequence are separated only by their own
membranes (Fig. 6 or 11,*), 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
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. Lach 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). 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 hematoxylin, and by continual aggregation

1 Strictly speaking, Figure 8 represents the /ast 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 hzema-
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; and 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 papille, 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? The papillz 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

oS

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.?

The multicellular glands present a type easily distinguishable from that
just described. They are to be met with everywhere, not only in the
papille, 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 hematoxylin 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. Its 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.?

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 (gl.!"" n. fbr., 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 to 90 X 150 p in the same region of the body, that the
probability of a muscular capsule snggests itself strongly.

Allusion has already been made to the relation of the glands to the
papilla. 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 papille 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 of 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 be 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 papilla in any region are markedly
wider than those more recently formed, while the length remains nearly
constant throughout the entire zone.

Sense Papille. — The papille of the anterior zone are thickenings or
modifications of the hypodermis, rather than typical papillz like the
posterior ones; they correspond probably to the “ Wimperdriisen” 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 papille, 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
aduct. 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- 406), and to regard them as simple sensory organs. The retraction of
the apical area would then be a simple method for protecting the Jong 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 (Hisig,
"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 Capitellide, 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 Capitellidze 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 papillae 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 » in thickness. This rem-
nant passes over into the retractors in a manner to be decribed 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 cay-
ity, traversed perpendicularly by numerous trabecule 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
2m, 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 condition of the introvert led Selenka ('83,
p. Xvil) 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 11 by 4 xu.
These cells are exactly similar to those contained in the sensory organs
before described. Some such cells are seen in Figure 21, d, f (Plate IT.).
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 a 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 in diameter. My attention was first called to
them in a preparation stained by Hamann’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 may be 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.

3eneath 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 COMPARATIVE 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 indefinite mass of confused fibres.
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 the 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 tentacnlar
fold shows a few circular fibres, immediately adjacent to the blood cav-
ity, which turn into the trabeculz and cross into the corresponding layer
of the opposite wall of the fold. In addition to these, the trabecule
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 trabeculee and the longitudinal muscles, whereas the circular muscles,
which are comparatively scanty, are only of secondary 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 lacunz. 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.”

Lastly, lining the blood cavity and covering the trabeculz 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 ccelomic 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 ccelomic fluid, and not far from the average (cf. 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.

Tn 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 trabecule 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 csophagus, 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, e¢ al. All of these forms pos-
sess, according to the same author, long thin filamentous tentacles
(ef. 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 See Addendum.

164 BULLETIN OF THE

The numerous dermal canals close under the hypodermis of 8. 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 coelomic
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. On the other hand, the muscular walls of these vessels serve to
force the fluid out into the lacune 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 lacune, 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
general 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

+

_— ae erp si

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. Lrain.

The supracesophageal ganglion (gz. su’e., 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. zzfra).

The entire ganglion is covered by a capsule (Fig. 25, cps. enc.), 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 retractors is here fused with the dorsal wall of the cesophagus, in
which lies the dorsa] 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. d.), 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, con’t. 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 p,
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 off tangentially to neigh-
boring cells.. (Cf. Rohde, ’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 fibres (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 10) 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
Rohde (’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. JJ.) 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 », 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 mztome,
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? cells, represented in Figure 28,
Plate III. These vary considerably in size and shape ; the mean longi-
tudinal diameter is 55 y, the transverse 40m. 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 w in diameter, is
usually found nearer the end of the cell from which the nerve fibre
emerges, 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 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. Nuclei 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 al.). 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 20 and its nucleus
7 by 11 » 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 znternal 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 > asmall 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 (coms. 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. J., 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 (c/. gn. J/., 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. J/J.) 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 ventral commissure, where they are lost to view,
either because they are split up into a number of small ones, or from
some other cause suffer a diminution in diameter. This crossing of
fibres from cells on one side of the body to the connective? 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 (x. ta. 1, Fig. 22,
Plate II.) ; the posterior contains the second, third, and fourth tentac-
ular nerves and the cesophageal 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 portions of the fold. I was unable to trace the
ultimate termination of the nerves in this region.

The cesophageal 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,® as stated by other writers,’ — and into

1 IT 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.

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

MUSEUM OF COMPARATIVE ZOOLOGY. eet

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 cesophageal 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 well (Plate
II. Fig. 22, 7.). 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, JZ), 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 outward 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 (plz. x. drm., Plate
I. Fig. 4) with lateral anastomoses. From this network, fibres (7m. g/.)

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 (7m. mu.)
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 (7’gl., Fig. 5), in which discoid
nuclei (n’gl. nl.) measuring 2 by 4.5 by 6 w 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 fibrille appear in the transverse sec-
tion as dots. These are also the fibres which are connected with the
cells of the multicellular glands (g/.’” 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 8.
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 2mm., to the anterior ventral
surface of the brain, where it terminates blindly (0. 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 (can. o. ceb., Plate II. Fig. 26) 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 are usually seen crowded
in masses along the summits of the ridges (cl. pig., Fig. 26). It is prob-
ably this canal which was found by Keferstein und Ehlers (761, p. 47) in
S. tesselatus. The canal is correctly figured (Taf. VII. Fig. 1, 2, «, w’),
but they evidently mistook its true character, since they say: “ Ausser-
dem sieht man vom Hirn zum Tentakelkranz einen aus zwei Hadlften
bestehenden, dicken Strang verlaufen, der dort endet, und an dem End-

MUSEUM OF COMPARATIVE ZOOLOGY. 1 es

punkte, wie man bei der Betrachtung von aussen her wahrnimmt, in
der Haut von einer Gruppe kleiner Falten umgeben ist als wenn er eine
Rohre ware und hier nach aussen mundete.”? 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.”

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 (sp/t.), 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 ftihrenden, 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-
eal character. In the same paper he says: “Das Gehirn stellt sich als eine knopf-
artige Verdickung des diesen Canal auskleidenden, mit der Epidermis zusammen-
hingenden 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
(0. ceb.) which I regard as the cerebral organ proper. Macroscopically,
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 » 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.?) and fibres extend at right an-
eles to the surface into an irregular group of fibres (transsected in Fig.
27, coms. 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 brain. The cells of the inner limb of the tubes
secrete 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 8. 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 protection, or perhaps due to the
development of the glandular area, 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. I 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 (90) for
S. Gouldii, it will be noticed that, while there is a general similarity, a

MUSEUM OF COMPARATIVE ZOOLOGY. yay

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 8. Gouldii, and the multicellular of 5. nudus agree with neither
group described for 8. 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 lack of figures. On the other hand, Andrews has emphasized
the fact that a close agreement exists between the dermal bodies of
S. Gouldii and those of various Phascolosomes. Again, in the arrange-
ment of the musculature, in the uniform unbanded 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 (’83) to Sipunculus, which include 8. 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-
chets 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 Annulata, 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 Cheetopods, and by Birger
(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 Birger (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 Birger in Nemertines, and with the first type in Sipunculus. The
first class of Rohde agrees in general with the second of Biirger ; but

VOL. 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 Polychets 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 Nemertines and
Class II. of the Chetopods. In both Chetopods and Nemertines 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
(Hisig, 87, and Friedlander, ’89). Now, either these “giant cells” are
neomorphie in both groups, and hence not at all homologous, or the
Sipunculids were separated from the primitive stem before the separa-
tion of Nemertines 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
heen 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 Spengel (’80, p. 487), and more recently
for Thalassema by Rietsch (’86, 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
by Shipley (91) on Phymosoma (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 cecal 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

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77. Anatomische Mittheilungen titber Gephyreen. Amtlicher Bericht der
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vom 17. bis 22. Sept., 1877. p. 189.

’'80. Beitrage zur Kenntnis der Gephyreen. I1.. Die Organisation des
Echiurus Pallasii. Zeitschr. f. wiss. Zool., Bd. XXXIV. Heft 3, pp.
460-538, Taf. XXIII-XXVI. 30. Jul, 1880.

’81. Oligognathus Bonelli, eine schmarotzende Eunicee. Mitth. aus d.
Zool. Station Neapel, Bd. III. Heft 1, pp. 15-52, Taf. IZ-IV. 9. De-
cember, 1881.

Vogt, C., und Yung, E.

’88. Lehrbuch der praktischen vergleichenden Anatomie. Bd. I., 906 pp.

mit 425 Holzschnitten. Viehweg und Sohn, Braunschweig, 1888.

EXPLANATION OF FIGURES.

All figures were drawn with the aid of an Abbé 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.
cl. gn. I., II., III. Ganglionic cell L., I,

el. fil.
cl. pig.
cl. sns.
coms. a.
coms. d.
coms. @.
COMS. v.
con't. tis.
cps. ene.
ernu. d.
ct.

cta.

gl.”

gl.” dt.
gl.” env.
gl.” nl.

gl.” dt.

gl” env.

or III.
Filamentous hypoderm cell.
Pigment cell.
Sensory cell.
Anterior commissure of brain.
Dorsal « bo
Cisophageal connective.
Ventral commissure of brain.
Connective tissue.
Capsule of brain.

Dorsal horn of tentacular fold.

Cutis.
Cuticula.
Bicellular gland.

Duct of bicellular gland.
Envelope “ a
Nucleus se
Vacuole s rs

Multicellular gland.
Duct of multicellular gland.
Envelope o s

gl’ n. fbr’ Nervous fibrilla to multicel-

gl” nl.

gn. swe.

lular gland.

Nucleus of the multicellular
gland.

Supraesophageal ganglion.

Wdrm.
lew’cy.
mb. ba.
mit.

mu. cre.
ngl.

n’gi. nl.
n. mu. ret.
nl. sns.

n. spl.

n. ta.

o. ceb.

or.

pa mit.
pap.

pli. ta.
plx.n. drm.
pr’e. dq.
rm. gl.
rm. mu.
spht.

va, sng. d.
va. sng. v.
z. lev.

Hy podermis.

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 i ss
Sphincter of cerebral canal.
Dorsal blood-vessel.
Ventral blood-vessel..
Smooth zone of introvert.

z. pap.a. Anterior papillate zone of

2. pap. p.

introvert.
Posterior papillate zone of
introvert. ;

i i

asi oi ay

ly yore eee Ot ee

e

Rsk cg ‘i. y

ia iv sn . vt ti ios
EN ae
Wh geld atrrl el eal Ghar ire etl

> 7 wD

Le alodeitn 4) earn
ie : pay i

Owe atilfvnis
She gS avi “3 civ 1% iy Mp: ‘
; ine dc aa”
Rib x. ft aw. itis vaciu ti
vag, bs iter A ois Latte a)

» De i | ‘

re : Ven
; ng to ot bie phy
Wieden iyo

Warp. — Sipunculus.

Fig.

Oa

PLATE it

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.

Anterior aspect of tentacular fold. The figure is diagrammatic only to
the extent that secondary folds are omitted. Camera outline. Simple
microscope. X 4.

Sagittal section of introvert. Diagrammatic in regard to details.. Simple
microscope. X 8.

Longitudinal section of body wall of introvert in the anterior portion of
the posterior papillate zone. Muscles diagrammatic. Boéhmer’s hema-
toxylin. Zeiss 3. A. X 98.

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.

Sections of bicellular glands in the three dimensions of space. Kleinen-

berg’s hematoxylin. Zeiss apochr. 4mm. Oc.6. X 425.

Soon after the beginning of secretion. ‘The membrane dividing the two
cells is shown at *.

At the period of greatest activity in secretion.

At the close of secretive activity.

Transverse section of duct of bicellular gland immediately below the
cuticula. Zeiss apochr.4mm. 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 428.

12-14. Multicellular glands. Zeiss apochr.4mm. Oc.6. 400.

12.

13.

14.

Longitudinal section. The duct is filled with a secreted material. Klei-
nenberg’s hematoxylin.

Transverse section. At the left centre of the section a cell has fallen out.
Hamann’s carmine.

Longitudinal section to demonstrate nuclei and connection of gland cells
with nerve fibres. Mayer’s cochineal.

WARD .— SIPUNCULUS.

Pr. I.

<_< 00.000)

10. 2 2G

y 7 way &

ke
| \ bd r fo
fi | \e\-.gllen

j | *
Crna.d. ==
‘

B Meisel ith. Boston.

WarD. — Sipunculus.

PLATE IL

Fig. 15. Transverse section of epithelium of tentacular fold to show the leucocytes

in situ. Hamann’s carmine. Zeiss 1. E. 500.

16-18. Sense papillze from anterior papillate zone of introvert.

16.

i:

18.

no:

20.

21.

Tangential section through a single papilla. Hamann’s carmine. Zeiss
LD ex 300:

Transverse section. Apical area retracted. Hamann’s carmine. Zeiss
1D ssl:

Transverse section. Papilla fully expanded. Orth’s picro-litho-carmine.
Zeiss 1. D. X 220.

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.

Median sagittal section of brain. Plane of section indicated on Figure
22. Mayer’s cochineal. Zeiss 1. A. X 50.

Cells of tentacular epithelium isolated by maceration ; a, c, and e, sensory,
b, d, and f, filamentous cells. Zeiss apochr.4mm. Oc.8. X 725.
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.— SIPUNCULUS.

= _.CAN.O.Ceb,

7a,

AS FE

B Meisel lth Boston.’

m7 : ast
j re) eres —
peta! ”~

‘- hig

Warp — Sipunculus,

26.

27.

PLATE III.

Transverse section of hypodermis of tentacular fold with sensory cells.
Weigert’s picro-carmine. Zeiss apochr. 4mm. Oc. 8. X 725.

Transverse section of brain. Plane of section shown in Figure 22, Plate
II Grenacher’s alcoholic borax carmine. Zeiss 1. A. X 50.

Transverse section of brain. Plane of section shown in Figure 22, Plate
Il The sertion was cut somewhat obliquely, and the right half lies
posteriad. Grenacher’s alcoholic borax carmine. Zeiss 1. A. X 60.

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 80.

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 cta. 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.4mm, Oc 8. X 510. Fig. 28, Class

32.

III. Fig. 29, Class II. Fig. 30, Nuclei of Class I. Fig. 31, Class I.

Oblique section through a ganglionic cell of Class II., showing the
regular arrangement of the paramitone. Hamann’s carmine. Zeiss
apochr. 4mm. Oc. 8. X 728.

WARD.— SIPUNCULUS.

Ay, CPS ene, : NN
y et) hae i :
K oe ng,
Me nether., Mh }

f ee

B Meisel, lith Boston.

No. 4.— Three Letters from ALEXANDER AGASSIZ to the Hov.
MarsHaLL McDonatp, 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 U. S. Fish Commission Steamer “Albatross,”
LIEUT. COMMANDER Z. L. Tanner, U.S.N., Commanding.

1

STEAMER ALBATROSS, Panama, U. S. oF CoLomBra,
March 14, 1891.

My pEAR CoLoneL McDona.p : —

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
quite 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
from 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 very
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 precretaceous 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.

Iam 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
Salpze, 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
raade on the Atlantic side of the Isthmus did we come upon such masses

en

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
“ Albatross” tell me they have not met before on the west coast of
North America. The Crustacea have supplied us with a most remark-
able type of the Willemoesia group. The paucity of Mollusks 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 find 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 Apiocrinide. The number
of Ophiurans was remarkably small as compared 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.
Gorgoniz 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 Globigerine on the surface.
Only on two occasions during a calm did we come across any number

188 BULLETIN OF THE

of surface Globigerine and Orbulinz. On one occasion the trawl came
up literally filled with masses of a species of Rhabdamina closely allied
to R. lineata. Thus far no pelagic Algz 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 faunz, 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 faune, 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 toa 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

EEE

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 brought 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 neither 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 Medusz, 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 on the 22d of February, and returned to Panama
after an absence of twenty days.

Il.

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, Asterina, and Archasteride 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 Sigsbea 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 Nephrops, a
genus we had not previously obtained.

Among the Worms the Maldaniz 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) Nemerteans
and Planarians.

The Mollusks were very scanty, and the absence of Comatule or other
Crinoids was 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

ceous 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 comes to the surface, it 1s 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 half way between Cape
San Francisco and the Galapagos, in 1,832 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 Sshizopods 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 Melamphes, 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 Meduse, several specimens of Atolla and
Periphylla were 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. {ts 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 Hyalzas 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 Acapulco
(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 things 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 Acapulco, 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 Schizopods,
and of Rhizopods, mainly Collozoun and Acanthometra.

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 depth 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 do not rise to any great distance, as we found
no trace of anything within 60 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 bathymetrical range 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 Galapagos at the beginning of a remarkably
early rainy season, I could not help contrasting the green 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 as 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 propetly 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 odlitic, identical with the formation found back of the
beach at Wreck Bay on Chatham Island. I 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 odlitic limestone, which
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 Admiralty
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 find true coral reefs, — 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 Jand 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 600 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 West 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.

a

MUSEUM OF COMPARATIVE ZOOLOGY. 197

As soon as we have reached Guaymas, I shall be able to give you a
better résumé 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.

Guaymas, 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 Umbellule, Pennatule, Trochoptilum, An-
thoptilum, and a fine Antipathes, a few Comatule, 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 Gasteropods, 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 Jimicolous Annelid also were found
here, Halinzcia, 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 Argonauta, 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 Periphylla, which
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

oaks.

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 depths 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 Nettastoma, and some Pene-
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 Penzid, 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 fige 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 Kast 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 jadge 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 Behring 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 haying 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 résumé
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.

a

No. 5.— The Development of the Pronephros and Segmental Duct
in Amphibia, By Hersert H. Frevp.}

ConTENTS.

PAGE PAGE
iwintroduction .) < << « «<< 201 CaAmblystoma.2s 5 1. 247
II. Descriptive Part . . . . . 208 Sitges. sya ole 248
PUMGAA Ban are te. 2) Faerie DO Stage . . . «.. . 250
Be cs at 3) DOE Staves. st 200
pee SS. StageIV. ..... . 262
SapeMie ye , s my e 20S SIAGCRURME + fs) 0 ss) Boe
eee. Ce aes 2 DIS Seaeewe My Tee es at) QB
Rey nn 58 Syd a,c | 227 III. General Discussion . . ; 262
ROOM oe x DOT The Kidneys of apiiienia 262

PER a hy 3 BAT The Pronephros of the Cra-
EE se) wea) 2A niota. . . | oe
eee eS es - S S SAD The Segmental Date te 285
ems ees 5). 8 248 Organogenetic Conclusions . 295
OS a ee: © Phylogenetic Conclusions . 307
rage a ew s, 24551 V.. Bibliography ....3 « « « 823
Stage V. . « . - . . - .246| V. Explanation of ena esa

I. Introduction.

Tue 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 Zodlogical Laboratory of the Museum of Comparative
Zodlogy, under the direction of E. L. Mark, No. XXVII.
VOL. xxI.—wno. 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 Rana, Bufo, and Amblystoma. The study of the problem was
begun with Rana 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 Rana sylvatica Le Conte.!
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 protovertebre 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 (Giinther, ’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 Rana 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
Rana 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. Rana.

Stace 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 ('75, 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 (/a. med.), which is produced by a proliferation of the deeper of
these 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. Lach cell contains scattered
pigment granules, which are especially massed along its external face.
Small yolk spherules (sph. 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 el. 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’cd.) 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, Ja. ms’drm.) 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 Ja. 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 splanchnopleure (so’plu. and spl’plu.) 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 the 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
consequence 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; mesoderm, mean diameter, 5; ectoderm, diameter
rarely exceeds 3. 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 cceelom (cel.). 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 (ccel.), 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 iine
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 ccelom 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, im 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.’ 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 (cel.’) 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 celom 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
lamelle.

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 protovertebre 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 protovertebral 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.? The somatopleural thickening may be traced
to a point about 80‘u 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, cras. so’plu.),
which is greatest immediately lateral to the protovertebral plate. An
anterior coelomic 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, cras. 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 1s 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
cranial 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 1s
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

Stace II.
Plate I. Figs. 4, 5. Plate Il. 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 protovertebrz 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 2.5mm.
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, Ja. pr’vr.), has a triangular out-
line in cross section, and is readily distinguishable from the lateral plate
(da. (.), 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 protovertebre are to be observed. Passing
farther forward, it is difficult to assign boundaries to the protovertebre.
There is certainly one which is partially broken up into mesenchymatic
tissue.’ Still farther forward the series of the protovertebre 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 Iam 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
has 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
protovertebree 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? 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 protovertebre. 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 protovertebre (com-
pare Plate IL. Figs. 15, 16), and tapers both dorsally and ventrally.
It is to be remarked in this connection that the protovertebre 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 ccelom 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 protovertebree. 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 shghtly 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 exprcssion 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.

[ == [ae

MUSEUM OF COMPARATIVE ZOOLOGY. Pallet

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’plw.) 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 for 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
protovertebre, 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
protovertebrz are sharply marked off from one another, and the somatic
layer is relatively thin. Near their ventral margins, however, the suc-
cessive protovertebre are in close contact, and the somatic layer shows a
pronounced lateral thickening (Fig. 13, cras. pr’nph.).

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 (spl’plu.) 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 (cras. 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 protovertebre II., IIL, 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 2.6 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 Il. 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 in 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 as
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 protovertebre, 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 protovertebre
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 protovertebre 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 .
protovertebra. 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» 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.
[ 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 protovertebree takes some part.

Stace IIT.
Plate I. Fig. 6. Plate I. 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 protovertebree 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 Jayer 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.!) is the fundament
of the first nephrostome, a later stage in the development of which is
shown in Plate III. Fig. 18 (nph’stm.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 layer represents the underlying peritoneum. In the region be-
tween somites II. and IIL. 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
nephrostome, 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 (cras.
prnph.). 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 0.37mm. 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 prorimal 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 anteriorly or posteriorly, medially or laterally, dorsally or
ventrally.

MUSEUM OF COMPARATIVE ZOOLOGY. 215

the thickening (cras. pr’nph.) near its posterior termination. The mass
is evidently a thickening ém sctw 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 protovertebre 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 LV. 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 pronephric 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. In the intersegmental regions, the outline of these two layers is
that of an elongated ellipse, the nuclei being disposed, for the most 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 I. Fig. 6, ca/. This figure demonstrates very
clearly the relations of the pouch to the lateral mesoderm and the over-
lying somites.

In the case of the younger set of embryos which have been con-
sidered in this stage, it will be remembered that the boundary between
the lateral mesoderm and the protovertebrae 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 protovertebre 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 protovertebre 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-
tovertebree 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 course of the earliest fundaments of the three tubules which
emerge from the somatopleure beneath protovertebre 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
ccelom 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. I am of opinion that, in the cases referred to, the separation is
largely artificial, and that the ruptures take place most frequently at the
nephrostomes 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 protovertebre. 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. Im 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 protovertebre from
the lateral plates.?

1 In sections from the posterior end of the embryo, it is necessary to guard
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 [X. 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).

Stace 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
killmgs. 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 3} to 34 mm. long; they have about 14 protovertebree and the
fundaments of 3 pairs of gills. The embryos of the older set are from
34 to 33 mm. long; they possess about 17 protovertebrz 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 protovertebre. 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 this section in the reconstruction is indicated by the dotted
line 78, in Figure 39. The lateral plates are here wholly cut off from
the protovertebree, 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 rvzght 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 I have observed such groups of cells lying entirely free
in the lumen of the pouch (Plate V. Fig. 45). In the present case,
however, the mass is very intimately connected with the adjoining walls.
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 occurrence
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 which 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 protovertebre 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 7 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 (cd.) is wider than an average cell of the fundament,
and probably represents the anterior ends of two cells lying in the fol-
lowing section (Fig. 25, c. and d.). Dorsal to this mass of protoplasm is
a nucleated cell (6.), and above this a small area of protoplasm with a
faint nucleus (a.) 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 d.), 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 (>. 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, f.). 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 uct 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 XI. 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 protovertebree 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 iz situ of that layer. In almost all of my prepa-

MUSEUM OF COMPARATIVE ZOOLOGY. 220

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 ot
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 protovertebre
is faintly indicated (above the letters ce/.'’) in the same transverse

224 BULLETIN OF THE

plane as the third nephrostome; and Figure 6, 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 protovertebre 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 protovertebra. Laterally this peripheral part
is represented by the entire somatic layer, which is separated from the
kernel by the protovertebral cavity (ca/.). 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 celom, 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 protovertebree
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-
vertebra, 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 layer, 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 protovertebree, but in anterior regions considerable portions of the
kernels of the protovertebree 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
protovertebre, 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 protovertebre and lateral
plates (Fig. 6). 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-
tovertebre. 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 ot
the pronephros, but also because the process is somewhat modified in the
protovertebra next in front of it (somite II.). Somite IT. is ope 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-
phrostome, and develops from this point backwards. It is the fundament
of the glomus or pronephric glomerulus.’ In the earliest trace of this
organ that I have been able to find (Plate I. Fig. 8) there were already
a few small mesenchymatic 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 sare connective-tissue elements.

1 The 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’chy.), 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 extent 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 LV. At this period almost all of the
eggs are hatched ; and, the duct having opened into the cloaca, the pro-
nephros becomes functional. The larve 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 LV., 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 superpositioa.

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.' A second change which is apparent in Figure 40

‘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.
{n 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 are 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
collecting trunk is usually marked by an abrupt change of direction ;
where this does not occur, however, the distinction between the two
portions must be somewhat arbitrary. The collecting trunk forms the
continuation of the first nephrostomal tubule, it receives in its backward
course the second tubule, and may be 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 duct. 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 in 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 larve. 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 larve.

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 three 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 celom 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 proctodeal 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 wrodeum.
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 seein Stage LV. 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 »; 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 the 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 larva, 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 p
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 jarge 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.?
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 Tn order to ascertain, if possible, what vein of the adult this vessel represents, it
will 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
the 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 Kcker 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°) 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°, p. 286) 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 larve 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 present 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 I 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
protovertebre, 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 genera]
it may be stated that the ridge constituting the organ under consideration
extends continuously from the first nephrosteme 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, /a. pi’ton., 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, fnd. glm.). In certain favorable re-
gions I have seen a thin structureless membrane lying directly within the
outer cellular layer (compare Figs. 49, 50, mb. 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 or two cells in each section (com-
pare Figs. 49, 50, en’th.) 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 LV. as occurring within the fundament of the glomus.
Their origin 1 have already discussed in connection with that stage,
where | 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 of 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, rz. 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.

Stace VI.
Plate III. Fig. 2%. Plate IV, Figs. 28, 30, 35-38, 41. Plate VI. Fig. 51.

The larve 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 aleng 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 ouly 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 employed 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 R. 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 folds, 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. 36, 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 oceurs 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 off 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, dé. sg.) are now situated at
the bottom of a depression in the dorsal wall of the cloaca (clce.). 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, but
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 (vm. erd.),
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 (tr. com.) which is adjacent to the collecting trunk (tra. elg.).
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. (Hig. 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 larvee 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 larve as well, and J shall there-
fore merely refer here to the account given in that connection."

1 Tn all the larve 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 fora length of two or three
myotomes, and appears to be closed upon all sides. The sac lies in a mass 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 undistinguishable 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, fnd. pul.) forms a
ridge on the splanchnic side of the celom. 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.

Stace VII.

The age of the larve 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 larvee are studied merely for the pur-
pose of observing the process of degeneration in the pronephros. In the
larvee 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. Ina larva of intermediate age, the sinus
communicates with the cardinal vein by means of a very narrow canal. Respect-
ing the fate and the significance of this singular structure, I have no suggestions to

offer.
VOL. XXI. —NO. 5. 16

249 BULLETIN OF THE

duct pursues a direct course through the gland. 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 canal 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 nearly 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.

Stace I.
In the case of Rana, this stage included embryos which showed an ill
defined somatopleural thickening lying immediately posterior to the
cranial ganglionic mass. This proliferation proved, on comparison with

MUSEUM OF COMPARATIVE ZOOLOGY. 243

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 about 2 »; 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 ccelom, 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.

Stace 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 protovertebre. 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 IT.

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 protovertebre, 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 pronephric 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 pronephric
thickening and the protovertebree that I have described for Rana, but
in Bufo the ceelom is entirely obliterated by the growth of the prone-
phric thickening, and consequently the pronephric 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-
topleure 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.

SracE 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 protovertebre 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
around a lumen. Segmentally, the pronephric thickening has in general
the form of a close fold of somatopleure, whereas intersegmentally 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

Stace 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-
tovertebre 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 celom 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 protovertebre IL,
Miprand: TVA

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, fnd. dt. sg.) in both cases re-
sembles very closely the adjacent mesoderm both in 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 protovertebrze. 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 body
cavity, however, has not yet appeared, the somatopleure and splanchno-
pleure being in other regions in close contact.

1 T have preserved in the enumeration of the body somites of Bufo the same
designations that were employed in the 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

Stace V.
Plate V. Figs. 42, 46. Plate VI. Figs. 47, 49, 52.

At this stage the larve were hatched and swam about freely in the
aquaria. The larve measured from 4 to 6 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 the extreme posterior portion of the bend,
which im 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 embryos 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 Rana. 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. Cwv.),
makes a rapid ventral descent to open into the sinus venosus. The

MUSEUM OF COMPARATIVE ZOOLOGY. 247

other vein (Fig. 42, wn. jg/.) 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), und 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 cel." (Fig. 52).
If the cells at the apices of such invaginations were to become detached,
this condition would 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.

Cc. Amblystoma.

Plate V. Fig. 44. Plate VI. Fig. 48. Plate VII. 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 will 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.

Stace 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, Planche 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.’

The general arrangement of the germ layers (Plate VI. Fig. 48) is
similar to that which I have described for Rana and Bufo. The ecto-
derm (ec’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 Rana 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 Rana, 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 O. 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.,15-20days. 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 ventralward 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 (/a. so.) and splanchnic (da. sp/.) mesoderm, and
the slight space which separates them is the ccelom (c@/.). 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 Jayers; 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.

Stace 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 protovertebree 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. Iam also unable 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-
tebree and the pronephric thickening. The latter may be traced for a
distance of about 0.5 or 0.6 mm. Each protovertebra at this stage
measures about 0.27 mm. in length, so that the thickening extends
through a length of about two protovertebre.

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 coelom between the layers.

Stace 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 protovertebre.

In all the embryos 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 celom are
opposite the middle of protovertebree 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 protovertebre III. and IV. and extending outward as coelomic
diverticula into the pronephric mass. From this condition the hasty
conclusion might 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 fignre cited, a horizontal section a
little below the level of the letters cv/.’) 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 protovertebre 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. 10c., 10 d., tr, and tr,).

fe BULLETIN OF THE

Stace IV.

Larvee of Amblystoma do not possess a conspicuous widely open
pronephric pouch, such as has been described in Anuran 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 celom in segmental
regions.

In slightly older embryos, the dorsal half of the pronephric thicken-
ing is no longer continuous through the region between protovertebre
II]. 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 which
approximates very closely to the condition which forms the starting
point for the next stage (Plate VIII. Fig. 58).

Stace 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 VIII. 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 Anuwran pronephros can be answered only by a con-
sideration of the relations which the several nephrostomes in the two
groups bear to the overlying protovertebre, 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 more 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 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
not 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 proctodzal 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 proctodeal 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 yw, while the parietal
peritoneum has an average thickness of only about 15 pp. 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 larve 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 Rana and Bufo, the pronephric capsule in Amblystoma develops
in the form of a downgrowth from the somatic layer of the protover-
tebree. In Amblystoma the two-layered condition of the capsule and
its connection with the overlying protovertebre are maintained in the
oldest larve of this stage. It seems probable, moreover, that the down-
growth from the protovertebre 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 larvee, but it is in general approximately equal
to that of the peritoneum in the same individual. In the older larve,
moreover, the pronephros, and especially the segmental duct, become
partially covered by a downward extension of the myotome. In such
larvee 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 mesenchymatie 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, 9, 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 are 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 larvee 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 » 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. 25

them they were clesely associated with the yolk entoderm, which lies
medio-ventral to the region of the glomus. It is probable that they
arise from the entoderm and migrate into the interior of the splanchno-
pleural fold. Ihave been unable to find in either Rana 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 larve 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.

Stace VI.
Plate VII. Figs. 53, 54. Plate VIII. Figs. 61-65.

The larve 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. 62). The convolution in this stage is
no longer confined to the common trunk, the nephrostomal tubules un-
dergoing slight contortion (Figs. 63-65).

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 larvee 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 larve 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 Rana. 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
spherules 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 the snow-white yolk-eutoderm.

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 (vn. erd.). In
the older larvze 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 maxillee 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 v. 1.). Iam 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 raass 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 lateral 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 larvee 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.

ee eye — oe “cc TT:
ee V.— Tubules absent.

(73 Vil foe (a3 ee

ee VII. 2s (73 73

(73 Wale eat: 6c (73

& IX.— 1 mesonephric tubule.
73 NE —= jj “ “

73 7G Is Esai “ “

es XII. — 2 a tubules.
sc SY OL 83 a ee

“cc MTV. 5 és “

“ec eV. ut 4 “ “

ae peVvile——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, however, 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-
nephric 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.

I 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 cireum-
stance seems to me to favor the second view. Furthermore, I have found
nuclear mitoses (Fig. 54) 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.

Recent 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°, Rabl, ’88, Ayers, ’90)

MUSEUM OF COMPARATIVE ZOOLOGY. 263

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. Miiller (42, p. 101, see also Langerhans, ’76,
p-. 322, and Rolph, ’76, 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 Vertebrates. The
same is true of the glandular structures described by Owen (’66, p. 533,
Fig. 169, 4), and the epithelial bands of Wilh. Miller (’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 excretory 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 Miiller, 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 ceelom,
but Weiss was unable to detect a continuity between their lumen and
the ceelom. Since the relations of these tubules to the ccelom are not
ascertained, I am of opinion that the observations of Weiss do not afford
satisfactory 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. 36 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 fiir Morphologie und Physi-
ologie in Miinchen, 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 of 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 ceelom. 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 gonadial pouches of Amphioxus correspond to the seg-
mental duct and mesonephros respectively of Craniota. The attempts
of Haeckel (’74*, p. 37, and 774°, 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*, et a/.). 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?) 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 pazr 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 (85, 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 Ammoccetes larva, but
aborts in the adult. The number of nephrostomes and of tubules is
small (4, according to Wilh. Miiller; 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*, 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 hy 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 Miiller 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.*

In Myxine nothing is known of the early development ; but in late
stages an organ has been made known by the studies of Wilh. Miller

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 system.

MUSEUM OF COMPARATIVE ZOOLOGY. 267

(75) and of Firbringer (’78*, 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. nephrostomes 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. 46).

According to Rosenberg (’67, pp. 42 et seg.) and Oellacher (’73, pp. 97-
100), the excretory organs arise as a pair of grooves of the somato-
pleure directly beneath the protovertebree. 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 a better
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 Fiirbringer
(’78*) confirms Goette’s view, Hoffmann (’86, p. 621 et seg.) 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 pronephrie 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°, 82; Grosglik,
85 and ’86; Emery, ’80, ’81, and ’85 ; Calderwood, 791).

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 of 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 ccelom, nor of a division into protovertebral
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 mesenterii. 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
répresents 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 Riickert (’88) and van Wijhe
(89). According to Riickert, the development begins with the forma-
tion of a pronephros as an outgrowth towards the ectoderm from the
ventral portions of several protovertebre, 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 that 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 Riickert (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 Riickert 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 Riickert 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 Riickert bases his statement seems to me far from conclusive, nor
has his observation been confirmed by any subsequent investigator.
Riickert described the pronephric thickening as a product of the proto-
vertebrae. 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. OT 1

ferent criteria for determining the boundaries of the protovertebre.
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 protovertebre. On the
contrary, I am of opinion that the ventral extent of the protovertebree
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 (790) 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>, ’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 nephrostomes. 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

des 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. iickert says
(88, p. 239), ‘Es [ein Paul Mayer’sches Quergefass] zieht dicht an der
medialen Grenze der Vornierenanlage vorbei und gelangt, indem es die
Leibeshéhle durchbricht, d. h. ihre Wandung vor sich herstiilpt, an die
Aussenfliche des Darmes, wo es zwischen Ectoderm [soll 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-
talblischen of Braun), which have a strictly metameric arrangement.
Throughout a region of five protovertebre (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-
tebree, 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 Mihalkovies (’85) upon Lacerta agilis differ from
those of Weldon mainly in two particulars. In the first place, according
to Mihalkovies (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 celom 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 off from
the ventral portions of the protovertebre, and gain secondarily a con-
nection with the body cavity ; the duct does not appear until the vesicles
are evident.

Ostroumoff (’88°, 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 protovertebre pass over into the lateral plates. The organ
extends over a variable number of somites (6-7 in Lacerta and 5-6 in Tro-
pidonotus). As protovertebree 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 protovertebre and
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 of the evagination belong-
ing to the immediately preceding somite. These backward processes are
described by Riickert for Selachii. Ostroumoff’s (’88, pp. 78, 79) state-

VOL. XXI.— NO. 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 ccelom, 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 colom, 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.?
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 héchst wahrscheinlich vor, dass
die Vermuthung von Mihalkovics, nach welcher die proximalen Urnieren-
kaniilchen 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, 268)
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 (790%, p. 413) in this connection,
do not relate to Reptiles at all. They represent sections of Duck embryos.

MUSEUM OF COMPARATIVE ZOOLOGY. Hig fe

Wijhe, the mesoderm which produces the mesonephric tubules in Selachii
belongs to a region dorsal to that which gave rise to the pronephros
(see the diagrams appended to van Wijhe, ’89, Taf. XX XII.).

In view of the difficulties to which I have alluded, it seems to me that
Hoffmann’s position cannot be regarded as satisfactory. Furthermore, if
Hoffmann’s observations? 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
(7907, 790”), 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 with its contained glomus breaks up into a series
of capsules containing glomeruli, each of which then appears to form the
blind termination ofa 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. 825-327).

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 a vascular 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°, p. 20), Siemerling (’82, p. 29), Jano&ik (’85, p. 43), and
Mihalkovies (’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 Sedgewick in their view respecting the nature of the vascular
body, and Sedgwick (’80°) 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*, p. 590).

(2.) The second view is set forth in the recent account of Felix (’90),
who describes in a chick embryo with eight protovertebre a series of
outgrowths, which, emerging from the lower hinder portions of protover-
tebree 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. Did er

pronephros as detailed in the preceding pages. Moreover, Felix pro-
duces no 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
distally 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 K6lli-
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 stage the space between the most anterior Wolffian tubule and

278 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 protovertebre), remains
connected with the peritoneal epithelium by short cords of cells. Be-
tween the 8th and 15th protovertebre, 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 dnet until their differentiation is
somewhat advanced. The cords of cells in the region between the 7th
and 11th protovertebree acquire lumens which may be 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 protovertebre 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 he
hoped to be able to harmonize such a view with the position (cf. page 276)
formerly taken by himself and Balfour (Balfour and Sedgwick, ’79).?

In the foregoing description I 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,
781, p. 468).

In the second edition of Foster and Balfour’s (’83, p. 218) Elements of Embry-
ology, revised by Sedgwick and Heape, the anterior end of the Miillerian 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
protovertebre and arises in the same region in which the duct first
appears.

Lockwood (’87, pp. 657-663) 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 region 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. Ina 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. He 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. 662, 663), and for Arvicola by Spoof (’83, p. 86, foot-
note). It is difficult to arrive at a satisfactory estimate of Renson’s posi-
tion. There would be no difficulty in classing him with Sedgwick, were it
not for the circumstance that he describes for the pronephric region (6 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 14th
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 I 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. m.), 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
celom, 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 /ater [!] 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. Near 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 separate buds from the peritoneum, or are differen-
tiated from a blastema which is directly derived from the peritoneum.
Janosik was able to confirm Renson’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 I have endeavored to present a comprehensive
résumé 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 Riickert (88) and van Wijhe (89), in Petromyzon by
Kupffer (’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 Wilh. Miiller (’75), was first described in detail
by Goette (’75) for Bombinator, and was later extended to other Am-
phibia by the researches of Firbringer (’77). It has been entirely con-
firmed by Wichmann (84), by Hoffmann (’86), and still more recently
by Marshall and Bles (90°).

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
prouephric thickening.’

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 Wilh. Miller 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 slit-like opening into
the body cavity, and that by the imperfect closure of this opening the
successive nephrostomes 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 I have described.
His statements seem to me in general correct,” 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, inclined 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,
Riickert 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 eutirely 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.

2 I 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. 283

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 protovertebral
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 protovertebre and the lateral plates; in a slightly older
embryo, however, the protovertebr 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 Riickert and Mollier seem to attach great
morphological significance to this feature of their account. This relation
to the protovertebree 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*, 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 protovertebree. Marshall and Bles confirm this
statement, and contend that, in the case of Rana, 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 protovertebre ; but since the most anterior two
protovertebrz are reckoned to the posterior region of the head, these
represent the 3d and 4th protovertebre 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 IIL,
whereas in Rana and Bufo 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 and 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. ;? 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 Rana and Bufo; 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
protovertebre present in Urodeles as compared with Anura is largely
accounted for by additional protovertebre 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 abortion 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 (’90, p. 224) has recorded the occasional occur-
rence of three pairs in Triton, and I have made similar observations in
Amblystoma. Spengel (’76, 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 Cecilia ; the recent observations of Semon (’90,
p- 462), on the other hand, have shown that there exist in Ichthyophis
on each side of the body ten pazrs of nephrostomes, therefore forty in all.

1 Somite I. of this enumeration probably corresponds to the one which has
been called somite XI by Houssay (’91). Houssay believes that he can identify
in Amphibia the somites which have been observed in the head region of Selachui.
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 nephrostomes belonging to any pair, one opens freely 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 protovertebre.

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 precise 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. ‘lhe discoverer of the organ, Joh. Miiller (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 Fiirbringer
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,! 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 corivolutions 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 larvee 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 protovertebre ; 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 Joh. Miiller (’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 seg.), 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-

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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 pronephric
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 considerable 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 body cavity. Such a
mode of origin was advocated by Rosenberg (’67, pp. 42 et seg.) 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 (’86) and Henneguy (’88, 789). Ac-
cording to Wilh. Miiller (75) and Fiirbringer, the duct arises in this way
also in Petromyzon, and a similar claim has been made for Ganvids by
Kowalewsky, Owsjannikow, and Wagner (’70). 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 (’75).

In Amniotes also such an account of the early development has not
received general acceptance ; it was first claimed in this class by Romiti
(74), and was adopted, with some modification it is true, by R. Kowa-
lewsky (’75), 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 Fiirbringer (’78*), Spoof (’83,
p. 84), and the earlier writers (Remak, 55, Kdlliker, ’61, Bornhaupt,
67, Waldeyer, ’70, and Foster and Balfour, 74), the duct arises in the
chick by a proliferation i situ of the subjacent mesoderm, and a similar
origin is maintained for Petromyzon by Scott (’82). The more recent

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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 Jano’ik (’85),
in Birds ; Renson (’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. 226) 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 Remak 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
(65, pp. 160-162) maintained that, in the Chick, the Wolfhan and
Miillerian ducts both arise as folds of the ectoderm ; but he abandoned
this position later (’68, p. 119), when it had been shown by Bornhaupt
(’67) and Dursy (’67) 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 Miillerian 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 protovertebre. 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. 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 Flemming (’86), did not clearly recognize the fact
that the Wolffian duct and the mesonephros 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.

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292 BULLETIN OF THE —
he

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-
peared 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 Cyclostomes, the only papers that have appeared during
this period have been preliminary notices ; that of Kupffer (88) main-
tains an ectodermal, those of Goette (’88) and Owsjannikow (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 bas 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 appeared 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, and 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), Mihalkovies (’85), and
Hoffmann (’89). Peculiarly significant, however, is the fact that Graf
Spee (86) 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, Ostroumoff (’88*, ’88) asserts that it is derived from the
ectoderm in Phrynocephalus. It seems to me, however, that Ostroumofi’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", Tab. IIL. Fig. 56) 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 (’86), 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. This latter feature is also
dwelt upon by 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 Mihalkovies
(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
the duct is here claimed to originate, viz. as a conspicuous fold of ecto-
derm, is so different from the method of origin described in other Mam-
mals that one cannot regard this observation based on a single specimen
as conclusive evidence. ?

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, An/age d. Urniere, Fig. 8°) which he
believes to be 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 protovertebre, 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. Keibel’s (’88*, p. 635)
studies on Erinaceus led him at first to accept Martin’s attempt at har-
monizing the two views; but in Cavia (’88°, 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. All 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 Riickert (’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 congeption 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 adjacent 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 definite 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 cireumstance 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? (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 ampulle of the vasa defer-
entia, since these are derived from the Wolffian duct and consequently may
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 body.
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 urogenitalis, or that of Nagel (’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 ;* in the region of the fundus and corpus, however, are
numerous long tubular ccecee 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
east in menstruation. In the cervical region occur glands (glandule
Nabothi, Schleimkrypten) 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,? 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.* It
is almost certain that the cervical glands produce true mucus. Not
merely, then, does the mesoderm give rise to glands, but zt produces
glands of the same nature as those found in mucous passages of ecto-
dermal origin.

A second view was that formulated by His (’65*), 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.

8 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
(784). 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. Thus His (’65°) saw in the mode of
development which he described for the Wolffian and Miillerian 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.*

The suggestion has recently been made by Sutton (’86, 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-
derma! 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 cases of fetus in fetu, 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 pronephros 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 distinguisked 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,

300 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 pronephric 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 body. 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 mesonephrie tubules in Ambly-
stoma (see page 261), 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- TV. 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
ereat 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 emboly 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 ceelom immediately
ventral to the lower boundaries of the protovertebre 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 protovertebra. 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*), Van
Wijhe (’88*, ’89), Riickert (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 protovertebre, 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
imner, 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
celom, 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 ccelom, and that the
latter ground is equally untenable because it simply results from the fact
that, before the glomeruli appear, the space into 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 into 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 protovertebre. 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
prenephros, 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 producing
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.

L conclude, therefore, that pronephros and mesonephros are parts of one
ancestral organ; that the glomeruli are strictly homodynamous with 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 with the body cavity are detached portions of the
calom, 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.

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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 (’51) ; but this view received no
recognition until it had been reasserted by Wilh. Miiller (’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
Plagiostomes. 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-
nephros. 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 Furbringer, 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). first 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 haye
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, 66) denied that they are wholly homologous, on the ground that the
,pronephric tubules are peritoneal evaginations, 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 (’88*, ’89), Riickert (’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
(Metacélom). 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-
stomal 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. (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 [
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-
cdlom).” 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 protovertebree. 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 as 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 protovertebre 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. 96) 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 (’90) 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 cham-

MUSEUM OF COMPARATIVE ZOOLOGY. 307

ber, from which a portion of the nephrostomes (“ inner” nephrostomes)
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” nephrostome. According to Semon, the
pronephric chamber, as well as the cavity of a mesonephric Malpighian
capsule, is a diverticulum of the celom; 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 can 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
entbryological research; it is dependent upon the principle that on-
togeny is in part an abbreviated recapitulation of phylogeny; its
method is to eliminate ccenogenetic 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 coenogenetic 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.

I 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 zodlogy (Hatschek, ’88, pp. 25, 26) 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 celom. This relation is peculiarly 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, 780, pp.
248-253); but I am of opinion that in the higher Invertebrates and in
Vertebrates the coclom 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 ceelom. 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 enterocelous
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 ccenogenetic; 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 cndependent 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 ceelom. 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 celom. As a further differentiation, it is to be conceived
that this groove became at intervals constricted off from the cclom,
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

310 BULLETIN OF THE

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 series 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. Iam 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 are 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 Cecilia, 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-

9

MUSEUM OF COMPARATIVE ZOOLOGY. oilek

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 Rickert (’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

oe 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 7 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

MJSEUM OF COMPARATIVE ZOOLOGY. Sls

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 that 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
tae 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 mesonephros 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 mesonephros, 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 foregoing 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-
ored 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 has hitherto proved im-
possible to find any organs in this group which can be regarded as
homologous to Vertebrate nephridia.* 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. =

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 Mollusks 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 Nemer-
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,
on 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 trache, 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
tubnles of Plathelminthes and certain interior canalicular spaces, which
they interpret as a rudimentary ccelom. 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 (’82%, pp. 106 et seg., ’82°, p. 80), Lang
(81, p. 208, ’84, p. 167), Tijima (’84, p. 400), Zschokke (’87, p. 165),
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 esophageal 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 esophageal blood lacunz 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. JBiirger (’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
celom ; they develop from its epithelium, and even in the adult arise
from chambers which must be regarded as detached portions of the
celom. The excretory organs of Nemertines lie between the vascular
trunks and the exterior, and show no such relations with the ecelom.

Among the Annelids, on the other hand, the Cheetopods 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 Cheetopod 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 ccelom 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 Claparéde (’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

9

MUSEUM OF COMPARATIVE ZOOLOGY. 317

been confirmed by a large number of observers,’ 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 (Roule, ’89,
pp. 201, 252, 290). 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. 816), and cases may be
found among Polychetes in which definite peritoneal glands are present
(Grobben, ’88, pp. 255 e¢ 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.? (5.) The
efferent conduits take their origin from the cceelom 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
(Kisig, 87, p. 594). (7.) The development of the Cheetopod 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 Polycheetes this is evidently
the mode of origin of the nephridia. In Oligochetes the development is
more doubtful, but the method of origin described by Bergh (’90) is in
essence the same as that known in Polycheetes, and the mode of devel-
opment maintained by Wilson (’87, pp. 185, 186, and ’89, pp. 419 e¢ 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-

1 E. g. Timm (’83, pp. 122, 123); Kiikenthal (’85, p. 336); 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 celom, 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 fact, 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 has been recently shown by
E. Meyer (’87, pp. 618-625) and Cunningham (’87*, 87°, pp. 248-253)
that in Lanice conchilega, a terebelloid Annelid, certain nephridia open
into a longitudinal trunk, and only secondarily communicate with 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 zz 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
nephridial 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, Q) that the
group of animals whicn presents nephridia most closely resembling those
of Vertebrates is unquestionably that of the Cheetopod 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 Wijhe (’89, pp.
506 et seg.). 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 Wijhe also employs two arguments
which are independent of his position in regard to this point: (1) 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 !— 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 Reptiles arises
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. 628), and Fiirbringer
(78%, pp. 95 et seg.) 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*) and others.

Riickert (88, p. 262) 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 iu 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 seg.) 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 (76, pp. 387, 388) suggests the possibility that the duct
(pronephric system so far as present in Selachians’) may represent the
unsegmented excretory tubules developed in the larva of Nephelis, or
may even represent an inheritance from the Plathelminthan water-vas-
cular system ; and Balfour (’81, p. 607) was led to accept this view.

In addition to his hypothesis of an origin of the excretory system of
Vertebrates from that of Plathelminthes, Fiirbringer 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*), and is offered as a suggestion in
Wiedersheim’s (’86, p. 731) text-book.

None 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 Analschliuche of Echiurids. I am unable to
see the slightest evidence that the Vertebrate excretory system is made

1 By this suggestion, Semper allows some justification to W. Miiller’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 Analschliuche 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 seg.) 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; 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. MRiickert (’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 Riickert
(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.

i Here belongs also the view of Boveri (’90), which has already been discussed
(page 265).
VOL. xxI.— No. 5 21

one 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.

CamBnIDGE, April 25, 1891.

MUSEUM OF COMPARATIVE ZOOLOGY. 323

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Hyrtl, J.

"51. Das uropoetische System der Knochenfisthe. Denkschr. d. k. Akad.
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’84. Untersuchungen iiber den Bau und die Entwicklungsgeschichte der Siiss-
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’85. Histologisch-embryologische Untersuchungen uber das Urogenital-
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’°87. Zwei junge menschliche Embryonen. Arch. f. mikr. Anat., Bd. XXX.
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Kastschenko, N.

87. Das Schlundspaltengebiet des Huhnchens. Arch. f. Anat. u. Physiol.,

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883. Zur Entwicklungsgeschichte des Igels (Erinaceus europeus). Anat.
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88>. Die Entwickelungsvorgange am hinteren Ende des Meerschweinchen-
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Kellogg, J. L.

90. Notes on the Pronephros of Amblystoma punctatum. Johns Hopkins

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’°89. Die allgemeine Pathologie oder die Lehre von den Ursachen und dem
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’61. Entwickelungsgeschichte des Menschen und der hoheren Thiere.
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"79. Entwickeiungsgeschichte des Menschen und der héheren Thiere, 2te
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K6llmann, Jul.

’822. Die Doppelnatur des excretorischen Apparates bei den Cranioten.
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’°82>. Ueber Verbindungen zwischen Coelom und Nephridium. Festschr. z.
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91. Die Rumpfsegmente menschlicher Embryonen von 13 bis 35 Urwirbeln.
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88. Ueber die Entwicklung von Petromyzon Planeri. Sitz.-Ber. d. k. b.
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’'81. Der Bau von Gunda segmentata und die Verwandtschaft der Platyhel-
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"76. Zur Anatomie des Amphioxus lanceolatus. Arch. f. mikr. Anat., Bd.

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'75. On some New Points in the Structure of Amphioxus and their Bearing
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’89. Contributions to the Knowledge of Amphioxus lanceolatus Yarrell.
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’'87. The Development and Transition of the Testis, Normal and Ab-
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©
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Qo

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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 développement des parties génitales et uropoiétiques chez les Ba-

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90%. The Development of the Kidneys and Fat-Bodies in the Frog. Stud.
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90. 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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’87. Ueber die Entwickelung des Herzens und der grossen Gefassstamme
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82. Zur Anatomie und Histologie von Polyophthalmus pictus Clap. Arch.
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Meyer, Hans.

90. Die Entwickelung der Urniere beim Menschen. Arch. f. mikr. Anat.,

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84. Untersuchungen tiber die Entwickelung der Harn- und Geschlechtsorgane
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88. The Ectoblastic Origin of the Wolffian Duct in Chelonia. (Prelim.

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'90. Ueber die Entstehung des Vornierensystems bei Amphibien. Arch. f.
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87. Contribution to the Embryology of the Lizard. Jour. of Morph., Vol. I.

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'88*. Zur Entwicklungsgeschichte der Hidechsen. Zool. Anzeig., Bd. XI.
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89. Etudes sur le développement des Annelides et un particulier d’un Oli-
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’°88. Ueber die Entstehung der Excretionsorgane bei Selachiern. Arch. f.
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Riickert, Joh., und W. Mollier.

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81. Studier over Testis og Epididymis Udviklingshistorie. 96 Sid., 3 Taf.
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’°88. Die Entwickelung der Keimblatter und der Chorda dorsalis von Rana
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81. Beitrage zur Entwickelung der Petromyzonten. Morph. Jahrb., Bd.

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7802. Development of the Kidney in relation to the Wolffian Body in the
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*81. On the Early Development of the Anterior Part of the Wolffian Duct
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Semon, Richard.

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74. Die Stammesverwandtschaft der Wirbelthiere und Wirbelosen. Arb.
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°87. On Some Points in the Development of Petromyzon fluviatilis. Quart.
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Schlesw.-Holst. Aerzte, Heft 11, Stiick 2,2 pp. Kiel 1886.
Spengel, J. W.
"76. Das Urogenitalsystem der Amphibien. Arb. zool.-zoot. Inst. Wiirz-
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90. Beitrag zur Kenntniss der Kiemen des Amphioxus. Zool. Jahrbicher,
Abt. f. Anat. u. Ontog., Bd. IV. Heft 2, pp. 257-296, Taf. XVII., XVIII.
30 Sept., 1890.
Spoof, A. A.
°83. Beitrage zur Embryologie und vergleichenden Anatomie der Cloake
und der Urogenitalorgane bei den hoheren Wirbelthieren. Akad. Habil.-
Schr. Helsingfors. xix + 116 pp., 5 Taf. Helsingfors. 1883.
Strahl, H.
'86. Ueber den Wolff’schen Gang und die Segmentalblaschen bei Lacerta.
Sitz.-Ber. naturf. Gesellsch. Marburg, Jahrg. 1886, Nro. 3, pp. 46, 47.
Aug., 1886.
Strahl, H., und E. Martin.
°86. Anlage des Wolff’schen Ganges beim Kaninchen. Sitz.-Ber. naturf.
Gesellsch. Marburg, Jahrg. 1886, Nro. 3, pp. 46, 47. Aug., 1886.
Sutton, J. Bland.
°86. An Introduction to General Pathology. xvi + 390 pp., 149 Figs.
Philadelphia: Blakiston. 1889.
Timm, R.
°83. Beobachtungen iiber Phreoryctis Menkeanus Hoffm. und Nais, ein
Beitrag zur Kenntniss der Fauna Unterfrankens. Arb. zool.-zoot. Inst.
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Veith.
°89. Vaginalepithel und Vaginaldrusen. Virch. Arch. f. path. Anat., Bd.
CXVIL. pp. 171-192, Taf. VIJ. 1 Juli, 1889.

Waldeyer, Wilhelm. .
°70. Lierstock und Ei. Ein Beitrag zur Anatomie und Entwickelungs-
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Weiss, F. Ernest:
°90. Excretory Tubules in Amphioxus. Quart. Jour. Micr. Sci., Vol.
XXXI. pp. 489-497, Pl. XXXIV., XXXV. Nov., 1890.

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°83. Note on the Early Development of Lacerta. Quart. Jour. Mier. Sci.,
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°86 = ~= Lehrbuch der vergleichenden Anatomie der Wirbelthiere. 2te Auflage.
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840 BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY.

Ziegler, H. Ernst.
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Centralbl. f. Bakt. u. Parasitenk., Bd. I. pp. 161-165, 193-199. 1887.

EXPLANATION OF FIGURES.

All the Figures, unless otherwise stated, were drawn with the aid of an Abbé
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, d, c, d, e, f, in Figures 24-26, see the explanation
of those Figures.)

ao.

can. comn.
ed. spi.

cl. ms’drm.
cl. vt.

cle.

cel.

cael.’

col.”

cp. sng.
cps. pr’nph.
cras. gn.
cras. pr’nph.
cras. so’plu.
d.

dt. sq.

dt. Cuv.
dz.

ec’drm.
ec’drm’
ec’drm.”
en’th.

Jnd. are. vr.

Jnd. eps.

Jnd. dt. sq.

find. glm.
Jnd. gl.’
Jnd. gn. spi.
Jnd. mbm.
Jnd.

ms’nph.

fnd.

nph’st.1

fnd. pul.
glo.

Aorta,

Communicating canal.

Spinal cord.

Mesodermal cells.

Yolk cells.

Cloaca.

Ceelom.

Protovertebral cavity.

Body cavity.

Blood cells.

Pronephric capsule.

Ganglionic thickening.

Pronephric thickening.

Somatopleural thickening.

Dorsal aspect.

Segmental duct.

Ductus Cuvieri«

Right side.

Ectoderm.

Superficial layer of same.

Deep layer of ectoderm.

Endothelium.

Deep layer of a vertebral
arch.

Fundament of the pro-
nephric capsule.

Fundament of the segmen-
tal duct.

Fundament of the glomus.

Fundament of glomerulus.

Fundament of a spinal gan-
glion.

Limb bud.

Fundaments of mesoneph-
ric tubuies.

Fundament of first pro-
nephric nephrostome.

Lung bud.

Glomus.

gn. nd.
gn. Spi.
hp.

m.

la. I.

la. med.
la. ms’drm.
la. pv’ton.
la. pr’vr.
la. so,
la. spl.
m.

mb. ba.
ms’drm.
ms’chy.
my’tm.
foils
n’cd.

npl’stin.

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.

nph’stm.t U-, M1. 1st, 2d, and 3d pronepbric

pvton.
pror.

rx. ao.

rx. vag.
sb.-n’ed.

Sn. Sng.
sot. IL ete.
so’plu.

sph. vt.
spl’plu.

nephrostome, respectively.
Peritoneum.
Protovertebra.
Aortic root.
Root of the vagus nerve.
Sub-notochordal rod.
Blood sinus.
Somites I. II., etc.
Somatopleure.
Yolk spherules.
Splanchnopleure.

tbl. nph’stm.1., 1, M1 1st, 2d, and 3d nephro-

tbl. pr’nph.
trn. elg.
trn. com.
va. sng.
vn. erd,

vn. jgl.

stomal tubules respectively.
Pronephric tubule.
Collecting trunk.
Common trunk.
Blood-vessel.
Posterior cardinal vein.
Jugular vein.

ro al lee
Aye proeree en!

o Me that, a9 ic ; ye Tagg ae

até ite 3 ries o> (Wibsce

Hadar Wu
hairs me Pyvacthel sipewads
wallz « gLirre : : pol ee autre
OA Bate Lime: by apiees ¢ ‘ie

z gg amg wid)liat mee

S. . =
shh *°Pb5 »

FieLp. — Pronepliros in Amphibia.

10.

PLATE I.

A portion of a cross section through the anterior trunk region of one
of the older embryos included under Stage I. X 110.

A cross section through the same embryo in the middie trunk region.
X 26.

A portion of a cross section through the middle trunk region of one
of the younger embryos in Stage I. XX 92.

A portion of a cross section through the hinder trunk region of one of
the younger embryos belonging to Stage IL. xX 92.

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. > 110.

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.
e110:

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. 615.

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.

A portion of a cross section through a slightly older embryo, showing
the glomus in a more advanced stage of development. X 110.

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 160.

B Meisel Jith Boston.

Fre.p, — 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 oné of the younger embryos
of Stage II.

Fig. 13 shows the ventral ends of the anterior protovertebre.

Fig. 14 shows a section through the dorsal portion of the pronephric thickening.

Figs. 15-17. Portions of three cress 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 Il.

Fig. 16 shows the pronephric thickening in the region of Somite V.

Fig. 17 shows the thickening near its posterior termination.

Fistp. — Pronephros in 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. 39) is
shown by the series of lines bearing corresponding numbers.

Fig. 18 shows the first nephrostome.

Fig. 19 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; cd., portions of the two cells c. and d., which are to be
seen in the following section.

Fig. 25 shows the duct one section in front of its termination ; c. and d., cells in the
rudiment of the duct; 6. 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.

FIELD.-PRq

Babi

B Meisel lith Boston.

Fisitp. — Pronephros in 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. > 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. 31-88. 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 larve of Stage V.

Fig. 32 is a diagram of the recongtruction 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 larve of Stage VI.

Fig. 36 is a diagram of the reconstruction shown in Figure 41.

Fig. 88 is from a larva of Rana halesina.

Figs. 39-41. A series of reconstructions from cross sections of larve 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 66.

Fig. 89. 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.

D
. FIELD de. PL.

B Meisel Jith Boston.

3 _
— at

Fre_p. — Pronephros in Amphibia.

Fig.

Fig.

. 42,

ig. 44.

46.

PLATE V.

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 nephrostome. X 110.

A section through the rudiment of the duct near its hinder tip, from an
embryo of Bufo belonging to Stage IV. X 400. Zeiss apochr.
4mm. Oc. B. ;

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.
xX 65.

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.

An embryonic blood corpuscle occurring in the glomus of a larva of
Bufo belonging to Stage V. x 956.

PEM

.- PRONEPEROS IN AMPHIBIA.

B Meisel lith Boston.

ee

7
‘
ns

FieLp. — Pronephros in Amphibia,

Fig.

ig. 47.

g. 50.

oll

52.

PLATE VL.

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.

A portion of a cross section through the middle trunk region of an em-
bryo of Amblystoma belonging to Stage I. > 65.

Anterior face of a cross section through the glomus of a larva of Bufo
belonging to Stage V. X 470. Zeiss apocr.4mm. Oc. 6.

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. 4mm. Oc. 12.

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.

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 cal.””) of the outer peritoneal layer of the glomus. Xx 500.

thlaphstm2Z MO) gi \,
{/ 6

trn.cly. eae 4) Sa .

or

———

eo

7; nt

af,

t
t
| .

XN.

=

Fieip. — Pronephros in Amphibia.

Fig.

Fig.

Fig.

53.

50.

g. 56.

PLATE VII.

A portion of a cross section through the posterior trunk region of a larva
of Amblystoma belonging to Stage VI. The section shows the fun-
dament of the second primary mesonephric tubule (dé. sp.). x 90.

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.
4mm. Oc. 8.

A portion of a cross section through the anterior trunk region of an
embryo of Amblystoma belonging to Stage III. The pronephric
thickening is shown in the region of the middle of Somite II.
x 90.

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.

FIELD.-PRONEPHROS IN AMPHIRIA.
Sid anew: ee

umytin.

. BMeisel lth Baston.

FreLp. — Pronephros in Amphibia.

PLATE VIII.

Figs. 57-60. Reconstructions of several pronephridia of Amblystoma larvz be-
longing to Stage V.
Fig. 57. Reconstruction of a pronephros showing three nephrostomal tubules.

Figs. 61-65. Reconstructions of several pronephridia of Amblystoma larve be-
longing to Stage VI.

FIELD.- PRONEPHROS IN AMPHIBIA. PL. VIII.

il

BULLETIN

OF THE

MUSEUM OF COMPARATIVE ZOOLOGY

AT

HARVARD COLLEGE, IN CAMBRIDGE.

VOL. XXII.

CAMBRIDGE, MASS., U.S. A.
1891-92.

UNIVERSITY PRESS:
JoHN WILSON AND Son, CAMBRIDGE, U.S. A.

CONTENTS.

No. 1.— Contributions from the Zodlogical Laboratory. XXVIII. Observa-
tions on Budding in Paludicella and some other Bryozoa. By C. B.
Davenport. (12 Plates.) December, 1891

No. 2. — Contributions from the Zodlogical Laboratory. XXIX. The Gas-
trulation of Aurelia flavidula, Pér. & Les. By Frank Smitu. (2 Plates.)
December, 1891

No. 38. — Contributions from the Zoélogical Laboratory. XXX. Amitosis in
the Embryonal Envelopes of the Scorpion. By H. P. Jonnson. (38 Plates.)
January, 1892

No. 4.— A Fourth Supplement to the Fifth Volume of the Terrestrial Air-
Breathing Mollusks of the United States and Adjacent Territories. By
W. G. Binney. (4 Plates.) January, 1892

PAGE

127

163

eee
-~ Theva oy
oa
© es"

gar,”
a
mire

}
-& @
}

No. 1.— Observations on Budding in Paludicella and some other
Bryozoa. By C. B. Davenrort.?

ConTENTS.
PAGE PAGE
A. SPpEcIAL Parr. III. Budding in Marine Gymno-
lemoatar ie . 40
I. Introduction 1

1. Architecture of the ‘Steck . 40

2
ee DUG Patudicella ‘ 2. Origin and Development of
1. Architecture of the Stock . 2
aE talon £ the .Buddin the Individual. . . . 53
Ree oe NE g 3. Meterctationatthe Poly pide 64
Region . . 4
IV. Origin of the Gemmiparous
peo emot the Polypide is Tissue in Phylactolemata 66
the Terminal Bud ... 7 y ‘
4. Origin and Development of B. GeneraL ConsIDERATIONS.
5 27 ah meena Body : lesbawsiotebuddings .. 3 is) +a. 901
are penn O& the xvody: II. Relation of the Observations on
Wallies. utero be Sti) 22
Budding in Bryozoa to the
6. Development of the Balepida 18
Germ Layer Theory . . . 8&8
7. Origin of the Museles . . 27 afte
, III. On some Characteristics of
8. Formation of the Neck and P :
Gemmiparous Tissue . . 98
Atrial Opening . . 31 é .
IV. Relationships of Endoprocta
9. Development of the Com-
and Ectoprocta . .. . . 102
munication Plate .. . 382
10. Role of the Mesodermal SENG yaa meme: Gih- Weems. LOG
Vacuolated Cells .. . 34/|Literaturecited . .... . . 109

A. SPECIAL PART.

I. Introduction.

THE somewhat heterogeneous studies here brought together have been
prosecuted at different times and in different places, as opportunity for
getting light on the problem of non-sexual reproduction as exhibited in
the group of Bryozoa has presented itself.

While studies on the fresh water species were pursued chiefly here at
Cambridge, those on marine Bryozoa were made while occupying one of
_ the tables of the Museum at the United States Fish Commission Labora-

1 Contributions from the Zodlogical Laboratory of the Museum of Comparative
Zovlogy, under the direction of E. L. Mark, No. XXVIII.
VOL XxII.—wno. 1. i

2 BULLETIN OF THE

tory at Wood’s Holl, Mass., during the summer of 1889, and while at
Mr Agassiz’s Newport Laboratory during the summer of 1890. To my
instructor, Dr. E. L. Mark, for many valuable suggestions during the
progress of my work and the writing of this paper, to Mr. Alexander
Agassiz, for the kind hospitality accorded me at his Newport Laboratory;
and to Hon. Marshall McDonald, United States Commissioner of Fish
and Fisheries, and Dr. H. V. Wilson, Assistant at Wood’s Holl, for fa-
vors shown me while at the Wood’s Holl Laboratory, I make grateful
acknowledgment of my indebtedness.

A word as to localities. The marine Bryozoa were found especially
abundant at Newport on floating eel-grass in the cove and on the piles
of the wharf. The embryos of Cristatella and Plumatella were found in
colonies which literally covered the bottom of some parts of the south
or shady side of Trinity Lake, Pound Ridge, New York. . They occur
especially in densely shaded and fairly deep water near the shore.

The Gymnolemata present many difficulties to finer technique. They
possess a chitinous covering, often very thick, and frequently, in addition,
a calcarous skeleton. When the latter is present, picro-nitric acid mixed
with sea water is a fairly good fixing reagent ; when it is absent, hot cor-
rosive sublimate was most serviceable. The objects must be transferred
through the grades of alcohol with extreme caution, to prevent the col-
lapse of the ectocyst. I used the chloroform-paraffin method of em-
bedding in order to make transfers more gradual at this stage. Some
difficulty was experienced in staining such small objects on the slide, since
the tissues are very loosely associated ; and on the other hand zn toto
staining is unsatisfactory in some cases, owing to impenetrability of the
ectocyst. Often it was necessary to open the body cavity of each indi-
vidual by means of a sharp knife or needle. The best results were
obtained with alcoholic dyes like Kleinenberg’s hematoxylin and
Mayer’s cochineal; although Ehrlich’s hematoxylin was often used
with success.

II. Budding in Paludicella.

1. ARCHITECTURE OF THE STOCK.

Paludicella, as is well known, occurs in quiet streams and forms
stocks on the under surfaces of stones and other objects. Seen with the
naked eye these stocks appear as a fine lacework, composed of constantly
branching lines of individuals. Some of the stocks which I have meas-
ured are over 25 mm. in length along their greatest diameter.

MUSEUM OF COMPARATIVE ZOOLOGY. 35

When the stock is studied more carefully, it is seen that the individ-
uals which compose it are arranged one in front of the other, forming
lines. (Figs. 1, 2, 2.) We may distinguish (1) a single primary branch,
which forms a continuous line from the oldest individual, which has been
derived directly from the egg, to the terminal one; and (2) secondary
branches, which arise from the individuals of the primary branch and at
right angles to their axes. Typically, a secondary branch arises from both
the right and left sides of each adult member of the primary branch. but
in some cases the secondary branch of only one side appears to be formed.
The secondary branches are composed, like the primary, of a continuous
line of individuals placed end to end. ‘These in turn give rise to ter-
tiary branches, which run out at right angles to the right and to the
left of the secondary ones, and hence parallel to the primary branches.
Quaternary branches may occur in like manner, but I have never seen
branches of a higher order than the fourth. All of these branches may
lie in one plane, but frequently some of the lateral buds are so placed
that they give origin to secondary branches which rise above the plane of
the object upon which the stock lies. A study of Figure 1 and the cor-
responding diagram, Figure 2, reveals some additional facts. The two
lateral buds of an individual do not arise at the same time, and there is
a tendency for the first, and therefore oldest and most developed, sec-
ondary branches to arise alternately on opposite sides of the primary
branch. This last rule has many exceptions, however.

The long axis of the individual coincides with that of its branch ; the
sagittal plane lies in that axis, and at right angles to the substratum.
The atrial opening is near the distal end of the individual in the sagit-
tal plane, and is turned away from the substratum. The anal aspect of
the polypide is placed nearer the tip of the branch, — hence distad ;
the mouth, on the contrary, proximad.

A very casual observation shows that not all branches nor all individ-
uals are of the same size. The shortest and therefore youngest branches
are placed most distally, and are seen as small buds. The terminal indi-
viduals of the branches are also evidently less well developed than the
more proximal ones. The adult individuals measure from 1.5 to 2.0 mm.
in length and from 0.50 to 0.35 mm. in width. The younger individ-
uals differ from the older in form also. The outline of the adult branch,
looked at from the side, and disregarding the atrial opening, is formed by
a series of beautiful sigmoid curves (Fig.9). The concave and convex
points of the upper and lower sides of an individual are not placed exactly
opposite each other, and the lower (abatrial) side approximates more

4 BULLETIN OF THE

nearly to a straight line. The point at which the upper and lower
curves most nearly approach each other is where the separation of two
individuals takes place; that at which they are farthest apart is the
middle of the zocecium, occupied by the polypide and sexual organs.
The outlines of the young zoccia are straighter, and their breadth is
considerably less than that of the adult.

From what we have already seen, the method of growth of the stock is
perfectly evident: it is by the formation of new median buds at the tips
of existing branches, and of new branches from lateral buds. In order
to understand the origin of the individuals of the primary branches, to
which subject we will first turn our attention, we must study the tips
of the branches.

2. HistoLtogy oF THE Buppine REGION.

Figures 7-9 will serve to show more in detail the method of formation
of new terminal individuals. We find in these cases one polypide already
pretty well developed and attached to the body wall by means of the kamp-
toderm at about the point at which the pyramidal muscles (mu. pyr.)
are seen to be forming. That portion of the animal which extends from
about the region of formation of the muscles to a point a little proxi-
mad of the tip represents the region which will go to form the new in-
dividual. The tip itself, for reasons which will presently appear, is not
to be included in the terminal individual. The tip of the branch is to
be regarded as homologous with the margin of the corm in corm-building
genera of Gymnolemata. Figures 7-9 (gn.) also show the position of
the bud which is to produce the polypide. By consulting first Figure 9,
in which the polypide bud is apparent, the significance of the swellings
of the body wall in Figures 8 and 7 becomes clear.

Figure 14 (Plate II.) represents a stage in the development of the
polypide bud, somewhat later than that shownin Figure 9, and this may
serve us as a starting point in our study of the origin of a new individual,
and, first of all, of the new polypide. The whole of Figure 14, from the
tip down to the neck of the older polypide (cev. pyd.), may be divided, for
convenience, into three zones: first, that distad of the young bud, which
may be called the tip of the branch (Fig. 14, a to B) ; secondly, the region
of the bud itself, which may be called the gemmiparous zone (6 to y) ; and
thirdly, the region between this last zone and the neck of the older poly-
pide, which, for want of a better name, may be called the proximal zone
(y to 8). In the formation of a new polypide between a and B, that
region will in turn become divisible into the three zones just named,

MUSEUM OF COMPARATIVE ZOOLOGY. 5

exactly as the region a to 8 represented the tip of the branch when the
older polypide, whose neck is shown at cev. pyd., was of the age that the
younger bud is now. It will be necessary first of all to study carefully
each of these three regions before treating of their origin and fate.

The tip of the branch consists of the two layers of cells which are found
in other parts of the body wall, —the ectoderm and the mesoderm, as the
coelomic epithelium may, for brevity’s sake, be called. The cells of the
ectoderm at the extreme tip (Plate I. Fig. 6) are greatly elongated, form-
ing a columnar epithelium. There are about 25 or 30 of the larger cells.
They have a length of 28 » to 32 p, and a diameter of about 4p. They
possess an ovoid nucleus averaging 5.7 » by 2.6 p, which lies in the middle
of the cell but slightly nearer the ccelomic epithelium than the cuticula.
It possesses a large nucleolus over | » in diameter, which often appears
stellate owing to the threads of plasma surrounding and proceeding from
it and forming a nuclear network. As the figure shows, the plasma of
the cell is filled with large, apparently deeply stained granules, some of
the largest being over 0.6 » in diameter. The coarser granules lie chiefly
in the immediate vicinity of the nucleus, but are also found arranged in
long lines at right angles to the surface throughout the greater part of the
cell, becoming finer the farther they lie from the nucleus. A fine network
can sometimes be made out between the large granules, but this appear-
ance is more evident at the peripheral portion of the cell, where there are
no large granules. At the outer and inner ends of the cells one finds large
vacuoles, the largest of which are of about the same size as the nucleus ;
these become smaller the nearer they lie to the nucleus. In many cases
the larger vacuoles are each seen to be partly filled by a body which stains
slightly, and, as focusing determines, is more highly refractive than the
plasma. Similar highly refracting, slightly stainmg granules are found
in, and in fact often composing, the smaller ‘‘ vacuoles.” Owing to the
fact that the deeply staining granules lie near the nuclei, and that the
vacuolated and finely granular plasma lies more remote, there is a very
marked deeply stainmg band occupying the middle of the ectodermal
layer, and having about four tenths the thickness of the whole layer.

At the outer ends of the cells, and doubtless secreted by them, there is
a cuticula about 1» thick. Its inner surface is sharply marked off from
the underlying plasma; its outer surface is less sharp, and there are
usually very minute particles of dirt attached to it (not represented in the
figure). The whole cuticula forms in section a continuous band of
substance, which stains deeply in Ehrlich’s hematoxylin (but not at all
in alum cochineal), and covers nearly the whole tip. Looked at from

6 BULLETIN OF THE

the surface after staining in hematoxylin, it appears uniformly dark.
The mesoderm of the tip is highly modified, and a description of it
will be more instructive after I shall have described the normal ccelomic
epithelium, as I shall do later.

Passing from the extreme tip towards B (Fig. 14), one finds the ecto-
dermal cells gradually changing in form, size, and structure, and becoming
slightly broader, and very much shorter. Their nuclei lie near the inner
ends of the cells, possess a thick ‘nuclear membrane,” and are more
nearly spherical than those of the columnar cells, but of about the same
size. They each possess one very large, centrally placed nucleolus, whose
diameter equals and sometimes exceeds one third that of the nucleus, and
whose outline is often somewhat stellate. Outside of the nucleus in the
cell body there are fewer and fewer vacuoles as we pass from the tip, but
the plasma is still coarsely granular, and here, as before, these stained
granules surround the nucleus. It is now the regions between cells
rather than those at the inner and outer ends which remain unstained,
so that the cells are separated from one another by light spaces.

The mesodermal layer becomes somewhat thinner than at the tip, that
is to say, its cells are flattened. he nuclei are elongated in the axis of
the branch, and average about 4m by 2.2». They possess one spheri-
cal nucleolus, whose diameter is about two thirds of the minor axis of
the nucleus. Small, clear vacuoles often with highly refractive spherical
bodies are abundant in the cell protoplasm, which stains as a whole less
deeply than does the ectoderm. Such highly vacuolated elements will
be called reticulated cells.

If we study the gemmiparous zone at astage considerably earlier than
that shown in Figure 14, in fact at a stage in which a polypide is about to
arise, we find an appearance of the layers represented by Plate I. Fig. 3.
In such a region the ectoderm consists of cuboid cells about 7p high by
6.5 w broad. The nuclei are large, nearly spherical, and vary in size from
3.5 to 6.0p. The largest nuclei are those in the region from which a bud
is about to arise (ev.). One in this region (to the right of ew.) is 6.5p
by 6.0 » in diameter, with a nearly spherical, eccentrically placed nucleo-
lus of about 3.0 in diameter. This nucleus is the largest which I have
found in the whole tissue of Paludicella, and the same is true of the nucle-
olus. From the examination of many regions from which buds are about
to arise, I can assert that such regions always, in Paludicella, possess large
nuclei and large deeply staining nucleoli. I shall have occasion to de-
scribe similar conditions elsewhere, and to point out the probable signifi-
cance of these facts. The cell body possesses a highly granular, deeply

MUSEUM OF COMPARATIVE ZOOLOGY. 7
staining plasma; the inner ends of the cells, however, do not stain so
deeply as the middle or peripheral portions.

The cuticula (omitted from Fig. 3, see Fig. 5) is usually somewhat
different in appearance from that at the extreme tip. In section we can
distinguish two layers : an outer, thicker, deeply staining layer, which is
not continuous but appears broken into larger or smaller bits; and an
inner, thin, non-stainable and highly refractive portion, from which the
first layer is often slightly separated. This second layer is closely applied
to the underlying cells, which doubtless secrete it. Looked at from the
surface (Fig. 10, a.) the deeply stainable layer is seen to be broken into
irregular polygonal pieces ranging from 2 » to 17 » in diameter and sepa-
rated from one another by spaces ranging from 0 to 6 pz.

The mesoderm forms a loose epithelium, whose average width is less
than that of the ectoderm (Fig. 3, ms’drm.). As a whole, moreover, it
stains less deeply. In a portion of the gemmiparous zone, which lies
about 180° from the budding region, the mesoderm has become so delicate
a layer, if it exists at all, as not to be easily distinguishable. In the vicin-
ity of the bud its cells have irregular outlines and extend out into the
ccelom as though possessed of the power of amceboid movement. The
nuclei are spherical or ovoid, smaller than those of the ectoderm, and on
the whole have smaller nucleoli. The cell body is highly vacuolated.
The vacuoles are not large and clear in outline, but whole regions of the
cell body seem to be reduced to a non-stainable condition, and in some of
these regions a finenetwork may still be observed.

The proximal zone (Fig. 14, y to 8) is distinguished, soon after the:
first rudiment of the bud appears, by the diminished thickness of the
ectoderm. The cells have become transformed from a columnar to a
pavement epithelium. The nuclei are smaller, the nucleoli less prom-
inent, and the cell body stains much less deeply. The cuticula is of
two kinds, as before, but with this difference: the deeply staining outer
part is less conspicuous, and the pieces are smaller and more widely sep-
arated. Looked at from the surface, we find an appearance like Figure
10, ¢., in which the dark bodies represent the deeply staining cuticula.
These pieces are much smaller than those of the gemmiparous zone,
ranging from 0.6 » to 9.5 m in diameter, and separated from each other
by spaces ranging from 0 to 13 x.

3. ORIGIN OF THE POLYPIDE IN THE TERMINAL Bubp.

Observation having shown that budding in Paludicella follows definite
laws, we ought to be able to discover the place and time at which buds

8 BULLETIN OF THE

will arise ; and it is necessary to do this in order to study the origin of
the gemmiparous cells, and the changes which they undergo preparatory
to an actual involution.

The study of tips of branches shows that the necks of the polypides of
any branch all lie in one plane, and that this plane also includes the
youngest polypide ; also that the youngest polypides always arise distad of
the next older. Knowing these facts, our observations may be confined to
a short line running from the neck of the youngest apparent buds to the
tips of the branches studied. The time at which to search for incipient
buds and the place in the line where they will be found is illustrated by
Figure 7 (Plate I.). The youngest developed bud is one the axes of whose
tentacles are approximately parallel to the axis of the branch, and whose
brain cavity, gn., is not yet constricted off from that of the cesophagus.
The place of origin is near the tip, immediately beyond the point at which
the ectoderm changes rapidly from a columnar to a pavement epithelium.

Figure 3 is from a section across the branch in the region of an incip-
ient bud. I have already described the conditions of the cells of this
region. Those near ex. are larger than the surrounding ones, and show
signs of cell division both in the ectoderm and mesoderm. In both cases
shown in the figure, the direction of division is such as will tend to
increase the superticial area of the layer in which it ocenrs. The ecto-
derm seems to be the most important layer of the two in the process of
invagination which is about to take place. I think one is led to this
conclusion if one considers a folding of an epithelium to be due to an
increase in the area of the epithelium within a certain circumference
without a correspondingly great increase in the circumference itself.
Such a conception implies, first of all, mutual pressure of the cells of
the invaginating epithelium. The cells of the mesodermal layer do not
seem to be under mutual pressure; in some cases they are barely in
contact. The cells of the ectoderm are evidently closely applied, and
probably, therefore, under mutual pressure.

The one case of cell division which is occurring in the ectoderm is at
the inner end of the cell. In fact, the centre of the nuclear plate is much
nearer the deep end than are the centres of the adjacent nuclei. The
effect of this division is to increase the area on the inner surface of the
ectoderm more than that on the outer, as appears from a study of the
sections shown in Figures 4 and 5. In Figure 4 certain cells lie already
below the niveau of the surrounding ones, very much as though they had
moved downward on account of this being the direction of least resist-
ance. <A later stage of this process is shown in Figure 5. Here the

\

MUSEUM OF COMPARATIVE ZOOLOGY. 9

nuclei are already arranged in a deep saucer-shaped layer. The transi-
tion to the U-shaped arrangement of Figure 37 (Plate IV.), in which
the invagination of the inner layer of the bud is completed, is not a diffi-
cult one to understand. It is to be observed, however, that the folding
is of such character that it can hardly be termed a typical invagination,
Comparing Figures 4, 5, and 37, it appears rather to be of a type some-
what intermediate between typical invagination and typical ingression.
The cavity of the bud first arises through a rearrangement and reshap-
ing of the cells of the inner layer of the bud. At this stage the nuclei
of the invaginated region stain very deeply, and have large nucleoli.
Figure 21 (Plate ILI.) shows the condition of the bud at this stage as
seen in longitudinal section. The proliferation which gave rise to the
rudiment of the bud is shown, by a comparison of Figures 37 and 21,
not to have been confined to one point, but to have occurred along a line,
so that the resulting bud is boat-shaped, and not cup-shaped. The whole
mass is therefore bilaterally symmetrical. Even at this early stage one
can distinguish a difference in the form of the bud at the anal and oral
ends. At the oral end (Or.) the bud passes more abruptly into the body
wall than at the anal end. Later, this feature becomes more marked.
This is an indication of a fact for which later stages will bring better
evidence: that the formation of the bud proceeds from the oral towards
the anal end, and that the increased length of the bud that one finds in
the stage represented by Figure 22 is due to growth at the anal end.

4, ORIGIN AND DEVELOPMENT OF THE LATERAL BRANCHES.

The first lateral branch appears as a prominent protrusion of the lat-
eral walls of an individual of the primary branch when the ganglion of
that individual has already nearly closed, and when the bud of the next
younger individual has attained a stage somewhat later than that shown
in Figure 37. The zone in which the lateral buds arise Jies about mid-
way between the neck of the median polypide and the tips of its. tenta-
cles at this stage. The place of appearance in this zone is approximately
90° to the right or left of the neck of the polypide of the median indi-
vidual. In one case measured, however, that shown in Figure 20 (Plate
II.), the centres of the two lateral buds seemed to be unequally distant
from the neck of the polypide, and each over 90° from it (approximately
100° and 110° respectively. (Compare page 3.)

A cross section of the branch through the region in which the lateral
bud is arising shows that the condition of the body wall at the bud is
quite different from that of the rest of its extent. Figure 19 represents

10 BULLETIN OF THE

a longitudinal section of a portion of the body wall passing through the
non-budding region. The wall seems to consist of one layer only of cells,
and a fine, non-stainable cuticula. This layer of cells is the ectoderm,
for it can be traced directly into the outer layer of the tip. The meso-
dermal layer is not represented in the region from which the figure
was drawn, but I believe it is not entirely absent from this part of the
individual, for occasionally extremely flattened cells, spindle-shaped in
section, may be seen lying inside of the ectodermal layer, quite sharply
marked off from it by a distinct line. Further evidence of the existence
of two layers is found in the fact that one occasionally sees in the flat-
tened body wall two nuclei lying together, one nearer the ccelom than
the other. The cells of the ectoderm are seen to be very much flattened
(average 2.5 »), and their nuclei are widely separated (35 »). The nuclei
are oval, and rather smaller than those near the tip. They possess a
single, rather large nucleolus, which does not stain intensely. The cell
protoplasm stains very little. The cuticula is about 0.5 p thick.

If we study the body wall in the budding region, when the latter is
first indicated on the surface by a marked protrusion of the outline of
the zocecium (Plate II. Fig. 15), we find that this protrusion is due to
an elongation of cells. There are about twenty-two cells in this section,
which are more or less thickened. Since the section figured passes
through the centre of the circular thickening, and is about one sixteenth
the diameter of the circle in thickness, it follows that there are over 250
cells of the ectoderm which have already at this stage become somewhat
enlarged previous to evagination. The highest of these cells are the
central ones, of which the largest is 22» high. The largest nuclei are
4 by 6.3 p, which approximates the size of those in the gemmiparous
region (page 6). They are placed nearer the coelomic epithelium than
the exterior, are nearly spherical, and each possesses one large nucleolus
and a quite apparent network with deeply stainable nodal points. The
cell body is stained as a whole rather deeply by Ehrlich’s hematoxylin,
but particularly around the nuclei. The outer parts of the central cells,
however, are stained very little, and the deep ends of some of the lateral
cuboid cells not at all. The network of plasma contains only fine
granules, and these seem to lie in rows parallel to the long axis of the
cell. The structure of the outer-layer cells, at a somewhat earlier stage,
is shown in Figure 18, under a higher magnification. The network is
very apparent in these large spherical nuclei, and the plasma of the cell
is seen to contain coarse granules, which lie near the nuclei and stain
deeply.

MUSEUM OF COMPARATIVE ZOOLOGY. i1

While the cuticula of Figure 18 is seen to be that of the normal body
wall in this region, that shown in Figures 15 and 16 appears under the
microscope after staining in hematoxylin to be of two distinct kinds :
(1) that outside of the central region, which is highly refractive and not
at all stained; and (2) that which lies immediately over the central elon-
gated cells of the bud, which is also highly refractive but stains deeply.
In fact, the central cuticula resembles in every way that already described
for the tip of the branch, and shown in Plate I. Fig. 6. Moreover, it
has other points of resemblance to the latter. It does not stain at all
in alum cochineal ; the outer boundary of the branch is often uneven
at this place (Fig. 16) ; and particles of dirt are often found adhering to
it, while the rest of the cuticula is comparatively free. The difference in
staining properties of the central and lateral cuticulas indicates that the
former undergoes with age a change in its chemical properties ; the irreg-
ular outer boundary and the adhesion of dirt particles seem to indicate
that the newly formed cuticula is viscid. The mesoderm of the stage of
Figure 15 consists of a single loose layer of subspherical cells of the two
kinds already noticed, reticulated and non-reticulated. The series of
Figures 18, 15, and 16 shows the behavior of columnar cells in the forma-
tion of a typical outfolding as distinguished from the slipping in of cells
to form the polypide (Figs. 3, 4, and 5).

In stages later than that of Figure 16, the tip of the branch becomes
further removed from the body wall of the median branch. The cells at
the tip always retain their elongated columnar condition. A polypide
is soon formed on the upper part of the body wall immediately behind
the tip, exactly as in the case of the median branch. A septum is
very early formed, cutting off the lateral from the median individual,
and the lateral secondary branch becomes the median primary one of
new individuals (Plate VI. Fig. 58).

We have already traced out the origin of the polypide of the median
branch from the mass of cuboidal cells near the tip; it remains to de-
termine whether the cells which give rise to the lateral branch can be
traced directly back to the cuboidal cells of the tip, or whether they have
arisen from the flattened epithelium of the general body wall and sec-
ondarily acquired their plump “ embryonic” character.

Figure 18 ( Plate IT.), to the cellular conditions of which I have already
referred, shows an early stage of the lateral branch, and Figure 20, gm. l.,
shows on a smaller scale the different cellular conditions in the body wall
in the region of two lateral buds which are yet far from showing ex-
ternal signs of evagination. ‘The cells are cuboid and much higher than

v2, BULLETIN OF THE

those of the adjacent body wall. Have they been so ever since they were
derived from the tip, or have they secondarily become so ¢ I believe
that these cells have never been flattened pavement epithelial cells, for
the following reason. All ectodermal cells of the body wall near the tip
are cuboidal; if these cells have only secondarily acquired this form,
they must have passed through a stage in which they were flattened epi-
thelium. Now, if these cells could be distinguished by greater thickness
from the cells of the surrounding body wall, at a time at which the lat-
ter cells had only just begun to emerge from the cuboidal condition to
become differentiated into the pavement epithelium of the body wall, it
would follow that, even though they had secondarily increased in size as
a result of an impulse preparatory to evagination, and even though they
would have been at a stage only a very little earlier indistinguishable
from the other cells of the body wall, yet they would never have passed
in this case through a flattened condition, because at a stage only a very
little earlier the whole body wall was composed of cuboid cells.

The conditions which I have set as the criterion of our problem are
fairly realized in Figure 17, which represents a portion of the body wall
of a median branch which extends from the gemmiparous region above to
the thickened body wall of the nascent lateral bud below (gm. /.). It will
be seen, by a comparison of the body wall of this regien with that shown
in Figure 19, which is taken from the same individual farther from the
tip, that even the most differentiated part of the body wall of Figure 17
is in a relatively indifferent condition as compared with the pavement
epithelium of the ectoderm of Figure 19, in which the mesoderm, indeed,
has become so thin and insignificant as scarcely to be visible. We may,
therefore, maintain that the ectodermic cells of the body wall have only
just begun to lose their cuboidal condition to become pavement epi-
thelium, and therefore conclude, in accordance with the argument just
presented, that the cells of the lateral bud (gm. /.) have never passed
through a stage in which they were flattened epithelium. It is evi-
dent, also, that the Anlage of the second lateral bud is also derived
from near the tip, because, as in Figure 20, we find two lateral regions of
cuboidal cells.

5, DEVELOPMENT OF THE Bopy WALL.

It is, of course, almost impossible to gain direct evidence upon the
place of origin and method of development of the body wall, and one is
therefore forced to the collection and weighing of circumstantial evi-
dence. Braem (’90, pp. 127, 128, 131) believes that the body wall (the

MUSEUM OF COMPARATIVE ZOOLOGY. 15

eystid, in Nitsche’s sense) has a double origin in Paludicella: “ Ein
Theil des Cystids zwar vor dem Polypid, ein anderer aber erst spiiter
angelegt und zwar aus der polypoiden Knospe selbst entwickelt wird.”
The part developed from the bud of the polypide is the elliptical region
of the body wall, whose main axis lies in the sagittal plane and which
has the neck of the polypide at the distal focus and the attached ends
of the retractor muscles and the parietovaginal (or pyramidal) muscles
lying in the proximal circumference, —the greatest part of the ellipse
thus lying oral of the atrial opening. The evidence for this conclusion
Braem finds in the following facts, which my own observations confirm:
The great retractor first appears in the angle between the oral part of
the polypide bud and the cystid wall (cf. Figs. 23, 24, cl. mu. ret.J; then
its cells gradually become elongated, and as its point of origin retreats
farther and farther from the polypide, it finally appears as a bundle
which joins a point lying between the mouth of the polypide [neck of
the polypide| and the inferior septum with the pharynx, and, as I be-
lieve, also with the cardial part of the stomach” (p. 125). Compare
the muscles at the left end of Plate I. Fig. 8. Further on he says:
“Die Parietovaginalmuskelu [pyramidal muscles] erscheinen an der
Knospe zuerst in Form zweier seitlichen Leisten, in welchen die einzel-
nen jugendlichen Fasern senkrecht zur Lingsaxe verlaufen. Indem sich
alsdann lateral von der Knospe das Cystid durch Neubildungen erweitert,
werden die Fasern verlangert und die beiden Biindel treten in Fliigelform
deutlicher zur Rechten und Linken der Miindung hervor. Ihr Ursprung
an der Cystidwand riickt nun von der Miindung immer weiter ab und
gelangt schliesslich auf die gegeniiberliegende Seite, wo er anal und late-
ral seinen definitiven Platz findet.” Compare Plate I. Figs. 7-9, mu. pyr.,
and Plate VI. Fig. 63, mu. pyr. From these observations Braem (90,
pp. 127, 128) concludes: “ So scheint es sicher, dass auch hier ein grosser
Theil des definitiven Cystids, das ja zum anderen Theil schon vor der
polypiden Knospenanlage entwickelt war, aus dem Material dieser letz-
teren hervorgeht. Das folgt namentlich aus der Art und Weise, wie
sich die Muskeln bilden. . . . Auch hier wiirde, wie bei den Phylac-
tolamen, oral vor der Knospe nach dem Retractor hin, ein grisseres
Gebiet der Leibeswand der Knospenanlage entstammen, als seitwirts
und hinten.”

An analysis of the facts has led me to conclusions differing somewhat
from those of Braem ; namely, that all or nearly all of the cells of the body
wall (cystid) are derived from the tip of the branch or from the immedi-
ate descendants of cells so derived. The number of cells contributed to

14 BULLETIN OF THE

the formation of the body wall by the neck of the polypide is much
smaller than Braem has suggested, and probably insignificant in amount.
The retreat of the points of origin of the retractor and parietovaginal (py-
ramidal) muscles may be in part accounted for by the normal growth in
area of the body wall, and in part by the actual movement of the point
of origin with reference to the cells of the body wall. These conclusions
rest upon the following circumstantial evidence.

Owing to the small number of cells in the body wall at the tip, and
the comparatively slow growth of the cystid, karyokinetic figures are
much less frequent than in the polypide. Quite a long search has there-
fore not afforded cases enough to enable me to draw any perfectly satis-
factory conclusions as to just where, and where only, growth was taking
place. I have, however, seen nuclear division occurring in the elongated
cells of the extreme tip, rather more abundantly in the cuboidal cells
between the extreme tip and the gemmiparous zone, and most abun-
dantly in the gemmiparous zone, but here evidently having to do with
the origin of the polypide, muscle cells, etc. Proximal to the gemmipa-
rous zone, I have noted few cases of nuclear division excepting about
the neck of the polypide. It seems probable that the cells of the tip
of the branch are not to be regarded as forming a differentiated organ
whose elements rarely divide, but as quite capable of adding new cells to
the body wall. On the other hand, there is by no means a Schevtel in the
botanical sense, but the cells added to the body wall continue for a time
to divide vigorously, and finally give rise to the polypide, to the Anlage
of the lateral branches, and to the body wall. The cells belonging to
the proper cystid then cease to divide rapidly.

I have already shown how the cells of the tip secrete a cuticula, which
becomes gradually replaced by a second cuticula secreted beneath it as
the body wall attains its adult dimensions. It appears as though the
first cuticula were secreted by the cells of the tip only. This being so,
since the area of the body wall increases, this first cuticula must either
stretch to cover the enlarged area, or else it will fail to cover it and
appear as isolated patches upon the body wall, and these isolated patches
will become more and more widely separated as the area of the body
wall increases. This latter condition seems to be the one realized in this
case. The presence of the old cuticula is easy to demonstrate, since it
stains deeply in hematoxylin ; and it may be easily distinguished from
that formed later, for with the same reagent this stains not at all. Figures
6, 11, 12, and 13 show different appearances of the cuticula at different
parts of the body wall. At the extreme tip (Fig. 6) there is a continu-

MUSEUM OF COMPARATIVE ZOOLOGY. 15

ous deeply stained band of cuticula. In Figure 11 it no longer appears
quite homogeneous, but is darker at some places than at others. The
ectoderm is here composed of cuboidal cells. At a later stage of devel-
opment the ectodermal cells have become very much flattened. A thin,
unstainable, more deeply lying cuticula has already begun to form, and
the outer deeply stainable cuticula is seen to be broken up into bits.
Figure 13 is from the adult body wall. The ectoderm is flattened. The
inner cuticula has attained a great thickness, and the outer cuticula is
represented by only a few deeply staining patches. One attains a simi-
lar result by studying the surface of a stained individual. Figure 10
shows the condition of the outer cuticula at intervals along the same
branch from the gemmiparous region a@ to a nearly adult region, d. The
bits of cuticula become more and more widely separated and smaller, as
I have already described in detail on page 7. Here, then, we have not
merely an interesting case of replacement of one cuticula by another to
meet the needs of the enlarged body wall by a method which has no par-
allel, so far as I know, in any other group of animals, but for the specific
purposes of our problem a criterion of growth of the body wall quite as
satisfactory as karyokinesis, and much easier of application.

Let us apply this criterion in our attempt to answer the question, Is
that portion of the body wall lying between the neck of the polypide
and the points of origin of the pyramidal muscles (Plate VI. Fig. 63,
b—a, b-c) derived wholly from the neck, or is it merely the result of
interstitial growth of that part of the*original cystid which was pre-
formed in the neck region? If the first condition is true, we should
expect to find no indications of the outer cuticula secreted by the tip of
the branch; if the second, we should expect to find the outer cuticula
broken into bits, and underlaid by the inner lately formed cuticula.
Figure 63 shows clearly the deeply stained outer cuticula here sep-
arated into bits, and, to my mind, thereby proves that this part of the
eystid has had an origin similar to that of the rest of the body wall.
Moreover, a comparison of the portion of the section figured with the
remainder (and this comparison has been made on many sections from
several individuals) shows that the parts of the cuticula about the neck
are indeed rather smaller and farther removed from each other than at
the opposite side of the branch; but the difference in this respect. is not
very marked, and may well only signify that there is a more rapid
growth of the body wall in the vicinity of the neck of the polypide than
at the opposite side.

But how then do the points of origin of the acetal muscles come

16 BULLETIN OF THE

gradually to move away from the neck of the polypide at which they
arose, in order finally to lie so that the muscle fibres are nearly parallel ?
If the points of origin remain fixed with reference to the surrounding
cells, they can hardly come to lie absolutely closer together, but only
relatively so by growth of the body wall between these points and the
neck. If, however, we find that in older individuals the points of origin
are not only relatively but absolutely closer together, we are driven to
the conclusion that these points move relatively to the surrounding cells.
To decide whether the points of origin come to lie closer together behind
the neck absolutely or only relatively, I measured cross sections of four
individuals through the region of the neck in which the muscle fibres
showed evident differences of length, and therefore of age. I may
preface a table of these measurements with the statement that the mus-
cles first appear plainly differentiated at a stage when the polypide is
well formed (Fig. 7, mu. pyr.), and that the growth of the body wall in
circumference is not very considerable after this time. The numbers
indicate measurements in micra:—

Distance on periphery between ae of mug. NOPE Nome, INGE eve eelo-

cles; atrialiside” <2) =) : = 150 154 187 260
Distance on periphery ietween origins of mus-

cles, abatrial side . . . Poe yet ie case ee 286 264 220

Total length of Der nHEET lates) Urge eae ata 440 451 480

The distance on the “atrial side” signifies the distance measured over
a, b, c, Figure 63 (Plate VI.). The length of the remainder of the sec-
tion is the distance on the “ abatrial side.”

From these measurements it appears that the “origins” of the py-
ramidal muscles approach each other absolutely, —a condition which
Braem’s hypothesis cannot explain, and which can be reasonably inter-
preted, it seems to me, only by assuming, however unique and difficult
of conception such a condition may be, that the points of origin move
relatively to the surrounding cells of the body wall. (Compare also
the movement of parietal muscles referred to on page 29.)

It is not necessary to assume that the increase in extent of the body
wall after the polypide is first formed is due to the addition of cells from

1 Professor Mark has called my attention to a discussion of the movement of
the fixation-point of a muscle in Mollusks by Tullberg (’82, pp. 26, 27, 44). This —
author says that he has undertaken no special investigation of the method of
migration, but concludes that this motion must result from the absorption of the
inner muscle fibres as new ones are formed on the outside. I do not find any
evidence of such a process in Paludicella.

MUSEUM OF COMPARATIVE ZOOLOGY. LZ

the neck. The change in form of the ectodermic cells from a columnar
to a pavement epithelium must alone cause a great increase in the ex-
tent of that layer. Some measurements that I have made seem to me
to prove that the area of the body wall does increase greatly, even out-
side the region whose growth Braem attributed to the addition of cells
from the neck of the polypide. Thus, in one case, the distance from the
distal end of the polypide bud, which becomes the neck of the adult,
to the point of origin of the young retractor muscle was 0.17 mm.; from
the same point to the septum separating the young individual from the
next older was 0.27 mm. In the next older individual, from the neck to
the origin of the retractor muscle was 0.72 mm.; from the neck to the
septum was 2.0mm. Thus assuming that the older individual passed
through a stage exactly equivalent to that in which we find the younger,
the distance from the neck to the origin of the retractors has increased
0.55 mm., and from the origin of the retractors to the septum 1.18 mm.
The first distance is that in which Braem has assumed the body wall to
grow by additions from the neck of the polypide, and this assumption was
apparently made to account for the increase in extent of this region ; but
the area between the origin of the retractors and the septum, which is
outside the region to which additions such as Braem contemplates could
have been made, has grown in this case very considerably more in extent.
This case is not a typical one, however, for we rarely find the distance
from the origin of the retractor to the septum to be so great. In gen-
eral, from observation of a number of cases, I should say that in the
adult the distance between the neck of the polypide and the origin of the
retractors, is to the distance between the latter and the septum about as
5:4, and that therefore the growth of the first region is slightly greater
than that of the second. From the fact, however, that the cells around
the neck of the polypide for a long time retain a somewhat embryonic
character, and may quite frequently be seen in division, this was to have
been expected. The conclusion which I draw from this last series of
conditions is, then, that it is unnecessary to suppose the addition of cells
from the neck of the polypide to account for the fact that the origin of
the retractors is carried backward from the polypide. Normal growth of
the body wall, such as occurs elsewhere, is quite sufficient to account

for it.
To recapitulate. That portion of the cystid lying in the vicinity of

the neck can hardly be derived from the neck alone, for the cells still

show adhering to them the cuticula which they derived from the tip of

the branch. It is not necessary, in order to account for the movement
VOL. XXII. — No. 1. 2

18 BULLETIN OF THE

of the origins of the muscles away from the neck, to suppose that the
circumcervical region is derived in that way; for (1) the origins of the
pyramidal muscles actively migrate away from the neck to a certain
extent, and (2) the normal growth of the body wall is sufficient to ac-
count for the carrying backward of the origin of the retractors.

From the facts already gained it seems clear that the ectocyst (cuticula)
is first formed at the tip, and then, to meet the wants of the growing
colony, this is replaced later by a cuticula of different chemical compo-
sition, which becomes thicker as the body wall grows older. At a late
stage we find a separation of the thick cuticula itself into two layers, of
which the outer one is much the more highly refractive.’ (Plate II.
Fig. 13; Plate IIT. Figs. 26, 29.)

6. DEVELOPMENT OF THE POLYPIDE.

We have already (pages 8, 9, Figs. 5, 14, 37) seen how the foundations
of the polypide are laid by the ingression of cells of the outer layer of
the body wall pushing before them the mesoderm, and how, finally, those
cells arrange themselves in a boat-shaped mass to form the inner layer
of the bud (Plate III. Fig. 21), which possesses no actual cavity, and
is constantly separated from the external world by the ectoderm which
remains behind to form the neck of the polypide. Even when a cavity
is formed later, it does not communicate with the exterior until the
permanent atrial opening has arisen. The earliest differentiation in the
bud is, as mentioned by Allmann (’56, p. 36), the formation of a cavity
which is to become that of the atrium. This cavity is first formed
at an early stage as an extremely slight fissure in the midst of the inner
layer. Figure 22 shows a longitudinal section of this stage. Cell di-
vision is taking place throughout the whole mass, but especially at the
neck of the polypide, cev. pyd. The position of the cavity is represented
by the central non-nucleated space, and this gives rise, as the later his-
tory of development shows, to the atrium and the pharynx..

Figure 23 represents a stage which is doubtless of short duration, for
I have found it only twice. The bud is much more developed at the’

1 Such a two-layered condition of the cuticula was long ago described by Reichert
(70, pp. 265, 266) for Zodbotryon. He distinguished “ eine dussere, festere, starker
lichtbrechende und sprédere Schicht und die innere weichere.” Realizing that the
“ectocyst” or cuticula undergoes many changes in form, — formation of lateral
buds, of septe: or communication plates, and increase in size of the stolon, — he
suggested, without having observed the process, that probably during these
changes the more rigid outer layer disappeared and was replaced by the inner
softer one.

MUSEUM OF COMPARATIVE ZOOLOGY. 19

anal end than in the last stage, and there is a second cavity below the
atrium, from which it is separated by a line of nuclei. This is plainly
an early stage in the formation of the alimentary tract, which thus first
appears at the anal side of the bud, as in Phylactolemata, and is progres-
sively formed towards the oral end. An appearance similar to the one
figured would be given by a slightly oblique section of a later stage; but
this section is strictly sagittal, and no trace of the lumen appears in adja-
cent sections. I have found a similar condition in a series of longitu-
dinal sections at right angles to the sagittal plane of the bud (Plate LV.
Figs. 39 and 40). Figure 39 shows that the atrio-pharyngeal cavity is
first developed at the anal end, and in Figure 40, which is three sections
(about 15 ») below Figure 39, the anal end only of the alimentary tract
is formed. It is worthy of notice that the cells of the mesodermic layer
of the bud are often greatly vacuolated at this stage, as in Figures 39 and
40, vac. Braem (90, p. 126) says of this stage: “Die der Resorption
dienenden Darmabschnitte, Magen und Enddarm, werden gemeinsam an-
gelegt, indem auf jeder Seite der Knospe eine Liingsfalte die Wandungen
nach innen und gegen einander zu einbiegt, worauf die benachbarten
Theile des inneren Blattes verschmelzen und so durch eine Art Abschniir-
ung das primaire Knospenlumen in den vorderen Atrialraum und die
hintere Darmhohle getrennt wird.” While I thoroughly agree with this
statement, the additional fact of the formation of the tract progressing
from the anal towards the oral end is interesting, in that it shows that the
process of formation of the organ in Paludicella is fundamentally similar
to, although differing slightly in detail from, that of the Phylactolemata.
Figure 24 shows in sagittal section a still later stage in the development
of the alimentary tract. A cross section of this stage is seen in Figure
30 (Plate IV.), in which the separation of atrial and gastric cavities is
demonstrated. The inner layer of the bud is here seen to be separated
from the ectoderm by a distinct line, and, to a certain extent, even by
the mesoderm. The distal (oral) part of the cavity of the alimentary
tract next becomes considerably enlarged to form the stomach (Fig. 25).
The outer layer of the bud, ms’drm., penetrates between the stomach
and the atrium, and a depression is formed at the bottom of the atrial
chamber which will give rise to the cesophagus. Even at this stage the
cesophagus is not in communication with the stomach, but their cavities
are separated by two layers of cells of the inner layer of the bud. These
two layers become those of the cardiac valve (Plate IV. Fig. 36, vlv. cr.).
By a further comparison of Figures 25 and 36 it will be noticed that,
whereas in the earlier stage, as in Endoprocta, there is no ccecum to the

20 BULLETIN OF THE

alimentary tract, in the later stage the coecum has already begun to form,
as in Phylactolemata, by an outpocketing of nearly the whole of the
lower wall of the stomach. (Compare also Plate I. Figs. 7, 8, and 9.)

Very soon after the establishment of the alimentary tract, and between
the stages shown in Figures 24 and 25 in sagittal section, there begin
to appear organs which have a very considerable phylogenetic signifi-
cance; namely, the lophophoric ridges, ring canal, and tentacles.

The lophophorie ridge is a fold which surrounds the mouth, and from
which at intervals tentacles arise. The ridge, however, arises before the
tentacles. The general position of the ridge, as well as its method of ori-
gin, may be learned from an inspection of a series of sections of the age
of those shown in Figures 31-34. In a section lying near the oral
end of the bud (Fig. 33), one finds two spaces, — a lower, which is that
of the stomach, and an upper, the cesophagus and atrium. This upper
space is broader above than below, and the cell layer which lines it is
thick below, but above, or nearer to the body wall of the budding
individual, it is thinner. The transition from one condition to the
other is quite abrupt, and is marked by a salient curve (Joph.). Ina
section near the anal end of the bud (Fig. 31), it will be seen that here
too the inner layer is thick below and thin above. The characters men-
tioned are still more strikingly shown in the median section, Figure 32.
That the differences in thickness of different parts of the inner layer are
recently acquired modifications of an earlier simpler condition is indi-
cated by comparing Figure 32 with Figure 30, which is from a younger
bud. The series of points (loph.) of transition from thick to thin epi-
thelium forms on the reconstructed polypide a curved line, convex above.
This line is the ridge of the young lophophore (compare Fig. 25, loph.).
I have said that the lophophoric ridge arises before the tentacles. The
evidence for this assertion is found in a series like that referred to above,
where, although the ridge exists along the entire side of the atrium,
one finds nascent tentacles in the middle region only (Figure 32, left
hand).

As Figure 25, of a later stage than Figures 31-33, shows, the lopho-
phore curves downwards rapidly at the anal end, so that it here lies at
right angles to the axis of the rectum, but does not extend at all beyond’
the anus. Orally, there is in the median plane only the slightest trace
of the lophophoric ridge. By the formation of this ridge in the wall on
each side of the atrial chamber, the original atrio-pharyngeal cavity has
become separated into two regions. The space lying within or below the
ridge forms the pharynx and the intertentacular space ; that lying with-

MUSEUM OF COMPARATIVE ZOOLOGY. 21

out and above, the atrium of the adult. (Plate III. Fig. 25; Plate IV.
Fig. 32, atr.) Since the lophophore curves rapidly downward to the
anus and does not extend behind it, the act of cutting off the lower part
of the atrio-pharyngeal cavity from the upper (atrium proper) does not
continue behind the anus, which therefore opens directly into a part of
the atrium. This part has the form of a compressed funnel, and is
bounded behind and laterally by the kamptoderm, and orally by the
hinder ends of the lophophoric ridges, and also, since the latter do not
meet in the median plane, by the pharyngeal cavity. Thus it has come
about that the anus, which at first opened into the common atrio-pha-
ryngeal cavity of the bud, has now, in the separation of the two regions,
come to lie near their point of division posteriorly, but to open distinctly
into the atrial cavity. The more pronounced separation of the part of
the atrial cavity into which the anus directly opens from the remainder
of the atrium takes place much later, and will be described further on.
In Figure 33, the ring canal (can. crc.) is seen to be already formed. At
this stage it is found on one side only, the left, if one looks at the poly-
pide from the tip of the branch. It occurs in only four sections (each 5 pu
thick), being found on the next section behind Figure 33, and on two sec-
tions nearer the oral end. At its oral extremity, it terminates blindly
as a thickening of the outer layer of the bud; at its anal end, one sees
cells of the outer layer extending out partly over the canal, but failing
to enclose it ; in the next section the mesoderm is undisturbed. In sim-
ilar sections of an older polypide (corresponding in age approximately to
Plate IV. Fig. 35), the canal is found on both sides, and near to the oral
end, but at about the middle of the series (cf. Fig. 35) it is found to
open again into the body cavity. I therefore conclude that the ring
canal makes its first appearance at the base of the lophophore in a
region just oral of the middle of the polypide. Exactly how it arises,
whether by a growing together of the lips of a shallow furrow formed
from the mesodermal layer, or by the formation of a pocket, which, elon-
gating, penetrates between the inner and outer layers of the polypide at
the base of the nascent lophophore, I have not been able to determine.
Two facts induce me to believe that the later formation of the canal
oralwards results from the penetration of a sac-like mass of mesodermal
cells between the two layers of the polypide at the base of the nascent
lophophore. One usually finds, (1) as in Figure 33, can. erc., a double
mesodermal wall between the lumen of the canal and the ceelom, and
one layer between the former and the inner layer of the bud; and
(2) at the oral blind end of the ring canal a number of loose cells

ae BULLETIN OF THE

(occasionally dividing) representing the blind end of the pocket and
lying between the inner and outer layers, both of which are intact.

Braem (90, p. 50) describes the formation of the ring canal in Phylac-
tolemata as taking place in the manner just suggested for Paludicella.
His studies were made, he says, preferably on statoblast animals. Nitsche
(’75, p. 858) concluded that in Phylactolemata the ring canal was first
a furrow, whose lips fused, and my own study (’90, p. 129) has led me to
the same conclusion. Since reading Braem’s account I have looked over
some of my own sections of Cristatella again. Certainly the process is
not so clear in the buds of the adult colony as in the statoblast embryo
which Braem figures. Nevertheless the series of sections (’90, Plate IV.
Figs. 33-38) given as evidence of my statement still seem to me capable
only of the conclusion I drew from them. Perhaps the processes may
be different in detail in the two cases; certainly the two explanations
are not fundamentally dissimilar.

The ring canal being established in the oral part of the polypide, it
grows forward, as I have said, and, secondarily, the canals of both
sides meet in the median oral line and their lumina become confluent
(Plate VI. Fig. 52, can. crc.). From what has already been said, it is
clear that the lateral parts of the ring canal are not now continuous
with each other behind. ‘They become so only after the formation of
the tentacles.

The tentacles arise upon the lophophoric ridge at a stage a little later
than that represented in Plate IV. Figure 32. At the stage represented
by Figure 35, however, the tentacles have begun to form, as indicated by
the fact that in the series from which this figure was taken the fold into
the upper part of the atrium appears now deep, now shallow, according
as the section passes through the length of a young tentacle, or only
through the lophophoric ridge between the tentacles. The position of
the section (Fig. 35) is about the middle of the series, corresponding
to Figure 32.

By a comparison of Figure 35 with Figure 32 in respect to the tenta-
cles, it will be apparent, first of all, that the lophophoric ridge itself has
been heightened and that this heightening has been effected, not by a
deepening of the fold existing in Figure 32, the lips of the fold remain-
ing quiescent, but by a movement downwards of the outer lip (*) of the
groove which is to form the ring canal. The movement is of course ac-
companied by an increase in the length of the kamptoderm, kmp. drm.
This growth of the lophophoric ridge naturally does not result in making
the tentacles project farther above the ridge. Their elongation must

MUSEUM OF COMPARATIVE ZOOLOGY. 2B

take place quite independently of the former’s. The lophophoric ridges
have now become elongated folds lying upon the right and left of the
polypide, which at this stage has a very compressed appearance (Plate LV.
Fig. 41). The folds occupy the position of the ridges, and therefore do
not lie throughout their whole extent in one plane, but oralwards are
nearly parallel to the body wall (Plate III. Fig. 25), analwards trend
nearly at right angles to it. It results from this fact, that one cannot
see the anal tentacles when looking at the polypide from the side of
the body wall to which it is attached. Figure 41 (Plate IV.) shows also
that no tentacles have yet made their appearance at the oral ends of the
two lophophoric ridges.1 The tentacles are here seen to be arising in
two long rows, and so that those of one row are placed opposite the in-
tertentacular spaces of the other. ‘There are six tentacles in each row.
The rows are not continuous with each other oralwards or analwards.

The separation of the atrial and oral cavities, begun by the first
formation of the lophophore, is, now that the tentacles have arisen, much
more pronounced. Other changes now occur in this region, which pro-
duce an extensive modification in the form of the polypide.

One of the first of these changes is the close approximation and
finally fusion of the anal extremities of the lophophoric ridges oralward
of the anus. A stage in this is shown in Figures 43 and 44 (Plate V.),
which are sections in the position of the lines 43, 44, of Figure 25
(Plate III.), but through a slightly older polypide than that represented
by Figure 25. The section shown in Figure 43 passes across the rec-
tum, grazes the outer lip of the ring groove of the anal tentacles, and
finally cuts, nearly longitudinally, one of the middle tentacles of the
row. The two lophophores are not yet completely fused in front of the
rectum. In Figure 44 (compare Plate IIL. Fig. 25, 44) this break in
the continuity of the lophophore is more prominent.

By the completion of the union of the lophophores in front of the
anus, the rectum is quite cut off from communication with the inter-
tacular space. It now opens only into the thin-walled, funnel-shaped
depression of the atrial cavity.

Pari passu with this operation the stomach and rectum are being
more completely separated from the pharyngeal cavity by the penetra-
tion of a double layer of mesoderm between these regions from each side,
and a fusion of the corresponding layers of the two sides. Finally, the

1 Compare Plate IX. Figure 77, which is a superficial view of the young lopho-

phore from Flustrella, in which the process is similar to that in Paludicella, only
the down curving of the anal tentacles occurs later than in the latter case.

24 BULLETIN OF THE

walls of the stomach and pharynx become separated from each other by
a part of the ccelomic cavity, as in Plate IV. Figure 36. This process
of separation of the alimentary tract proceeds analwards, and finally the
rectum is far removed from the cesophagus.

The anus thus comes to le farther outside of: the anal tentacles.
Finally, the ring canal, which is formed progressively farther and farther
analwards, follows the fusion of the anal ends of the lophophores, and
thus completes the canal behind the esophagus. (Plate IV. Fig. 36 ;
Plate VI. Fig. 53, can. cre.)

The anal part of the ring canal is doubtless not merely a groove,
but a tube; but the ring canal is not closed at this, and probably not
at any stage throughout its entire extent, for in Plate VI. Figure 52,
two sections below Figure 53, an opening is shown to exist on each
side (at can. erc.), putting the cavities of the ring canals and the
celom into communication with each other. These openings lie at
the sides of and slightly above the ganglion (gn., Fig. 52); a position
exactly comparable with that of the openings in the ring canal of Phy-
lactolemata, which leads from the ccelom into the lophophoric arms on
the one hand, and into the circumoral part of the ring canal on the
other.

By a comparison of Figure 41 (Plate IV.) with the sections shown in
Figures 60-62, it will be seen that the row of tentacles has undergone
a change of form: from being laterally compressed, it has become cir-
cular. This change of form has not resulted from an increase in the
number of the tentacles, for at the stage of Figure 41 there are six ten-
tacles on each side already formed (the sixth not visible), and there
are in front of the mouth spaces already reserved for the two additional
tentacles. There are also, probably, two nascent tentacles at the anus,
although these are little developed, making a total of 16. In Figure 61
there are only 15 tentacles ; moreover, the actual diameter of the ten-
tacular corona in the sagittal plane is less than at the earlier stage
of Figure 41. This change of form is perfectly normal, all young
polypides having tentacles arranged in two parallel rows, and adult
polypides having a circular lophophore.

These changes in the form of the tentacular corona are correlated
with important changes in the direction of the axes of other organs.
These changes may be understood by comparison of Figures 25 and 36,
together with the assistance of Figures 7—9, all of which are oriented
in the same manner. In Figure 25 the points fixed by the cardiac valve
(vlv. cr.) and anus (an.) lie in a line which is approximately parallel to

MUSEUM OF COMPARATIVE ZOOLOGY. 25

the body wall. In Figure 36 the line passing through the same points
makes an evident, but not very large, angle with the body wall. This
line has undergone, then, a slight change of position only. The axis
of the anal tentacles lies in both cases nearly parallel to the body wall,
and so does the neural wall of the pharynx. The oral tentacles, on the
contrary, whose axes in the earlier stage are directed perpendicularly
to the body wall, lie in the later stage with their axes parallel to the
wall; and the base of the lophophore, which in the earlier stage trended
at its oral end parallel, at its anal perpendicular to the body wall, in
the later lies throughout its whole extent in one plane perpendicular
to the body wall. The axes of the oral tentacles have rotated through
an angle of nearly 90° relatively to most of the other organs of the poly-
pide. The cause of this rotation must evidently be sought in unequal
growth in different parts of the polypide. A comparison of the length
of the kamptoderm on the anal side in Figures 25 and 7 indicates
that it has grown more in length than on the oral side. This excessive
growth would tend to rotate the line wv. er. — an. to a position perpen-
dicular to the body wall. Since this rotation has not occurred to so
great an extent as was to have been expected, we must look for a com-
pensating growth on the oral side of the polypide, between v/v. cr. and
the neck of the polypide, which shall be nearly equal to the excessive
growth of the anal kamptoderm, and which must be owtside of the oral
kamptodem. These conditions of location are fulfilled only by the
oral wall of the cesophagus, and it is by change of position and growth
of this wall that the extension of the anal kamptoderm is nearly com-
pensated for on the oral side of the polypide. By this growth in the
wall of the cesophagus the oral part of the ring canal has been brought
to lie over the anal part, the sagittal diameter of the tentacular corona
has been reduced, and the compressed lophophore has been transformed
into a circular one.

Concerning the number of tentacles, Dumortier et van Beneden (’50,
p- 46) observe that in the adult there are ordinarily 16, although
individuals with 18 tentacles occur not infrequently, an observation
which Kraepelin (’87, pp. 98, 99) confirms. In addition to these num-
bers, I have found 15 and 17. The growth of the odd tentacle is quite
interesting. The sections reproduced in Figures 60-62 (Plate VI.) will
serve to illustrate a condition which I have quite frequently found in
a polypide with 17 tentacles. In this particular series there are only
15 tentacles. The successive sections abundantly demonstrate that
the odd tentacle (*) is anal in position, and that it is younger than any

26 BULLETIN OF THE

of the others; thus in Figure 61 its tip is cut, in Figure 60 there are
only 14 tentacles visible, and these are found in the two following sec-
tions. Since there are six sections which pass through the tentacles,
aud the odd tentacle is found in only three of these, it follows that it
is only about one half as long as the others.

The nervous system arises, as in Phylactolemata, by a depression in
the floor of the common atrio-pharyngeal cavity, in the region which later
becomes the anal surface of the pharynx. As in Phylactolemata, we
first see a shallow pit (Fig. 25, gn.). This appears to become deeper,
sinking downward and somewhat toward the cardiac valve (Fig. 78, gn.).
Finally it becomes constricted off from the wall of the cesophagus, and
then appears as a cellular mass closely attached to it and surrounded
exteriorly only by mesoderm. (Plate I. Fig. 8; Plate VI. Figs. 52,
53, gu.) Even before the closure of the ganglionic pocket is completed,
the formation of the circumcesophageal nerve, first described by Krae-
pelin (87, pp. 62, 63) in the adult, begins.

Figure 52 (Plate VI.) shows a transverse section of a young polypide
in which the ganglion is solid, and not provided with a large cavity as
in Phylactolemata. There is a small cavity in the upper part of the
ganglion, and this is not yet wholly closed from the cesophagus. The
ganglion is continuous with a pair of hornlike processes (7.) which
partly enclose the cesophagus, and at a later stage do so wholly
(Plate IV. Fig. 36, 2’.) The cells of these horns are found dividing in
unusual abundance. ‘The horns lie next to the digestive epithelium,
and between it and the mesodermal lining of the ring canal. From the
method of growth, and from the sharp line of separation between the
tips of the horns and the surrounding tissue, there can be little doubt
that the circumcesophageal nerve of Paludicella, like the lophophoric
nerves of Phylactolemata, arises as an outgrowth of the brain.

Serial sections show that the ganglion suddenly diminishes in size
immediately below the point at which the circumoral nerves arise, but
one can trace a layer of cells continuous with the brain downwards for
ten or fifteen micra farther, to near the cardiac valve. At this point
one can still see nuclei of a third layer lying between the digestive epi-
thelium of the valve and the mesoderm. It seems to me, therefore,
that this may be regarded as a gastric nerve, which seems to originate
by a single root and later to give rise to two nerves, one of which lies
on either side of the cardiac valve.

MUSEUM OF COMPARATIVE ZOOLOGY. on

7, ORIGIN OF THE MUSCLES.

a. Retractor. — After its first formation the bud becomes elongated
in the direction of the axis of the branch. The derivation of this elon-
gated stage from the much shorter earlier one might be effected in one
of two ways: either, first, by the ingression of cells from the ectoderm
at points successively more and more remote from the point of primary
invagination, the additions to the length of the bud being made by a
continuation backwards of that process by which the first foundations
were laid ; or secondly, by cell proliferation at the point of first invagi-
nation pushing the oral end of the buds farther and farther from the
neck of the polypide.

I think there can be little doubt that the second is the method
by which the bud becomes elongated; and for the following reasons.
(1) The oral end of the bud, on the supposition of continued invagina-
tion of the body wall, should become very gradually of less diameter, and
transverse sections at that end should exhibit the ingression (potential
invagination) of cells which were observed in the earliest stage; but
as a matter of fact the oral end is abrupt (Plate III. Fig. 22, 23, Or.),
and no stages of ingression are to be found there. (2) On the first
assumption, the inner layer of the bud should be at all points in equally
close relation to the ectoderm of the body wall; on the second, the
inner layer should be closely connected with the ectoderm at the neck
of the polypide (Plate III. Fig. 22, cev. pyd.), but elsewhere it should
be sharply separated from it. As a matter of fact, a sharp line can
be distinguished, in a sagittal section, separating the inner layer of the
bud from the overlying ectoderm at all points except at the neck (anal
part) of the polypide (Plate III. Figs. 22-25). Moreover, cross sections
of the anal part of the bud show the inner layer passing directly into
the ectoderm, and oralward the outer layer of the bud tends to pene-
trate more and more between the ectoderm and the inner layer.
Therefore I conclude that the inner layer of the bud is constantly
augmented by cell proliferation in its mass, and especially at the neck
of the polypide, and this explanation also accounts for the active cell
proliferation observed at the neck in Plate III. Figure 22, cev. pyd.

Since the polypide later becomes attached to the body wall by the
comparatively narrow “neck” only (Figs. 7, 9, cev. pyd.), a separation of
the oral part from the body wall has to take place. This process begins
at the oral end. In its earliest stages it is indicated by the sharp sep-
aration of the inner bud-layer from the overlying ectoderm, and the

28 BULLETIN OF THE

partial penetration of the mesoderm on each side into the space between
these two layers (Plate IV. Fig. 30, ms’drm.). At a later stage the
mesoderm may be seen as a single cell layer lying between the ectoderm
and the inner layer of the bud midway between the oral and anal ends
(Plate IV. Fig. 32, ms’drm.), and as a double cell layer at the oral end of
the bud (Fig. 34, ms’drm.). It is from these cells at the oral end of the
bud that the retractor muscles are to arise (Plate III. Figs. 23-25, el.
mu. ret.). As the oral end of the kamptoderm and esophagus to which
their inner ends are attached moves away from the ectoderm, and as the
area of the latter itself increases, the two ends of the cells move farther
and farther apart, and the young muscle cells become drawn out into
spindle-shaped muscle fibres. (Plate III. Fig. 25, cl. mu. ret.; Plate IV.
Fig. 36, mu. ret.) The retractor thus arises unpaired and remains so
at its origin, but nearer its insertion in the ring canal and cesophagus
one can distinguish a division into right and left masses. The adult
muscle fibres consist of two parts at least, the inner contractile portion
and an outer less modified protoplasmic portion, which can be traced
over the whole of the first part, but is most evident around the nucleus,
where it has a granular appearance.

b. Pyramidalis. — At about the stage of Figure 25 (Plate III.) one
finds, on cross sections of the branch which pass through the neck of the
polypide, that the mesoderm of the body wall on each side of the neck
is greatly thickened, and that its closely packed cells, which lie three
or four deep, have become somewhat elongated. Cell division is quite
common in the ectoderm of this region, and by it the area of the circum-
cervical region is increased and the two ends of the muscle fibres are
carried farther apart, one end remaining attached to the neck of the
polypide and the other moving towards the abatrial surface. . I have
given reasons above (page 16) for believing that the abatrial ends of the
muscles are not carried towards the abatrial side passively, and solely by
the growth of the body wall, but that the ends move relatively to the
cells of the body wall. A somewhat late stage in the development of
the pyramidalis is shown in Figure 63 (Plate VI.). Nearly the whole
of the mesoderm of the body wall has here been transformed into
muscle cells. The insertion of the muscles is in the mesoderm of the
neck of the polypide. (Plate VI. Fig. 63; Plate V. Fig. 45.)

c. Parietal muscles first make their appearance at about the stage
of the terminal individual of Plate II. Figure 14, immediately below
the bud and to the right and left, i. e. so that the muscles, which
usually arise paired, have their long axes parallel to the sagittal plane

MUSEUM OF COMPARATIVE ZOOLOGY. 29

and perpendicular to the long axis of the branch. They arise from cells
of the mesoderm, most of which in this region are filled with vacuoles,
and often project into the celom. But in my opinion the muscle cells
do not themselves arise from such vacuolated cells, for at even an earlier
stage (corresponding to Figure 21, Plate III.) one can distinguish thick-
ened patches of elongated cells in the mesoderm which are undoubtedly
the young muscle cells; but they do not show the slightest traces of
being vacuolated, and in fact are sharply distinguished from the adjacent
cells by their uniformly granular appearance and their deeper coloration.

Braem (’90, pp. 124, 125) has already stated that the parietal muscles
arise in pairs, and come to traverse the ccelom, not remaining in the
body wall. The truth of this statement I can confirm in the case of
the parietal muscles first formed, which lie near the future septum.
Plate V. Fig. 42 shows the origin of the muscle fibres on both sides
of the branch. They have already migrated into the celom. As Braem
plainly states, the component parts of this pair of muscles, developed
from the mesoderm, migrate towards each other and finally fuse into
one unpaired mass, as we see in Plate III. Figure 26. It is perfectly
evident, in this case at least, that both ends of two muscles originating
far apart migrate in some manner towards each other so that the cor-
responding ends come to lie close together. Such a migration cannot
be accounted for merely by growth of the body wall. The ends of the
muscle fibres must move relatively to the body wall.

When the muscles have reached their permanent positions in a
diameter of the branch, we find their ends attached to the cuticula.
As the muscle fibres stain deeply in hzematoxylin, they can be distinctly
traced through the vacuolated and poorly stained cells of the body wall
(Plate III. Fig. 26). Figure 29 shows a bit of the wall mechanically
separated from the cuticula, the end of the muscle fibre remaining in
place. Fine lines can be distinguished in the contractile, deeply stain-
ing portion of the fibre. The surface by which attachment is effected
appears very slightly crenulated on longitudinal sections of the muscle
fibre. I could not distinguish any structural peculiarity on the part
of the cuticula to which the muscle was attached, — nothing to indicate
how attachment is effected.

Freese (88, pp. 15, 22, Fig. 11) has described a similar method of
attachment of the muscles to the cuticula for Membranipora.?

1 My friend, Dr. G. H. Parker, tells me that a similar method of attachment of mus-
cle fibres to the cuticula occurs in Crustacea. According to Tullberg (’82, pp. 27, 44,

45), the adductor muscle fibres are in Mollusks attached to the cells of the ectoderm.
The same condition as in Mollusks seems to exist in Annelids (Hisig, ’87, pp. 25, 36).

30 BULLETIN OF THE

At a later stage smaller bundles of muscles arise successively toward
the neck. These muscles are free from the body wall at their middle
region. They do not usually pass through the calom in a diameter of
the branch, however, but rarely subtend as chords an are of more than
120°. As Braem supposed, such: muscles, although arising later than
the most proximal pair, originate in a similar manner to them (Plate
VI. Fig. 55). The mesoderm is very thin at the region at which they
are first seen, and they are quickly discerned by their larger nuclei
and prominent cell body. At a later stage they have grown much
longer, and become freed from the body wall at their middle part.

As is well known, there are two fwniculi in Paludicella, called by
Allman respectively anterior (nearer the atrial opening) and posterior.
The origin of the funiculi of Paludicella was observed by Dumortier
et van Beneden as long ago as 1850. They say (p. 54), “‘ La couche
muqueuse une fois formée s’etend rapidement dans V'intérieur et touche
bientot par son extrémité inférieure les parois opposées de la loge.
Les cellules muqueuses dont le tout est encore composé contractent de
Vadhérence dans cet endroit, et c’est ce qui donne naissance au muscle
rétracteur de V’estomac [= funiculi].” Allman (56, p. 36, Plate XI.
Figs. 7-9) also describes and figures very clearly and correctly this pro-
cess, and Braem (’90, p. 127) has recently confirmed their observations.

It is perhaps unnecessary to redescribe the more evident part of
this process, the contact of the polypide with the abatrial wall of the
branch. The mesoderm of the bud comes into contact with that of
the body wall, the cells of each of the two layers become attached to the
other, and by the withdrawal of the polypide the attachment persists
at two points forming a long drawn out string of tissue. Figures 36°
and 38 (Plate IV.) are contributions to a knowledge of the finer details
of this process. Apparently the upper funiculus is developed earlier
than the lower, as I have always found it longer at about this stage.
The lower funiculus at present consists of only the two mesodermal
layers of body wall and polypide intimately united. The funiculus
itself consists of a cord several cells thick ; but I believe these cer-
tainly to be derived from the mesoderm only. Very early some of
these cells show an appearance of highly refractive and deeply staining
fibres, which I interpret as muscular differentiation (Plate IV. Fig. 38,
fun. su.), so that the funiculi must be regarded as partly muscular
in function. As in Phylactolemata, these fibres lie near the axis of the
funiculus. Braem (’90, pp. 66, 67,) has demonstrated that the muscular
fibres of the funiculus of Plumatella pass directly into the muscularis

MUSEUM OF COMPARATIVE ZOOLOGY. at

of the body wall. It is interesting to find them persisting in the fu-
niculus of Paludicella, beneath the mesodermal covering, although there
is apparently no muscularis developed in the body wall of this region.

8. Tue Formation oF THE NECK AND ATRIAL OPENING.

This is the last act in the history of the polypide that I shall con-
sider. The body wall around the neck of the polypide continues to
possess a less differentiated character than the remaining portion for
some time after the oral tentacles have undergone their revolution.
One still sees the cells of this region dividing, and the body wall is
gradually protruded at this point above the general level. (Plate II.
Fig. 14, cev. pyd.) The neck of the polypide to which the kamptoderm
is attached consists, at a somewhat earlier stage than that just referred
to, of a disk of greatly elongated columnar cells in the centre of which
there is a distinct notch caused by the presence of shorter cells at that
point. (Plate VI. Fig. 63 6.) At the inner ends of the columnar cells
of the neck lies a flat epithelium quite sharply marked off from the
latter, but which is nevertheless undoubtedly derived from the same
source as the columnar ceils and the inner layer of the bud. This flat
layer is directly continuous with the inner layer of the kamptoderm.
At a later stage, the columnar cells of the ectoderm become elongated
still more, and lose their staining capabilities at their outer ends. Still
later one sees them arranged in the form of a cup whose cavity is sep-
arated from the outside world only by a cuticula which becomes slightly
invaginated at this point. The cells are soon found with their long
axes perpendicular to the edge of the cavity they line.

There is one point that I have not been able to determine; namely,
how the new cuticula, which is certainly formed at the ends of the cells
which lie next to the cavity, becomes continuous with the old cuticula
of the non-invaginated body wall, as it is in Figure 50 (Plate V.). The
presence on the new unstainable cuticula of the remains of the stainable
one, whose origin I have already discussed at length, may serve as a
guide to the limits of the old cuticula. The new cuticula is being secreted
by cells lying deep in the inner end of the neck, and apparently in one
rod-like mass. Unfortunately, I lack stages between this figure and
Figure 45 (Plate V.), which shows the neck of a nearly or quite adult
polypide cut lengthwise. The solid cuticular rod has now become a hol-
low cylinder, whose inner (deep) edge is embedded in the deep-lying cells
of the neck. Moreover, one finds superficial to the cuticula of the gen-
eral body wall a second cuticular cylinder, which is free at its outer end,

Be BULLETIN OF THE

but at its inner end fuses with the surrounding cylinder of cuticula, This
inner cylinder, which is probably formed, as Kraepelin (87, p. 40) sug-
gested, by splitting of the delicate cuticula at the base of the marginal
thickening (Randwulst), has been compared by Kraepelin to the “ collare
setosum” of Ctenostomes. The /andwulst itself I believe to be the equiv-
alent of the Diaphragma of Nitsche, as I shall try to show later.

At the deep end of the neck (Vig. 45), the inner layer of the bud is
seen to be continuous with the ectoderm. ‘The region of transition may
be called the atrial opening, of. atr. Surrounding the atrial opening
is a fold in the ectoderm, and between the layers of this fold is a thin,
non-stainable homogeneous layer, slightly more refractive than the sur-
rounding protoplasm. This membrane extends also a short way into
the kamptoderm, and here lies between its two cell layers. Embedded
in this homogeneous membrane in the fold, one can distinguish still
more highly refractive bodies, spht. On account of their form and
high refractivity, I believe these to be muscle fibres cut across. The
homogeneous membrane has also the same general appearance and
relation to the muscularis as the so-called supporting membrane of
Nitsche, and it is the only representative of that structure that I
have found in Paludicella.

9. DEVELOPMENT OF THE COMMUNICATION PLATE.

In their description of Paludicella, Dumortier et van Beneden (50,
p- 40) say : “Il se compose de plusieurs loges ou cellules placées bout a
bout . . . en sorte qu’il n’y a aucune communication entre les differents
animaux.” Also Allman (’56, pp. 114, 115) refers to the presence of a
perfectly formed septum separating the cavities of adjacent “cells.” To
Kraepelin (’87, p. 38) belongs the credit of having first carefully studied
this structure in the adult by means of sections. He came to the con-
clusion from the appearances which he figures (cf. my Plate V. Fig. 49),
that there are small canals passing through the nearly homogeneous
central mass, and therefore “dass wir in dem ganzen Apparat eine Vor-
richtung zu erblicken haben, durch welche Nihrstofflésungen des einen
Tieres mittels siebartig wirkender Cautelen in die Kérperhdhle des
Nachbarindividuums tbergefiihrt werden.”

The descriptions of Kraepelin concerning the structure of the “ Roset-
tenplate” are confirmed by my own observations, and seem to justify his
conclusions concerning its function. The development of the organ has
not, however, been carefully observed heretofore. Korotneff (’74, Plate
XII. Figs. 1 and 2) gives figures to show this process, but I have never

MUSEUM OF COMPARATIVE ZOOLOGY. 30

seen any such circular groove surrounding the branch as he figures. In
all cases the two layers of the body wall form a circular fold, in which,
however, there is never, even at the earliest stages, a space between the
ectodermal layers, nor any infolding of the cuticula as Korotneff (’75, p.
369), according to Hoyer’s rather incomplete abstract, maintains (Plate
V. Fig. 47). When the circular fold has advanced until only a small pore
remains, by which the cavities of the older and younger individuals are
kept in communication, the mesodermal cells at the angle of the fold
begin to undergo a metamorphosis both in form and histological charac-
ter. In the first place they become much elongated and extremely
attenuated, passing from one surface of the septum to the other, and
forming the lips of the pore. In the second place their plasma becomes
first deeply stainable, and later, in addition, homogeneous and highly
refractive. These metamorphosed cells form what may be called the
teeth of the plate. They are derived wholly from mesoderm.

The cells in the upper mesodermal layer next increase rapidly in
number and size, and the number of teeth is also angmented (Plate V.
Fig. 48). The metamorphosis of the cells extends still farther away
from the communication pore, and involves the lower mesodermal layer ;
but, apparently, each cell of the latter is metamorphosed only to a
slight depth within its cell wall (Fig. 51), whereas in each of the upper
cells the ends which project into the communication pore are modified
through and through (Fig. 46). At a later stage (Fig. 49) the meta-
morphosed part of the cell seems quite sharply cut off from the active
part, and the slits between the metamorphosed teeth are considerably
reduced. Nevertheless, I believe a transfer of fluids may still occur
between them, for even in the adult communication plate one can trace
continuous lumina when the cells are by accident torn off from the
“teeth” which they have produced. It is important to note that the nu-
clei are not destroyed in the cell metamorphosis. Some lie above, others
below the pore, and become deeply stainable. The ectodermal layers of
the communication plate secrete a cuticula between them. This is thin-
ner than that of the body wall, and does not extend, of course, to the
centre of the communication plate, but ends in a thickened ring, whose
diameter is about one tenth the diameter of the plate, or, absolutely,
about 9.4 p.!

1 Reichert (’70, p. 267) first carefully described the Rosettenplate of Cteno-
stomes in Zobbotryon, and the organ in Paludicella must be regarded as homologous
with it. The central circular hole in the cuticula of Zodbotryon is from 7 to 10 u
in diameter, and from one ninth to one seventh that of the entire plate. Similar

VOL. XXII. — NO. l. 3

34 BULLETIN OF THE

10. ROLE oF THE MESODERMAL VACUOLATED CELLS.

Allman (56, p. 36) observed that at the time a lateral branch was
well formed, and before the origin of the polypide, the internal outline of
the body wall was uneven, and he figures (Plate XI. Fig. 4) very large
cells lying on the inside of the body wall. Korotneff (74, Taf. XII.
Figs. 1-3, ’75, pp. 369, 370) progressed a step farther, and recognized
a distinction between large, coarsely granular cells projecting into the
cavity of the bud, especially near the tip, and the surrounding epithelial
cells. Braem (’90, p. 126), finally, has described them more accurately.
He finds cells filled with numerous granules in the youngest branches of
the colony. Immediately around the bud, such cells are less abundant ;
probably, he says, because their granules have been absorbed in the
process of formation of the polypide. He compares the granules with
the yolk spherules of the statoblast cells, and believes that they are to
be regarded as food matter.

My observations and conclusions, achieved independently of Braem’s,
fuily confirm his. I have succeeded, moreover, in obtaining some addi-
tional evidence as to the function of these cells, a subject to which I
have paid some attention.

First as to the distribution of the cells, and their frequency in different
regions. We can best get an approximate idea of this by counting the
number of the reticulated cells in each section of a series which in-
volves a young polypide and the regions immediately above and below
it. It is not possible to do this with perfect accuracy, because there is
no sharp line of distinction between reticulated and non-reticulated cells ;
but I have made the count without prejudice, and I believe as fairly as
possible. When the bud of the polypide has reached about the stage
shown in Plate III. Figure 28, the number of reticulated cells seems to
have nearly reached a maximum. In the series from which this figure
was taken there was an average of 4.8 reticulated cells to the section in
the ten sections distal of the bud. There was an average of 11.2 reticu-
lated cells to the section for the twenty sections which passed through the
bud, and 11.2 for the eleven sections proximal of the bud in the region

perforated organs have been described by Smitt (’67, p. 426), Nitsche (’71, pp.
420-422), and Vigelius (’84, p. 26) for Flustra, by Freese (’88, p. 7, 13, 14) for
Membranipora, by Ostroumoff (’86%, p. 13) for Lepralia, by Claperede (70, p. 160)
for Bugula and Scrupocellaria, by Ehlers (’76, p. 14) for Hypophorella, and by
Joliet (’77, p. 222) for Bowerbankia. Nitsche alone (’71, p. 455) has had anything
to say upon their origin, and this apparently not the result of direct observation.

MUSEUM OF COMPARATIVE ZOOLOGY. a)

at which muscle fibres were arising. A similar series through a slightly
older bud gives for the same regions respectively 5, 14, and 13 cells per
section. In series through older buds, a rapid decline in the number of
these cells occurs so that at the stage of Figure 30 (Plate IV.) there is an
average of only about 3.1 cells per section through the bud, and about
2.2 immediately below. These reticulated cells are not very numerous
in the region of the bud at the time this is sbout to arise, as a look at
the sections Figures 3 and 4 shows. One finds reticulated cells in the
mesoderm at the tip, and most abundantly at a rather early stage in
the development of the bud. The number of these cells diminishes as
one leaves the young individual to pass into the next older of the same
branch. In the adult such cells are rather rare; so rare, in fact, that
Kraepelin (’87), who studied with care the body wall of the adult indi-
vidual, makes no mention of them. Nevertheless they do occur in the
cells which are to go into the lateral branch (Plate II. Fig. 15), as well
as elsewhere on the body wall. The place in which one finds the reticu-
lated cells most abundant, however, is in the young /ateral branches near
the time when the polypide bud is about to arise. Here every cell of the
mesoderm is greatly enlarged, and filled with the vacuoles (Plate VI.
Fig. 58). These are very apparent upon a surface view of the branches.
Reticulated cells occur not only in the mesodermic cells of the body
wall, but also in those of the polypide bud, which were, indeed, only
lately a part of the mural mesoderm (Plate III. Fig. 28, Plate VI. Fig.
56). Thus, in general terms, we may say that the reticulated cells of
the mesoderm are chiefly confined to regions in which there are young
buds developing; and since these arise at intervals only, there is a
periodicity in their appearance,—a time of maximum development
followed by one of decline, then one of reproduction of such cells in
the ends of branches culminating in another maximum, and so on.
Turning our attention now more particularly to the structure of these
reticulated cells at the period of their best development, we find (Plate
VI. Figs. 56, 57, 59) that they possess a large nucleus lying at the
deep end of the cell and containing a relatively large nucleolus, and that
this is surrounded by a granular protoplasm with included vacuoles. It
is very common to find the nuclei in various stages of division, and thus
it is frequently seen as a mass of chromatic substance without any nu-
clear membrane or nucleochylema. The vacuoles, which in the more reg-
ular cells lie in a semicircle nearly peripheral (the nucleus being at the
centre), are highly variable in number, some of the cells containing as
many as 20 to 30. They often appear as perfectly clear homogeneous

36 BULLETIN OF THE

spaces, but more frequently at this stage contain a spherical body, which
frequently fills the entire vacuole and is more refractive than the sur-
rounding plasma (Fig. 59). Not unfrequently one sees a less refractive,
clear space, surrounding the highly refractive body (Fig. 57).

The description just given corresponds to the condition seen in a ter-
minal branch whose polypide has attained the development of that shown
in Figure 28 (Plate III.). At the time immediately preceding the ori-
gin of the bad, the cuboidal cells of the mesoderm show traces of vac-
uolation, but their form and size have suffered no appreciable disturbance.
This vacuolation of cells proceeds hand in hand with the development of
the bud, and one first notices the homogeneous, highly refractive bodies
in the vacuoles when the bud is well established. At about the time the
alimentary tract has become formed, the reticulated cells begin to show
signs of degeneration. The highly refractive bodies have disappeared,
and the skeleton of the cell which remains becomes very irregular. As
already stated, the number of reticulated cells also decreases, until, at
about the time of “rotation” of the polypide, there are few reticalated
cells in the mesoderm, but these few are filled with vacuoles and their
highly refractive bodies.

The conditions of the mesodermal cells at the tip are slightly different
from those found elsewhere. Usually, instead of many small vacuoles,
one finds only one or two which fill almost the entire cell, — sometimes
perfectly homogeneous in structure, sometimes containing small highly
refractive granules.

These appearances I believe to be explicable only upon the assumption
that the mesodermal cells are capable, at the time at which the young poly-
pide is arising, of imbibing the fluids of the body cavity and storing them
up for the purpose of supplying the rapidly growing cells of the bud with
nutrition. It is desirable to show reasons for believing, first, that the
contents of these cells are nutritive matter ; secondly, that this has been
taken up from the body cavity; and, thirdly, that it is supplied to the
bud for its nutrition.

It must be admitted that the strongest argument for the belief that
these are absorbing cells is derived from a comparison of the appearances
which we find in these cells with those described for Protozoa, and by
Metschnikoff (83, Taf. I. Figs. 18-35) for mesodermal trophic cells.
At the same time, it must be acknowledged that) similar cells are found
in other cases where the function is believed to be not ingestive, but
excretory, as in the chlorogogen cells of Annelids, as shown by Kiiken-
thal (’85), Eisig (’87, pp. 751-762), and others, and indeed even in the

MUSEUM OF COMPARATIVE ZOOLOGY. 37

cells of coelomic epithelium. Eisig (’87, p. 752) has already clearly ex-
pressed how, in view of the many cases of high excretory activity of
peritoneal and blood cells demonstrated by him, “ kiinftighin bei der
Beurtheilung gewisser Zelleneinschliisse erst genau festzustellen sein wird,
ob man est mit von aussen aufyenommenen (gefressenen), oder aber mit
von der Zelle ausgeschiedenen Producten zu thun habe.”

A criterion for judging this matter may be found, in the first place, I
believe, in this: that the products of excretion increase with the activi-
ties of the cells, and are thrown out, usually in the shape of concrements,
either from the cell or with the cell into the coelom ; whereas bodies
taken in from without for digestion decrease with the activities of the
region. In the second place, vacuoles are less characteristic of excretory
tissue than of imbibitory. But vacuoles are the important feature of the
reticulated cells in Paludicella, and the highly refractive bodies are less
constant phenomena. As for the latter, they are not found in the later
stages, nor in the earliest. Moreover, these bodies differ from excretion
concrements in this, that they are always transparent, often almost indis-
cernible in the vacuole, except by their higher refractiveness, and there
is no sharp demarcation between cases of vacuoles filled by such bodies
and those the contents of which are less highly refractive. The degree
of refractiveness is variable, at one end of the series grading off into the
undifferentiated fluid of the vacuole. What significance is to be assigned
to these highly refractive bodies in the vacuoles? There are two reasons
why I do not believe that they represent solid food particles devoured
as such by the mesodermal cells. First, I do not find such highly
refractive bodies lying loose in the body cavity before the stage at which
they first appear in the cells ; and, secondly, one can find all gradations
between less highly refractive vacuoles and highly refractive ones (which
I have assumed to be entirely filled by one highly refractive body), and
between the latter and vacuoles containing a small body surrounded by
a broad, clear area. I believe, therefore, that the vacuoles are rather
cavities filled with chemically different nutritive fluids, which are acted
upon differently by the reagent.

I have assumed that the contents of the vacuoles represent material
taken up from the body cavity, because it seemed most reasonable to
look there for the source of their supply. The ectoderm is covered on
_its outer surface by an apparently continuous cuticula, so that food
cannot be gained from the outside world directly. It is, moreover, not
unreasonable to suppose that some of the products of digestion elabo-
rated by the adult polypides of the colony pass through the wall of the

38 BULLETIN OF THE

alimentary tract in solution, and thus into the body cavity, from which
they may be taken up by the mesodermal cells at the growing part
of the body wall. Nor is there anything unreasonable in insisting that
the body cavity functions, in these animals without blood-vessels, as a
heemo-lymph system, for in many animals with incomplete vessels, such
as Arthropods, Hirudinea, etc., it evidently does so to a certain degree.
Moreover the constant motion of the fluids of the body cavity of Bryozoa
points to the same thing. It is conceivable that the food in the digest-
ive cells might be distributed throughout the body wall without passing
into the body cavity, since all parts of the body wall are continuous
with the digestive epithelia of the polypides of the colony. Two consid-
erations make it improbable that the cells of the tip gain their nutri-
tion in this manner from the digestive cells of the youngest functional
polypide: first, the considerable distance of the rapidly growing, and
hence rapidly consuming tip, from the youngest functional polypide ;
and, secondly, the fact that the tip is separated from that polypide by
one or two septa, whose central cells are highly metamorphosed, and
apparently cuticularized, thus serving to break the continuity of the
cell wall. An objection to the assumption that the mesodermal cells of
the tip derive their nourishment from the products of digestion which
have been elaborated by the alimentary tract of the youngest polypides
and passed into the body cavity, might be based on the fact that the
communication plates are always fully formed between the bud and
the next older polypide before the older polypide has become functional.
If the communication plate were a closed septum, this would be a fatal
objection. But it is not closed to fluids carrying food in solution. The
very persistence of an opening indicates that it has a function, and favors
the hypothesis here presented.

Positive evidence for the conclusion that the reticulated mesodermal
cells take up food material from the body cavity is derived from the
fact that these cells often show evidences of being ameeboid. Thus they
are sometimes found with pseudopodia-like prolongations of the cell body
(Figs. 54 and 59). <A large percentage of all reticulated cells of this
stage show similar appearances. Although they here seem to keep their
places in the mesodermal epithelium, their movements being confined
to their free surfaces, the cells derived from the homologous Jayer in
marine Bryozoa are migratory. Therefore these may be considered as
morphological equivalents of migratory cells, which have come to remain
in or have never departed from the mesodermal layer, although possess-
ing some of the characters of these notoriously trophic elements.

MUSEUM OF COMPARATIVE ZOOLOGY, a9

That the nutritive matter in the coelomic cells is supplied to the
young bud is what we should expect, since the cells of the bud, being
most actively engaged in growth, will require most nutriment. The
actively dividing cells of the outer layer of the bud are thick and cuboid,
and are rarely so highly vacuolated as the more passive ones of the body
wall; yet occasionally one finds one or two huge cells in this layer full
of vacuoles, which contain highly refractive bodies. In most cases these
cells send out processes into the ccelom, and in a few instances I have
seen them united with similar processes from cells on distant parts
of the body wall. This remarkable phenomenon, shown in Figure 54
(Plate VI.), may possibly signify that cells of the ceelomic epithelium at
times directly communicate with those of the outer layer of the bud to
supply it with nourishment. Nutrition of the bud is also probably
effected through the presence of large reticulated cells at the angle
between the bud and the body wall. A condition like that shown in
Figure 56, cl. ret., is very common.

Every author from Dumortier et van Beneden to Braem, who has
studied the origin of the polypide in Paludicella, has mentioned the
presence of highly refractive bodies in the alimentary tract at the time of
its formation. These are very striking in some living specimens, and
in whole animals after killing. I have found that this highly refractive
substance in the bud is exceedingly variable in amount and position,
and that sometimes it is apparently absent. When present, it usually
occupies the lumen of the forming alimentary canal ; but, as sections
show, it is often located in large vacuoles in the future digestive cells
of the alimentary tract. It seems highly probable that, as Braem sug-
gests, this is nutritive substance, and it has doubtless come from the
body cavity through the agency not only of the outer layer of the bud,
but also of other parts of the ccelomic epithelium.

I am inclined to interpret the phenomenon of cells filled with nutri-
tive material as an adaptation to the peculiar conditions of Paludicella,
in which the individuals are early separated from one another, except
for the communication plate, through which at best fluids can pass only
slowly, and in which a rapid growth of the body wall to produce the
polypide takes place periodically. The mesodermal cells rapidly absorb
the nutritive fluids of the body cavity and store them in their substance
before the formation of the communication plate, and give them out
again during the period of the polypide’s most rapid growth chiefly to
this part of the individual. This hypothesis has been mainly derived
from considering the fact of the great development of the reticulated

40 BULLETIN OF THE

cells in the lateral bud and the very early completion of its commu-
nication plate, the immediate needs of the polypide, which arises only
after the formation of the plate, being met by this supply of stored
nutriment.?

But why is the septum (communication plate)-formed so early, if it
is desirable for the species that the growing tip should be well nourished
by the fluids of the body cavity? Here again I must resort to pure
hypothesis. I assume that the early formation of the septum is a pro-
vision for the protection of the stock against a rapid influx of the sur-
rounding water in case the branch is broken. One can understand how,
if the body wall and growing regions depend upon the fluids of the body
cavity for nutrition, an open communication of this cavity with the out-
side world would be a serious obstacle to regeneration of the body wall in
the lost part, or the growth of the stock in any other direction. There is
a fact which ought to be mentioned in this connection, as bearing on this
hypothesis of the function of the septa. One frequently finds that in
stocks which have been handled with reasonable care the median branches
are broken off at either end, and in almost every colony one or more lat-
eral branches are missing from the parent branch. Apparently, then,
the lateral branches are unusually subject to destruction, and we find
the septze developed at a much earlier period between them and the an-
cestral branch than between individuals of the median branch. Compare
Plate IT. Figure 14, in which the communication plate has not yet begun
to form, with Plate VI. Figure 58.

III. Budding in Marine Gymnolemata.

1. ARCHITECTURE OF THE STOCK.

I have already described the process of stock-building in Paludicella,
and have attempted to show that it follows a certain law. I desire now
to present a few observations upon the architecture of certain stocks of
marine Gymnolemata, which will aid in arriving at some general con-
clusions later on. Other observers have worked out the architectural
laws of single species or groups, and I shall refer to their studies either

1 Similar conditions to those in Paludicella exist in some marine Bryozoa, and in
one of these cases, Bowerbankia, I find them fulfilled by a similar arrangement. The
young buds of the stolon which give rise to the “nutritive zooids ” are, at an early
stage, loaded with food grannies. As in Paludicella, so in Bowerbankia the
communication plates are formed early. '

MUSEUM OF COMPARATIVE ZOOLOGY. 41

in connection with the species which I have used in common with them,
or in the general part of this paper, in considering the process of budding
in Bryozoa as a whole.

I will begin my description with Bugula turrita* of Verrill, which I
gathered in the summer of 1889 at Wood’s Holl, where it occurs abun-
dantly on the piles of the wharf. The stock is bushy, and, when its
polypides are active, of an orange color. In its simplest form the
stock consists of a central axis, which is somewhat zigzag, and gives off
lateral branches like the trunk of a tree. The lateral branches are in-
serted on the trunk in a spiral line. Each lateral branch is fan-shaped
(Plate VII. Fig. 64), the part corresponding to the handle of the fan
being the point of attachment, and the fans are smaller the nearer they
are to the tip of the trunk. The attachment of the branch to the
trunk is effected by one primary individual. Each fan-shaped branch
extends from its point of attachment obliquely upward and outward,
and, although it is slightly concave on its upper inner surface, the
concavity is not sufficient to prevent its being spread out upon the slide
for study without materially disturbing the interrelation of the indi-
viduals in the stock.

I have studied several branches flattened in this way (one of 400
individuals), and have made camera drawings of them. Since the
results in the different cases are substantially in agreement, I have con-
cluded that they are significant. One of these camera drawings is shown
in the figure just referred to.

To designate individuals in the stock, I have adopted a simple no-
menclature. The forty-four terminal individuals are numbered from
1 to 44. The successive generations (if I may be allowed to use this
word in a loose way) are indicated by the Roman numerals from I. to
XIII. Any one individual is indicated by placing the numbers of the
radial line or lines to which it belongs first, and following this by
the Roman numeral of the generation to which it velongs. Thus,
27-30 IV. is an individual near the base of the twig 27-30 and of
generation IV. Figure 64* (Plate VII.) is a diagram showing the

1 This species is very similar in general habit to B. avicularia, Linneus, and to
B. turbinata, Alder (Hincks, ’80, pp. 75-80). It differs from the first named spe-
cies by possessing only one spine, on the outer upper edge, as described by Leidy
(’55, p. 142), instead of having three, — two outer upper and one inner and upper.
It differs from Hincks’s diagnosis of the second in having only two “cells” in each
branch, instead of 3-6 in the upper portions. The form of the avicularium would
seem to ally it more closely to B. avicularia.

42 BULLETIN OF THE

arrangement of the individuals in Figure 64. The radial lines repre-
sent the rows of individuals; the concentric lines separate adjacent
individuals of the same radial row. ‘The same nomenclature is used
as in Figure 64.

In studying Figures 64 and 64%, one of the first facts which attracts
our attention is that (1) the individuals of the twigs are in pairs, and
the adjacent individuals of the two rows “ break joints.” In general, one
finds that the individuals of the same twig are of the same length ; but
since the two rows of any twig ultimately rest upom one, either the prox-
imal two individuals of these rows must be of unequal length, or else
they must arise on different parts of the individual which supports
them. Both of these cases occur. Sometimes one individual (26 IX.)
has nearly twice the length of the other (25 IX.), and in other cases
(9, 10 VI, 11, 12 VI.) the more proximal of the two individuals
(9, 10 VI.) arises so far proximally on the side of the supporting indi-
vidual 9-12 V. as to have a total length quite equal to that of the more
distal (11, 12 VI.). Owing to their different positions upon the indi-
vidual 9-12 V., these two individuals may be designated as lateral
(9, 10 VI.) and terminal (11, 22, VI.). The terminal individuals con-
tinue the ancestral row, the lateral individuals are the first of lateral
branches.

This distinction is an actual, and by no means a meaningless one.
The constant difference in position of the two individuals which rest
upon one shows conclusively that this branching cannot be regarded as
dichotomous, and I may say parenthetically that I shall try to show
in the general part of this paper that true dichotomy is not com-
mon in Bryozoan stocks, if indeed it exist at all. Now, since in
the rows of individuals in which there is no lateral budding the
distal lies directly terminal to the proximal individual, that individual
which fulfils this condition at the region of bifurcation of the twig
must be regarded as continuing the ancestral branch; and that
individual, conversely, which arises from the side of the single prox-
imal individual must be regarded as the lateral one. Thus we have
the stock composed of ancestral and lateral branches as represented
in Figure 64°.

(2) When two lateral branches are given off from two ancestral ones
which have had a common origin (and are consequently themselves re-
spectively ancestral and lateral branches), they are given off towards
each other. This is equally true whether the two lateral branches in
question arise in the same generation (32 X., 33 X.) or in different

MUSEUM OF COMPARATIVE ZOOLOGY. 43

generations (24 X.,25 IX.). This may be expressed by saying branches
are given off on the side towards the axils.

By consulting Figure 64* and tracing out the finely dotted lines which
connect the second, third, ete. axils of all branches counting from the
proximal end of the fan, it will be seen that (3) lateral buds tend to arise
on two closely related branches in the same generation. There are several
slight deviations from this rule. The less closely related the branches,
the less marked the tendency, although it is still discernible. (Cf.
branches 9-16, 23-30.)

This rule does not hold, however, so well on the margins as in the
middle region of the fan, for here another and a superior rule seems to
obtain. This is that (4) lateral budding occurs more frequently at the
margins of “ fans” than elsewhere. Thus in Figure 64* there is at the
margins, on the average, 1 case of lateral budding to 4.3 cases of median
budding. Elsewhere the average is as 1 to 6.5. In larger fans the
difference is even more pronounced. This is true not only for the
“fans,” but also, to a less degree, for the two ‘‘subfans” which arise re-
spectively from the two individuals of generation II. (but 17, 18 is very
anomalous in this respect). In general, any rule deduced for the mar-
gin of the fans holds true also for subfans to any degree of Buu awiciees
but the less perfectly, the higher the degree.

By consulting again the diagram, it will be seen that the branches
have attained different lengths. Thus 9, 10, 29, and 30 contain repre-
sentatives of generation XIII., while the terminal individual of branch 1
is of generation X., and those of branches 35-44 are of generation XI.
So the curve which connects the tips of the branches (see dot-and-
dash line, Fig. 64*) would rise from 1 to 9-10 as a maximum, and fall
again till it reached the margin of the first subfan; then rise again,
reaching a second maximum in the middle at 29-30, and finally fall
again to the other margin. In general, then, (5) the marginal branches
are shortest, the intermediate ones longest, i. e. give rise to the greatest
number of generations.

Although the marginal individuals of say generation TII., IV., or V.
do not support branches with so many generations as the intermediate
ones, yet they are not therefore necessarily less prolific in individuals,
because the number of branches arising distally of such individuals is
greater according to rule 4 than the number arising distally of the
intermediate ones. Thus, if we count the number of individuals borne
on each of the eight individuals of the fourth (IV.) generation of
Figure 64, we find in the given case :—

44 BULLETIN OF THE

Outer. 1. 1-8 IV. (an outer individ.) gives rise to 8 branches and 34 individ.
2. 9-12 IV. (an inner $f hae) aie 60) 4 “ Sees Qe tubes
. 138-16 IV. (an inner ff via eine ca sf ee
Benes: c 4. 17,181V.(asubouter “ ) “ “© 2 «& ety: ce
rence 5» 19222 TV. (a eubouters =~)" (ad s si MN as
23-26 IV. (an inner TD) ee N= AS (hoe wes 230 Mall
Outen. 7. 27-380 IV. (an inner TR a) Vn Beta Lene: “ “26 ob
8. 31-44 LV. (an outer ems) a) Pee Sou ae: Ss 0G «

According to the rule that inner branches are slightly prolific, we should
expect cases numbered 4 and 5 in the above table to contain the fewest
branches and individuals; in accordance with the rule that marginal
branches even of subfans are more prolific, we should expect them, on
the contrary, to contain more branches and individuals than cases num-
bered 3, 6, etc. The result is usually a condition intermediate between
that of the middle and outer branches, such as is partially realized in
case number 5. Case number 4 seems to present an unusual condition,
which may be correlated with the fact of its close approximation to
number 5. (See Fig. 64, 17-20.) From the consideration of this and
other cases, I think this conclusion may fairly be drawn: (6) Of the
four proximal individuals from which a fan arises, the outer two will bear
the greater number of individuals, the inner two the lesser.

Since from rule 2 median individuals (ancestral branches) occupy the
margins of fans (or subfans of any degree) and the lateral branches are
intermediate, it follows, as a corollary to rule 5, that, in general, the an-
cestral branches are the shorter, the lateral branches the longer ; and, as
a corollary to rule 6, that from any axil the ancestral branch will of the
two give rise to the greater number of individuals ; the lateral branch,
conversely, to the less, other conditions being aatal,

We have deduced the laws of lateral budding on different parts of the
circumference. We find also that there is a regular variation in the
frequency of lateral budding, dependent upon the distance of the region
from the primary individual of the fan. This rule, like any other, is not
invariable, whatever the other conditions may be ; but it is more or less
dependent upon them. A small and regular fan having seven genera-
tions gives this result.

No. of Generation. Number of Individ. Increase per Cent.
iL 1
TI. 2 100
We 4 100
1G 8 100
Ve 12 50
WAG 16 834

MUSEUM OF COMPARATIVE ZOOLOGY. 45
In this table the first column, gives the number of the generation, the
second the whole number of individuals in the generation, and the third
column the increase per cent of individuals in each succeeding genera-
tion over the last. In this specimen the increase underwent a very
regular diminution.
With larger colonies so great a regularity as that just shown is hardly

to be expected, nor is it found. The following table is based on

Figure 64, and is like the preceding; but in addition the percentage
increases have been averaged —i. e. the means of successive increases
taken in pairs have been given—to eliminate what may be called
accidental variations.

| | |
Gener Number of. Increas Genera- |Number of} Increase
an Individ. ‘ per Cent. Average: tion. Individ. | per Cent. | ANEESZO:
ee 1 29
VIII. 28 27
Te 2 100 23
100 IX, 33 18
Iil 4 100 18
100 XK: 39 18
LV. 8 100 16
88 XI. 44 13
AVE 14 75
49 XII 20 |
VI 17 22 | Incomp lete.
26 XIII. 4 |
VIL 22 30 | |

Hence we conclude, There is a diminution in the rate of increase of in-
dividuals in the “fan” as it grows older,

In searching for an explanation of this phenomenon, I first drew
a line from the centre of the primary individual of the fan to the
periphery, and divided it into four equal parts. I then described
ares with the primary individual as a centre, and with radii equal to
4, %, 2, and # of this line respectively. Counting the number of in-
dividuals cut by these arcs respectively, and dividing those numbers
by the length of the corresponding arcs, I found that there is almost
exactly the same number of individuals per unit of arc for each of the
four arcs. (Rule 7.) The previous conclusion, that there is a dim-
inution in the rate of increase of individuals in the fan as it grows
older, may then be considered as a corollary to’this rule, as it ob-
viously follows from it.

46 BULLETIN OF THE

Bugula flabellata, J. V. Thompson.1— I have studied this species for
the purpose of confirming the results obtained in B, turrita, and have
found the architecture of the two species alike in all essentials.

The entire colony of B. flabellata (Plate VII. Fig. 66) may be com-
pared to a single “fan” of B. turrita, only there are usually many more
individuals in the former, and of course there is no central stem to which
it is attached ; but the fan is fastened directly by its rhizoids to the
object which supports it.

Usually about four rows of individuals are united, instead of two as
in B. turrita, —a condition which can be easily derived from the latter
by imagining adjacent branches to become fused together. Here as
there adjacent individuals break joints. Here as there lateral branches
are given off towards the axils.

Rule 3 is not true for B. flabellata. This is entirely annulled by the
establishment of a new rule, which depends upon the new conditions
found in this species ; namely, that more than two rows cling together,
and that consequently one or more rows of individuals are enclosed be-
tween outer marginal rows. In any such twig composed of more than
two rows (Rule 3a) lateral branches are given off only from the marginal
rows. (See Fig. 66, 49-54 XVII.) It might possibly result, then,
that certain of the middle rows of the twig should never give rise to
lateral branches. But I do not believe that this ever occurs in very
long rows, for by the splitting up of the twigs the middle rows sooner or
later become marginal (so 46-51 XV.). In one stock that I have
drawn, consisting of 17 to 21 generations, every middle row occurring
as such up to the 13th generation had become at the periphery a
marginal row.

As in B. turrita, so in B. flabellata lateral budding occurs most
frequently at the margins of fans, —in a fan of about 800 individuals in
the ratio of 1:10 for the margin, and 1:14 for the remainder of the
fan. By a comparison of these figures with those given on page 45 for
B. turrita, it will also appear that lateral budding is less frequent here
relatively to terminal budding than in B. turrita.

The fifth rule deduced for B. turrita holds equally well here. In one
case the curve of the tips of the rows rises from the margin of the fan at

1 The species which I have studied is identified by Verrill (73, pp. 711, 389)
under this name, and my specimens also agree fairly with Hincks’s (’80, pp. 80-82)
diagnosis. The two pairs of spines, one longer than the other, could be distinctly
seen. Hincks says, “The rows of cells. . . are never, I believe, fewer than four,
and range as high as seven.” But his Figure 66 shows three rows only in some
places.

MUSEUM OF COMPARATIVE ZOOLOGY. 47

generation XXIV., reaches 3 maxima of XXVI., XXVII., and XXVIII.
respectively, and falls again at the other margin to generation XXIII.
In the subfan from which Figure 66 was taken, the curve begins at the
outer margin with generation XVILI., rises to generation XXII. at two
points, and falls again to XX. at the inner margin of the subfan.

Of the four proximal individuals in any fan here, as in Bugula turrita,
the outermost, ancestral give rise to the greater number of individuals.
In one case, for instance, the marginal individuals lie at the base of 31
rows with 184 individuals, while the inner ones support only 7 branches
with 65 individuals. Similar results were obtained from other stocks.

With the middle of the primary individual as a centre, I passed an
are of a circle through the extremities of the branches of a large camera
drawing of a fan of B. flabellata, divided the radius into eighths, and
passed arcs through these points. The number of individuals cut by the
different arcs was then counted and tabulated; the arc with the longest
radius cut through 87 individuals. By measuring the length of the arcs,
the number which should be cut by each arc on the assumption that the
number of individuals per unit of are is constant for all radii was deter-
mined. This was then compared with the actual number found, with
the following results : —

Length of No. of Individu- Theoretic Length of No. of Individu- | Theoretic
als observed. No. Radius. als observed.

In this instance, then, the 7th rule deduced for B. turrita evidently
holds true for B. flabellata.

While at Mr. Agassiz’s laboratory at Newport, during the summer of
1890, I had frequent opportunity to examine other stocks of Bryozoa,
which occur there very abundantly. I will take four species as typical
examples of the groups they represent, and treat of the architecture of
their colonies.

Lepralia Pallasiana, Busk.1—It is not at all easy to determine

1 TI do not feel perfectly certain that the specimen shown in Figure 71 (Plate
VIIL) belongs to this species, because the characters of the young stocks differ some-

48 BULLETIN OF THE

from a young stock what has been the order of succession of individuals.
One has to view the object from both sides, make a careful examination
of the walls of the zocecia and of the relation of the polypides to one
another, and, when he has done his best to determine what are the facts,
he must feel that his conclusions are after all more or less subjective.
By a careful study of the colony shown in Figure 71, I have constructed
the diagram shown in Figure 71%

The stock of Lepralia is a creeping one, and all of its rows of individ-
uals are in juxtaposition. This juxtaposition is continued into the adult
stage. Even the young stock begins to show evidence of a quincunx
arrangement of individuals. This is less evident in the youngest indi-
viduals than in the older part of the stock, and is most evident in old
colonies. That there is not here a true dichotomous division of rows of
individuals, resulting in the annihilation of the ancestral row and the
establishment of two new ones, is evident from a glance at the youngest
generation in rows 11, 12, or, better, 2, 3, in which the relation of ter-
minal (11, 3) and lateral (12, 2) individuals is very different. The for-
mer continue the ancestral line, the latter establish new rows. Lepralia
differs from Bngula in this: that two lateral branches may be given off
from the ancestral row in the same generation, as at B, C, and a, a
(enclosed in circles), Figure 71°.

In contradistinction to the conditions in Bugula, when only one branch
arises, it is not given off towards the axil, but away from it.

The synchronism of the budding process noticed in B. turrita is hardly
distinguishable in the adult stock of this species; in the young, however,
it is quite marked, and gives to the whole a very symmetrical form. The
cleavage of eggs does not proceed by more regular steps. Of the three
individuals a, C, a (in circles), which follow 5, each has given rise to
three others, a median and two lateral. From each of the three individ-
uals derived from the two individuals a, a@ (in circles) has arisen a
lateral branch. Rule 3 is therefore well marked in the young stock of
Lepralia.

Rule 4, concerning the greater frequency of lateral budding at the
margin, is also exemplified in Lepralia. The ratio of cases of lateral
to median budding being 1:1 on the margin (rows 1-6 and 15-19)
and 1: 2.8 in the middle (rows 7-14.)

In Bngula, as will be recalled, it was concluded that the marginal

what from those of older ones. Yet it isan Escharine closely allied to Lepralia, and
Ihave seen in some cases the broad-based spine on the proximal border referred to
by Verrill as being found in L. Pallasiana.

MUSEUM OF COMPARATIVE ZOOLOGY. 49

branches were possessed of fewer generations than the intermediate ones.
Since by Rule 2 the lateral branches were given off towards the axils,
and the ancestral branches therefore always remained marginal, it re-
sulted that the ancestral branches were the shorter, the lateral branches
the longer. But in Lepralia lateral branches are turned away from the
axils, and here we find the conditions concerning the relative number of
generations in marginal and intermediate rows correspondingly reversed.
Thus, in Figure 71, the terminal individual D of row 10, a median but
ancestral row, belongs to generation IV. while the lateral branches 6
and 15 have five generations of polypides. Thus it is true here, as in
Bugula, that the ancestral branches are the shorter, the lateral branches
the longer (page 44).

That the outer individuals a, a, of rows 6 and 15, have given rise to
more individuals than the inner C, is clear without further comment.
Finally, since the individuals retain a nearly constant width, the neces-
sity of the rule established for Bugula, — viz. that there is almost ex-
actly the same number of individuals per unit of are for all radii, — and
of its corollary, — that the increase of individuals in sucessive gener-
ations undergoes a regular diminution, — is apparent.

Flustrella hispida, Fabricius.‘ — This stock is a very dense corm-like
one. The primary individual becomes surrounded on all sides by the
younger zocecia. It is very evident from an inspection of the position
of this primary polypide with relation to the periphery, that growth
occurs most rapidly on each side and in front of the primary polypide.
In making any diagram of such a stock, it is not very difficult to decide
upon the origin of the more peripheral individuals of the stock, but it is
wellnigh impossible to say with any certainty what are the relations
of the individuals of the second generation to those of the first. Bar-
rois (77, pp. 227-229) has, however, determined this for this species,
and my diagram (Fig. 67) is based in part upon his observations. I
do not desire to insist that the diagram represents the exact method
of growth of the stock. It is an attempt to represent it, founded princi-
pally on careful study of Figure 69. The quincunx arrangement of in-
dividuals is already apparent in the young stock (Fig. 69) ; it becomes

1 Hincks (’80, pp. 504-506) makes the existence of a larval bivalve shell a char-
acteristic of this genus, and therefore I assign to it a very common Alcyonidium-like
form which was extremely abundant on Fucus at Newport. F. hispida is the only
species of this genus. I found the bivalve shell still adhering to the primary indi-
vidual of a young colony (Plate VIII. Fig. 69, 0.). In Verrill’s (’73, p. 708) catalogue
this species is referred to under the name “ Alcyonidium hispidum, Smitt.”

VOL. XxII —No. l. 4

50 BULLETIN OF THE

more evident in the adult, and when new individuals arise distad to any
two, one of the new ones is median (ancestral branch), the other lateral.
(So terminal individual of rows 11 and 10; 22, 23; 43, 44; etc.) In
the diagram, however, I have not always indicated which is the median
and which the lateral branch, for in the older parts of the colony, owing
to a shoving of individuals, it is not easy to distinguish them.

Lateral branches appear usually to be given off towards the axis.
Here, as in Bugula, the lateral branches tend to be longer ; the ances-
tral, shorter.

It is evident from the diagram that lateral budding is most frequent
at the margins of the corm, i. e. that part lying posterio-dextral or poste-
rio-sinistral of the primary individual, and that the descendants of the
two lateral individuals of the four belonging to generation II. are more
numerous than those derived from the middle two. Finally, it is evi-
dent that the number of individuals per unit of are will be the same for
ares of all radii, and therefore the rate of increase of individuals will
diminish through successive generations.

In Crisia eburnea, Linn.,! we find the same laws illustrated. The
architecture of the genus has been carefully treated of by Smitt (’65%, pp.
115-142) as forming the basis of classification. Barrois (77, pp. 76-85)
has described in a masterly way the formation of the young stock of
tubuliporid Cyclostomata, and the relationships of the different types
of budding in this group. Harmer (791, pp. 145-173) has recently dis-
cussed the architecture of the stock in British species, adopting Smitt’s
graphic method of showing it. I have found his paper of great value
for my purpose. :

This species grows as a shrub-like stock upon floating eel-grass, etc.
I was wrong in saying, in my Preliminary (91, p. 282), that Crisia has
its branches united in pairs. The comparison of this species made by
Barrois (77, p. 82) with the “geniculata form” is conclusive evidence,
to my mind, that the apparent double row is in reality a single one, and
that such a branch as 18, Figure 65, is to be represented by a single
line in the diagram Figure 65%. We find here terminal and lateral
branches ; no true dichotomy. Branches are given off on the side away
from the axils, as in Lepralia, not as in Bugula. (But branch 11 is an
exception to the rule.) They are given off, as Harmer (91, p. 131)
has shown, alternately to the right and left. .

1 This is the only species of Crisia given by Verrill, and, since my species

is very common, it must be the one to which he refers. Moreover, it agrees
fairly well with Harmer’s diagnosis (91, p. 131).

MUSEUM OF COMPARATIVE ZOOLOGY. 51

There is something of a tendency for lateral branches to be given off
in the same generation from closely related branches. Thus (Fig. 65*)
from the primary individual, 0, of the stock, two individuals, a median
and a lateral one, arise. Each gives rise in its first generation to two
individuals, a median and a lateral. Of these four individuals each
gives rise at the end of three generations of median buds to two buds,
a median and a lateral. Comparing 2 and 10, first descendants of the
two branches arising from the second individual of 8, we find that each
gives rise to lateral branches from their first individual and from their
fourth. Comparing 14 and 19, first descendants of the two branches
arising from the first individual of 8, we find each giving rise to lateral
branches from their first individuals. The law breaks down, however,
when an attempt is made to carry it to extremes.

The fourth rule is not always so pronounced in Crisia eburnea as
elsewhere, although lateral budding seems to be slightly more frequent
at the margin.

The extreme marginal branches usually attain far fewer generations
than the more intermediate ones; thus, in Figure 65°, branch 20 ends
in the 7th generation and branch 13 in the 7th also, while the more
intermediate branches 15 and 18 attain 12 and 14 generations respect-
ively. So, too, while the outer branches 6 and 1 contain respectively
10 and 11 generations, the inner branches reach 12 and 14.

It is very noticeable that the outer branches give rise to more indi-
viduals than the intermediate ones. Figure 65* will serve to illustrate
this also. Here the outer branch 4, the intermediate 8, and the outer °
15 possess, together with the branches arising from them, 33, 28, and 40
individuals respectively. Harmer (’91, p. 168) finds this true for his
Crisia ramosa, for he says, “It is frequently remarked that the longest
and most branched parts of the colony are lateral branches, and not
parts of the main stems.”

There is, in the long run, a decrement in the rate of increase of indi-
viduals in successively older generations, yet it is not so regular a one
as that which we found to exist in Bugula. Thus, in the seven gen-
erations which even the shortest branches shown in Fignre 65* had
attained, the average increase of the number of individuals in the
second, third, and fourth generations over the number in the preceding
is 67% ; in the fifth, sixth, and seventh, 44%. The generations beyond
the seventh are not complete; they would have contained more indi-
viduals at a later period, when the branches which have now attained
only seven generations had grown. ‘Thus the number of individuals

52 BULLETIN OF THE

in successive generations beyond the seventh increases more and more
slowly, and finally decreases to zero. Thus the average rate of increase
of individuals in the generations 7 to 10 over those in the preceding is
only 16%.

One finds here, as elsewhere, that the number of individuals cut by
any unit of arc, the primary individual being taken as a centre, remains
practically constant, whatever the radius of the are.

In studying the creeping stocks of Cheilostomes (Plate VIII. Fig. 71),
young corms have been chosen because they exhibit fewer irregularities
of formation than old ones. Such irregularities are chiefly due to some
unevenness of the surface on which the corms lie, but sometimes
apparently to a crowding of individuals. Old rows of individuals are
occasionally entirely cut off and end in the middle of the stock; some-
times two rows running side by side, perhaps derived from a common
ancestor, suddenly merge into one again. In one case, Escharella varia-
bilis, Verrill, I have seen three rows thus merge into one at the margin,
suggesting the existence of a samknopp (common bud) in the sense of
Smitt (65, pp. 5-16). Ostroumoff (’86%, pp. 338, 339) has observed
a case in Lepralia Pallasiana. He says: ‘‘ Dans quelques cas, qu’on
peat considérer comme des anomalies, il arrive parfois que deux bour-
geons, provenant de loges differentes, viennent a se fusionner.” It seems
to me, therefore, that while Nitsche (’71, pp. 445, 446), who opposed
with such vehemence and success the idea of Smitt that zocecia arise from
an undivided marginal zone of cells, was quite right in affirming (71,
p- 447) that even the smallest marginal zocecia are sharply marked off
from the adjacent ones, yet he overlooked the possibility that under
certain circumstances the lateral walls might fail to develop, and thus
one zocecium might arise in the place of two, or even three.

I have not read Smitt’s Swedish paper, but I do not find anything
in the translation given by Nitsche to warrant the latter’s conclusion
(71, p. 446) that Smitt believed the “Gesammtknospe” to be “ formed
from the sum total of the mature peripheral zocecia.” If I understand
Smitt, he conceived the samknopp not to be derived from the most
peripheral mature zocecia, but to be self-proliferating, and to give rise
to the rows of zoccia, not to arise from thém. It is the “bud of
the colony,” not the sum of the buds of the peripheral individuals of the
stock. In this I would agree with him exactly. Although wswally one
finds the marginal gemmiparous tissue forming the lateral walls at the
extreme edge of the corm, and thus apparently separated into wholly
distinct adjacent gemmiparous masses; under certain conditions, the

MUSEUM OF COMPARATIVE ZOOLOGY. 59

lateral wall may not be formed between two or more rows, which will
then merge into one.

2. ORIGIN AND DEVELOPMENT OF THE INDIVIDUAL.

My studies on this subject, which were undertaken for the purpose of
showing the unity of the type of budding throughout Ectoprocta, have
been very fragmentary.

Figure 72 (Plate IX.) has been introduced for the sake of orientation.
It represents a longitudinal vertical section through the peripheral part
of a stock of Lepralia Pallasiana. The body wall is thicker at the mar-
gin (marg.), and gradually becomes thinner as one passes backward. A
septum (sep.) has already arisen cutting off the youngest zocecium from
the more proximal one, which contains a young polypide; proximal to
this is another septum, and the distal end of a third zocecium.

Nitsche (’71, pp. 445-456) has already well described the process of
forming the zowczvm in Flustra membranacea. In fact, he has studied
the organogeny more thoroughly in many respects than I have. Nitsche
(71, p. 452) showed that the wall of the advancing margin of the colony
was composed of two layers of cells, — an outer, “ Cylinderepithelschicht,”
which secretes a cuticula, and an inner, ‘‘ Spindelzellschicht mit anliegen-
den Kornerhaufen.” As the body wall, formed directly from these cell
layers is left behind by the advance of the margin, it becomes continually
thinner. ‘‘ Die Cylinderepithelzellen der Wandung platten sich weiter
nach dem proximalen Ende zu ein wenig ab, besonders die der Unterseite
verkiirzen sich, die einzelnen Zellen riicken auseinander, die Zellgren-
zen werden undeutlicher, die Kerne jedoch bleiben deutlich erkennbar.”
Vigelius (’84, p. 76) could not find the inner cell layer in Flustra, even
at the youngest stages, and consequently he believed that only one ex-
isted at the margin, and that this went to form the “ Parenchym-
gewebe” of the adult. Ostroumoff (86%, p. 336) seems inclined to doubt
the existence of any mesodermal /ayer at the distal portion of the bud-
ding zocecium in Cheilostomes, and Seeliger (’90, p. 580) has failed to
find in Bugula “eine zusammenhdngende dem Ectoderm dicht anlie-
gende Schicht von mesodermalen Spindelzellen.” Both Ostroumoff and
Seeliger, however, believe in the existence of dsolated mesodermal ele-
meats at the budding end.

According to my own observations, there is usually only one continu-
ous layer at the budding margin of the stock. Thus, in Flustrella
(Plate IX. Fig. 79) one can usually distinguish a continuous ectoderm,
but the mesoderm (ms’drm.) is represented by scattered cells only. At

54. BULLETIN OF THE

the margin in Lepralia (Fig. 73) one finds a thick ectodermal layer,
composed of columnar cells, but the mesoderm consists of an irregular
thick mass of cells, some of which appear to be amceboid. They how-
ever show no signs of having been derived from the outer layer. The
condition of the budding margin of Escharella resembles that of Lepralia.
In older parts of the body wall, where the ectoderm is reduced to an ex-
tremely thin layer, only scattered mesodermal cells appear, and these are
amceboid or mesenchymatoid.

On the other hand, one finds in the body wall, around the nascent
neck of the polypide (Plate X. Fig. 88), even to a late stage, both ecto-
derm and mesoderm well formed as layers. The ectoderm is a columnar
epithelium ; the mesoderm is flatter, and often its cells are not sharply
delimited from one another. It is thus perfectly evident, to my mind,
that the mesoderm has in general lost its original epithelial character
in the marine Bryozoa, although it has retained it in Phylactoleemata.
Whenever it does exist in the former group as an epithelium, it is at the
budding regions (neck of polypide, and Figures 74, 75, 78, 79, ew.).

Origin of the Polypide. — There are very few problems in modern
morphology, I fancy, the history of whose investigation shows a less
satisfactory aspect than that of the origin of the polypide in Gymnole-
mata. It is hardly to be wondered, however, that investigators have
sought for another interpretation of the process than the most obvious
one, because that seemed to oppose many long cherished and wellnigh
universally held dogmas. While the first recognition of the animal
nature of marine Bryozoa, which we owe to the studies of Bernard de
Jussieu in 1742 and John Ellis in 1755, brought with it a knowledge of
their colonial nature, yet it was not until much later that the most
characteristic part of this process—the formation of the polypide—
was clearly observed. Grant (27, p. 115) and Farre (’37, pp. 400, 409,
415) first described the process by which is formed this complex of or-
gans, and settled once for all the controversy which had sprung up as to
whether these animals were truly stock-builders. Under the influence
on the one hand of the endosare theory of Joliet (’77), and on the other
hand of the view promulgated by Hatschek (77), that similar organs
in larva and polypide are equivalent as far as regards their origin from
the germ layers, the more important papers ! between *77 and ’90 main-
tained either that the polypide arose independently of the body wall,

1 Excepting those of Barrois, who, from the study of the favorable material

presented by metamorphosing larve, has persistently maintained the correct
interpretation.

MUSEUM OF COMPARATIVE ZOOLOGY. 55

and secondarily acquired connection with it, or that it had a double
origin.

To Nitsche (’71, pp. 456-463) belongs the credit of having first described
the histological changes in the origin and development of the polypide
of marine Bryozoa, particularly with reference to the part which the
germ layers play in that process. He says (’71, p. 456): “ Die Anlage
des Polypids erscheint zunachst als eine Wucherung der Zellschicht der
Endocyste in der Mitte der Hinterwand der Knospe, und zwar in dem
Winkel, den die Hinterwand mit der oberen Wand macht. Bald ordnen
sich die Bestandtheile des regellosen Zellhaufens in zwei deutlich geson-
derte Schichten, und wir sehen nun einen rundlichen Korper, beste-
hend aus einer dusseren einschichtigen Zellschicht, welche sich scharf
absetzt gegen die das Innere des Korpers bildenden Zellen.”

This stood until a year ago as the most satisfactory description of this
process in the adult stock. The appearance within the last year of the
two papers of Prouho (90) and Seeliger (90) marks a distinct epoch in
the advance of our knowledge concerning the origin of the polypide in
Gymnolzemata. The paper of Prouho treats of the process in the case of
the primary polypide of the metamorphosing larva of Flustrella, that of
Seeliger in the case of the young (practically adult) stock of Bugula.
According to both authors, the polypide arises from the body wall by an
invagination of it, and its two layers are from the first distinct and
separate, and go to form the two layers of the adult polypide, and the
whole of those two layers. The outer layer of the body wall gives rise
to the outer layer of the tentacles and the lining of the alimentary tract,
and the inner layer of the body wall gives rise to the mesodermal lining
of the polypide. Prouho alone is cognizant of the method of origin of
the ganglion, and in addition there are several points of difference be-
tween these two authors concerning the development of other organs, to
which I shall refer in the proper place. Thus the latest studies have
confirmed the assertions of Nitsche, that the polypide arises from a
single centre of proliferation of the body wall; they have made an ad-
vance in this, that they have shown that the two layers of the bud do
not become secondarily differentiated from a single cell mass, but are
respectively derived from the two cell layers of the body wall. My own
studies have led me to the same conclusion on this point.

Figure 75 (Plate IX.) is a vertical radial section through the margin
of an adult Flustrella stock. The ectoderm is relatively thick at the sole
(sol.) and margin, and very greatly thickened at the point marked gm.
Here two layers, sharply separated, are apparent. The cells of the outer

56 BULLETIN OF THE

layer are columnar and full of granular protoplasm, the mesodermal cells
cuboid. The body wall has clearly begun to invaginate in this region.
Figure 79 is a similar section, and shows a later stage in this process.
The lumen of the bud is apparent, and has been formed by invagination,
not, as in Paludicella or Phylactolemata, by ingression. The two layers
of the bud are apparent; they have been derived from those of the
body wall.

Figure 73 (Plate IX.) shows a stage in the development of the poly-
pide which is intermediate between that of Figures 75 and 79, but from
another suborder, Cheilostomata. ‘The mesoderm has here a mesenchym-
atous character, and is loosely attached to the inner layer of the bud ; it
is not always sharply marked off from it by boundaries, but is quite dis-
tinct in its reaction with staining reagents. This bud has evidently
arisen by invagination of the body wall. Seeliger (’90, p. 581) also finds
that there is an actual invagination of the ectoderm in Bugula, the open-
ing to which he calls “ blastopore.”

From what has been already shown, it is evident that in Flustrella, as __
well as in Cheilostomata, the first appearance of the young polypide is
near the margin of the stock, not near the proximal part of the young
zocecium. This will also be apparent at 6 and 9, Figure 71 (Plate VIIL),
where the accumulation of nuclei immediately behind the margin indi-
cates the neck of the polypide, — the point at which the bud arose. To
be sure, at quite an early stage, but very much later than that of Figure
73, the polypides are found near the proximal wall of the zoccium, but
a delicate funnel-shaped sheath of tissue runs from the polypide to the
distal part of the zocecium, where the polypide is attached to the body
wall.

After invagination the pocket closes at its attached end by a growing
together of its lips (Figs. 79, 78). Thus the body wall becomes contin-
uous again over the lumen of the bud, and this union is first broken
when the fully formed polypide is ready to evaginate itself. Seeliger
(90, p. 582, Taf. XXVI. Figs. 8, 10) has described and figured a similar
condition in Bugula.

The young bud now becomes elongated (Fig. 80), the walls of the bud
sometimes becoming closely approximated. A little later it begins to
pass backwards relatively to the distal wall of the zocecium. A trans-
verse section through the young polypide and the neck of the colony
shows that the connection has become a less intimate one (Fig. 81, cev.
pyd.). The tissue by which the connection is still effected is that from
which the kamptoderm will be formed. It is apparently the existence

MUSEUM OF COMPARATIVE ZOOLOGY. a9

of this stage, in which the kamptoderm is long drawn out and easily
overlooked in optical as well as actual sections, that led to the belief that
polypide buds may arise independently of the body wall and only sec-
ondarily become connected with it.

At about this time the lumen of the alimentary tract begins to be
separated from that of the atrium. Thus, in the series from which Fig-
ure 81 was taken the more oralward lying sections show that the cavities
of the lower and the upper parts of the bud, which at the anal end are
broadly confluent, have here become separated by a constriction. A
sagittal section of a somewhat later stage is shown in Figure 76, which
is from Flustrella. Here we find the alimentary tract represented by a
space in the lower part of the bud, broader at its anal than at its oral
end and separated from the upper cavity — the common atrio-pharyngeal
cavity, @. + atr.— by a line of nuclei which represents the line of ap-
proximation of the inner layers of the two sides of the bud. The bud is
attached to the body wall at its marginal (anal) end, and is free from it
oralwards. (Compare with Paludicella, Plate III. Fig. 24.) It seems
to me highly probable from these and other series of sections that the
alimentary tract is separated from the rest of the lumen of the bud, not
by an approximation of the inner layers of the bud along the whole ex-
tent of the future alimentary tract at once, but that the rectal part is
first formed and constitutes a large cavity, at first broadly open to the
atrium above, and that the gastric portion is formed somewhat later by
a progressive enlargement of the lower cavity of the bud, which now
becomes constricted off from the atrium and cesophagus above. This
process is like that found in Paludicella (page 19), which forms a sort
of transition to that of Phylactolemata, described by Braem (’90, pp.
45, 46) and myself (90, p. 112).

Prouho (’90, p. 448, Fig. 6) shows that the rectum at first appears
as a blind sac open to the atrium at its posterior end, although later
this opening is greatly reduced. Hence in the Flustrella larva also the
space from which the lumen of the future rectum is to arise is formed
before that of the stomach, although this part of the alimentary tract
is the last to be cut off from the atrium. Seeliger (’90, p. 585) says
concerning the formation of the alimentary tract in Bugula: “ Der ganze
Basaltheil des Polypids sich in der Mittelpartie durch zwei immer tiefer
werdende Furchen von dem vorderen abschniirt, wihrend er an zwei
Stellen, einer oberen und einer unteren, mit ihm in Verbindung bleibt.
Die obere Verbindung entspricht dem Anus, die untere dem Mund.” The
author here seems to imply that the whole alimentary tract is formed at

58 BULLETIN OF THE

one time ; but as he has not attended particularly to this point, this can
hardly be said to militate against my view.

There is, however, in my opinion, a more important error in Seeliger’s
description of the origin of the alimentary tract, — an error into which
Nitsche (71, p. 457) also fell. As in Phylactolemata and Paludicella,
so also in marine Bryozoa in general, so far as I have studied them, the
posterior and anterior parts of the alimentary tract are formed indepen-
dently, and their cavities coalesce only secondarily. The constriction
which separates the lumen of the bud into a cavity nearer, “ vorder,” and
one more remote from the body wall, “ basal,” does not separate off the
whole alimentary tract from the atrium. Neither does that constriction
result in the formation of a space opening into the cavity nearer the
body wall, “ Vordertheil,” at an upper [distal] point (anus) and lower
[proximal] point (mouth). Thus if one examines a complete series of
sections through a polypide even of so late a stage as Figure 92
(Plate X.), one finds that, while there is an open connection between the
anal end of the alimentary tract and the atrium, the oral end is at all
points sharply separated from the cavity above by a double-layered wall
of cells, as is shown in Figure 92, between @. and ga. Such a condition,
moreover, has been found by Barrois (’86, pp. 73-76) in the primary
polypide of Lepralia, and by Prouho, as just stated, in the primary
polypide of Flustrella.

Origin and Development of the Ring Canal and Tentacles. — Nitsche
(71, p. 4380) first described in Flustra a ring canal surrounding the
mouth-opening and lying at the base of the tentacles, but did not refer
to the origin of it. Seeliger (90, p. 588) describes it in a young pol-
ypide of Bugula, as derived from the mesodermal layer.

My own sections also show that it arises on each side of the cesopha-
gus as a groove lined by mesoderm (Plate X. Fig. 92, right). This
canal, which is shown cut along its course in Plate IX. Fig. 82, can.
erc., is not wholly separated from the body cavity, but communicates
with it below the brain. This communication occurs in the section
below that shown in Figure 82, near the point can. ere. This ring
canal at an earlier stage is shown in Figure 87. It has not yet been
formed backwards nearly so far as the brain; anteriorly the section has
traversed the tentacles under which it runs. The canal is also shown
cut across in Figure 86 at the base of a tentacle, with whose lumen its
cavity is directly continuous.

The formation of the tentacles is closely connected with that of the
ring canal, from the upper wall of which they arise. Since the upper

MUSEUM OF COMPARATIVE ZOOLOGY. 59

wall of the ring canal is two-layered, the tentacles are two-layered also.
The outer layer of the tentacle is thus derived from the inner layer of
the bud ; the inner layer, on the contrary, from the outer layer of the
bud. It would be hardly necessary to make this statement, which
agrees both with early and the most recent observations, had not Bar-
rois (’86, p. 75, Fig. 48) referred to and figured the tentacles as having
been formed from the inner layer of the bud only.

My observations fully confirm Seeliger’s (90, p. 587) description of
the manner of growth of the tentacles ; that is, that the outer edge of
the ring canal, together with its tentacles, moves downward and outward
along the sides of the polypide, turning the axis of the tentacle from a
nearly horizontal to a vertical position, and increasing the area of the
kamptoderm. Thus in Figure 92 this process has progressed farther on
the left side than it has on the right.

Nitsche (’71, p. 458) lays some stress upon the statement that the ten-
tacles are not at first few in number, gradually becoming more numer-
ous ; on the contrary, he says, “Ich sah stets, beim ersten Auftreten von
Tentakelanlagen, 16, 17, oder 18 Stiick gleichtzeitig erscheinen.” See-
liger (90, p. 584) agrees with Nitsche in this respect ; but Prouho (’90,
p. 449) finds the conditions different in Flustrella. Here the tentacles
“ne se développent pas simultanément sur tout son pourtour, mais ap-
paraissent d’abord de chaque cété du plan de symétrie, puis se multi-
plient vers Varriere.” As I have shown, 14 of the 17 tentacles arise
nearly simultaneously in Paludicella, for here there are few of them ;
and this is the case also in Escharella variabilis with its 17 tentacles,

As the tentacles of both Flustra and Bugula are few in number,! the
statements may easily be considered to be correct for these genera. The
tentacles of Flustrella hispida are much more numerous (30-35), and
Prouho’s statement may well be true for his form. In fact, my own ob-
servations on this species are fully in accord with those of Prouho.
Figure 77 (Plate IX.) represents a young polypide of a Flustrella corm,
viewed from the roof as an opaque object. Six tentacles were visible on
each side of the bud, but the oral and anal parts of the corona were yet
incomplete. The remaining nine or ten pairs of tentacles subsequently
arise oralward and analward of these rudiments.

Much disagreement has prevailed concerning the number of layers in-
volved in the kamptoderm of marine Gymnolemata, in both the adult
and the developmental stages. As in so many other cases, we owe to

1 Bugula avicularia has 14 or 15 tentacles, and Flustra (Membranipora) mem-
branacea 20, according to Hincks (’80, pp. 76 and 140).

60 BULLETIN OF THE

Nitsche (71, pp. 431, 432) our first intimate knowledge of this organ.
He believed it to consist in the adult of Flustra of a single cell layer,
in which are imbedded (or applied?) longitudinal and circular muscle
- fibres. He believed the kamptoderm to be formed gemmigenetically only
by the outer cell layer, the derivative of the mesoderm. Repiachoff (75,
pp- 138, 139) observed in Tendra (Membranipora) “ die Doppelschichtig-
keit der Tentakelscheide nicht nur bei den jungen Knospe sondern auch
bei den ganz ausgewachsenen, offenbar schon lingst functionirenden, in
ihrem mittleren Theile ganz braunen ‘ Polypiden,’” and later (’76, p. 152)
a similar two-layered condition of the kamptoderm (Tentakelscheide)
in Membranipora and Lepralia. Ehlers (76, p. 37) finds a single layer of
cells in the kamptoderm of the adult Hypophorella (Ctenostome), which
he believes is continuous with the endocyst of the body wall, and thus
is ectodermal. He finds neither longitudinal nor circular muscle fibres.
Haddon (’83, p. 517) believes the kamptoderm to be derived from both
the inner and outer layer of the polypide bud. Vigelius (’84, pp. 33, 82)
describes it as arising from the mesoderm only (Parenchymegewebe), and
as being essentially one-layered, both longitudinal and circular muscles
lying in this layer. Barrois (86, p. 74) derives the kamptoderm from
the mesodermal layer only. Ostroumoff (’86*, p. 15) believes the kamp-
toderm to be two-layered and provided with muscles ; it is in his opinion
derived from both layers of the bud. Freese (’88, pp. 18, 19) studied
only the adult of Membranipora. He admits the presence of muscle
fibres, but believes the kamptoderm one-layered. Pergens (’89, p. 507)
states only that in the Cheilostomes studied by him the tissue of the
kamptoderm is composed “aus abgeplatteten Zellen, zwischen welchen
Liings- und Ringmuskelfasern eingebettet sind.” Prouho (90, p. 451)
states that in the primary polypide of Flustrella this organ is early
differentiated, “et les deux couches de rudiment prennent part a sa for-
mation.” Finally, Seeliger ('90, p. 587): “Es kann danach keinem
Zweifel unterliegen, dass die Tentakelscheide ektodermalen Ursprungs
ist. . . . Das Mesoderm erscheint auf allen gelungenen Schnitten von
der Tentakelscheide scharf abgesetzt.”

It is my belief that throughout the group of marine Gymnolemata, as
in Paludicella and Phylactolemata, the kamptoderm is derived from both
of the two layers of the polypide bud, is provided with a strong system
of longitudinal and a slight one of circular muscles, and contains in the
adult two layers, or at least modified representatives of two layers. I
have arrived at this conclusion from a careful study by sections of the
following genera: Bugula, Lepralia, Escharella, Flustrella, Bowerbankia,

MUSEUM OF COMPARATIVE ZOOLOGY. 61

and Crisia. The existence of two layers was easily demonstrated in all
cases in the young polypide by cross sections of the “neck.” The two
layers are of nearly the same thickness, and distinctly separated from
each other. The presence of two layers in the adult is more difficult to
determine, but it was always indicated by the occasional presence of
two nuclei lying side by side, and especially at the attachment to the
diaphragm. The presence of muscles was demonstrated in all cases
(except Bowerbankia, where my few sections did not show the proper
region) upon tangential sections of the sheath. I may add, that the
existence of muscles is wellnigh conclusive @ priort evidence of the
existence of the mesodermal layer, since nowhere else in Bryozoa, so far
as I know, do muscles arise from any other layer. Prouho’s evidence in
support of his position is perfectly satisfactory to my mind, certainly
more so than the negative evidence of Seeliger in support of his. In
further support of my statements I may refer to the condition of the
kamptoderm (kmp’drm.) in Figures 92 and 83, Plate X.

Nervous System. — Since Dumortier discovered, in 1835, a ganglion in
Lophopus, there has been seen in marine as well as fresh water Bryozoa
a body which has been considered, with greater or less certainty, to con-
stitute the central nervous system. Overlooked by Farre, it was, I be-
lieve, first described for marine Gymnolemata in 1845 by van Beneden,
co-worker with Dumortier, for Laguncula (Farrella). Nevertheless, up
to the present the evidence of its being a ganglion homologous with that
of Phylactoleemata has not been satisfactory. The homology can be estab-
lished only by determining its similar origin with the brain of Phylacto-
leemata ; its function can be best established by showing the existence of
ganglionic cells and fibres. I hope to have advanced our knowledge in
both of these directions.

At about the time that the cesophagus and stomach have become con-
fluent, one notices a papilla-like elevation of the floor of the atrio-pha-
ryngeal cavity. This has been noticed by Korotneff (’74) in Paludicella,
and by Nitsche (’71, p. 459) and Seeliger (’90, p. 586) in Cheilostomes.
It has been called by them “ Epistome,” and compared with that of Endo-
procta or Phylactolemata. In my own opinion, it is merely a structure
brought into prominence by the sinking down of the floor behind it to
form the ganglion (Plate X. Fig. 86, gv.). This depression has been
seen by Barrois (’86, pp. 74, 75) and Prouho (’90, p. 450), and rightly
interpreted by them as probably destined to giye rise to the central
nervous system. That this is the correct interpretation is shown by
later stages from different species, as Figures 89 and 83, in which we see

62 BULLETIN OF THE

the ganglion gradually assuming the position it has in the adult, on the
anal side of the pharynx at the base of the anal tentacles.

A section across the pharynx in such a stage as Figure 83 is shown
in Figure 87. A comparison with Figure 51 (Plate V.) of my Crista-
tella paper (Davenport, ?90) will show a great similarity of conditions at
about the same age, and can leave no doubt concerning the homology of
the regions marked in both cases du. gm. ; or compare Taf. VIII. Fig. 100,
nh., of Braem’s (90) magnificent work. A section through a later stage
is shown in Figure 82. The brain has already sent out cirecumeesopha-
geal nerves, as in Paludicella. The central part of the ganglion does
not stain; one sees only a granular mass, sometimes with signs of short
fibres. In the cornua (x’) one occasionally sees very large clear nuclei
with a single nucleolus, lying in the midst of a cell mass which is
spindle-shaped and stains more deeply than adjacent cells. These remind
one strongly of bipolar ganglionic cells, but fibres could not be traced
far from their pointed ends. Series of sections of Flustrella parallel to
Figure 82 show, as one passes below the level of the ganglion, a con-
tinuous band of cells extending down from it towards the cardiac valve
and between the cell layer lining the cesophagus and the surrounding
mesoderm. One is reminded of the exactly similar conditions in Palu-
dicella (page 26), and of the “linienartige Zeichnung” seen by Nitsche
(71, p. 431) and Vigelius (84, p. 42) in the same place in Flustra.
These facts go to indicate the existence of a gastric nerve.

At about the time at which the ganglion arises, the cavities of the
stomach and the csophagus become confluent (Fig. 86 @.). At this
stage (somewhat earlier than Figure 86) the alimentary tract consists
of a U-shaped tube of nearly uniform calibre, and without any indica-
tion of the ceecum. The tentacles lie in two parallel rows in the middle
of the bud, the corona being incomplete both in front and behind, but
less so oralwards than towards the anus (Fig. 77, atr.). In fact, while
new tentacles are formed later towards the oral median line, they never
appear behind the line afr. This hinder region has another fate. Its
wall increases very greatly in area, diminishes correspondingly in thick-
ness, and forms a large part of the kamptoderm lying behind the post-
oral tentacle in Figure 86. With this growth of the kamptoderm the
anus is carried backwards, and farther and farther from the posterior
ends of the rows of tentacles, immediately behind which it formerly lay.

As the kamptoderm grows in area, the polypide comes to lie in the
proximal part of the zocecium, Pari passu with this process occurs the
rotation of the oral tentacles, as in Paludicella. The oral tentacles which

MUSEUM OF COMPARATIVE ZOOLOGY. 63

at first lie perpendicular to the roof of the colony (Fig. 86) gradually
come to lie parallel with it (Figs. 89 and 83). The esophagus loses its
elongated, laterally compressed form, and becomes circular, and the gan-
glion lies just below the mouth-opening. Not until now, in fact, can one
speak ofa mouth. It was not at all formed synchronously with the anus.
To illustrate this process I have taken three different genera represent-
ing different stages. Similar stages could have been obtained from each
genus. By using three genera, the similarities as well as the dissimi-
larities of the process are indicated. Among other things, the larger
size of the polypide and shorter kamptoderm of the Ctenostome Flus-
trella (Fig. 89) is noticeable.

Lastly, the coecum is formed as a wholly secondary differentiation of
the alimentary tract. This arises in some species relatively earlier than
in others; thus it is better developed in Figure 86 than in the later
stage of Figure 83.

The lining cells of the alimentary tract now rapidly undergo the dif-
ferentiations characteristic of the different regions, The most extreme
modification takes place in the pharynx. In Cheilostomes the cells of
this region gradually become vacuolated, until finally very little stain-
able protoplasma remains. The nucleus lies at the deep end of the cells.
A very peculiar modification of the cell walls takes place, in that they
become plainly perforated by holes through which the adjacent cells
are in communication (Fig. 85). It is in a region similar to this that
the cells become cuticularized in Bowerbankia to form the so-called
gizzard. The pharyngeo-cesophageal region is also provided with a very
powerful musculature of circular muscles (mw., Figs. 85, 86).

Concerning the orzgin of the muscles | have made very few studies.
The parieto-vaginal muscles seem to arise, as in Paludicella, from
around the neck of the polypide, and the retractors from the oral end
of the polypide bud (mu. ret., Fig. 89).

The neck of the polypide sinks below the general level of the body
wall by an infolding of the latter, as described for Paludicella, and the
mass of columnar cells which passes down with it forms, I am confident,
the diaphragma of Nitsche (71, p. 432), which is thus exactly com-
parable with the mass of cells around the atrial opening of Paludicella
in Figure 45, of. atr. (Plate V.). According to this view, then, the dia-
phragma is not placed at about the middle of the kamptoderm, but at
its proximal end, and all that lies between it and the outer body wall —
the non-evaginable portion — has been formed in the elongated neck,
exactly as the non-evaginable portion is formed in Phylactolemata (see

64 BULLETIN OF THE

Davenport, 90, Plate IX. Fig. 77, Plate XI. Fig. 98) and Paludicella
(Plate V. Figs. 50 and 45).

As my purpose is not so much to present a complete organogeny of
Bryozoa as to show the method of origin of the bud and the fate of the
layers, | have had to desist from carrying on my studies further in the
organography, and have left many interesting and important questions
unsolved ; such, for instance, as the development and structure of avi-
cularia, the presence of an excretory system, and the degenerative pro-
cesses which occur with regularity in the polypides.

3. REGENERATION OF THE POLYPIDE.

I have been led to study the regeneration of the polypide because
Ostroumoff seems to believe that in regenerating buds the digestive epi-
thelium of the stomach is derived from an extraneous source, — the
brown body. Thus he says (’86*, p. 340) the brown body appears as a
coecal appendage of the young digestive tube. ‘C’est sur ce dernier
[tube digestif] qu’on trouve un groupe de cellules affectant la forme
d’un bonnet et se réunissant trés tot a langle proximal du rudiment
ectodermique. A mesure que les cellules du bonnet, ainsi que la masse
brune, sont employées 4 la formation de la portion moyenne du tube
digestif, ces dernieres se débarrassent de leur contenu,” etc.

The external phenomena of regeneration are well known. In the
Membranipora stock, for instance, one sees polypides being produced at
the margin, and one finds them older and older as one passes backwards,
until finally they are seen to be wholly degenerate, and to be replaced by
young polypides. Thus, in passing backward along a single row of indi-
viduals in a Membranipora stock about 18 mm. long, I have seen this
process of regeneration recurring four times. In Alcyonidiun, too, one
finds an apparently regularly recurring degeneration and regeneration
of polypides. In the mat-like Cheilostomata the regenerating polypide
(Plate VIII. Fig. 71, pyd. rgn.) is always found at one place, — namely,
on the operculum, — that is, proximal of the opercular opening.* In
Flustrella it is found in a similar position on the dorsal body wall, proxi-
mal of the cuticularized introverted portion. My studies have been
chiefly made on the Cheilostomata. Figure 91 (Plate X.) represents an
early stage in the formation of a regewerating polypide. Here, as in the
marginal polypides, there is a typical invagination involving the two

1 Haddon (’83, pp. 522, 523) has found the regenerating polypide arising from
the same place in Flustra membranacea and in Eucratea, and Ostroumoff (’86+,
p 389) in Cheilostomes in general.

MUSEUM OF COMPARATIVE ZOOLOGY. 65

layers of the body wall (7, ew.). Owing to the reagent, the body wall is
shrunken from its contact with the operculum (op.).

If one inquires what has been the histological conditions of this region
antecedent to this stage, one must look to younger adjacent and mar-
ginal zocecia, since they reproduce these conditions. I will again call
attention to Figure 88, which represents a cross section of the body wall
through the region of attachment of the kamptoderm of a young pol-
ypide of about the stage of Figure 83. This, then, represents the neck
of the polypide, and it is from about this region that the operculum and
finally the regenerating polypides will arise. The cells are columnar,
and stain deeply about the nuclei, and both cell layers are well devel-
oped. Elsewhere in this same individual the body wall is composed of
smaller, flatter cells, and two layers are not easily distinguished. The
region of the future operculum possesses at an early stage some of the
largest, most columnar cells of the body wall. The cells of this region
do not, however, retain their peculiarly large size throughout life, but
in the adult we find the same region occupied by a flat epithelium,
nearly as thin as the epithelium shown in Figure 90. Meanwhile
the epithelium of the rest of the body wall has become still more
attenuated. The difference between the body wall of the operculum
and that of adjacent regions is best shown by the greater abun-
dance of nuclei under the opercular region when the stained stock is
looked at in toto from the roof (Plate VIII. Fig. 71). The regions of
the future opercula are seen, in young zoccia (Fig. 71, 4, 6), to be
patches of densely packed nuclei. The opercula of older zocecia show a
slight preponderance of nuclei, and thus indicate more numerous cells.
It is from such a region, then, that the young regenerating polypide
arises.

As in the case of the marginal polypides, so here, the lips of the
invagination pocket close and become fused to form the neck of the
polypide (Plate X. Fig. 84). The later stages of the development of
the regenerating polypides seem to be the same as those of the marginal
buds. Figures 74 and 89 are, indeed, regenerating polypides. I cannot
find any evidence that the alimentary tract, or any part of it, is formed
in regenerating buds by a method differing in any essential particular
from that in marginal buds.

It is well known, however, that the degenerated polypide which forms
a “brown body” in the old zocecium eventually disappears. Haddon
(783, p. 519) maintains that in the developing regenerated polypide
“the walls of the stomach, or, more strictly, that portion of the stomach

VOL. XXII.— No. 1. 5

66 BULLETIN OF THE

which forms the gastric ceecum, grow round and envelop the brown body,
so that the brown body passes as a whole into the alimentary tract of the
young Flustra.” It seems to me that the burden of proof of such a
remarkable occurrence lies with him who asserts its existence, and cer-
tainly sufficient evidence is not presented by Haddon,

To settle this question in my own mind, I cut a series of thin sections
through a part of a stock of Escharella (which in budding shows a prac-
tical identity with Flustra), in which all stages of regenerating polyp-
ides were to be found. From complete series, at critical ages, I utterly
failed to find any indication of the inclusion im toto of the brown mass
by the polypide. But I found the alimentary tract of the polypides
usually applied to the brown body (pyd. dgn.), as shown in Figure 92.
At this stage the degenerated mass is surrounded by spindle-shaped cells,
and just within these by a homogeneous or lamellated sheath. At later
stages the elements of the degenerated mass were seen to be more loosely
associated. The cells of the alimentary tract at the same time appear
highly granular, and a granular coagulum often partly fills the alimentary
tract. Before the new polypide is ready to expand itself, the brown body
as such has often wholly disappeared. Just as my sections leave no
chance for the brown body to be included en masse by the alimentary
tract, so too do they yield no evidence of the addition to the latter of
new cells from this degenerate mass, as Ostroumoff, in the sentence
quoted above, implies.

The interesting facts of degeneration in Bryozoa deserve a more careful
study than I have been able to give them. We are quite ignorant of the
physiological significance of the regularly recurring degeneration and
regeneration in certain Bryozoan colonies. Ostroumoff (786%, p. 339) has
offered an interesting hypothesis, to the effect that the degeneration of
the polypides, the remains of which are taken into the stomach of the
regenerated polypide and the undigested portion of which is cast out with
the feces, is a method of excretion, made necessary to these animals from
lack of urinary tubules.

IV. Origin of the Gemmiparous Tissue in Phylactolemata.

After having found that in Paludicella and the marine Bryozoa, as in
Phylactoleemata, the growth of the colony takes place at the margin or
tips, and that it is here primarily that buds originate, and after having
thus found that throughout the group all of the organs of the polypide
are derived from two layers, of which the inner gives rise to organs so

MUSEUM OF COMPARATIVE ZOOLOGY. 67

dissimilar in origin as the central nervous system and the alimentary
tract usually are, it becomes a matter of no little importance to solve
the two problems, what is the origin of these growing regions, and what
that of the two layers. Through the works of Barrois (’86), Ostroumoff
(87), Vigelius (88), and especially Prouho (90), on the metamorphosis
of the larva and formation of the first polypide of Gymnolemata we
are fairly well acquainted with the facts in this group; but a careful
study has not heretofore been made of the Phylactolemata with reference
to the points mentioned above. Korotneff (’89) and Jullien (90) have
published quite extensive papers on the ontogeny of Phylactolemata,
which describe too incompletely the stages which should reveal the
required facts.

In order to throw a little light on these questions, I undertook the
study of the embryology of two species of Phylactolemata. But before
beginning the account of what I have found, it is necessary to remind
the reader of some facts concerning the origin of the polypides in the
adult colonies. For our knowledge of these we are chiefly indebted to
Braem (’90, pp. 18-32) ; it has also been my privilege to confirm many
of them.

The details of the budding process are slightly different in Plumatella
and Cristatella. In the latter genus the body wall becomes highly mod-
ified as it grows older by the formation of secreted masses which nearly
fill most of the ectodermal cells. In Plumatella, on the contrary, the
ectodermal cells retain, for the most part, a more primitive, unmodified
condition. Here, moreover, by a rapid growth at the neck of the pol-
ypides, the individuals are carried to considerable distances from one
another, whereas in Cristatella there is a less rapid growth resulting in a
compact stock.

In Plumatella, the whole of the embryonic tissue from which any bud
arises does not go to the formation of a polypide, but a part of it re-
mains as the neck of the polypide, and gives rise by cell proliferation to
the body wall and the Anlage of a new bud. Thus the Anlage of each
bud is part of that of a preceding bud. The question remains yet un-
solved, Whence came the Anlage of the first polypide? Since the em-
bryonic tissue of the inner layer of the bud, which seems to take the
most active part in the formation of the bud, gives rise to both the lining
of the alimentary tract and the wall of the brain, it becomes an ex-
ceedingly interesting question, From what germ layer is this inner bud
layer derived ?

In Cristatella, as in Plumatella, not all of the embryonic tissue from

68 BULLETIN OF THE

which any bud arises goes to form that bud; but some of it is, appar-
ently, passed along under the highly metamorphosed cells of the ecto-
derm, again to divide itself, one part going to form a new polypide, the
other to form the Anlagen of new buds. In Cristatella, this embryonic
mass of cells of the inner layer of the bud seems to be to a considerable
extent independent of the highly metamorphosed ectoderm, and to form
at places a sort of third layer, lying below the true ectoderm and above
the muscularis with the cclomic epithelium. Here, too, while it is
easy to see buds arise from preceding buds in the adult colony, we
cannot consider our question answered until we have discovered the
origin of the cells from which, as from a stolon, the Anlagen of polypides
successively arise.

I desire to say that I have avoided giving a full account of the on-
togeny of these species, both because it is not directly required for the
solution of the problems in hand, and because we are promised studies
in this field by Braem.

The eggs of Phylactolemata arise, as has long been affirmed, from the
ccelomic epithelium of the body wall. The evidence of this is conclusive,
for one often sees in a single section various stages in the development
of the eggs. (Plate XI. Fig. 93, ov.) It is also to be observed that
they do not arise indiscriminately from any region of the body wall, but
always close to the neck of a polypide. Sooner or later these eggs,
surrounded it may be by a few follicular cells, are enclosed in an o@cium,
and here undergo their development up to the stage of a young stock,
possessing perhaps a dozen immature polypides. In the figures on
Plates XI. and XII. the ocecium (ow.) has been usually drawn, but in
Figures 100 and 104 it has been omitted. As a result of cleavage, a
blastula is formed, and from one pole of this — the pole nearest to the
neck of the ocecium —cells are given off which move into the blastocel
(Figs. 94, 98) and finally come to line the cavity. It is important to
observe that in the earliest stage of this process found there were four
inner cells, of which two are represented in the section (Fig. 94, ms’drm.
+ en’drm.). Thus the two layers of the adult body wall are established.
Up to this stage the conditions are practically the same in Cristatella
and Plumatella. From now on, they are somewhat different in the two
genera,

The first difference to be noticed is in the ocecium itself. In Crista-
tella the cells composing this rapidly become a pavement epithelium
(Fig. 97); in Plumatella, on the contrary, the cells of the occium
remain columnar (Fig. 99). The neck of the ocecium also differs in the

MUSEUM OF COMPARATIVE ZOOLOGY. 69

two cases. In Cristatella it is long, thick, and filled with a dense mass of
large cells (Figs. 95, cev. ow., and 101 *, 102*). In Plumatella (Fig. 99)
it is very short.

The second difference concerns the embryo itself, and is connected
with the formation of the first polypide. In Plumatella (Fig. 99) the
first indication of the formation of the first polypide occurs at or
very near the neck of the occium, or, since the ingression of cells
into the blastoccel took place at the pole of the blastula nearest the
neck, we may say near to the pole at which ingression occurred,
The cells of the outer layer (7.) are elongated and contain large ellip-
soidal nuclei which are often pressed close together. All of the cells
of the larva stain more deeply at this pole than elsewhere, and those of
the inner layer rather more deeply than those of the outer. The nu-
clei are also very large, those of the outer layer being possibly more
prominent than those of the inner ; but the difference is not so marked as
in the drawing, where too the nucleoli of the inner layer are represented
relatively too small. Even at this stage one finds in another section
of the same embryo the beginning of a second polypide, whose position
is indicated at *. This second polypide is indicated merely by a con-
siderably thickened inner larval layer, and a very slightly thickened
outer one. The two polypides are thus seen to be wholly independent
of each other. The first invagination further advanced is seen in cross
section of the whole larva in Figure 96. The entire outer layer would
seem at first sight to be involved in this invagination ; but even in this
figure there are seen one or two nuclei which lie under the ocecium at
the place of invagination. I believe that they will not be involved in
it, for at a very little later stage (Fig. 104) one finds a layer of cells
lying over the invaginated bud, which I believe are destined to form the
ectoderm of the body wall at this place.

Later stages in the development of the larva in this species are
not shown. The bud follows, I am confident, the same steps that are
pursued by the bud in the adult colony. A placenta-like connection
of the larva with the ocecium, which was first described by Korotneff
(87, p. 194), begins at about this stage, and continues until two well
formed polypides are present. This “giirtelformige Placenta” begins
to form in about the middle of the young embryo, and the elongated
cell of the outer layer of the larva, in contact with the ocecium shown
on the left of Figure 99 below the *, is, I believe, the first indication
of it. The ocwcium and larva both continue to increase in size, and
the walls of the former become thinner with their increase in area.

79 BULLETIN OF THE

The attachment of the owcium to the body wall of the mother stock
always remains small, as in Figure 99, and the embryo, in my experi-
ence, does not come in contact with it.

The formation of the first polypide in Cristatella is preceded by
another process. Just as in the adult colony the inner layer of the
polypide does not arise by invagination of the ectoderm, but from the
stolonic cells lying at the base of the ectoderm (see Davenport, ’90,
pp. 108, 109, Figs. 4 and 15), so too in the embryo. The first process
then must be the formation of the stolonic cells. Figure 101 shows at
the point marked sto. (which is at the pole of the embryo whence the
inner-layer cells originated) that certain of the cells of the ectoderm ap-
pear to be arching over a disk, containing about six cells in section, and
thus coming in contact with the cylinder of cells (*) which projects
from the neck of the ocecium. By a continuation of this process, the
central disk of cells gradually comes to lie below the general level
of the ectoderm, and to be cut off from contact with the neck of the
ocecium (Fig. 97, sto.). The position of the stolonic mass with refer-
ence to the neck of the polypide in this last figure must be considered
abnormal; it is at any rate exceptional, as it lies at one side of the neck
of the occium, which does not, therefore, appear in this section. The
next later stage which I have found is shown in Figure 102. The sto-
lonic mass seen lying beneath the ectoderm in Figure 97 has here
already given rise to a young polypide (¢., ex.), and its area is increas-
ing in all directions by cell division (sto.). The beginning of a sec-
ond polypide is indicated on the right at sto. The ectoderm is seen
lying above this stolonic mass, and closely applied to the neck of the
ocecium (*).

Neither at this nor at any subsequent stage have I been able to
detect in Cristatella any “giirtelformige Placenta” such as exists in
Plumatella. I am therefore of opinion that the process of nutrition,
which is effected in Plumatella from the ocecium through its placenta,
is effected in the Cristatella larva by its attachment to the neck of the
occium. I am pleased to see that Jullien (’90, pp. 13, 14) has also
reached this conclusion in a paper which he has had the kindness to send
me. At a later stage, the embryo, or young colony, seems to become
detached from its intimate association with the neck of the ocecium, as
we see in Figures 95 and 103.

Figure 103 represents a stage in which there are two well developed
buds, both shown in the section. There is, in addition, on another section,
one less developed. The stolon is seen passing oralward of these twe

MUSEUM OF COMPARATIVE ZOOLOGY. al

primary polypides, or rather the primary and secondary one. Moreover,
as the series of sections shows, the stolon does not exist merely in this
section, but it is a disk which is cut here in one of its diameters, A sepa-
ration of the stolonic mass has occurred between the two oldest polypides,
so that the ectoderm is here in contact with the coelomic epithelium,
just as is the case between buds in the adult stock. As the colony in-
creases, the inner and outer margins of the stolonic tissue continue to
extend farther outward, and this tissue forms at first a broad ring of
ever increasing diameter. Later, as the area of the stock increases, the
ring becomes broken, so that, instead of growing along an infinite number
of radii, its growth is confined to a few, as in the adult colony.

I will defer a discussion of the significance of these facts to the gen-
eral part of this paper.

B. GENERAL CONSIDERATIONS.

I. Laws of Budding.

Carefully conducted studies on stock building have generally revealed,
just as these on Bryozoa have shown, a law in budding. This law in
budding results in the formation of a stock the interrelation of whose
individuals is a determinate one. I now propose to offer an hypothesis
to account for the existence of these laws, and then to show how facts
of budding in Bryozoa and other groups can be explained by means
of it.

And first of all I must acknowledge that this hypothesis, although
perhaps here first formulated, really depends upon observations and de-
ductions made long ago on this group, first by Hatschek, who from 1877
has maintained that individuals do not arise independently of one an-
other, and secondly and mostly to Braem, who in ’88 (pp. 505, 506)
declared of Phylactolemata “dass in dem Stock keine Knospe entsteht,
die nicht auf das embryonale, d. h. den specifischen Leistungen der
K6rperwand noch nicht angepasste Zellmaterial einer ulteren Knospen-
anlage zuriickgienge und dass somit in der ersten Knospe des keimen-
den Statoblasten siimmtliche Knospen des kiinftigen Stockes implicite
enthalten sind.” Not less is the following hypothesis indebted to the
ideas of Roux and Fraisse, and to Nussbaum, who has said (’87, p.
293): “ Ein lebendes Wesen ist somit als Ganzes oder in seinen Theilen
soweit individualisirt und vergiinglich, als die Gewebebildung und die
Theilung der Arbeit vorgeschritten ist ; das Ueberdauern der Einzelexis-

72 BULLETIN OF THE

tenz, die Theilbarkeit auf geschlechtlichem oder ungeschlechtlichem
Wege, spontan oder kiunstlich bedingt, ist an das Vorhandensein undif-
ferenzirter Zellen gebunden und ist um so grosser, je weiter im Organis-
mus diese Zellen verbreitet sind”; and, finally, to the idea which is
implied in the conclusions of Nussbaum (80, pp. 106-113) and Weis-
mann, that germplasma does not find its origin in the parent individuals,
but is merely borne by them in its unbroken passage from generation
to generation.

This hypothesis is simply that there is in every stock of Bryozoa a mass
of indifferent cell material which is derived directly from indifferent cells
of the larva or embryo, and whose function is to form the organs of the va-
rious individuals, including the polypides. This indifferent cell material
lies in the body wall, principally at the growing tip or margin of the
stock. By its growth and differentiation it gives rise to the body wall,
muscles, etc., and at intervals it leaves behind, as a portion detached
from itself, a mass of indifferent cells, which is capable of forming a polyp-
ide, or of becoming a new centre of growth, or of both. Which of these
possibilities will be fulfilled, where and when these masses of indifferent
cells will be left behind, depends upon the necessities of the species, and
the variations in these respects give rise to the peculiar characters of
the different stocks.

This hypothesis differs from that of Braem in that the pre-existence
of a Knospenanlage assumed by Braem is, according to my view, a non-
essential feature in the formation of the colony ; the pre-existence of an
indifferent cell mass, which does not itself constitute buds, but may give
rise to masses which can, is the only essential feature.

As a first application of this hypothesis I refer the reader to the con-
ditions of stock formation in Paludicella, already described. We find at
the tip of the colony a mass of large proliferating cells, which I regard
as histologically undifferentiated. These cells give rise to the body wall,
— the cystid, —and at intervals leave behind three masses of cells, which
I regard, from the fact that they retain their cuboid condition, as well
as from their ultimate fate, as indifferent or embryonic. The median
mass of each of these gives rise to a polypide, and to one only. The
lateral masses form centres of growth similar to the one from which they
were derived.

In order to reproduce the arrangement of individuals in the stock re-
sulting from this manner of budding, we may make use of some graphic
method of representation, as Smitt (’65%, pp. 139, 140) did long ago, and
as Allman (’70), Semper (’77, pp. 67-78), Chun (’88, pp. 1167-1180),

MUSEUM OF COMPARATIVE ZOOLOGY. ihe

Braem (’90, pp. 33 and 44), Ehlers (’90, p. 9), and others, have since
done. I shall represent the mass of indifferent cells by an asterisk,
and individuals (according to Chun’s nomenclature) by the use of the
large and small letters of the Roman alphabet, and, finally, by Greek
letters. The typical stock of Paludicella might then be graphically
represented thus (cf. Plate I. Figs. 2 and 2*) :—

¥
*C*
* *
*abax*
* * *
*b * SAR e282
AR Se *¥ BaaaBx
* xXaaa* SESE Be te
a *xax* oe
(1) 3 D C B A
* *a* x *
* x¥aaax KK KX
Se) 35 * BaaaB *
xb * ** **
* * *
*¥abax
* *
*C
*

Here the letters indicate polypides or their Anlagen, and the asterisks
indifferent tissue. The individuals represented by capital letters may be
called primary individuals ; they may be said to belong to the primary
series, and to have been derived from the primary indifferent mass.
The individuals represented by small Roman letters will then be secon-
dary individuals, belonging to the secondary series and arising from sec-
ondary masses, etc. It is to be observed that this indifferent tissue is
here found only at the tips of branches or An/age of such. No asterisks
are found adjacent to the adult polypides A, B, C, etc., which have
given rise to lateral branches, and these have therefore no power of pro-
ducing new parts of the colony. The asterisks must not be regarded as
having been descended from the letters which they adjoin, but from the
terminal asterisks only ; that is to say, in Paludicella embryonic tissue
has originated from terminal embryonic tissue, and not from indifferent
tissue left remaining alongside of the polypides.

Conditions differing in an interesting manner from these were found
by Braem (’90, pp. 18-32) and myself (Davenport, ’90, pp. 103-106) in
Phylactolemata. In Plumatella Braem has shown in the clearest man-
ner how some of the embryonic tissue around a polypide at the proximal

74. BULLETIN OF THE

end of a nascent branch is carried away to the oral side of the “ mother
polypide,” and lays the foundations of another polypide. In like man-
ner the embryonic tissue around the “ mother polypide ” may give rise
to one or several additional embryonic masses. He has also (pp. 29-
32) shown in the most convincing way that each. mass, particularly in
the case of secondary buds, consists of two parts, of which one goes to
form the polypide; the other contributes to the further growth of the
common cystid and the formation of new embryonic masses. Since
here every embryonic mass is in intimate relation with a polypide, and
since the polypides arise nearly in one plane, only secondarily moving
out from it, the relation of individuals may be expressed by a formula
occupying a single line. Braem has thus expressed it: —

ala Aeil
(2) Dec8Be BB A

According to the system adopted for Paludicella, this may be given
thus :—

(8) *a *a *b *A *a *B *C #

or, more developed, thus : —

(4) *a, *a *B *a *a *b *¥c *¥A *a *8 *b *B xa% C ¥D &

in both of which the right hand asterisk (3) takes the place of the A at
the right of Braem’s diagram. These symbols denote that we have a
mass of indifferent tissue connected with each polypide, or the Anlage of
such ; and this indifferent mass, as well as the adjacent polypide, was
derived from some other indifferent mass. Thus the masses connected
with A, B, C, D are to be regarded as having been cut off from the em-
bryonic mass at the extreme right ; and each of these secondarily gives
rise to the polypide buds a, b, etc., and their embryonic tissue. Thus
we have to do with centrifugal budding only.

In Cristatella the conditions are essentially similar to those in Pluma-
tella, the chief difference being that usually only two polypides with
their embryonic masses arise from each polypide. This condition may
be represented by the formula : —

(5) %a, *8 &B *a *%a #b [xJA 4a Xa ¥b Ue [3]

in which the embryonic masses originally attached to A, B, etc., are
bracketed to indicate that they are normally no longer active in giving

MUSEUM OF COMPARATIVE ZOOLOGY. 75

rise to new polypides. As a matter of fact, the secondary rows often
make a greater or less angle with the primary ones, and as a result
lateral branches are formed. Taking this character into account, the
Cristatella formula might be written : —

*
* b
*Q@, — a |
| [*]
*B — *a — A
|
(6) xb — a aie ere)
eS) =
*

This representation indicates the fact that the first formed buds (A, a, «,
etc.) are lateral ones; the second, median (Davenport, ’90, p. 106).
Intermediate stages between the condition in Plumatella, in which an
indefinite number of polypides and gemmiparous masses can be budded
off from pre-existing gemmiparous masses, and the condition in Crista-
tella, in which only two such arise, occur apparently in some species of
Plumatella, in which, as Braem (90, p. 31) has shown, few polypides
are produced from any gemmiparous mass, and all but two of these gen-
erally do not develop. In the young corms of Cristatella, on the other
hand, more than two polypides may thus arise.

Other Ctenostomata show a regularity in the budding process similar
to that of Paludicella, and exhibit instructive variations upon it.

Victorella, an interesting Ctenostome occurring in slightly brackish
water, and first described by Kent (70) in 1870, possesses, according to
the pregnant observations of Kraepelin (’87, pp. 75, 76, 154-157), a
stolon-like tube, from which at intervals polypide-bearing “ cylindrical
cells” arise. Kraepelin (’87, pp. 155-159) has shown it to be in the
highest degree probable that the protrusion of the body wall in the neck
region of the polypide of Paludicella is the homologue of the “ cylindrical
cells ” of Victorella, and that the remainder of the zocecia of Paludicella
is homologous with the “ stolon” of Victorella. While in Victorella the
cylindrical cell is developed to such an extent that the retracted poly-
pide is still included within it, and the stolon remains of small calibre,
in Paludicella, owing to its shortening, the retracted polypide must seek
refuge in the stolon, whose diameter is consequently increased to receive
it. Evidence for this is found in the stclon-like nature of the youngest
zocecia of a hatching winter bud of Paludicella Ehrenbergii, and in the
elongated cylindrical cell of the adult Palndicella Miilleri, Kraepelin,

76 BULLETIN OF THE

which must be considered a form intermediate between P. Ehrenbergii
and Victorella.

The architecture of the Victorella and Paludicella stocks is, then, sim-
ilar, in that they both consist of a row of individuals successively formed
at a stolonic tip. The resemblance is heightened by the fact that, as in
Paludicella, so also in Victorella, a pair of lateral buds is given off from
each zocecium to form lateral branches (Kraepelin, ’87, p. 157). As in
Paludicella, so also in Victorella, communication plates, Rosettenplatten,
arise early to separate the zoccia from each other. But Victorella
differs from Paludicella in this, that while in the latter the neck of the
polypide does not become the centre of origin of new buds, in the former
it does, just as is the case in Plumatella (Kraepelin, ’87, Plate III.
Fig. 75) ; that is to say, there are laid down from the tip of the branch
three masses of bud-producing tissue, besides that which goes to form
the polypides of the primary branch. The graphic representation of
this species will therefore be more complicated than that of Paludicella,

and has this form : —
\ *

* * *
* * * * *
«bx *Bx ax a*a *Be
* * * * * *
* * *
*a* *a* A*A* a
z * * * *
*
(7)% Dx C¥ax Bx bx ax ax Ax cx* b*¥ a* ax Be ax
*
* RI AS & x
*Ax *A%* A*A a
* * *
* * * * * *
xb x»Bxe ax a *a *Bx
* # * * *
* * *
xa* b *a*
* *
*
*Cx
cd

Compare with (1), page 73, and (4), page 74.
From around each individual of the series A, B, C, etc., which has
been derived from the tissue of the stolon tip, there arise series of

MUSEUM OF COMPARATIVE ZOOLOGY. eh

lateral and of median buds. From around each of the lateral buds, in
like manner, both lateral and median buds of a higher order arise. But
from each of the median buds only median buds arise. These median
buds, are not, however, all of the same kind. The one first produced (a *,
of Formula 7, * of Form, 8) differs from all formed after it (b *, c *,d *,
etc.) in this, that it bears no polypide, but forms the tip of a stolon
from which both median and lateral buds arise (8, a, near extreme right of
Form. 8). From the second and all succeeding median buds (a b, ¢, etc.,
Form. 8), there arise only median buds of a still higher order. Of the
latter, the first, as before, produces no polypide, but becomes the tip of a
stolon giving rise to both median and lateral buds ; the others give rise
to only median buds of a still higher order, and so on. Our former for-
mula assumed that all median buds were alike, and all incapable of giving
rise to lateral individuals. Their dissimilarity introduces a complication,
so that the species must be represented by some such formula as
this?: —

*
*C *
*
* *
*a* b *Q*
* * *
a
* * *
* a, * Bg * ay *
woh x &
*B* a* ay* seask *a, *a *B*
* * * et
* * a, * *@ *
*b* * B *
* *
(3) xe
* *
*#Q* &2 *Q@* a
* * * *
* a xe
*a* * * a, * *
* 3% * ‘ * 0 *
* 2 * * *
De tCxaxttBe be ax thax Axce bettax B* ax HH Bx* hax a, * Xe
* * * *
* x = * aj *
*a* * * a, * *
* a 3
etc. etc.

1 It must be borne in mind that such a graphic representation as this, while it
agrees with the descriptions and figures of Kraepelin and Hincks (’80, Plate 79) so

78 BULLETIN OF THE

in which the heavy asterisks represent the budding tips of the stock,
which give rise to new individuals (tips of the stolons), and a,, B,, ete.
indicate individuals of the fourth order. The lighter asterisks indicate,
as before, points of proliferation from which new buds may arise.

It seems highly probable that Victorella finds near allies in Mimosella
and other genera of the Stolonifera. ;

In Hypophorella expansa, according to Ehlers (’76, pp. 5-9) and
Joyeux-Laffuie (’88, pp. 137-139), the stolon is composed (as in Vic-
torella), of a number of internodes, each separated from the other by
communication plates, and bearing on the distal end typically a feeding
zovid (Nahrthier) and a lateral stolon. It seems to me that the jointed
condition of the stolon is reasonably accounted for in the same way as
that of Victorella, by supposing that each internode, together with its
zocecium, is comparable with the whole indvidual of Paludicella. The
“feeding zodids” of Hypophorella will then be comparable with the
Cylinderzelle of Victorella. Two facts are opposed to this view : first,
the polypide is not formed primarily in the stolon, coming only secon-
darily to lie in the Cylinderzelle as in Victorella ; and, secondly, there is a
Rosettenplatte in Hypophorella between the feeding zodid and the stolon,
while none exists in Victorella. But upon this assumption one can best
account for the fact that the stolon is composed of as many joints as
there are feeding zodids, —a condition which appears to occur in only a
few other genera, and these closely allied to Victorella. Thus, in Cylin-
dreecium pusillum and C. dilatatum of Hincks we have two species which
may be considered to represent two possible intermediate stages between
Victorella and Hypophorella, not only on account of the jointed stolon,
but also on account of the enlarged distal end of the joint, which is emi-
nently characteristic of the allies of Victorella. The first objection, that
the polypide is not developed in the stolon, but first arises in the well
formed zocecium of the feeding zodid, might result from the increased
importance of the zocecium over the Cylinderzelle. The formation of the
plate between the zowcium and the stolon might be accounted for by
the physiological need of such an organ resulting from the increased im-
portance of the zocecium (cf. p. 40). Such plates exist, in fact, between
the primary median individuals, and those secondary median ones in
Victorella which are budded from the Cylinderzelle. This hypothesis

far as they go, may not fit the conditions in all parts of the colony. Moreover, it
is to a certain degree idealized, i. e. subjective, for even in the figure of Kraepelin
(’87, Fig. 75) one of the individuals of the series a, b, c, etc. has given rise to no
stolon as its first bud.

MUSEUM OF COMPARATIVE ZOOLOGY. 79

is further supported by the fact that, as a stolon may arise from the
Cylinderzelle of Victorella, so in Hypophorella such a condition is not
uncommon, although hardly typical. In accordance with this hypothesis
the formula for Hypophorella might be given thus :—

*

c*
* bax

(9) . b * *
* aa* aaB*
* a* * * *

wD = =C B A

Ehlers (’76, pp. 127, 128), in founding the group of Stolonifera, clas-
sified the different methods of arrangement of the individuals in the
colony as follows :—

I. Many polypides (Vdhrthiere) on the single joints of the stolon
(Stengelgliedern).

1. On the entire length of the joints.
(a.) Arranged in two rows.
(b.) Arranged in a spiral.
(c.) Arranged in one row.

2. At the ends of the joints.

(a.) In rows.
(b:) Massed.

II. Only one polypide Nahrthier on a joint of the stolon.
1. Polypide lateral, near it one or many stolonic joints (Hypophorella).
2. Polypide terminal.

In the present state of our knowledge, it is very difficult to say how
the types of budding shown in those Stolonifera which possess more than
one Vdhrthier on a joint of the stolon are related to, or are to be connected
with, the types of Paludicella, Victorella, Hypophorella, or other genera
possessing only one Nahrthier to a joint. This could doubtless be deter-
mined, however, by studying the early stages in the development of the
stocks. Taking them as they are, however, we find a very simple condi-
tion in the stocks of Class I, in which the Ndhrthiere are arranged in a
single row, as in Vestcularia spinosa (cf. Hincks, ’80, Plate 73, Figs. 3—7).
The tip of the stolon consists, as I have myself observed in allied spe-
cies, of somewhat cubical cells of variable thickness, and it is from this
tip that the Anlagen of the individuals arise. Lateral branches occasion-

80 BULLETIN OF THE

ally replace a Vahrthier, and the latter seems never to produce secondary
individuals. The formula of the stock might be written : —

(10) OE ee ee fe

In Bowerbankia pustulosa we have two rows of individuals produced
side by side from near the end of the stolon. This condition would be
represented by

(11) x *« D mae *B «A
oe D #£C, * By A

provided the individuals of this primary series possess the power of giy-

ing rise occasionally to secondary buds, as seems certainly to be the case

in some members of this genus which I have seen. The spiral arrange-

ment of some colonies is striking ; it is of evident advantage to the stock,

but its cause in these cases is wholly unknown.

In every one of these cases, and, in fact, in all of those figured by
Hincks, which belong to the Stolonifera, there is no trace of dichotomy.
Throughout we have to do with linear series, which give rise to lateral
branches.

Turning now from the Stolonifera to the other grand division of
Ctenostomata, the Aleyoniide, we find the same prevalence of a law in
budding. In its typical expression it may be written as follows :—

*
x b *
* * (*) *
*a* * aaa *
* * *
(12) 3% C (*) B (*) A
* @* AAO wie. 00s oe
% * (*) *
* b *
*

Although secondary median individuals are not habitually formed,
yet, owing to the capacity of regeneration possessed by individuals
A, B, ©, ete., an asterisk is affixed in parentheses to show the probable
persistence of embryonic tissue. Of the lateral series one or both may
fail to be developed.

MUSEUM OF COMPARATIVE ZOOLOGY. 81

It might be difficult to determine whether in this group we have to do
with dichotomy, did not the tips of the margins at times reveal the fact
that there is no division of the ancestral series, but that a new one is
added at the side of an ancestral one (Plate VIII. Fig. 69), where of
the marginal individuals 4 is clearly median (ancestral) and 3 is lateral,
13 median and 12 lateral, etc. (see page 49).

The members of the group of Cyclostomata seem to be closely related,
and the method of budding is so similar throughout the group that it
seems fair to interpret the more compact Tubuliporide from the Crisiade.
In Crista, as we have seen, individuals are placed in rows, from which at
intervals lateral rows are given off to the right or left. One may say
that typically these are given off from each individual to both the right
and the left, although in some cases, as in Figure 65%, lateral branches
are typically given off alternately to the right and left, and are often
aborted. Perhaps the most general formula of all for Cyclostomes should
be that of two lateral branches from each individual, one or both of
which may remain undeveloped. Such a formula I believe to be also
the typical one for Bugula and its allies, and for the Flustrina and
Escharina. It would be written thus: —

*
* @ *
* (*) *
*a@ b a*
* * *
*« b *
* «x (x) « *
* * (*) * kop * Ga. a © Be
* @ * *@a aqQq* a * * *
* * *
(13) 3% D (x) C (x) B (x) A
* * *
x a * *aaa* * * * *
* + (+) « *B()a a als) B*
* * (*) « *
Dy s
*
*a b a*
2 (3) 3
AE hy ok
*

in which the parenthesized asterisks indicate the presence of regenera-
tive tissue. This is identical with (12) and similar to (1).
Braem (90, pp. 130-133) has already called attention to the differ-

ence between Phylactolemata and Gymnolemata in the orientation of the
VOL. XXII. — No. 1. 6

82 BULLETIN OF THE

polypide. In Phylactoleemata the oral aspect of the polypide is turned
towards the margin of the corm or the tip of the branching stock ; in
Gymnolzmata, on the contrary, the anal aspect is turned in that direc-
tion. This difference is a very striking and constant one. It is corre-
lated with another difference in the law of budding of the stock, which
will become evident upon comparing Formulas (4) and (5) on page 74,
of Phylactolemata, with Formulas (1) on page 73 and (7) to (13). In
all of these the margin or tip of the stock is at the left, the centre at the
right. In the formule of Phylactolemata the budding is centrifugal,
new individuals being produced from the embryonic masses towards the
margin; in the formule of Gymnolemata budding is centripetal, new
individuals being produced from the embryonic masses towards the
centre. In both Phylactolemata and Gymnolemata the anal aspect is
turned towards the gemmiferous region.

Braem calls attention to one other difference, namely, that, in the case
of the retracted polypide, in Paludicella the rectum lies next the at-
tached surface of the stock; in Phylactoleemata, the oesophagus. <A
mechanical cause of this is suggested when this statement is put in other
words : the polypide in its retracted position is stored in both Phylac-
tolemata and Gymnolemata proximad of the atrial opening ; i. e. away
from the tip or margin, and towards the centre of the stock. May not
this be explained, in part at least, as an adaptation to room?

I will here add four examples of regular budding taken from other
groups of animals, to illustrate the general applicability of this method
of representation. The first of these is that of the Siphonophore Halr-
stemma whose formula has been worked out by Chun (’88, p. 1169), and
expanded and illustrated by Korschelt und Heider in their recent text-
book (p. 39). It runs as follows : —

(14) D e bia ©» da ¢ ba) Bre Gee placa ee
According to my interpretation of the case, this formula might be

written (15) :—

wD xc eb ea *xC 3d ¥c xb xa *B 3 ¥e *d xc *b *a xa #y *B #a. * A,

in which the # behind B has been derived from the embryonic mass at

A, that behind C from B., etc. The 3%’s represent embryonic masses

from which a, b, ¢c, etc. are derived.

If we assume that the terminal individual (A) has not been derived
from the primary embryonic mass, at the extreme left, but has had its

MUSEUM OF COMPARATIVE ZOOLOGY. 83

origin in the embryo, the formula would have to be written somewhat
differently ; namely, thus (16) :—

2% Cz cx be ax Bus dx ce bx ax Aztex de cx bk ak ax y* Be at [A]

In a species of Pennaria, common on our coast, which is probably
Pennaria tiarella, McCready,’ I have noticed the presence of a similar
law of budding. The whole stock lies in one plane, the lateral branches
arising alternately from the right and left of a central stock, like the barbs
of a feather. These lateral branches give rise to a series of secondary
ones, which are all placed on the same (axial) side of the branch. Each
branch, of whatever degree, originates as a bud bearing a polyp. From
the elongating stalk of this terminal polyp, buds arise, — the beginnings
of branches of the next higher order. The stock may be represented by
the following formula : —

A

*

|

dx —

|

e*——

Ph |

ie

a
B x
ce * | |
* | Ppa) act
(17) See h

a*x——
|
bx —
|
cx
*
B

Expressed in a linear series, this formula may be written :—
(18) % Dx Cx Be cx bk ax Ax dex cx bx a* ax B+ a*

which is identical in form with the second formula (16) given for Hali-
stemma.

1 This species is figured by Leidy (55, Plate 10, Figs. 1-5) and Verrill (’73, Plate
XXXVII. Fig. 277). An allied species, P. gibbosa, is figured by Louis Agassiz
in the “ Contributions” (Vol. III., Plate XV. Fig. 1). In describing P. Carolinii,
Weismann (Entstehung der Sexualzellen, p. 122) says that the lateral hydranths
do not possess the capacity of giving rise to new lateral hydranth-buds (of a higher
order). But, as indicated above, P. tiarella seems to do this regularly. Leidy’s
and Verrill’s figures show the same thing.

84 BULLETIN OF THE

Lastly, this formula may be applied to certain cases of fission, as in
fresh water Annelids, As is well known, the fissiparous process is pre-
ceded by the formation of the so-called budding zones (Knospungszone).
These arise in Ctenodrilus, according to Kennel (’82, pp. 403, 404), be-
tween two dissepiments in the middle of a metamere, and new ones are
continually formed behind the others as the animal grows in length
by cell proliferation at the tail end. The budding zones are, according
to Kennel, regions composed of embryonic cells. I think it probable
that this embryonic tissue. has been derived from the embryonic tissue
of the anal end of the animal. There are as many budding zones pro-
duced as there are new metameres added by the anal growth, and since
the budding zones are intrasegmental, each zodid consists of four parts ;
viz. (naming them from anterior to posterior end) of the posterior half
of the preceding budding zone, of the posterior half of the metamere in
which the budding zone arose, of the anterior part of the next follow-
ing metamere, and, finally, of the anterior part of the following budding
zone. Zodids then are made up of parts of two adjacent metameres,
and the middle of each zodid is intersegmental. The zodid has progressed
little beyond the state of possessing two (half) metameres at the time it
becomes free. New metameres must become formed by caudal growth.
The animal is, then, according to my conception of the significance of the
process, derived chiefly from these budding zones. Evidently, the law
of production of new individuals (or new budding zones) is a simple
one, and may be written, in accordance with my nomenclature,

(19) a E (x) * D (x) * C (*) * B(x) * A

in which A, B, C, ete. represent successive individuals (adjacent halves
of two metameres), and the asterisks, as before, embryonic tissue. The
two adjacent asterisks together represent the budding zone, of which
the posterior half (parenthesized) proves itself the least active.

The conditions given by Semper (’77, pp. 69, 77) for Cheetogaster
(and Nais) are much more complicated, but may be expressed by
the use of a formula constructed upon the same plan. Cheetogaster
differs from Ctenodrilus in this: that young budding zones, and event-
ually young individuals, are produced between older ones, instead
of always at the anal end; and the new zodids often acquire several
metameres before becoming free. It seems to me probable that, as in
Ctenodrilus, the budding zones are derived ultimately from the anal
zone ; but here, in contradistinction to Ctenodrilus, new budding zones
may secondarily arise from other budding zones produced earlier, thus

MUSEUM OF COMPARATIVE ZOOLOGY. 85

giving rise to the phenomena of young individuals interpolated between
older ones. Representing, then, individuals by the budding zones from
which they have arisen, we may convert the following formula of Semper
into one based on our own nomenclature :

E D H B G C F A

_—_ _—_— _——_—- __—_ —_" —_—_ _——_—_ Ga_—_

az.—38+0—0 4+0—1+4+0—0 4+4+1—1+4+0—0 44+0—2+0—0 4+1
in which the succession of generations of zodids is
Bese ih! iw 0 ay Pana as ai

In the above formula A, B, C, etc. represent zodids; the numerals
below the letters, the number of metameres of which each is composed ;
0, an incomplete metamere about to be derived from a budding zone ;
az. the anal zone. Written in accordance with my conception of the
facts, this formula would read :

(20) 3 D(x) *C(*x) *a(*) *B(*) *b(#) #a(#) a! (x) # A,

which somewhat resembles Formula (15) of Halistemma, and signifies
that two embryonic masses are left behind by the anal zone, of which
the one anterior to the zodids proper (represented by letters) goes merely
to form the head parts, and is represented parenthesized. The second
is caudad the zodid, and may form a secondary ‘anal zone” giving rise
to new zodids. From one zodid two or more anal zones may take their
origin. Thus, from the embryonic mass caudad of A there have arisen
that caudad of b, which has given rise to b (*) * a (*), and that caudad
of a/, which has given rise to a! (x).1

The most general formula given on page 81 undergoes many modi-
fications in the different groups, but in the midst of these modifications
certain laws are to be discerned, to some of which I have already called
attention. I will now proceed to a discussion of the significance of
these laws.

The quincunx arrangement of individuals, which is so noticeable in

1 From a study of surface views of many specimens of Autolytus collected
at Newport during June, 1891, I am convinced that the sexual individuals are
produced by proliferation of cells in the metamere XIII. or XIV. of “parent
form,” —the last which remains behind after breaking off of sexual form. Rep-

resenting the proliferating metamere by (2), we may write the budding formula
of Autolytus thus:

(208) we E(*) D(*) C(*) B(s) A(x)

in which the parenthesized asterisks indicate the proliferating, but not gemmif-
erating anal metameres of the sexual form. (Cf. A. Agassiz, ’63, pp. 397-400.)

86 BULLETIN OF THE

the phytoid stocks of Bugula, and in creeping corms like Lepralia or
Cristatella, may be explained as affording additional strength on the one
hand, and as a device for saving space on the other.

The absence of true dichotomy, which I have sought to show charac-
terizes the budding of Bryozoa, is interesting as seeming to indicate
the fundamental similarity of the process of budding in Paludicella to
that found elsewhere. The tip of the branch does not divide equally
in the first nor in the other instances, but constantly maintains its
precedence, giving off parts of itself to form lateral branches. These
parts may grow out at right angles to the primary branch, as in Palu-
dicella, but generally they grow forward nearly parallel to it, as in
most marine Gymnolemata.

In Bugula (Plate VII. Fig. 64°) branches are always given off
toward the axils, and therefore an ancestral branch gives off all lateral
branches from one side and the successive orders of branches are given
off alternately to the right and left. In Crisia, on the contrary, branches
are given off abaxially, and they are given off not from one side only,
but alternately to the right and left. In both cases the two facts are
mutually dependent. The first case gives rise to a stock in which the
branching tends as greatly as possible towards compactness and the for-
mation of a closely built stock ; the second case gives rise to a diffuse
and loosely built stock (ef. Figs. 64, 65, and 64°, 65"). In the sec-
ond case there is a maximum space to each individual ; in the first,
a maximum economy of space.

The rule that lateral buds on two closely related branches tend to
arise in the same generation is one that, as has been shown, is more
or less apparent in some cases, but is easily obscured by other rules.
May not the tendency be due to the same causes that produce the
synchronism of division in related cells of a cleaving egg?

That lateral buds should occur in Bugula flabellata on the outermost
rows only is not surprising when we reflect that there is abundant room
on the margin, whereas the inner individuals are hemmed in from lat-
eral expansion by the pressure of adjacent rows. This is very marked in
certain repent colonies, as, for instance, occasionally in Membranipora
(Plate VIII. Fig. 70). Here the intermediate branches 6, 7, 8, and 9
have produced no lateral buds for many generations, while almost every
individual of the marginal rows has given rise to a lateral branch. It is
merely a result of the same cause, it seems to me, that lateral budding
occurs more frequently in Bugula turrita at the margins of fans than
elsewhere. Here there is room to spread.

MUSEUM OF COMPARATIVE ZOOLOGY. 87

The rule (5) that ancestral rows contain fewer generations of indi-
viduals than lateral ones may perhaps receive a partial explanation from
the further fact (rule 6) that of the two rows starting from any axil the
ancestral branch will give rise to a greater total number of individuals
than the lateral one will in the same time. We should expect a less
rapid forward growth if the lateral growth is extremely vigorous. One
might also say that the intermediate rows had grown abnormally in
length, since that is the direction in which there is most room.

The reason why the ancestral branches in Bugula give rise to the
greater total number of individuals is, to my mind, because they are
marginal. In Crisia it is the lateral branches which are the most
prolific, and for the same reason.

The existence of the 7th rule in mat-like species is a mechanical
necessity ; in the phytoid species, like Bugula and Crisia, it must be ac-
counted for on another ground ; namely, on the relations of food supply
to demand, — on the deterrent effects of overcrowding. And this, to my
mind, is the key to the significance of the 4th, 5th, 6th, and 7th rules.
The form of the stock is determined by the same law which has deter-
mined the form of the individuals, — the struggle for existence and the
survival of the fittest, — the fittest in the present case being those which
are most advantageously placed with reference to food supply. Abundant
food supply has made possible the rapid production of lateral individuals
at the margin, and less abundant food supply has retarded such produc-
tion in the middle. Therefore has lateral budding occurred more rapidly
at the margin ; therefore has the number of individuals produced at the-
margin been greatest ; therefore have the median rows grown in length
only with great rapidity ; therefore has the distance between adjacent
rows of individuals in phytoid stocks remained constant.

Many observations on different groups of animals agree in demonstrat-
ing a relation between rapidity of the budding or fission process and food
supply. Thus Zoja (90, pp. 25-27) has shown for Hydra, and Zacha-
rias (86, p. 274) and von Wagner (790, p. 360) for Turbellarians, that
abundant food supply results in an acceleration of the processes of
non-sexual reproduction, and Braem (’90, p. 24) has shown that bud-
ding in Cristatella proceeds less actively during the late fall. This
diminution in activity has been attributed by Braem to diminished
temperature ; but we know also that this period is one of scarcity of the
small fresh water organisms upon which the fresh water Bryozoa live
(cf. Parker, 90, pp. 597-600), and this fact also must be considered as
having an important influence in this case.

88 BULLETIN OF THE

II. Relation of the Observations on Budding in Bryozoa
to the Germ Layer Theory.

No question in Bryozoan morphology has been more thoroughly dis-
cussed than that of the part played by the germ layers in the production
of the polypide, and upon none has there been less agreement. Nitsche
first boldly opened the question, and concluded that we have in this pro-
cess a fatal objection to the idea of the homology of the germ layers, in
so far as their homology depends upon a similarity of fate throughout the
Metazoa. A single layer, the invaginated ectoderm, gives rise to the outer
covering of the tentacles, to the pharynx, and to the brain, — structures
elsewhere considered as ectodermal, — and also to the lining of the ali-
mentary tract, elsewhere universally accounted entodermal. In view of
these facts, “sind die Keimblitter,” concludes Nitsche (75, p. 398), “ kei-
neswegs mit einer besonderen histologischen Pradisposition ausgestattete
Zellschichten, sondern lediglich die flichenhaft ausgebreiteten Elemente,
aus denen die den Metazoenkérper zusammensetzenden, ineinander
geschachtelten Rohren sich bilden.” Prouho, although recognizing the
facts to be as stated by Nitsche, has not discussed the theoretical bearing
of the question. Seeliger (’89°, p. 204) finds in the budding process of
Endoprocta a shortening and confusion of the embryonic process. ‘“ Wie
die gesammte Knospenentwicklung verktirzt ist, erscheinen auch die
beiden Processe der Einstiilpung durch welche im Embryo zuerst Ento-
dermkanal, dann Atrium sich bilden, in einen zusammengezogen.” In
another place (Seeliger, 90, p. 595) the budding process is considered as
an “immer sich erneuerende Gastrulationsvorgang.” Braem (’90, p. 116)
regards the inner layer of the bud as entoderm, and the process of its
formation as one of gastrulation. In a preliminary notice published last
February (Davenport, 91, p. 279) I suggested that the embryonic tissue
from which the inner layer of the polypide arises is to be regarded as
“neither ectoderm nor entoderm, but as still indifferent, and capable of
giving rise to either.” A few weeks ago I saw for the first time the paper
of Oka (’90), in which he offers (p. 145) @ priori a similar suggestion
concerning the significance of the embryonic tissue from which the inner
layer of the polypides arises. I am pleased to find that our ideas, thus
independently arrived at, are so fully in agreement. My idea of the re-
lation of the germ layers to the layers of the polypide bud chiefly grew
out of my studies on the embryology of Phylactolemata as described in
earlier pages.

As there are two layers to the bud, the question of the part taken by

MUSEUM OF COMPARATIVE ZOOLOGY. 89

the germ layers in the polypide bud may be subdivided into two: What
is the significance of the outer layer of the bud? and, What is the sig-
nificance of the inner ?

The outer layer of the bud is derived from the coelomic epithelium.
The views of those who have studied the formation of this inner layer
of the cystid in Phylactolemata may be classed in two categories :
(1) those in which it is regarded as entoderm, and the process of its
formation as gastrulation ; and (2) those in which it is regarded as
mesoderm. ‘To the former class belong the views of Reinhard (’80,
p- 208), Korotneff (’89, p. 403), and Jullien (90, p. 19); to the latter,
those of Kraepelin (86, p. 601) and Braem (’90, p. 116), and in this
class the views of Barrois (’86, p. 68) and Haddon (’83, p. 543), founded
on @ priori considerations, must be placed.

It seems to me that, since, as Barrois has demonstrated, there is a great
similarity between the Phylactolematous and Gymnolematous larve,
and especially since the former show evident signs of degeneration, we
are bound to study the phenomena they exhibit in the light of our
knowledge of the ontogeny of Gymnolemata.

But first it is necessary to give reasons for believing that the larva of
Phylactolemata is to be regarded as homologous with that of Gymnolee-
mata; and to do this I will first name the points of similarity in the two
larvee, and then try to show that the differences which exist are not suf-
ficient to invalidate the attempt to establish a homology. And, first of
all, it may be said that, since the adult Phylactolemata and Gymnol:-
mata are strikingly similar to each other, and since no one doubts their
close relationship, we should expect @ priord that their larvee would be
homologous, especially since the larve of Gymnolemata are admitted to
belong to the trochosphere type, of whose ancient origin there can be
little doubt. In the second place, the very existence of a larval stage
in Phylactolemata is indicative of its inheritance from an earlier condi-
tion, for two reasons: (a) because in general fresh-water life tends to
eliminate larval stages from species which have inherited them from ma-
rine ancestors, and tends little to form them de novo (Hydra, fresh-water
Turbellarians, Rotifera, Oligocheta, Hirudinea, Astacus, and fresh-water
Mollusca); and (4) because, specifically, the early stages of development
of Phylactolemata are pasSed within a uterus-like sac, from which the
embryo is released only when a colony is already well established. In
the third place, the Phylactolematous larva possesses, in common with
all Gymnolematous larvee, the following characteristics. The primary
polypides arise in hoth at a pole, and this pole is in both a prominent disk,

90 BULLETIN OF THE

surrounded by a circular fold, — the so-called mantle fold, — from which
it is separated by a circular groove, — the so-called mantle cavity. This
organ has a similar origin and fate in the two groups, as shown by Barrois.

The following points of difference, however, must be recognized.
First, the absence of a definite ciliated ring, cowronne (Barrois), of an
internal sac, and of a pyriform organ. But, as Barrois (’86, p. 67) has
shown, these are absent, or at least (Ostroumoff, ’87, pp. 182, 183) little
developed, in Cyclostomatous Bryozoa. The ciliated ring and pyriform
organ are doubtless organs connected with a free locomotive larval life,
which is greatly abbreviated in Phylactolemata. A second difference
exists in the fact that, while most Gymnolematous larve possess either,
rarely, (1) a functional alimentary tract, or (2) a mass of loose tissue
lying inside of the ectoderm, the Phylactolemata possess (3) a central
space lined by an epithelium placed next to the ectoderm. However
great the difference between the first and third conditions mentioned
above, it is to a large extent bridged over by the widespread existence of
the second. In some Cyclostomes, moreover, a similar condition to that
in Phylactolemata seems to exist. Compare Metschnikoff (’82, p. 310,
Taf. XX. Fig. 62). Lastly, the origin of two primary polypides, instead
of one, at the aboral pole, upon which Barrois has laid some stress, can-
not be considered a very strong objection to the homology, because in
reality the two polypides do not arise at the same time even in Pluma-
tella, and in Cristatella this difference is still more pronounced. In fact,
it is not the formation of two polypides which requires explanation, but
that of a young stock before hatching.

There remains, therefore, to my mind, no serious objection to regard-
ing the larve of Phylactolemata and Gymnolemata as having been
derived from some common ancestral larva, possessing, of course, more
points of resemblance to the Gymnolematous than to the Phylactolema-
tous type; and therefore it is perfectly justifiable to interpret the latter
by aid of the former.

Admitting the larvee to be homologous, we should expect the process
of gastrulation to be comparable throughout Ectoprocta. As a matter
of fact, we do find a great similarity in the earliest stages. Thus, the
first indication of the inner layer is the ingression of four cells at one
pole, which by multiplication give rise to a-layer of cells lying inside of
the ectoderm.’ It is to the comparative study of the fate of this inner

1 This has been shown for Membranipora (Tendra) by Repiachoff (’78, pp.
416-420) ; for Alcyonidium polyoum by Harmer (’87, pp. 445, 446); for Bugula
by Vigelius (’86, p. 519); and for Cristatella in the present paper (page 68).

MUSEUM OF COMPARATIVE ZOOLOGY. 91

layer in the different Ectoproct larve that we must look for an explana-
tion of the layer in the specific case of the Phylactolemata.

For the purposes of this study, it is desirable to begin with species in
which there has been a minimum amount of degeneration. Such are
Membranipora (Cyphonautes), Alcyonidium, and Flustrella, to which
we must now turn our attention.

The studies of Repiachoff on Membranipora lead up to a stage in which
the entoderm lies as a solid mass inside the ectoderm, and is separated
from it at all points. Neither the origin of the mesoderm nor the forma-
tion of stomodzum or proctodzeum was observed at this time. As for
the fully formed Cyphonautes, it is certain, as I can confirm from personal
observation, that there is a well developed functional alimentary tract,
and that it is provided with a well developed muscular system, including
cross-striped muscle fibres. There is, therefore, every reason for believing
that typical entoderm and mesoderm have been formed in it.

In Aleyonidium (polyoum), Harmer (’87, p. 445) has shown that after
gastrulation a great mass of cells occupies the former blastoccel. This,
in the author’s opinion, represents entoderm and mesoderm. The young
larva possesses a mouth, an cesophagus, and a large stomach, but never
an anus. No evidence is presented that the oral pole corresponds with
the pole of ingression. ’

Flustrella, which is nearly related to the last species, possesses in
its young larval stages a pocket, which Prouho (’90, pp. 424-426) has
shown to represent the anterior part of the alimentary tract, directly
comparable with that of Aleyonidium polyoum, but less developed. Mus
cle fibres and an epithelial lining of the entoderm and ectoderm exist to
indicate the presence of mesodermal tissue.

These three genera, Membranipora, Alcyonidium, and Flustrella, are
the only Ectoprocta in whose larvze the presence of an alimentary tract
has as yet been demonstrated.

In Bugula, a very careful study of which was made by Vigelius (’86
and *88), one finds after gastrulation and cell multiplication a mass of
cells filling the whole interior of the larval body, at first appearing as
an epithelium surrounding a central space, but later without arrangement
and often showing signs of degenerescence. No definite separate meso-
derm could be found, and at no time was any trace of an alimentary tract
to be seen. Vigelius calls the mass derived from the four entodermal
cells Fiillgewebe, and he believes it to correspond morphologically to
both “hypoblast and mesoblast.” It is to be noted, however, as a
point of considerable importance, that in his figures of the metamorphos-

92 BULLETIN OF THE

ing larva Vigelius (’88, Taf. XIX. Fig. 6) represents this tissue as hay-
ing almost entirely disappeared ; that which remains giving rise to the
mesodermal lining —the outer layer of the bud —of the developing
polypide.

There can be no doubt that the so-called oral pole of the Bugula larva
corresponds to the mouth-bearing pole of Alcyonidium, but does it cor-
respond to the pole of ingression of entoderm’? This question has not
been answered by Vigelius. The existence of homopolar stages like that
represented in his (86) Figure 25, Taf. XXVI., makes it very difficult
to establish this doubtful point.

The formation of the inner layer of Cyclostomes has been studied by
Barrois (’82, p. 141). He says: ‘ Des les premiers stades les spheres
vitellines glissent les unes sur les autres de maniere a former une espéce
de gastrula par épibolie et ’on ne tarde pas 4 rencontrer des stades d’un
volume extrémement exigu et déji composés d’une couche exodermique
et d’une masse endodermique libre dans son intérieur. La masse endo-
dermique s’atrophie rapidement et l’on arrive a une petite blastula qui
succéde non pas a un stade composé de cellules radiaires dans lequel
se forme une cayité centrale, mais qui est issu, au contraire, d’une vraie
gastrula née par épibolie dans les premiers stades de la segmentation
et dans laquelle la masse endodermique est déja disparue.” I have
quoted Barrois thus at length, since his description will show forcibly at
least one thing, that the fate of the cells which by ingression had entered
the blastoccel is quite different from that of those in Bugula, where a
great Fiillgewebe is formed. Ostroumoff (87, p. 183), however, has
shown that the inner layer of the Cyclostome larva does not disappear,
but comes to line the ectoderm as a very thin layer. In the adult larva,
however, we find the contents of the ectodermal sac “filled with me-
senchymatous cells, which are commingled with yolk granules and glob-
ules of albumen.” It is these cells that produce the very considerable
mesodermal layer of the first polypide, which arises in the metamorphosis
of the larva. Here, as elsewhere, an apparently homopolar stage inter-
venes between gastrulation and the formation of larval organs, making
orientation difficult.

Thus, passing from Cyphonautes, through Aleyonidium and Flustrella,
Bugula, and finally Cyclostomes, we have a series in all of which the
inner germ layer is derived from one pole by ingression or by ‘ epiboly,”
and in which there is a gradual reduction of the functional entoderm until
it seems, in Cyclostomes, to be lost, and a gradual transformation of the
mesoderm from a cell mass nearly filling the larva, and producing muscles

MUSEUM OF COMPARATIVE ZOOLOGY. 93

and a lining to the body wall and alimentary tract, to a single thin cell
layer lying next to the ectoderm, or to mesenchymatous cells extending
through the ccelom.

This same series may be said, also, to be one in which there is a
gradual decline in the complexity of larval organs. These find their
maximum development in the bivalve Cyphonautes and Flustrella, and
the complicated and beautiful Alcyonidium larva. They find their mini-
mum development in the Cyclostomes, whose larve, instead of a girdle of
flagella, possess merely an undifferentiated clothing of cilia, are reduced
to a cylindrical or ellipsoidal form, lack the pyriform organ of other spe-
cies, aud in some cases possess only the rudiment of the internal sac.

If we were to imagine still another term at the degraded end of the
series, it would be a form in which the four inner-layer cells that arise
by ingression at one pole of the larva should give rise to little or abso-
lutely no entoderm, in which the mesoderm should come to form an
inner lining to the ectoderm, and in which the internal sac should be
entirely absent. It is just these conditions which are fulfilled by the
Phylactoleematous larva.

Of all these changes, the loss of the entoderm is the most striking.
What can be said in explanation of it? I would suggest this hypoth-
esis: that the entoderm of the Bryozoan larva has become rudimentary
through loss of the alimentary function.

In direct support of this hypothesis I have little experimental evidence
to offer. One observation, however, which I made last summer, seems
to favor this conclusion strongly. This is that larval life is of consider-
able duration in Cyphonautes, which possesses a functional alimentary
tract, but is very brief in Bugula, in which no alimentary tract arises.
As is well known, Cyphonautes occurs in enormous numbers in the
“tow ” at certain seasons of the year, and this is alone evidence of a con-
siderable length of life. I have taken Cyphonautes thus obtained from
the tow and have kept them for three or four days, at the end of which
time they died, or had settled to the bottom of the glass vessel to un-
dergo their metamorphosis. In fact, from several hundred Cyphonautes
which I collected, not more than half a dozen completed their full meta-
morphosis, the others apparently succumbing to unfavorable conditions.1

1 Just as the manuscript of this paper is going to the printer, after long delay
caused by an accident necessitating the re-engraving of the plates, I find that Dr.
Prouho read last summer (’90), before the Association Francaise pour |’ Avancement
de la Science, a preliminary communication on the development of Cyphonautes.
This is published in the printed report of the proceedings of that association. The

94 BULLETIN OF THE

The Bugula larvee, on the contrary, I have never found in the tow, but
they swarm out from stocks gathered in the morning and placed in a
glass vessel ; and I can confirm Nitsche’s (’69, p. 9) observation that they
settle and begin their metamorphosis within “a few hours” after hatch-
ing. One rarely or never finds these larve succumbing to the unfavor-
able conditions of the aquarium before metamorphosing. From these
observations I conclude that the Bugula larva has a very much shorter
life than Cyphonautes. Now, since the larva, owing to its shortened life,
has no need of functional entoderm, and since entoderm can be of use to
the larva only, no part of it going over into the tissues of the primary
polypide of the stock (except as food material), functional entoderm is
not developed. In other genera, its rudiments have become less and
less important in the ontogeny, and, finally, in Phylactolemata are
wholly lost.

That the entoderm should reach its last stage of degeneration in
Phylactolemata is easily understood when we consider that the larval
period is passed in a closed ocecium, from the wall or neck of which it
receives nourishment as a parasite does. Moreover, by the delay in the
period of hatching, as well as by precocious development of polypides, one
at least of the latter is usually functional in the just hatched stock, for
there is sometimes found at least one polypide in the newly hatched
larva, which is partly extruded, and therefore capable of feeding, and
thus of supplying the whole stock with nutriment. Of what advantage
to a species could be the development of a functional larval entoderm,
which should go to form no part of its adult tissue, provided the larva
was contained in a uterus during its early stages, and was provided with
the adult digestive organs in a functional condition before leaving the
uterus 4

Those who maintain that the inner layer is to be regarded as entoderm,
and are still unwilling to place the Bryozoa among the Coelenterata, must
account for the absence of mesoderm. Korotneff (’89, p. 400) finds de-
generating cells in the blastoccel before this is wholly obliterated by the
extension of the inner layer. These he seems to regard as degenerate
mesoderm. According to his view, then, the entoderm gives rise to
the muscularis, — for this arises from the inner larval layer, according

author does not there state whether stomodeum and proctodeum are formed on
the blastoporic side of the larva. He accounts for the existence of an alimentary
tract in Cyphonautes by the fact that it undergoes its development disconnected
with the parent, while almost all other Bryozoa pass their early stages in the
parent or some protecting zodid (ocecium, ovisac, ovicell).

MUSEUM OF COMPARATIVE ZOOLOGY. 95

to Braem’s (’90, Taf. VII. Fig. 89 mb.) observations, which I can abun-
dantly confirm, — and to the celomic epithelium of the adult stock. In
the few series of sections of the proper stage which I possess, I have not
found with certainty the degenerating cells of which Korotneff speaks ;
but even if they regularly occur, I should be inclined to regard them as
the degenerated entoderm, the mesoderm persisting to give rise to the
muscular tissue and the ccelomic epithelium.

From a consideration of these facts, —that the larve are homol-
ogous and the process of gastrulation is comparable throughout the
Ectoprocta, that in the least modified larvee both functional entoderm
and mesoderm are produced by that gastrulation, that one of these
two germ layers has become rudimentary in Phylactolemata, that
it is highly probable that the entoderm has disappeared from loss of
function, and that the layer which persists gives rise to the muscula-
ture, sexual cells, and ccelomic epithelium, —I conclude that the cnner
layer of the Phylactolematous larva, and therefore the outer layer of the
bud, ts mesoderm.

If we accept the point of view of Kleinenberg (86, pp. 1-19) and ad-
mit the existence in general of only two layers, ectoderm and entoderm,
a clearer conception of the modification undergone by the Phylactoleema-
tous larva may be gained. We may divide the entoderm arising in
Bryozoa*into. two parts; viz. (1) that which gives rise to the lining
of the midgut, as in Cyphonautes, and (2) all the rest of the inner
layer. Now, since no midgut is formed in the Phylactoleematous larva,
part (1) of the entoderm has ceased to be differentiated ; all which
remains, then, is part (2); but this is equivalent to “mesoderm” in
the sense in which I have employed it, and therefore I am justified
in saying that “mesoderm” only is produced.

The question has now to be answered, What is the significance of the
inner layer of the bud? Two different answers have been given to this ques-
tion. It has been maintained, on the one hand, that it is to be regarded
as ectoderm ; on the other, as entoderm. There are serious difficulties
in the way of accepting the first view, — so serious, in fact, that few
authors have maintained it, although at first glance it seems to be re-
quired by the facts. Although we have not yet sufficient grounds for
declaring that organs formed by budding must be built up from the
same germ layers as corresponding larval ones, — although we may ad-
mit that gemmigenesis recapitulates phylogeny and corresponds with
ontogeny only in an imperfect and confused way, — still, from the expe-
rience gained by tracing the development of hundreds of animals from

96 BULLETIN OF THE

the most widely separated groups of the animal kingdom, the idea that
a functional alimentary tract is ever wholly derived from differentiated
ectoderm will not be accepted by most embryologists without conclusive
evidence.

The second view is that the formation of the inner layer of the bud
is a process of gastrulation, giving rise to entoderm, and that the so-
called ‘ gastrulation ” of the sexual ontogeny of Phylactoleemata is to be
regarded as a precocious ingression of mesoderm only.

Two considerations are opposed to this view. In Membranipora there
is a gastrulation which gives rise to the entoderm and mesoderm of the
larva; and since the gastrulation of Phylactolaemata is similar, these
elements must be potentially present here also. The “ gastrulation”
in Bryozoa is a normal one; if there is any entoderm in the body wall
giving rise to the inner layer of the bud, it must have been ento-
derm which failed to become inyaginated. But what, in the second
place, is to be gained by assuming that the inner layer of the bud is
formed from entoderm? Here is as great a difficulty as before, since the
nervous system originates from this layer. It has been maintained in
many cases that the nervous system arises from mesoderm, and Seeliger
(789, p. 602) believes that it is formed from that layer in the non-sexual
reproduction of some Tunicates; but I know of no good evidence of its
origin in any of the Triploblastica from entoderm. ‘

Before going on to state my conception of the significance of the
inner polypide layer, I desire to call attention to the conditions in the
region at which it is first formed. I have shown above (page 69) that
the primary polypide or polypides arise from the pole of ingression in
Phylactolemata, and that therefore in this group the aboral pole (in the
sense of Barrois) corresponds to the pole of ingression. As I under-
stand Barrois, he means by oral pole merely the pole which in Cypho-
nautes, for instance, bears the mouth, —the pole also by which the
larva attaches itself. Braem (’90, p. 123, foot-note), however, interprets
‘oral side’ in Barrois’s sense to mean in the last instance the place at
which gastrulation takes place. Perhaps Barrois does somewhere state
such to be the significance of his term (I have not found the place),
but in that case I can only say that, to my mind, he has not produced
sufficient evidence to prove that the oral pole of the larva of Gymno-
lzemata is the same as the pole of ingression in the gastrula ; nor, in my
opinion, has any other investigator done so. Nearly all species studied
have a stage early in their development when their poles are very sim-
ilar, and orientation certainly would be exceedingly difficult. One of the

MUSEUM OF COMPARATIVE ZOOLOGY. 97

best figured series in which to trace the homology of poles is that shown
by Repiachoff (’80, Taf III.) for Bowerbankia. So far as the figures
go, one would conclude that Figure 10 A and its predecessors were
oriented in the opposite direction to Figure 11 and its successors,
whicn would result in placing the pole of ingression (Fig. 9) at the
aboral pole of the larva, — the pole which here, as in all ather Gymno-
lemata, and, I believe, in Phylactolemata also, gives rise to the primary
polypide. I have given above additional evidence for this conclusion,
in my argument to prove the homology of the larve and larval organs in
Phylactolemata and Gymnolemata.

The polypides arise in Phylactolemata at the pole of ingression, which
ts probably homologous with the aboral pole of Gymnolemata. The pole
of ingression, or the region of the lips of the blastopore, must be regarded
as being a region of less pronounced differentiation than the rest of the
gastrula. Its cells cannot be said to be either ectodermal or ento-
dermal. It is an interesting fact, that it is just these indifferent cells
—not yet either ectoderm or entoderm—that give rise to the inner
layer of the polypide, from which organs usually considered ectodermal
as well as those considered entodermal arise.

My conclusion, then, the objections to which I fully realize, may be
stated in the following words: The inner layer of the polypide bud is
composed of cells derived from the rim of the blastopore. Such cells are to be
regarded as still indifferent, and as first becoming differentiated into ecto-
derm and entoderm in the formation of the young polypide.

Just when and where, on this hypothesis, the differentiation into
ectoderm and entoderm occurs, is an important question ; but unfor-
tunately I cannot answer it decisively. It may be pointed out, however,
that it has now been shown for most Ectoprocta that the lining of the
middle part of the alimentary tract is formed independently of the
cesophagus, and by an actual or potential outpocketing of the primitive
simple sac of the bud. In Endoprocta there is a similar outpocketing,
which, however, arises in connection with the cesophagus, and is formed
independently of the rectum.

This is perhaps the proper place to call attention to the fact that the
mesodermal outer layer of the bud has a very embryonic character at
the budding region. This is indicated by the fact, that in Phylactolemata
(in which group alone I have studied the subject) eggs always arise
from that part of the ccelomic epithelium which lies in the budding
region (cf. Plate XI. Fig. 93). In Pyrosoma, also, according to the
researches of Seeliger (’89, pp. 598-602) the mesoderm of the budding

VOL. XXII. — NO. 1. 7

98 BULLETIN OF THE

region, the stolon, gives rise to eggs. The same condition seems to exist
in other Tunicates.

III. On some Characteristics of Gemmiparous Tissue.

In the preceding part of this paper the words “embryonic tissue,”
“undifferentiated tissue,” have often recurred, and they are terms in
wide usage in modern zodlogy. I do not know of any attempt to define
further the real character of this tissue, nor to give its more detailed
characteristics, other than that usually employed in the term plasma-
reich, or “‘rich in plasma.’’ The persistence of yolk granules is, as
Nussbaum (’80, pp. 2-14) and Goette (’75, pp. 31, 32, 831) have shown
in the case of amphibian embryos, indicative of the embryonic condition
of cells, when these have been derived from an egg filled with yolk.

It is very far from my purpose to go into a detailed discussion of the
significance of embryonic tissue, for which I am not yet fitted ; neverthe-
less, I wish to call attention to the minuter characters of gemmiparous
tissue as I have found it in Phylactolemata and Paludicella. I have
described it in some detail in preceding pages.

First, then, gemmiparous tissue seems to stain more deeply than non-
gemmiparous tissue in the same section. This character has been re-
peatedly observed before by others, and Braem calls attention to it
several times. I have already described how I found, by the use of
high powers, that much of this depth of stain was due to the unusually
large number of deeply staining granules scattered through the cell, but
chiefly gathered about the nucleus (Figs. 6, 17, 18, etc.). So marked
is the greater depth of the stain around the nuclei, that, with a power so
low that the nuclei are hardly distinguishable, their position is indicated
by a deeply staining band.

Secondly, gemmiparous tissue, as I have found it in the cases referred
to, is distinguished by the possession of large cells, nuclei, and nucleoli.
I had already noticed this fact in my studies on budding in Cristatella,
and I find that Braem has figured the nuclei in the budding region as
larger than the average (cf. Braem, ’90, Taf. VII. Figs. 86, 88-90).
My own figures show this repeatedly (Plate I. Figs. 3, 4, 5, 6, Plate ITI.
Figs. 15, 17, Plate XI. Fig. 99, etc.). I have also noticed this to a
certain extent in the marine Bryozoa, but, since the cells of the latter
are smaller, and as I did not succeed in obtaining from them sections so
satisfactorily stained, the results are not so reliable. In attempting to
obtain an explanation of this phenomenon one involuntarily recalls to

MUSEUM OF COMPARATIVE ZOOLOGY. 99

mind the condition in young egg cells, where the nucleus attains a rel-
atively enormous size. This great size of the nucleus in young egg cells
is explained by Korschelt (’89, p. 92) as due to its participation in the
trophic activity of the cell: “Sein grésster Umfang fallt in die Zeit des
energischen Wachsthums der Eizelle.”” So in the gemmiparous regions
the large size of the nuclei must be considered as connected with the
growth of the cells.

But if the growth of the cells is accompanied by a rapid ingestion of
food material (which the larger nucleus implies), some evidence of that
fact should be observed in the cells themselves in the presence of food
granules. Such food material in rapidly growing ovarian egg cells lies
near the nucleus, Stuhlmann (’87, pp. 13, 14) describes such a condi-
tion in the ovary of Zoarces. ‘ Neben dem Keimblaschen, jedoch ein
klein wenig von seiner Membran entfernt, bilden sich an verschiedenen
Stellen jetzt eigentiimliche Verdichtungen des Protoplasmas, die sich
ein wenig stirker mit Saffranin fiarben als das Zellplasma.” Such a
thickening of the protoplasma is represented in the figures as minute
granules. Korschelt (’89, pp. 123-125) mentions several other such
instances.

It has seemed to me possible to interpret the stainable granules lying
near to the nucleus in gemmiparous tissue as such food material,’ par-
ticularly since we know that food material does exist in the coelomic
epithelium lying next to the cells which are about to divide rapidly and
to give rise to the inner layer of the polypide. That food is being taken
in by the inner layer cells from the ceelomic epithelium is indicated by
the fact that the nuclei of the former cells lie near the latter epithelium
(cf. Figs. 15, 17, 18, 28, 56, etc.) ; for, as Korschelt has shown, the nu-
cleus tends to move towards the centre of activity of the cell. That these

1 Granules similar to these appear to exist in the protoplasm of all cells. It
is their extraordinary abundance in the gemmiparous tissue upon which I lay
stress. They have been variously interpreted by different authors. Biitschli (’88,
pp. 1469-1472) describes various kinds of stainable granules in Ciliata which are
food products, and the general character of which accords with that of the gran-
ules referred to above. ‘Excretion granules ” of Ciliata do not stain, according
to this author, which is an indication that the bodies in gemmiparous tissue are not
such. Iam particularly struck by the fact that the food products of Protozoa are
chiefly found in parasitic forms, — Gregarinide and parasitic Ciliata. These take
up food in solution from their hosts exactly as the cells of the body wall of Bryo-
zoa do from the body cavity. Altmann (’90) has recently interpreted similar
deeply staining granules in other cells, as “die Elementarorganismen.” I can
see no reason, on Altmann’s theory, for the peculiar distribution of the granules
that I have found.

100 BULLETIN OF THE

granules observed in the cells are food material is indicated by their
abundance in cells lying next to the reticulated cells of the ccelomic
epithelium (Figs. 6, 28, 56).

My conclusion, then, is this: Gemmiparous tissue is a rapidly as-
similating tissue, possessing large nucler because actively assimilating, and
staining deeply because full of food material.

While for Nussbaum, as already quoted (page 71), “ indifferent cells”
are essential to the reproduction of individuals by non-sexual as well as
by sexual methods, Seeliger (790, p. 596) has concluded that ‘ die Vor-
viinge bei der Knospung der Bryozoen uns zeigen, wie histologisch sehr
bestimmt differenzirte Gewebe einen ganz embryonalen Charakter wie-
dergewinnen kénnen. Mehr noch als bei der normalen Knospung am
freien Stockende ist dieses Vermégen bei der Regeneration der Polypide
der Ektoprokten oder der Képfchen der Pedicellinen ausgebildet. In
diesen Fallen sehen wir ein plasmaarmes, dusserest feines Plattenepithel,
das iiber sich eine miichtige Cuticula ausgeschieden hat, sich in kubische
und cylindrische plasmareiche Zellen zuriickverwandeln und durch eine
Einstiilpung ein neues Polypid bilden, in welchem schliesslich die man-
nigfachsten Gewebsformen vertreten sind.”

It seems to me that many facts in the budding of Bryozoa are strongly
in favor of Nussbaum’s hypothesis. On this assumption, we can best
understand why in Cristatella there is not an invagination of the ecto-
derm, and why instead a stolon is formed in the embryo, which passes
along at the base of the ectoderm and at intervals gives rise to the
inner layer of the body wall. I believe it is becanse the outer layer
of the body becomes so rapidly differentiated by the secretion of the

1 Other observers describe gemmiparous tissue as being either rich in food or
deenly staining. Seeliger (’85, p. 588) speaks thus of the mesodermal gemmiparous
tissue in Salpa: “ Die einzelnen Zellen sind grossblasig, enthalten einen runden
Kern und fiihren Oel- und Fettsubstanzen die als Reservematerial beim Aufbau
des embryonalen Leibes weiterhin in Verwendung gelangen.” Von Wagner (’90,
p- 377) says of the indifferent cells which are being transformed into the new
pharynx of dividing Microstoma: ‘‘ Dieselben nehmen an Grosse zu, . . . indem
gleichzeitig ihre SLES feinkOrnig granulirt und fiir Farbstoffe imbibi-
tionsfahiger werden.”

In some sections of gemmules of Esperella fibrexilis, H. V. Wilson, bE which
Dr. Wilson has very kindly sent me several slides, I find the outer layer of young
gemmules, in which the inner layer has been newly formed, stained very deeply.
Observed with a Zeiss Apochr. 4.0 mm., Ocs. 8 and 12, the cell contents are seen to
be evidently of two kinds, —light and deeply stained. The latter appearance is
due, in part at least, to small dark granules, which can be discerned without much
difficulty.

MUSEUM OF COMPARATIVE ZOOLOGY. 101

gelatinous balls in its cells as to be incapacitated for the work of build-
ing organs. In Plumatella the outer layer of the body wall, which is
derived, as Braem has shown, from the neck of the older polypide, re-
tains for a long time its embryonic condition, so that its deeper cells can
and do go to form the inner layer of the polypide bud.

On Nussbaum’s hypothesis we can best understand why in Paludicella
the Anlagen of the lateral branches exist from the beginning as cuboidal
cells, quite different from those of the rest of the body wall; we can
understand why the cell layers of the margins of the stock, the tips of
branches, and the ends of stolons from which buds arise, are thicker
and more rapidly dividing than the rest of the body wall (cf. Figs. 14,
71, 73, 75); and we can also understand why the regenerating buds
always arise from the region of the neck of the degenerated polypide,
—the same region from which that degenerate polypide had arisen by
budding.

There is no doubt, however, that at times buds do arise from tissue
which, as Seeliger says, has lost its enboidal nature only to regain it.
From such tissue apparently the polypide of Figure 79 has arisen;
from such tissue certainly, as Seeliger says, do regenerating polypides
arise. But is the process by which cuboidal cells become a pavement
epithelium one of so fundamental differentiation that, in accordance
with Nussbaum’s doctrine, we should not expect, under favorable condi-
tions, to see these cells regain their cuboidal form? No doubt we have
many other cases in the animal kingdom in which flat epithelial cells
regain their cuboidal form. Thus, for instance, among the Bryozoa,
Oka (90, p. 132) has shown how the flat cells of the outer layer of the
statoblast begin to thicken again at the return of warmth, and at the
beginning of the active assimilative processes, not only at the pole from
which the primary polypide is to arise, but also opposite to this.

Many facts indicate that cells may become flattened epithelia, and yet
not lose their embryonic character. Maas (’90, pp. 541-544) has re-
cently shown step by step how the columnar ectoderm of the fresh water
sponge is forced, on account of the great increase in surface which it is
called upon quickly to cover, to become broad and flat. It finally gives
rise to an epithelinm so flat that its existence was long overlooked, and
has been denied by so competent an observer as Goette ; and yet in its
flattened condition it possesses to a remarkable degree the capacity of
sending out pseudopodia-like processes, a condition indicative much less
of a high degree of differentiation or specialization, than of an unspecial-
ized, primitive or embryonic condition.

102 BULLETIN OF THE

I have already stated (page 65) that the region from which the re-
generating buds of Cheilostomes arise, although one of flattened epithe-
lium, is one in which many more nuclei persist than elsewhere in the
adult (cf. again Fig. 71). This fact, coupled with the constancy of
position of regenerating buds with reference to the degenerated polypide,
is to my mind evidence against the assertion that buds arise here from
“ histologically very definitely differentiated tissue.”

As for regeneration in Endoprocta, no one is more competent to speak
than Seeliger himself. JI am the more surprised, therefore, to find that
in Ascopodaria macropus, which is quite closely allied to the species
studied by Seeliger (’89), the cells at the part of the stalk immediately
below the ‘‘ head,” from which regenerated buds arise, are, as Ehlers’s
magnificent Pedicellina work shows, very large and cuboidal (Ehlers,
90, Taf. UL. Figs. 26-33). I think one may conclude that a similar
condition obtains in some cases in Pedicellina, even judging from Seeli-
ger’s own drawings, although they are drawn to a scale that is not quite
large enough to allow of settling this point (Seeliger, 89, Taf. X. Fig.
35, a, Fig. 41, etc.).

If the increase in size of the flattened cells, and their subsequent rapid
division and invagination to form a bud, are due to their more active nour-
ishment, it would be difficult to see why certain cells of any region should
quickly undergo this modification, while the adjacent cells apparently
as favorably situated with reference to the acquirement of food retain
their flattened, quiescent condition, if we assumed such favorable situation
to be the only requisite. Still less satisfactorily would such an assump-
tion explain the regular position of regenerating buds. It is taking only
one step farther back, but, to my mind, a helpful step, to assert that cell
proliferation in any region which produces invagination depends upon
the capacity of the cells of that region to become better nourished than
their fellows. This may evidently be effected by a diminution in the
feeding capacity of the surrounding cells, or by an increase in this respect
in the growing cells.

IV. Relationships of Endoprocta and Ectoprocta.

I discussed this topic in my earlier paper (Davenport, ’90, pp. 132,
133). I have only to add, that later studies have confirmed my
opinion of Nitsche’s correctness in placing these two groups close to-
gether, and in regarding the Endoprocta as nearer the ancestral types.
The stages of Figures 25 (Plate III.) and 77 (Plate IX.) probably rep-

9

MUSEUM OF COMPARATIVE ZOOLOGY. 103

resent roughly a phylogenetic stage ancestral to both groups of Bryozoa,
but most clearly allied to adult Endoprocta. ‘The formation of new ten-
tacles anteriorly and posteriorly in Figure 77, would reproduce the adult
Endoproct condition. Two changes lead to the Ectoproct stage: first,
the closure of the tentacular corona posteriorly 7m front of the anus (Plate
V. Fig. 43), and, secondly, the formation of the pharynx or anterior
part of the cesophagus by the growth of the oral tentacles over the floor
of the atrium towards the atrial opening. Thus the brain, which lies at
the floor of the atrium in Ectoprocta, comes to lie on the pharynx. The
pharynx would, upon this assumption, be a new structure, not found in
Endoprocta. Such appearances as are exhibited by Figure 77 lead me
to retract my former expressed opinion, in which I agreed with Nitsche in
saying that the earliest condition of the tentacular corona is a U-shaped
one. Rather, the tentacles are formed first on each side of the atrium,
and only secondarily grow around the mouth in front, as later they grow
in between mouth and anus. The U-shaped stage is therefore not the
primary one, but secondary.

The close relationship of Endoprocta and Ectoprocta has recently been
doubted by Cori (’90, p. 16), but his chief argument depends upon the
dissimilarity of the Endoproct and Ectoproct kidney. Unfortunately,
our knowledge of the latter is still very imperfect, and we may well hope
for renewed researches in the subject by this skilful investigator.

Ehlers (’90, pp. 149-154) has recently re-expressed his former (’76, p.
132) utterances concerning the lack of homology between the tentacles
of Ectoprocta and the “ cirri” of Endoprocta. He finds the homologue
of the latter in the “ Diaphragma” or “ Kragen ” of Ectoprocta. This is
the organ which I have believed to be homologous with Kraepelin’s
“ Randwulst” (which may be Anglicized as marginal thickening), —an
organ occurring in all Ectoprocta. It is nothing but the “neck of the
polypide,” which has sunk below the general level of the body wall. It
is always provided with sphincter muscles, and in Ctenostomes forms the
base of insertion of the cylindrical or comb-like “collare setosum.” It
ean hardly be that Ehlers refers to this latter structure by the term
“Kragen,” since this is merely cuticular. In my opinion the “ Dia-
phragma” of Nitsche cannot be homologized with the cirri of Endo-
procta, because it is merely a part of the body wall comparable to that
part from which the “ polypide” of Endoproctous Bryozoa arises, and
beneath which the tentacles or “cirri” arise. This part of the body wall
is provided with a sphincter in Endoprocta as well as Ectoprocta, and by
it the atrial cavity may in both cases be closed.

104 BULLETIN OF THE

To my mind, the most significant difference between the two groups
exists in the fact that the outpocketing to form the stomach arises from
the oral end of the future alimentary tract in Endoprocta, and from the
anal end in Ectoprocta. One is led to believe that in the ancestral form
either two nearly equally important outpocketings from both the oral
and anal sides existed, or that the two existing methods are remnants
of a method different from either (such as the formation of the whole
alimentary tract at once), or, finally, that the Endoproct condition repre-
sents the ancestral one, and that the rectal evagination has secondarily
become of greater importance in Ectoprocta, and that the oral evagina-
tion has become less significant. Oka (90, pp. 134, 141) has recently
asserted that in the polypide buds of the statoblast and adult colony of
a Pectinatella of Japan (P. gelatinosa) the cesophagus and stomach are
formed by one evagination, which acquires secondary connection with
the rectum. This condition reminds one, then, of Endoprocta. I must,
however, doubt the accuracy of Oka’s conclusions until more satisfactory
evidence is forthcoming; the more so, since Pectinatella magnifica,
Leidy, presents a method of budding exactly comparable to that in
Cristatella and Plumatella, as my own sections show with sufficient
clearness.

The homology of the Ectoprocta and Endoprocta implies a homology
of their larvee, and demands that the life history of the two groups should
be directly comparable.

It is well known from the researches of Hatschek (77) on Pedicellina,
and of Harmer (’85) on Loxosoma, that the surface of the larva which
bears the mouth and anus, i.e. its oral side, corresponds with that of the
blastopore. How, then, is the oral aspect of the Ectoproct larvee, which
I have tried to show is opposite to the pole of the blastopore, to be
homologized with this?

The month and anus of the Endoproct larva undergo a rotation after
the larva has settled, so that they come to occupy the pole opposite to
that at which the blastopore was. This stage of the Endoproct larva
is comparable to the whole larval stage of Ectoprocta. I believe the
two stages to be homologous, and that, just as polypides are pre-
cociously formed upon the Phylactolematous larva, its larval digestive
tract having dropped out from the ontogeny, so the mouth and anus of
Gymnolemata are precociously formed on the pole opposite the blasto-
pore, the primitive stage during which they existed at the blastoporic
pole having dropped out of the ontogeny.

It is well known from the works of Harmer (86) and Seeliger (89),

Ut

MUSEUM OF COMPARATIVE ZOOLOGY. 10d

that in Pedicellina, in which the metamorphosis of the larva has been best
studied, the stolon arises from the base of the stalk — that is, at the pole
where mouth and anus were first formed — at the pole of invagination.
I have shown that this is true for Phylactolemata, and probably for
Gymnolemata.

If the interpretation which I have put on Gymnolzmatous ontogeny
becomes confirmed, the larvee and the budding areas will be homologous
throughout all Bryozoa. The following diagrams will explain my idea
of the relation of the different ontogenetic stages in the two groups.

ENDOPROCTA. EcToPpRocTvA.

The left hand vertical series represents stages in the development of
Endoprocta ; the right hand one, stages of Ectoprocta. The blastopore (*)
is throughout turned upwards in the figures. Stage I. is in both cases a
young gastrula. Stage IT. is that of the free-swimming larva of Endo-
procta. This stage is lost in the ontogeny of Ectoprocta, in which, by °
abbreviation of larval life, the free-swimming stage corresponds to the
condition of the fixed Endoproct after it has undergone its rotation.
This stage, or one slightly later, is shown in III. Both larve are fixed,
the Endoproct by the blastoporic, the Ectoproct by the opposite pole.
The position of the stolon, or of the first polypide of the colony produced
_ by non-sexual methods, is represented at gm., near the blastoporic pole.

106 BULLETIN OF THE

Summary.

The following general scheme of the budding process in Ectoprocta,
derived from my own and other recent studies, may be now drawn up.
The references are to pages of this paper.

All Ectoprocta build stocks or corms. The individuals in these are
arranged in rows radiating from a centre, — the larva or statoblast, —
and are placed one in front of another (Figs. 2, 64", 65%, 67, 71%, etc.).

New rows or branches are constantly being produced peripherally.
There is no dichotomy in the branching (page 86), but the ancestral or
median branch gives rise to one or more lateral branches, which in turn
become median branches of their part of the stock.

The body wall and polypides of the median branch, as well as the
Anlagen of lateral branches, arise from a pre-existing mass of embry-
onic tissue, the gemmiparous mass (pages 72-82). This may exist cen-
trally of the forming region, as in Phylactolemata, or peripherally, as in
Gymnolemata.

The anal aspect of the polypide is turned towards the gemmiparous
mass (page 82).

The outer layer of the body wall in the budding region is one
of rapidly assimilating and rapidly dividing tissue ; the inner layer of
the body wall becomes filled with food taken from the body cavity in
species in which the latter is early cut off by a partition (Paludicella,
Bowerbankia, Lepralia ?) ; it shows no tendency to do so in species with
a ccenoceel (Phylactoleemata, Aleyonidium).

The first impulse to the formation of the polypide is found in the
outer layer of the body wall (excepting when this is highly modified, as
in Cristatella), and many cells seem to be involved in its formation from
the beginning (pages 8, 56).

This outer layer of the body wall is embryonic tissue, derived from the
tip of the stock (margin of the corm) as in Gymnolemata, or from the
neck of pre-existing polypides, as in Phylactolemata. It is the direct de-
scendant of the gemmiparous tissue of the larva, which in turn has been
derived from the region around the blastopore, —in Phylactoleemata cer-
tainly, in Gymnolemata probably (pages 8, 11, 12, 69).

The inner layer of the body wall is also embryonic in the budding
region, as indicated by the fact that ova arise near the neck of the
polypide, in Phylactolemata at least (page 68).

The outer mural layer becomes the inner bud layer by invagination,
with or without the formation of a cavity. In the former case (many

MUSEUM OF COMPARATIVE ZOOLOGY. 107

Gymnolemata) the mouth of the invagination pocket rapidly closes to
give rise to the neck of the polypide (page 56). In the latter case, the
cavity of the bud arises only secondarily by a separation of its walls
(page 18).

By a rapid growth of the walls of the bud, its distal part, in which the
alimentary tract is to arise, is formed. Since this rapid growth occurs
earlier at the anal side than at the oral, the rectum is formed first, the
stomach last (pages 19, 57).

By an approximation of the lateral walls, alimentary tract and atrio-
pharyngeal cavity become separated.

The cesophagus arises as a pocket of the atrio-pharyngeal cavity, and
secondarily unites with the stomach (pages 19, 58).

The lophophore arises first as two lateral thickenings of the atrio-
pharyngeal wall, then as two lateral folds, whose cavity becomes the ring
canal (pages 20, 58),

Tentacles appear on the ridge of the lophophorie fold thus established,
and like it are formed first at the sides of the polypide, then anteriorly
and posteriorly (pages 22, 59).

The posterior end of the lophophoric ridge is the last to be formed,
and, in forming, it cuts off the anal part of the atrium from the inter-
tentacular cavity (pages 23, 62).

‘The compressed intertentacular cavity becomes circular by change in
position of the oral tentacles (pages 24, 62).

The ganglion arises as a depression in the floor of the intertentacular
room, and becomes included in the pharynx, which is differentiated by
the change in position of the oral tentacles (pages 26, 61).

Muscles and funiculi arise from the ccelomic epithelium of both the
body wall and the bud (pages 27-31, 63).

The neck of the polypide may sink to a considerable distance below
the general level of the body forming the “ Randwulst” of Phylactole-
mata or “ Diaphragma ” of Gymnolemata (pages 31, 63, 103).

The atrial opening first arises at a late period by separation of the
cells of the neck.

The communication plate arises in Paludicella as a circular fold
of the layers of the body wall, the mesodermal cells at the centre of
which become cuticularized. It is not so completely closed as to pre-
vent communication between the celomata of the two individuals it
separates.

The mesodermal cells of Paludicella become stored with food mate-

108 BULLETIN OF THE

rial before the formation of the communication plate, and yield it up to
the rapidly growing bud.

The regenerated polypides, like the marginal ones, arise in Cheilo-
stomes in a definite position, — on the wall of the operculum from tissue
left behind to give rise to the polypide, but not wholly used up in its
formation. They arise wholly from the body wall, come to lie next to
the “ brown body,” and cause its disintegration.

The more important theoretical conclusions to which I have arrived
are :—

a. There is in every stock or corm of Bryozoa a mass of indifferent
cell material, which is derived directly from the indifferent cells of the
larva or embryo, and whose function is to form the organs of the different
individuals, including the polypides. This mass by constant growth and
division affords the embryonic material for lateral branches.

6. The form of the stock and interrelation of individuals is in large
part controlled by food supply.

c. The inner layer of the Phylactolematous larva represents meso-
derm only: the entoderm has become rudimentary through loss of the
alimentary function.

d. The polypides arise in Phylactolemata at the pole of ingression,
which is probably homologous with the aboral pole of Gymnolzmata.

e. The inner layer of the polypide bud is composed of cells derived
from the rim of the blastopore, and they are to be regarded as still
indifferent, and as first becoming differentiated into ectoderm and ento-
derm in the formation of the young polypide.

jf. Gemmiparous tissue is a rapidly assimilating tissue possessing
large nuclei because actively assimilating, and staining deeply because
full of food material.

g. The Endoproct and Ectoproct larvee are to be compared by assum-
ing that the act of rotation of the axes occurring in the former has been
leaped over in the ontogeny, the mouth and anus arising at once on the
pole opposite the blastopore.

Campripce, Mass., June 1, 1891.

MUSEUM OF COMPARATIVE ZOOLOGY. 109

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VOL. XXII.— No. 1. 8

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f ven
nae a
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SR One De .
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5 |

Davenport. — Budding in Bryozoa,

Fig.
Fig.

Fig.

PLATE. EF.
ABBREVIATIONS.
cev. pyd. Neck of polypide. gn. Ganglion.
cta. Normal cuticula of adult i. Inner layer of bud.
body wall. kmp’drm. Kamptoderm.
cta.’ Cuticula secreted by tip. ms’drm. | Mesoderm.
ec’drm. Ectoderm. mu. pyr. Pyramidal muscles.
ex. Outer layer of bud. @. Césophagus (pharynx).
ga. Stomach. rt. Rectum.
gm. Bud. ta. Tentacle.

All figures are of Paludicella Ehrenbergii.

1. Stock of Paludicella Ehrenbergii, viewed as an opaque object. X 4.5.

Diagram representing the interrelations of individuals in stock shown in
Figure 1. A-H are individuals of the ancestral (median) branch;
a, b,c, ete., lateral branches given off from the ancestral branch to the
right; a’, b’, branches given off to the left ; a, B, etc., lateral branches
of second order given off in the direction of the distal end of the
ancestral branch; a’, B’, etc., given off in the direction of proximal end;
a,’, lateral branches of third order — to left.

ig. 28. Diagram of another (smaller) stock. Letters have same significance as

in foregoing.

3. Cross section of branch near tip, showing the first trace of the bud of the
polypide at er., 7. 635.

4, Cross section of branch near tip, showing bud of polypide slightly older
than in Figure 3. X 635.

5 Cross section of slightly collapsed branch near tip, showing ingression
of cells at ex. to form inner layer of bud. 635,

6. Longitudinal section of tip of branch to show cell structure. Zeiss, ;
oil immersion, Oc. 1. > 1000.

Figs. 7, 8,9. Optical sections (nearly in sagittal plane) of three tips of branches

in successive stages of development, showing relations of young bud,
gm., to next older polypide. In Figure 8 the branch is slightly
shrunken. X 87.

Pit

msdrm.

Beisel Jith Boston.

Davenport. — Budding in Bryozoa,

PLATE II.
ABBREVIATIONS.
cev. pyd. Neck of polypide. ec’drm. Ectoderm.
cta. Normal cuticula of body gm. l. Anlage of lateral bud.
wall. kmpdrm. Kamptoderm.
cta.’ Cuticula secreted by tip. ms’drm. Mesoderm.

All Figures from preparations of Paludicella Ehrenbergii.

Fig. 10. Surface view of cuticula near the end of a branch at intervals, a being
nearest the tip, and d farthest from it. The branch was stained in
Erlich’s hematoxylin, the color being taken up by superficial cuticula
only. X 820.

Figs. 11, 12, 13. Cross sections of the cuticula taken at different distances from
the tip, to show the stainable and non-stainable cuticule. Figure 11
is from near the tip, Figure 13 farthest from it. >< 1000.

Fig. 14. Longitudinal median (sagittal) section through the tip of a branch show-
ing cells of tip and an early stage in the development of the polypide.
x 410.

Fig. 15. Cross section of branch showing origin of lateral bud. X 636.

Fig. 16. Longitudinal section of body wall of branch through the point at which
a lateral bud is originating. Polypide of ancestral branch is nearly
adult. > 636.

Fig. 17. Longitudinal section of body wall from near the tip through the Anlage
of alateral bud. X 410.

Fig. 18. Cross section of branch showing histological conditions of Anlage of
lateral bud. The polypide has reached a stage of development cor-
responding to that of Figure 36, Plate IV. 1000.

Fig. 19. Longitudinal section through body wall from the same branch as Figure
17, but farther from the tip. Histological conditions are to be com-
pared with those of Figure 17, which represents a less differentiated
condition. X 410.

Fig. 20. Cross section of branch in which the polypide has reached a stage slightly
younger than that of Figure 386. To show An/age of two lateral buds
with their cuboidal undifferentiated cells. Xx 410.

|
|

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k
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c
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DAVENPORT

DAVENPORT. — Budding in Bryozoa.

PLATE III.

ABBREVIATIONS.

An. Anal side of polypide. kmp’drm. Kamptoderm.

atr. Atrium. loph. Lophophore.

cev. pyd. Neck of polypide. ms'drm. Mesoderm.

cl. mu. ret. Young cells of retractor mu. par. Parietal muscle.
muscle. e. (Esophagus.

cta. Cuticula. Or. Oral side of polypide.

ec’drm. Ectoderm. rt. Rectum.

ga. Stomach. vlv. cr. Cardiac valve.

gn. Ganglion.

All figures from preparations of Paludicella Ehrenbergii.

Tigs. 21-25. Longitudinal sections through buds of polypides at successively older

stages. The tip of the colony, and therefore the anal aspect of the
polypide, is to the right in all cases. All figures < 410.

Stage of Figure 37 (Plate 1V.). Few nuclei in central region.

Shows rapid growth of bud, chiefly at neck of polypide. ‘The two inner
cell layers are about to separate to form the common cavity of atrium
and esophagus.

Beginning of formation of alimentary tract at rectum, 7t. The row of
nuclei separating the atrio-cesophageal cavity from the alimentary tract
is due to the fusion of the two inner layers of the bud along this line.

Rectum and stomach completed. Retractor muscles begin to form.

Lophophore and young tentacles have made their appearance, and
cesophagus and pharynx are separated from atrium. Beginning of
formation of brain at gn.

Part of cross section of a branch of stage of Figure 30. Parietal mus-
cles, mu. par., occupy a diameter of the section, and are attached to
the cuticula. < 635.

Young parietal muscle at stage of Figure 28. This is one of the pair
which in a later stage are found lying together in Figure 26. 635.

Cross section of branch showing young polypide, and reticulated vacuo-
lated cells. 410.

Bit of body wall, with cuticula separated from underlying ectoderm to
show ends of parietal muscles. X 690.

DAVENPORT. — BUDDING IN BRYOZOA. PLIIL.

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PLATE IV.
ABBREVIATIONS.

An. Anal side of polypide. i. Inner layer of bud.

an. Anus. loph. Lophophore.

atr. Atrium. ms’drm. _Mesoderm.

can.erc. Ring canal. mu. Muscle fibre in funiculus.
ec’drm. Ectoderm. n’ Circumesophageal nerve.
ex. Outer layer of bud. @. (Esophagus.
Jun. inf. — Inferior funiculus. Or. Oral side of polypide.
Jun. sup. Superior funiculus. rt. Rectum.

ga. Stomach. vac. Vacuole.

gn. Ganglion. vlv. er. Cardiac valve.

All figures from preparations of Paludicella Ehrenbergii.

Fig. 30. Cross section of polypide bud of stage of Figure 24, Piate III. The posi-
tion is indicated by the line 30, Figure 24. > 410.

Figs. 31-34. Four cross sections of a branch through a young polypide, some-
what younger than that of Figure 25. Figure 31 is nearer the anal,
Figure 34 nearer the oral surface. In Figure 34 that part only of the
section of the polypide which lies near the body wall is represented.
x 410.

Fig. 35. Cross section of branch through polypide of age of Figure 25. To show
origin of tentacles and ring canal. X 410.

Fig. 36. Sagittal section of young polypide at period of closure of ganglion, gn.
x 410.

Fig. 36. Bit of same polypide a few sections to one side of plane of Figure 36,
showing origin of inferior funiculus. X 410.

Fig. 37. From cross section of branch showing early stage in development of the
bud. x 410.

Fig. 38. From a sagittal section of nearly adult polypide, showing the two funiculi
and their muscles. X 410.

Figs. 89 and 40. Two neighboring sections parallel to the body wall through a bud
of the stage of Figure 23. Figure 40 lies three sections below Fig-
ure 39. Figure 39 shows the atrial cavity, formed as yet only on the
anal side. Figure 40 shows the beginning of formation of the ali-
mentary tract at the anal end. Note the vacuolated condition of the
mesoderm. X 410.

Fig. 41. Polypide of about the stage of Figure 25 looked at en face. The anal
tentacles, being turned under, do not appear. ‘To show compressed
condition of polypide, and alternating position of tentacles. Cf.
Figure 77, Plate IX. X 3820.

DAVENPORT — BUDDING IN BRYOZOA.

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PLATE V.
ABBREVIATIONS.
an. Anus. kmp’drm. Kamptoderm.
cev. pyd. Neck of the polypide. loph. Lophophore.
clr. set. | Collare setosum. ms’drm. Mesoderm.
cta. Normal cuticula of adult mu. par. Parietal muscles.
body wall. mu. pyr. Pyramidal muscles.
cla.! Cuticula secreted by the tip of. atr. Atrial opening. *
of branch. rt. Rectum.
ec’drm. Ectoderm. spht. Sphincter.

All figures from preparations of Paludicella Ehrenbergii.

Fig. 42. Cross section of branch of age of Figure 387, Plate 1V., to show origin of
primary parietal muscles. > 410.

Figs. 48 and 44. Successive sections through a polypide slightly older than that
of Figure 25, cut perpendicularly to the long axis of the branch. Dur-
ing this period the lophophore becomes more nearly circular, and its
aboral ends meet oralwards of the rectum, 7t. Figure 44 is nearer the
tip of the branch. X 410.

Fig. 45. Axial section of neck and atrial opening of polypide just sufficiently devel-
oped to be capable of extrusion. Shows the collare setosum in place.
x 410.

Fig. 46. Section of communication plate cut across the branch. Two sections
(10 w) above Figure 51. x 635.

Figs. 47-49. Three stages in the development of the communication plates. Lon-
gitudinal sections of the branch. In Figure 47, the polypide has
reached the stage of Figure 22; in Figure 48, the stage of Figure 23;
and in Figure 49, the stage of Figure 24. x 635.

Fig. 50. Longitudinal section through neck of young polypide, showing the sink-
ing of the neck below the general surface of the body, and the method
of forming the inner cuticula of neck. X 3890.

Fig. 51. Cross section of branch through communication plate. The left side
of the section includes the cuticula and the underlying flat ectodermal
layer. The right side cuts a little lower into the mesodermal cells.
X 635.

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Fig. 53.
Fig. 54.
Fig. 55.

Fig. 56.

Fig. 57.
Fig. 58.

Fig. 59.

— Budding in Bryozoa.

PLATE: VEZ
ABBREVIATIONS.

Anus. i. Inner layer of bud.
Ring canal. kmp’drm. WKamptoderm.
Neck of the polypide. la. comn. Communication plate.
Reticulated cells. ms’drm. | Mesoderm.
Normal cuticula of adult mu. par. Parietal muscles.

body wall. mu. pyr. Pyramidal muscles.
Cuticula secreted by tip. nv Circumeesophageal nerve.
Ecetoderm. a. (Esophagus.
Outer layer of bud. rt. Rectum.
Bud. vac. Vacuole.
ganglion.

All figures from preparations of Paludicella Ehrenbergii.

Cross section of a branch through a polypide slightly older than that
shown in Figure 36. The section passes through the brain and whole
extent of the ring canal, together with its opening into the ccelom.
x 635.

Next section below Figure 52 of same series ; showing the beginning of the
circumcesophageal nerve ring. X 6969.

Shows connection of mesodermal cells of body wall, ms’drm, with those
of the outer layer of bud, er. X 1080.

Origin of the secondary parietal muscle cells from mesoderm of body
wall. X 635.

Histological conditions of the budding regions. The cells have large nu-
clei, the mesodermal cells are vacuolated and rapidly dividing; the
cells of the bud are densely granular. Zeiss, ;'; oil immersion, Oc. 1.
x 1070.

Normal vacuolated cell, full of food particles. > 10380.

Longitudinal section of young lateral branch, showing highly reticulated
character of mesoderm, and nearly complete formation of communi-
cation plate. X 410.

Reticulated cell, showing, one of the pseudopodia-like processes which
frequently appear on them, projecting into the celom. X 1030.

Figs. 60-62. Three successive sections from a series across the tentacles of a pol-

Fig. 63.

ypide which has 15 tentacles, and is of about the stage of Figure 36.
The odd tentacle (*) is shorter than the others, and lies opposite the
rectum, rt. XX 296.

Cross section of branch through neck of polypide of about the age of Fig-
ure 36. Shows also the young pyramidal muscles. > 410.

DING IN BRYOZOA.

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DAvENPORT. — Budding in Bryozoa.

Fig.

Fig

Fig

Fig.
Fig.

64.

PLATE VII.

For explanation of notation employed on this plate, see page 41.

Outline drawing of one of the lateral “ fans’

?

of Bugula turrita, taken

from the axis of the colony and spread out flat on the slide.

< casl2:

. Diagram showing arrangement of individuals in Figure 64.

Outline drawing of one of the lateral branches of a stock of Crisia eburnea,

spread out flat on the slide.

Part of stock of Bugula flabellata.

x 16.

x 10.

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Davenport. — Budding in Bryozoa.

Op.

PLATE VIII.

ABBREVIATIONS.

Operculum. pyd. rgn. Regenerated polypide.

pyd. dyn. Degenerated polypide.

Fig. 67.
Fig. 68.

Fig. 714.

Diagram to show interrelation of individuals in the corm, Figure 69.

A part of a corm of Membranipora pilosa, to show regular arrangement,
with a single median branch, each of whose individuals gives rise to
two lateral branches. The * indicates margin of frond on which
stock was growing. X ca. 8.

Young corm of Flustrella hispida, to show arrangement of individuals.
x 10.

Young corm of Membranipora pilosa, with several median branches, show-
ing regular arrangement. The marginal ones alone give rise to lateral
branches. X 10.

Young corm of Lepralia Pallasiana, showing arrangement of individuals.
On the left, the nuclei of the cells of the body wall are shown, to
indicate the inequality of their distribution. On the right, nuclei are
omitted. At pyd. rgn. a regenerating polypide is seen, on the opercu-
lum. X 45.

Plan of Figure 71.

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DAVENPORT. — BUDDING IN BRYOZOA. PL.VIIL.

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DAVENPORT. — Budding in Bryozoa.

PLATE IX.

ABBREVIATIONS.
An. Anal side of polypide. ~ lu. gm. Lumen of bud.
atr. Atrium. marg. Margin of corm.
can. cre. Ring canal. ms'drm. Mesoderm.
cev. pyd. Neck of the polypide. n’ Circumeesophageal nerve.
cla. Cuticula. @. Cisophagus.
ec’drm. Ectoderm. Or. Oral side of polypide.
ex. Outer layer of bud. rt. Rectum.
ga. Stomach. sep. Wall of zocecium in the corm.
gm Bud. sol. Sole of the corm.
gn. Ganglion. tet. Roof of the corm.
7. Inner layer of bud.
Fig. 72. Longitudinal vertical section through the peripheral part of the corm of
Lepralia Pallasiana, showing the margin of the corm and two zoecia,
the older of which contains a polypide. > 160.
Fig. 73. Longitudinal vertical section through the margin of a corm of Lepralia
Pallasiana, showing the two layers of this region and the origin of the
polypide. x 410.
Fig. 74. Young regenerating polypide of Flustrella hispida. The section passes
through the sagittal plane. > 380.
Fig. 75. Vertical section through margin of corm of Flustrella hispida, to show
origin of polypide. x 410.
Fig. 76. Sagittal section through young polypide of Flustrella hispida, to show
early stage of development of alimentary tract. X 410.
Fig. 77. Superficial view of young polypide from upper surface of corm of Flus-
trella hispida, showing young tentacles and their relation to the anus
(at atr.). X 820.
Fig. 78. Bud of polypide of Flustrella hispida at the time of closure of the pore of
invagination. x 390.
Fig. 79. Radial section through margin of corm of Flustrella hispida, showing bud
of polypide. » 410.
Fig. 80. Young polypide of Flustrella hispida. 880.
Fig. 81. Bud of Lepralia Pallasiana immediately before the formation of alimen-
tary tract, showing relation of the rectal pocket (7t.) to the atrio-
pharyngeal cavity above. X 410.
Fig. 82. Section through polypide, through lately formed brain and circum-

cesophageal nerves (n.’) growing around esophagus (@.). X 410.

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Davenport. — Budding in Bryozoa.

PLATE X.
ABBREVIATIONS.

An. Anal side of polypide. kmp’drm. Kamptoderm.

an. Anus. lu. gn. Lumen of the ganglion.

atr. Atrium. ms’drm. Mesoderm.

can. ere. Ring canal. mu. Musculature of esophagus.

cev. pyd. Neck of polypide. mu.ret. Retractor muscle of polypide.

ce. Cecum. @. (Esophagus.

eta, Cuticula. op. Operculum.

di’sep. Wall of zocecium in the corm. Or. Oral side of polypide.

ec'drm. Ectoderm. or. Mouth.

ex. Outer layer of bud. pyd. dgn. Degenerated polypide, “ brown

Sun. Funiculus. body.”

ga. Stomach. rt. Rectum.

gn. Ganglion. ta. Tentacle.

Ue Inner layer of bud.

Fig. 83. Sagittal section through young polypide of Escharella variabilis. X 820.

Fig. 84. Regenerated polypide of Lepralia Pallasiana on operculum (op.). > 3880.

Fig. 85. Cross section of pharynx of adult polypide of /scharella variabilis, show-
ing perforated cell walls. 635.

Fig. 86. Sagittal section of young polypide of Lepralia Pallasiana, showing forma-
tion of brain. X 3820.

Fig. 87. Section parallel to sole of a corm of Escharella variabilis at about the stage
of Figure 86, showing atrium, ganglion, and rectum. X 430.

Fig. 88. Vertical section through a bit of roof of corm of Lscharella variabilis at
neck of polypide, showing also the region of future operculum and of
origin of future regenerated buds. Compare with Figure 90. X 410.

Fig. 89. Sagittal section of young regenerated polypide of Flustrella hispida inter-
mediate in age between Figures 86 and 83. Shows the origin of the
ganglion and rotation of the oral tentacles. X 820.

Fig. 90. Vertical section of a bit of body wall from same individual as Figure 88,
to show the comparatively less embryonic condition of cells here than
at neck of polypide. > 410.

Fig. 91. Operculum of Lepralia Pallasiana cut perpendicularly to surface, show-
ing origin of aregenerating polypide. Body wall somewhat shrunken
from cuticula. > 410.

Fig. 92. Section through a regenerated polypide of Escharella variabilis, showing

relations of alimentary tract to “brown body” (pyd. dgn.). X 410.

DAVENPORT. — BUDDING IN BRYOZOA.

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DavenPoRT. — Budding in Bryozoa.

PLATE XI.

ABBREVIATIONS.
cev. ow. Neck of ocwcium. ms’drm. Mesoderm.
cel. Ceelom. ow. Occium.
ec’drm. Ectoderme ov’ Ooblasts.
en’drm. Entoderm. pyd. Polypide.
ex. Outer layer of bud. sto. Stolon.
2. Inner layer of bud. tet. Roof of stock.

lu. gm. Lumen of the bud.

Fig. 93.

Fig, 94.

Fig. 95.

Fig. 96.

Fig. 97.

Fig. 98.

A portion of a longitudinal section through a young stock of Plumatella
polymorpha, about two weeks after hatching from statoblast (killed
12th May, 1890), showing the body wall just analward of the neck of
a young polypide (pyd.), at the oral side of which a younger bud has
already arisen. The inner (mesodermal) layer of the body wall
shows odblasts (ov.’) in various stages of development. > 600.

Longitudinal section of ocecium of Cristatella showing embryo which is
giving rise to the coelomic epithelium by ingression of cells at its
proximal pole, —i. e. the pole nearest the neck of the oecium. There
are in the next section two other cells in the cavity of the blastula,
one of which appears degenerate in that it contains a huge vacuole,
and has no distinct nucleus, the chromatic substance lying scattered
loose near the cell wall. X 600.

Longitudinal section through occium of Cristatella and its contained em-
bryo. One polypide bud and the stolon (sto.) are shown here. There
are two other buds in the embryo further developed than this one,
lying to one side of it, and on the side of each of these buds is the
Anlage of another. The stolon is seen to be well developed, lying be-
tween the ectoderm and mesoderm throughout the region bounded
by the three older buds, and extending as a zone beyond them, and
even beyond the An/age of the youngest polypides. The embryonic
tissue thus forms a disk about 75 & 150 win extent. X 890.

Transverse section of ocecium of Plumatel/a, showing origin of first pol-
ypide. Compare with Figure 99, which represents an earlier stage.
x 390.

Longitudinal section through ocwcium and contained embryo of Cristatella.
The stolon is already cut off from the ectoderm. This stage imme-
diately follows that of Figure 101, Plate XII. The forming bud is
that of the first polypide. X 890.

Oblique section through owcium of Plumatella, showing a later stage in
development of the inner layer of the larva (cf. Fig. 94). > 600.
Longitudinal section of occium and contained larva of Plumatella. The
bud shown at 7., ex. is the first in the colony. An incipient (second)
bud is shown five sections to one side in the region indicated by an

asterisk. X 410.

PLXI.

DAVENPORT. — BUDDING IN BRYOZOA.

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Davenport, — Budding in Bryozoa,

PLATE XII

ABBREVIATIONS.
ec’drm. Ectoderm. ms’drm. Mesoderm.
ex. Outer layer of bud. ow. Occium.
v Inner layer of bud. sto. Stolon.

lu. gm. Lumen of the bud.

Fig. 100. Longitudinal section of a larva of Plumatella polymorpha, in which the
two layers are established; the pole of ingression is directed upward,
on the plate.

Fig. 101. Section of upper part of zocecium of Cristatella mucedo, with its contained
larva. Showing the formation of the stolon at the pole of ingres-
sion and the attachment of this pole to the placenta-like neck of the
oecium (*). x 390.

Fig. 102. Section through an occium of Cristatella, with its contained larva. One
polypide is already established, and a second is arising. The two
are the only buds in the larva. On the left of the older bud the
stolon is seen to be intruding itself between the ectoderm and meso-
derm of the larva. X 390.

Fig. 103. Section through the two oldest polypides of the Cristatella larva, to-
gether with the stolon. This larva contains one other less developed
bud at one side of these two. X 390.

Fig. 104. Plumatella polymorpha. Stage of first bud later than that shown in
Figure 96, exhibiting pore of invagination closed by overgrowth of
ectoderm. X 390.

DAVENPORT. — BUDDING IN BRYozoa. Pr xi

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No. 2. — The Gastrulation of Aurelia flavidula, Pér. & Les.
By Frank Smiru.!

PRECEDING the appearance of Goette’s (’87) publication in 1887 upon
the development of Aurelia aurita and Cotylorhiza tuberculata, the yas-
trulation of Aurelia had been regarded, in the light of the studies of
Kowalewsky, Haeckel, Claus, and others, as the result of invagination or
at least of a process nearer to invagination than to any other method of
gastrulation.

Gvette’s work seemed to show, however, that, instead of an invagina-
tion, there is an ingression of cells to form the entoderm, and that the
first result of this ingression is the production of a solid gastrula, or
sterrogastrula, which is only subsequently hollowed out, and is put into
communication with the exterior through the formation of a prostoma
at a still later period. Recently, in a paper dealing especially with the
development of Cotylorhiza tuberculata, Claus (’90) reaffirms the posi-
tion taken in his previous paper (’83), in which the gastrulation in
Aurelia was represented as being simply a modification of invagination.
In recent papers by Hamann (’90) and McMurrich (’91), Goette’s views
are adopted, and form part of the basis for statements that, in the devel-
opment of the Scyphomeduse, invagination, instead of being the rule, is
the exception.

This want of agreement among those who have given the subject
most attention makes the determination of the actual method of gastru-
lation in Aurelia a matter of considerable interest, and it may be
assumed that any contribution to the solution of the question will not
be unwelcome.

Early in the current year, at the suggestion of Dr. E. L. Mark, I
undertook to investigate the method of gastrulation in A. flavidula.

Through the kindness of Mr. B. H. Van Vleck of the Boston Society
of Natural History, I was enabled to spend two months of the summer
of 1887 at his seaside Laboratory at Annisquam, Mass., where I then
collected the material used in the present study. The embryos were
killed with picro-nitric acid, and preserved in 90 per cent alcohol, in

which they have been kept during the three intervening years. Of the

1 Contributions from the Zodlogical Laboratory of the Museum of Comparative

Zoology, under the direction of E. L. Mark, No. XXIX.
VOL. XXII. — NO. 2.

116 BULLETIN OF THE

various staining fluids tried, Erlich’s acid haematoxylin gave decidedly
the best results for sections. For examination of the whole embryos,
Grenacher’s alcoholic borax-carmine and Czokor’s alum-cochineal each
gave good results. The latter stain possesses the peculiarity of stain-
ing embryos of different ages with corresponding degrees of intensity,
the youngest stages being stained the least, the degree of intensity in-
creasing with the age of the embryo up to the planula stage.

The result of segmentation is a one-layered blastosphere, as in
A. aurita. Although the diameter of the blastoceel, or segmentation
cavity, presents some individual variations at a given stage of develop-
ment, it in general corresponds very nearly with that of A. aurita, as
described by Goette (’87, p. 3). It increases slightly as the process of
gastrulation advances. ‘The cells of the blastosphere are usually some-
what shorter at one pole than elsewhere, and it is from this region that
the entoderm is formed. The nuclei of all the cells are situated very
near the outer surface of the blastosphere. Small spheroidal bodies con-
stitute the greater portion of each cell; they are very evenly distributed
through its substance, except in the vicinity of the nucleus, where they
are somewhat jess abundant. Vacuoles of variable sizes are usually
found in some of the cells. The nuclear region stains a little more
deeply than the remaining portion.

The method of gastrulation in A. flavidula is similar to that in A.
aurita as described by Claus (’83, pp. 2 and 3), although it resembles
even more closely a typical invagination. When the process of cleavage
has resulted in the formation of a blastosphere composed of somewhat
more than four hundred cells, a depression of limited extent appears in
the portion of the wall which is composed of the shorter cells. From this
depressed region is formed the entoderm, which develops as a single con-
tinuous layer of cells surrounding a small cavity, the coelenteron. At the
beginning of the process, and throughout its duration, the coelenteron is
in communication with the exterior by means of a narrow passage, the
blastopore, or blastoporic canal. See also Explanation of Figures (Plate
I. Figs. 1-4). From these figures it is apparent that only a small por-
tion of the wall of the blastosphere is concerned in the invagination,
and to that extent it must be regarded as deviating from the typical
invagination, where one half of the wall of the blastosphere is infolded
to form the entoderm. The colenteron is, however, at all stages of gas-
trulation, an open sac-like cavity, and therefore noticeably different from
that of A. aurita, of which Claus (’83, p. 3), says: “ Mit dem weiteren
Nachriicken der die Mundspalte begrenzenden Zellen in das Innere des

MUSEUM OF COMPARATIVE ZOOLOGY. Dwi

Larvenleibes iindert sich jedoch allmahlig das friihere Verhiltniss zu
Gunsten der Entodernifiillung, die noch immer keine wahre Hohle, sondern
eine schmale lineare, mit der Hauptachse des Levbes zusammenfallende Spalte
besitzt.” 1 With the growth of the entodermal layer, the celenteron
enlarges, and the cleavage cavity is diminished, until finally it is
entirely obliterated and the entoderm everywhere comes into contact
with the ectoderm (Plate I. Figs. 4-6, Plate II. Fig. 11).

During the process of gastrulation, and also for a short time after its
completion, the thickness of the entoderm, which is much less than that
of the ectoderm, does not increase. Figures 5 and 6 (Plate I.) are
from sections of two embryos at different stages of development.
Figure 5 is from an embryo soon after the completion of gastrulation ;
Figure 6 is from an older stage. Since in each case the section is from
the middle of its series, it follows that a decided thickening of the ento-
derm takes place between the stages represented by these Figures. This
thickening is apparently due to an increase in the number of the cells,
which are soon unable to find room for themselves except by elongation.
The entodermal cells are quite different in appearance from those of
the ectoderm; they are approximately spherical, and do not have as
numerous spheroidal yolk bodies as the latter. Their nuclei, however.
closely resemble those of the ectoderm, and usually lie in the portion of
the cell nearest the ccelenteron.

As is to be seen from Plate II. Fig. 7, —a section nearly perpendicular
to the blastoporic canal,— the blastopore in A. flavidula is very small.
A similar condition has been shown by Claus to exist in A. aurita, and
by Metschnikoff (’86, Taf. X. Fig. 14) in Nausithoé marginata.

The nuclei of the cells composing the wall of the blastosphere are sit-
uated, as has been stated, near the surface of the sphere. But at about
the time of the beginning of the invagination, sometimes a little earlier,
a few of the nuclei are found in the deeper portion of the wall. At first
there are only one or two such displaced nuclei to be observed in the
whole embryo, but as development progresses they increase in number.
A careful examination of sections shows that the cells to which they
belong do not extend, like the remaining cells of the wall, through its
whole thickness, but that they are wedged in as it were between the
bases of the ordinary cells. The latter are much elongated, and from
mutual pressure are prismatic, whereas the deep cells are spheroidal and
project in some cases into the segmentation cavity. Since these cells
are found at various intermediate positions between the outer and inner

1 The original is not Italicized.

118 , BULLETIN OF THE

surfaces of the wall, I infer that they result from a process of migration
inward, either at the time of cell division or independently of that pro-
cess. Indeed, there is obviously no other possible source whence these
cells could come, but the exact process of transfer is not easily determined.
I believe that this increase in number is at first for a considerable time
due exclusively to the migration of cells which once shared in forming
the external boundary of the sphere, but later the division of cells which
have already migrated into the deeper portion of the ectoderm undoubt-
edly contributes to this increase.

We have now to turn our attention to a phenomenon of considerable
importance, the study of which from preserved material is, however,
attended with difficulties. I refer to the ingression of cells from the
wall of the blastosphere into the cleavage eavity, which begins a con-
siderable time before the invagination commences. The latter does not
take place until the number of cells forming the wall of the blastosphere
has exceeded 400, whereas the ingression, as far as can be inferred from
the cases which I have studied, may occur at any time after the blasto-
sphere contains about 100 cells up to the period of invagination. The
phenomenon of ingression in A. flavidula is not of constant occurrence,
but when it does take place is similar to that represented by Goette
(87, Taf. I. Figs. 1-5) for the earlier stages of the blastula in A.
aurita. It consists of a migration into the cleavage cavity of one or two,
rarely more than three, of the cells of the blastospheric wall. With
the exception that they assume a spherical form, because relieved from
pressure, they are at first similar in size, as well as in nuclear and other
characters, to the cells remaining in the wall.

The study of ingression upon preserved material is attended with diffi-
culty, since in any one specimen we have the condition at only one stage
of development, and cannot say with certainty what its condition has
been in past stages, or what it might have been during some subsequent
period. This can be determined only by studying the conditions exist-
ing in other embryos killed at other stages, and arranging all in their
probable natural sequence. In view of this fact, I have sectioned and
examined several hundred embryos which were killed at different stages of
development. As faras possible the results obtained from these sections
have been verified by the study of embryos cleared and mounted whole.
Although this ingression occurs before invagination, I have deferred the
discussion of it until now, because invagination is constant in its occur-
rence, whereas the ingression does not appear to be so; indeed, the
majority of the specimens have shown no indications of it.

MUSEUM OF COMPARATIVE ZOOLOGY. 119

The subsequent history of these cells, as shown by the comparison of
specimens of succeeding stages of development is both interesting and
peculiar. I imagine that it is such cells as these to which Claus (90,
p. 3) refers when he says: “Ich habe den vereinzelt eingetretenen zwei
bis drei Zellen, weil sie nicht regelmiissig in jeder Blastula sich abldsen,
der am vegetativen Pole einwuchernden Zellenmasse gegenitiber keine
weitere Bedeutung beigemessen, so dasse ich dieselben zwar auf einer
Abbildung darstellte, im Texte aber nicht besonders erwiahnte, und bin
auch jetzt noch der Ansicht, dass diese auffallend kleinen Zellen wieder
riickgebildet werden und iiberhaupt nicht zur Bildung des Entoderms
beitragen.” In my judgment, a part of the difference of opinion be-
tween Goette and Claus is due to the fact that there are two kinds of
cells which find their way into the cleavage cavity. These are the large
cells described by Goette as beginning to be formed at an early stage of
the blastula, and much smaller cells, of which I shall have more to say
hereafter, that make their appearance only at later stages of develop-
ment. Claus seems to have seen “very small cells,” and to have
assumed that they were equivalent to the large cells figured by Goette.
I am unable to say with certainty that the cells seen by Claus are the
equivalents of those figured by Goette, but Claus assumes that they are,
and I have the more reason to believe it because the large cells are of
more frequent occurrence than the small ones. But if this be so, I do
not understand how Claus could speak of them as “diese auffallend
kleinen Zellen.’” But however that may be, I have reason to believe
that the supposition of Claus, that they ultimately degenerate, is
correct.

Soon after the ingression of a cell its nucleus undergoes changes:
which result in its disappearance as such, for instead of a nucleus there
can be seen only one or more small, isolated, deeply stained particles,
which I judge to be scattered portions of the nuclear chromatine
(Plate II. Figs. 8 and 10). Even these are often wanting. I have
said that this nuclear change follows soon after the ingression of the
cell, because out of the numerous instances in which these cells have
been present there is not one in which the nucleus retains its original
condition after the cells in the wall of the blastula have given evidence,
by their diminished size, that they have undergone division since the
ingression took place. This conclusion is in part based on the assump-
tion that at the time of ingression the ingressing cells are of about the
same size as those which remain in the wall of the blastula. The in-
gressing cells sometimes persist, without any further apparent changes

120 BULLETIN OF THE

than the disintegration of the nucleus, until the process of gastrulation
is completed. Such cases are not as common, however, as others, where
there is to be found in the cleavage cavity material which appears as
though it had resulted from the disintegration of similar cells. This
material has a spongy or vacuolated appearance, and contains faintly
staining bodies or granules similar to those found in the ectodermic
cells; it does not possess definitely circumscribed boundaries; on the
contrary, it fills the cleavage cavity more or less completely, but is not
of uniform density throughout. The fact that this material is not homo-
geneous, and that it contains granules, etc., prevents the conclusion that
it has been produced as a simple secretion into the cleavage cavity,
although it may have been formed in part by such a process. The fre-
quent association of this material with ingression cells in the same spe-
cimen (Plate II. Fig. 8), and the lack of other ways of accounting for
its presence, lead me to believe that it is produced by the disintegration
which I have suggested.

There is another peculiarity of the development which I believe to
be connected with this process of nuclear disintegration. It is this:
after having once entered the cleavage cavity the immigrating cells seem
to lose their power of division, and consequently do not become more
numerous, while the cells composing the blastospheric wall undergo
repeated divisions, as is shown by their increased number and dimin-
ished size.

The number of these immigrating cells is small, usually only one or
two, very rarely more than three, so that I have not been successful in
finding the ‘“ Verbindungsglieder” connecting the conditions shown by
Goette (’87, Taf. I.) in his Figures 5 and 6, which Claus (’90, p. 4) re-
garded as essential to the substantiation of Goette’s view of the method
of gastrulation.

Reference has been made to the fact that in some cases the ingrowing
cells persist both during and after the process of invagination. In the
latter case, they are to be found in the ecelenteron rather than in the
cleavage cavity. Figure 11 (Plate II.) is drawn from such a specimen.
Figures 9 and 10 represent two sections of one individual in which the
invagination is not completed, and furnish a hint as to the process by
which the cells pass into the ceelenteron from the cleavage cavity. The
entoderm being composed of less closely fitting cells than the ectoderm,
doubtless admits the passage of the large immigrated cells through it
more readily than the latter would (Plate II. Fig. 9). The immigrated
cell is of course passive in this process. Since it is prevented by the

MUSEUM OF COMPARATIVE ZOOLOGY. Lot

firm wall of the ectoderm from escaping, the pressure exerted upon it by
the enlarging entoderm is probably sufficient to cause it to be forced
through the entodermic wall into the ceelenteric cavity. From Figure 10
it is to be seen that one cell has already reached the gastral cavity. In
speaking of these peculiarly situated cells I have thus far assumed that
they are such as originally reached the cleavage cavity by an early
ingression, where, with changed nuclear condition, but apparently with
no further alteration, they have remained until the time of gastrula-
tion. That this is their source is evident from the following consid-
erations. First, the small diameter of the blastoporic canal (Plate II.
Fig. 7), which is from the same series as Figures 9 and 10, precludes
the assumption that they might have entered the gastrula cavity from
without. Secondly, in their large size and general appearance they are
unlike the cells of either ectoderm or entoderm at any time during
gastrulation, and so could not have been derived from those sources
during that process. ‘Thirdly, they do correspond in size and general
characters, except in their nuclear conditions, with the cells of the
blastospheric wall as the latter appear at the time when ingression
takes place.

It is difficult to state either the cause or the purpose of this immigra-
tion. That it is not essential to the welfare of the embryo, either by
affording nourishment to the developing cells of the entoderm, or in
any other way, is evident from the fact that in a large number of cases
it does not occur. That it is not an inherited tendency, derived from a
more primitive method of gastrulation by ingression, is probable from
the fact that the immigrating cells do not appear to have any share
whatever in the formation of the entoderm. On the other hand, its
occurrence seems to be much too frequent to be considered as acci-
dental.

I have stated previously (p. 119) that two very different kinds of cells
are to be found at times in the cleavage cavity. Besides the large immi-
grating cells already described at length, I have found in a much smaller
number of cases very small cells (Plate I. Fig. 2), one or two in num-
ber, that appear precisely like the deep-lying ectodermal cells already
described. Because of their strong resemblance to the latter, their
exceptional occurrence, and the fact that they do not appear until after
the beginning of the development of the deep-lying ectodermal layer,
I incline to the opinion that they are derived from that layer, and that
their occurrence is entirely accidental.

At first it appeared to me surprising that two investigators could

122 BULLETIN OF THE

reach such different conclusions as those published by Claus (’83 and
90) and Goette (’87), concerning the method of gastrulation in the
same animal, A. aurita. Since studying this process in A, flavidula, it
seems less strange. ‘The results obtained from my first sections led me
to think that the conclusions reached by Goette would be confirmed in
the case of A. flavidula. Better staining, thinner sections, and more
accurate orientation have made it certain, however, that the method of
gastrulation in this species is much more in accord with the description
given by Claus, and that the process really is one of invagination.

Certain considerations weaken my confidence in the position defended
by Goette. A comparison of his Figures 6-9 (787, Taf. I.) with some
of my thicker sections, or with those which were made when the gastrula
was so oriented as not to be cut parallel to the blastoporic canal, makes
it appear to me probable that his results are based upon similar inade-
quate sections. In Figure 8 (Plate II.) there are only about one half
as many nuclei visible as there are cells, the nuclei of a portion of the
cells being contained in adjacent sections. In figures of corresponding
stages of A. aurita as represented by Goette (87, Taf. I.), nuclei are
figured in nearly all the cells. I believe this to be evidence that his
figures were drawn from thick sections. The blastopore, because of its
very small diameter, is quite easily overlooked in thick sections, and
especially if the plane of sectioning is somewhat oblique to the longitu-
dinal axis of the blastopore. Since, as previously stated, the nuclei of
the entodermal cells are usually situated in the portion of the cell near-
est the ccelenteron, it is easy to find in thick sections of an invaginating
embryo conditions like those represented by Goette in his Figures 6-8.
My Figure 12 (Plate II.) reproduces a section of the same series as
that represented in Figure 3 (Plate I.). The intervening section (not
figured) is quite similar to Goette’s Figure 8. An examination of the
cells bordering the blastoporic canal in Figure 3 will show how sections
like Figure 12, or such as are a little oblique to the chief axis of the
embryo have the appearance of containing immigrating cells. Such
sections also exhibit the flattening in the region of the shorter cells to
which Goette (’87, p. 4) has called attention in the following words :
“Schon wiihrend der Gastrulation zeigt sich eine Stelle des Keims im
Bereich seiner kiirzeren Zellen etwas abgeplattet.”

Additional considerations increase the probability of the correctness
of the view which I have advanced to explain Goette’s error. With
advancing stages of development, I have found an increase in the num-
ber of the cells composing the ectodermic wall. This is undoubtedly

MUSEUM OF COMPARATIVE ZOOLOGY. 133

subject to slight individual variations, but the number of such cells is
nevertheless in quite close correlation with the stage of development. An
examination of Goette’s Figures 6-9 (’87, Taf. I.) reveals such a simi-
larity in the number and size of the cells composing the ectoderm in each
of the four supposed stages, that I am driven to the conclusion that
they represent sections from specimens of a single stage of development,
which may have been produced by cutting in planes having different
relations to the chief axis of the embryo.

When we consider that in the majority of embryos there are no signs
of ingression, and that in the cases where it does occur the immigrating
cells in some instances degenerate early, and in others persist undivided
throughout the process of gastrulation, and that they at no time show evi-
dences of even sharing in the formation of an entoderm, — and when we
further reflect that all the conditions shown in Goette’s Figures 6-9 can
easily be reproduced from sections of invaginating gastrulz of a single
stage of development, —it seems improbable that the entoderm of Au-
relia develops even occasionally by ingression. At present, therefore,
there seems to me to be no evidence that in this genus gastrulation
occurs by both methods, invagination and ingression.

The Scyphomedusz present several interesting variations in gastru-
lation. The anomalous development occurring in Lucernaria is as far
removed from the usual process as that group itself is from the other
Scyphomeduse. According to MeMurrich (91, p. 314), the solid plan-
ula in Cyanea arctica is formed by the immigration of certain of the
blastula cells. This plunnla is subsequently hollowed out, and gives
rise to a structure like an invaginate gastrula, but it is formed without
any invagination. In Cyanea capillata (Hamann, 790, pp. 16, 17) there
seems to be a solid ingrowth of cells from one pole of the embryo, and a
simultaneous development of the celenteron. The entoderm of Chry-
saora (Claus, ’83, p. 5, Taf. I. Fig. 21 h) is developed in a way which is
somewhat similar to that described by Hamann for Cyanea capillata,
According to Claus (’83, p. 2, and ’90, p. 4), the gastrulation of Aurelia
aurita approximates the method by invagination a little more closely
than that of Chrysaora, since its cells are arranged ina single layer
about the fissure-like ccelenteron. Aurelia flavidula exhibits a still more
nearly typical invagination, since the ccelenteron is from the beginning

_an open sac-like cavity. Cotylorhiza tuberculata (Cassiopea Borbonica)
“has an invaginate gastrula which closely resembles that of Aurelia
flavidula (Claus, 90, Taf. I. Figs. 2 and 3; Kowalevsky, ’73, Taf. IT.
Fig. 1). Finally, in Pelagia noctiluca and Nanusithoé marginata, as

124 BULLETIN OF THE

shown by Metschnikoff (’86, pp. 66-68, Taf. X.), there is a typical in-
vagination.

If the observations of MceMurrich (791, p. 314) on Cyanea arctica are
substantiated, we have among the Scyphomedusze one example of the
formation of a sterrula by ingression, with the subsequent formation of a
gastrula-like structure, without an invagination. From the preceding
summary it is to be seen that there are in Scyphomedusz two cases in
which the mode of gastrulation appears to be intermediate between
ingression and invagination, and at least four cases of unquestionable
invagination. If, in the light of so much variation in the mode of
gastrulation in this group as is shown by the few forms studied, it is
“safe to conclude that any one mode is typical, that mode would cer-
tainly appear to be invagination, and not, as Hamann and MeMurrich
have recently maintained, ingression.

CAMBRIDGE, June 20, 1891.

MUSEUM OF COMPARATIVE ZOOLOGY. 125

BIBLIOGRAPHY.

Claus, C.

’°83. Untersuchungen iiber die Organization und Entwicklung der Medusen.
Prag u. Leipzig, 96 pp.

790. Ueber die Entwicklung des Scyphostoma von Cotylorhiza, Aurelia und
Chrysaora, sowie tiber die systematische Stellung der Scyphomedusen. _ I.
Arbeiten a. d. zool. Inst. Wien, Tom. IX. p. 85.

Goette, A.

87. Abhandlungen zur Entwicklungsgeschichte der Tiere. Viertes Heft.
Entwicklungsgeschichte der Aurelia aurita und Cotylorhiza tuberculata.
Hamburg u. Leipzig, 79 pp.

Hamann, O.

°90. Ueber die Entstehung der Keimblatter. Ein Erklarungsversuch. In-

ternat. Monatsschr. f. Anat. u. Physiol., Bd. VII. pp. 1-28.
Kowalevsky, A.

73. Untersuchungen iiber die Entwicklung der Coelenteraten. Nachrichten
Gesellsch. Freunde Naturerkennt., Anthropol. u. Ethnog. Moskau, 1873.
(Russian.)

See also Hoffmann u. Schwalbe, Jahresbericht, Bd. IL. p. 279.
McMurrich, J. P.

°91. Contributions on the Morphology of the Actinozoa. II. On the Devel-
opment of the Hexactinie, Jour. Morphol., Vol. IV. p. 303.

91". The Gastrea Theory and its Successors. Biological Lectures delivered
at the Marine Biol. Laboratory, Wood’s Holl. Boston, p. 79.

Metschnikoff, E.
°86. Embryologischestudien an Medusen. Wien, 159 pp.

Akt Aint P
Ma Sas a fy i

= -
Limite”
P)

vas

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EXPLANATION OF FIGURES.

All the figures were drawn from sections with the aid of an Abbé camera. The
sections from which the figures were made were 5 wu in thickness.

Surra. — Gastrulation in Aurelia,

PLATE I.

ABBREVIATIONS.
bl’ po. Blastopore.
cav. sq. Segmentation cavity.
el, Immigrated cell.
celent. Celenteron.
cog. Coagulum.
ec’drm. Ectoderm.
en’drm. Entoderm.
nl, Chromatic portion of degenerated nucleus.

nl. ec’'drm. Nuclei of deeper portion of ectoderm.

Figures 1-4. Sections to illustrate the nature of the invagination.

Fig. 1.

or 9 ro

An early stage of invagination. 460.

A slightly later stage than that of Figure 1. x 540.

A stage in which the invagination is well advanced. 385.

A gastrula with invagination completed. 410.

Section of a gastrula cut in a plane (equator) perpendicular to the axis of
the blastoporic canal. X 3865.

Section of an older individual through the equator, showing increase in
thickness of the entoderm. X 586.

SMITH.— GASTRULATION IN AURELIA.

B Meisel, ith. Boston

Smira. — Gastrulation in Aurelia.

PEATE

ABBREVIATIONS.
bl’po. Blastopore.
cav. sg. Segmentation cavity.
el. Immigrated cell.
ceelent. Ceelenteron.
cog. Coagulum.
ec’drm. Ectoderm.
en’drm. Entoderm.
nl. Chromatic portion of degenerated nucleus.

nl. ec’drm. Nuclei of deeper portion of ectoderm.

Figures 7, 9, and 10 are from different sections of the same individual.
Fig. 7. Section through the blastoporic canal and nearly perpendicular to it.
x 410. F
“ 8. Section at a stage preceding invagination. It shows an immigrated cell
in which the nucleus has degenerated. X 385.
“ 9. Section before the close of gastrulation, showing an immigrated cell in
the segmentation cavity. X 410.
“ 10. Section from the same individual as Figure9. It contains an immigrated
cell in the ceelenteric cavity. 410.
“ 11. Section of a gastrula with two immigrated cells contained in the ccelen-
teric cavity. X 385.
“ 42. Section from the same individual as Figure 3, to show the appearance
when the gastrula is cut parallel to, but at one side of, the blastoporic
canal. X 385. \

JRELIA.

—
UU

SMITH.— GASTRULATION IN

coplent.~

bl po.

iS
nS
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Yet
MeSel, Ui

Bao:

No. 3.— Amitosis in the Embryonal Envelopes of the Scorpion.
By H. P. Jounson.?

In the fall of 1889, at the suggestion of my instructor, Prof. E. L.
Mark, I decided to work upon the problem of the so-called “ direct ” or
amitotic division of nuclei. While in search of suitable material, my
attention was called to a brief article by Blochmann (’85), describing a
very well marked amitotic division for the large nuclei of the embry-
onal membrane of the’ scorpion. A number of Centrurus embryos
were kindly given to me by my friend, Dr. G. H. Parker. These em-
bryos had lain in 90% alcohol since the summer of 1886. The mode
of fixation (for the purpose of studying the development of the eyes)
was somewhat unusual; for, immediately after their removal from the
mother, they were immersed in 35% alcohol, and thence carried up
quite rapidly, through 50 and 70%, to 90%. Notwithstanding this
rather crude method, the membranes were in excellent histological
condition, in no way inferior to material afterwards prepared by the
most approved methods of fixation.

In addition to the material above mentioned, I received from Mr.
Richard Goeth, of Burnet County, Texas, during the following winter
and spring, about three dozen live specimens of Centrurus (sp. incog.).?
A lot that arrived in the latter part of May contained several pregnant
females, with embryos in different stages. The scorpions were chloro-
formed, and the ovarian tubes with the embryos enclosed were dissected
out as quickly as possible. A number of killing agents were used,
including Flemming’s weaker chrom-aceto-osmic, Rabl’s chrom-formic,
Perenyi’s fluid, Kleinenberg’s picro-sulphuric, and Merkel’s fluid.

For staining, I have used chiefly Ehrlich’s hematoxylin. Grena-
cher’s alcoholic borax-carmine and Czokor’s alum-cochineal have given
fair results. Safranin, employed according to Flemming’s method, I

1 Contributions from the Zodlogical Laboratory of the Museum of Comparative
Zoology, under the direction of E. L. Mark, No. XXX.

2 This is the species used by G. H. Parker in his study on the development of
the eyes (see Bull. Mus. Comp. Zodl., Vol. XIII. No. 6, p. 175, 1887), and was then
undescribed. Iam not aware that it has since received a name.

VOL. XXII. — NO. 3. :

128 BULLETIN OF THE

have found less serviceable than the stains above mentioned. After
staining, the preparations were dehydrated, cleared with oil of cloves,
and mounted in benzole-balsam.

The embryo is enveloped by three epithelial membranes, the ovarian
capsule, the membrana serosa, and the amnion, —named in order from
without inward.

The serosa and amnion are strictly embryonic structures, analogous
to the foetal membranes of the higher Vertebrates. There are two
contradictory accounts as to the manner of their formation. Possibly
they do not arise in the same way in all genera of scorpions. In a
brief communication by Kowalevsky und Schulgin (’86, p. 526) upon
the development of Androctonus ornatus, it is stated that they originate
as a fold from the edge of the blastoderm, the outer layer of the fold
forming the serosa, the inner the amnion. The fold grows up over the
blastoderm, the edges coalesce, and the membranes finally separate from
the ovum. ‘The more recent account by Laurie (’90, p. 114) states that
in Luscorpius the serosa arises by a proliferation of the peripheral cells
of the blastoderm, extends as a delicate membrane forward and back-
ward over the egg, which it finally covers completely, and then becomes
entirely separate from the blastoderm. The formation of the amnion
begins when the serosa has covered about two thirds of the embryo, and,
like the serosa, its origin is ectodermic. The amnion, however, “ never
loses its connection with the epiblast as the serous membrane has now
done, but remains attached to its edges and only extends round the
egg as the epiblast extends” (p. 116). Unfortunately, I have not
obtained sufficiently early stages of Centrurus to ascertain how its mem-
branes arise, but, in removing the latter from the embryo, I have never
found the amnion attached to the ectoderm. The membrane which I
have called the “ovarian capsule” I at first wrongly took to be the
follicular epithelium, and under this supposition it was indicated as e’th.
fol. in Figure 2. Like the follicular epithelium, it arises from the
ovarian tube ;-but the follicle is formed as a diverticulum of the tube,
previous to the maturation of the ovum, and serves as a nutritive cap-
sule for the latter during its growth. The ovarian capsule, on the
contrary, is that part of the ovarian tube which receives the ovum after
fertilization, and enlarges to accommodate the growth of the embryo.

The fcetal membranes fit so loosely over the embryo that they can be
easily removed in a single piece. In late stages, the ovarian capsule is
readily separable from the membranes ; in earlier stages, it adheres closely
to them. It is rarely possible to separate the serosa from the amnion,

MUSEUM OF COMPARATIVE ZOOLOGY. 129

and a transverse section (see Fig. 2) shows only a trace of a dividing
wall between them, although in surface view the cell walls of both mem-
branes are clearly seen (Fig. 1). Metschnikoff (71, p. 219) describes
the membranes of Scorpio (Huscorpius) italicus as connected with each
other by delicate fibres, which terminate just over the amniotic nuclei.
I have found such fibres in the earlier stages of my material, but not in
the older ones, nor are they everywhere present in the younger mem-
branes. The membranes of the Brazilian scorpion examined by Bloch-
mann (’85, p. 481) were found closely applied to each other.

I. The Serosa.

Plate I.; Plate II. Figs. 14,15; Plate III.

The cells of the serosa have great superficial extent, measuring
half a millimeter or more in diameter; but proportionally they are
very thin. Their size is exceedingly variable, as may be seen by com-
paring Figure 3 with Figures 11 and 13 of the same magnification,
although the last two represent cells of only average size. Both
small and large cells are apt to be aggregated in certain parts of the
serosa, yet very small cells often occur sporadically in the midst of
large ones. The cell walls are extremely distinct in late stages of the
embryo, but in earlier stages are often difficult to trace in an ordinary
stained preparation. As remarked by Blochmann, they have a distinct
fibrous structure. The cells are irregularly polygonal in shape, usually
elongated, sometimes nearly square or triangular. Not infrequently
they are bounded by curved outlines (Fig. 13).

The nuclei of the serosa measure from 25 to 60, or more in
diameter, but as a rule are small in proportion to the cells (Figs. 1-3
and 11-15). In the membranes of young embryos the nuclei are larger
absolutely and in proportion to the cells than in old membranes. In
face view the resting nucleus is nearly circular; in section, it is seen
to be considerably flattened, in accordance with the thinness of the cell
(Fig. 2, n/. sr.). It occupies the full thickness of the serosa, and some-
times causes a bulging of the cell at the point where it lies, as is shown
in Figure 2. Blochmann states (’85, p. 480) that the nuclei of the se-
rosa always cause that membrane to encroach zxward upon the amnion ;
but a dividing line between amnion and serosa is so seldom visible in
Centrurus, that I am unable to say whether such is the case.

The nuclear membrane is thin, but clearly visible, except in nuclei that
have undergone degeneration. ‘The chromatic substance, or nuclein, is

VOL, XXII. — NO. 3. 9

130 BULLETIN OF THE

for the most part in the form of granules distributed evenly throughout
the nucleus. Indications of a reticular or filamentous structure are,
however, frequently present. I believe there is a chromatic network
throughout the nucleus, but the abundance of granular chromatin pre-
vents one from tracing it. Several nucleoli are always present. They
are extremely variable in size and shape, and in many cases appear to
be only aggregations of granular chromatin. They take a stain with
hematoxylin and carmine in no way different from the rest of the
chromatin, except that it is more intense.

A very large proportion (about four to one) of the cells of the serosa
contain two nuclei. These pairs of nuclei have all arisen from single
nuclei by amitotic division. It is obvious that division of the cell is not
contemporaneous with, and does not immediately follow, the division of
the nucleus. In many cases, especially when the embryo is far ad-
vanced, cell division probably does not occur at all. Very few cells out
of the thousands I have examined have had more than two nuclei; but
I have found several with three nuclei, and two cells with four. This
seems to be the maximum number. These cells of the serosa, therefore,
are not to be classed with multinucleate cells in which the nucleus
divides into a great number of irregular and unequal fragments. Here
the division takes place in an orderly fashion, and division of the cell
follows nuclear division in regular sequence, though not immediately.

In every serosa examined, nuclei were found in process of division.
Some preparations furnish many more examples of division than others ;
and occasionally three or four adjacent cells will contain dividing nuclei
(Fig. 45). Very frequently, however, only one or two dividing nuclei
will be found in the whole serosa. It cannot therefore be supposed that
nuclear division is frequent; and I have found that there are more cells
with dividing nuclei in the membranes of late stages of the embryo
than in the earlier ones.

The first sign of approaching division is an elongation of the nucleus
(Fig. 4), almost always parallel to the long axis of the cell. Naturally,
the elongation progresses by insensible gradations from the nearly circu-
lar form of the resting nucleus, so that one cannot say positively that
the nucleus is going to divide until the elongation has become marked.
The absolute amount of elongation varies greatly, and is less in the
membranes of young embryos than in those of older ones. The example
represented in Figure 4 is from an old membrane, and shows almost the
extreme of elongation. This stage, while giving not the slightest evi-
dence of ordinary mitosis, is characterized by a longitudinal arrange-

MUSEUM OF COMPARATIVE ZOOLOGY. 131

ment of the chromatic substance, as indicated in Figure 4. The effect
is most marked upon the nucleoli. Blochmann (’85, p. 482) found only
two nucleoli at this stage, and these were usually situated one at each
end of the elliptical nucleus. Where there are several nucleoli, as is
usually the case with the nuclei I have studied, there is an approxi-
mately equal distribution of them to the daughter nuclei. The nucleoli
vary so much in size and shape, that it is impossible to say how precise
is the apportionment of chromatin by this method.

Most nuclei in the elongated condition already show a slight constric-
tion, generally more marked on one edge than on the other (Fig. 4).
If no further elongation takes place, the constriction beeomes deep and
narrow, as represented in Figures 5 and 12. This style of division is
characteristic of young membranes, and gives rise to daughter nuclei
which lie close together, or even in contact (Fig. 13). It is doubtless a
more vigorous and rapid type of division than that found in the older
membranes, to be described directly. If the nucleus continues to elon-
gate while constricting, it assumes the dumb-bell form represented in
Figures 6 and 7. The daughter nuclei, at first ovate or pyriform, be-
come rounder as the connecting thread becomes thinner. Division of
this type is almost confined to old membranes; I have rarely found it
in those from young embryos.

The nuclei represented by Figures 6 and 7 show more clearly than
usual a peculiar arrangement of the chromatic threads. The filaments
have the appearance of a fascicle of slender rods, which lie very close
together in the connecting bridge, and thence radiate into both daughter
nuclei. They are stainable both with carmine and hematoxylin. Some-
times these threads can be resolved into rows of granules (Fig. 7, right-
hand daughter nucleus). The later stages also show traces of these
longitudinal threads (Figs. 8, 9, 10). In the example represented by
Figure 6, the nucleoli partook of the general longitudinal disposition of
the chromatic substance, but were probably arranged in this manner at
an earlier stage of division, as explained for Figure 4. In the later stages
of division, this arrangement of the nucleoli is gradually lost.

The final stages, represented in Figures 8, 9, 10, may be briefly de-
scribed. These stages are far commoner than the early ones; hence, it
must be supposed that they require more time. The constricted por-
tion is drawn out into a thin, deeply staining thread. This thread
undoubtedly contains chromatin, and in a peculiarly condensed form.
In this respect these nuclei differ from the nuclei of the Malpighian
vessels of Aphrophora spumaria, as described and figured by Carnoy

132 BULLETIN OF THE

(85, Plate I. Fig. 7) ; for the connecting thread in the dividing nucleus
of Aphrophora remains unstained, and therefore contains no chromatin.

The dividing nucleus represented by Figure 8 is peculiar in several
respects. In the first place, the daughter nuclei are very unlike in
form, though this is by no means unusual with dividing nuclei from old
membranes. All the stainable nucleoli are in one daughter nucleus,
while the other still shows a faint longitudinal arrangement of its
chromatic threads. The sharply stained connecting thread is notched
at a point midway between the daughter nuclei, probably indicating
the place where, at a later stage, rupture would have occurred. The
daughter nucleus on the left is nearly destitute of chromatin in the
crescent-shaped space lying next the connecting thread, and an inner
contour line is visible (#), from the central point of which a stainable
cord extends to the proximal end of the connecting thread. I have seen
a similar appearance in the late stages of other dividing nuclei, and it
undoubtedly indicates the manner in which the daughter nuclei some-
times attain a rounded form. Occasionally, however, daughter nuclei
entirely separate from each other have a conical or tapered form.

In the last stages of division, the connecting thread is drawn out to
extreme tenuity (Figs. 9 and 10), So exceedingly fine does this thread
become, that, with the highest power accessible to me (Zeiss’s homoge-
neous immersion objective 4), I could barely trace its course through
the cytoplasm, though in most cases I made out that it was continuous
from nucleus to nucleus. It is finally broken at or near the centre, and
the proximal tips, as Blochmann suggests, are probably absorbed by the
daughter nuclei. In even so late a stage as that shown by Figure 10,
the longitudinal chromatic filaments are still perceptible. The right-
hand daughter nucleus contains four loop-shaped bodies that strongly
resemble chromosomes. ‘They are, however, almost unstained by hema-
toxylin.

Blochmann states (’85, p. 482) that in no case did he find a division
of the cell following the division of the nucleus. As already said, the
great proportion of binucleate cells renders it certain that cell division
is not an immediate consequence of nuclear division. Although I have
carefully examined great numbers of binucleate cells, I have only
once seen a cell wall in process of formation (Fig. 27). Yet one finds
plenty of evidence that cell division does take place. Pairs of cells like
those in Figure 11 are of frequent occurrence. It is safe to infer, I
think, from the arrangement of the binucleate cells which surround
these, as well as from the correspondence in size and shape of this pair,

MUSEUM OF COMPARATIVE ZOOLOGY. 133

that they have arisen from an elongated binucleate cell by the forma-
tion of a divisional cell wall. In one instance, I have found a cell wall
fully formed before division of the nucleus was completed (Fig. 27). It
cuts across the fine connecting thread at about the middle point of the
latter. This must be considered as in some degree abnormal, especially
since it was found in a serosa the nuclei of which had evidently degen-
erated.

Although division of the cell is almost always accomplished by the
formation of a cell wall, I have found several constricted cells, showing
that division may be partly, or even wholly, effected in this manner.
Sometimes the constriction is so deep that the opposite walls meet
(Fig. 28); but it is more usual to find that, after the cell has become
considerably constricted, a cell wall is formed joining the inward curves
of the constriction, and completing the division. At first, I thought it
possible that the constriction was mechanically produced by the pres-
sure of growing cells on either side. But this would not explain the
invariable occurrence of the constriction at precisely the point where it
would take place in a free cell, — equidistant from the daughter nuclei.
Furthermore, the curvature of cell walls (see Fig. 13), which is almost
certainly caused by the growth of cells and consequent tension, has no
reference to the position of the nuclei.

As far as can be judged, the dauglter nuclei are, as a rule, of equal
size, and alike in shape. I have found many instances of beautifully
symmetrical division (Figs. 9 and 10); but the nuclei of the serosa are
not altogether exempt from the irregularities that seem to be inseparable
from amitotic division wherever it occurs. Sometimes the resulting
nuclei are obviously unequal (Fig. 13), even in young membranes; and
in old membranes, where the nuclei have undergone degeneration, not
only are the daughter nuclei extremely irregular in shape, but often
very dissimilar in size.

Relations of the Nuclei to the Cell. — A very brief examination of a
preparation of the serosa convinces one that the nuclei are symmetri-
cally arranged in the cells. When there is but one nucleus, it occupies
the centre of the cell; when there are two or three nuclei, each presides
over a half or a third of the cytoplasm. This arrangement is so con-
stant, that any marked deviation from it catches the eye at once. In-
stances of decidedly unsymmetrical arrangement of nuclei, one of which
Figure 13 represents, are very unusual. As regards elongated cells, the
daughter nuclei lie in the long axis of the cell, and at approximately
equal distances from its ends. Occasionally, however, the nuclei lie in

134 BULLETIN OF THE

the short axis (Fig. 12), and much more frequently are placed obliquely,
as in cell a, Figure 14. We would suppose that, in the event of division
of an elongated cell with nuclei lying transversely, the cell wall would
pass longitudinally between the nuclei; but I have not been able to find
evidence of longitudinal divisions. From the large number of cells with
nuclei lying obliquely, one would infer that oblique division of the cell
often took place. I am unable to discover, however, that such is the
case ; and it seems extremely probable that the divisional plane of the
cell does not always coincide with that of the nucleus.

I have found about 25 cells of the serosa with three nuclei. This
seems to be a matter of individual variation in the make-up of the
membrane, for all but three of the trinucleate cells were in membranes
from the brood of a single scorpion, and membranes from some broods
appear to have none. I have in one instance found a group of tri-
nucleate cells (Fig. 14, 4, 2, 3, 4). At this spot nuclear multiplica-
tion has outstripped cell multiplication. It is nearly always easy to see
which of the two original nuclei has divided, for we find two of the
nuclei smaller than the third, and nearer to each other than to the
latter. In cell 2, for instance, the pair of nuclei on the left have arisen
from a nucleus occupying a position about midway between them. The
same statement would doubtless hold true for the two nuclei on the right
in cell 3, and here the odd nucleus is elongated. When the cell is long
and the nuclei all lie in the longitudinal axis, as is the case in cell 7,
it is usually impossible to determine which of the two original nuclei
has divided ; for the nuclei are equidistant, and nearly alike in size.
Another type of equidistant nuclei is shown in cell 4,——a distribution
quite as characteristic of very large, broad cells as the linear arrange-
ment is of elongated cells. I have spoken of the division of one of the
two original nuclei as though it always took place after the nuclei were
completely separate, and had taken their positions in the cell. This
seems to be the usual method, for I have several times found one of
the original nuclei in the act of dividing. But it is possible, of course,
for them to arise by a tripartite division, in which the three nuclei
would be formed simultaneously. I have found only one instance of
a true triple division, represented in Figures 29 and 30, and as this
occurred in a serosa which had plainly undergone degeneration, I do
not consider it as altogether normal. It will be noticed that the origi-
nal nucleus became trilobed, and that the lobes became daughter nuclei
of approximately equal size by the formation of three divisional planes,
meeting at the centre of the original nucleus, The daughter nuclei on

MUSEUM OF COMPARATIVE ZOOLOGY. 135

the right are still united to each other by strands at the corners. Very
similar tripartite divisions were found by Overlach (85, Plate XI. Figs.
35 and 41) in the epithelium of the cervix uteri. In two other cases,
I have found one of the daughter nuclei in a late stage of division
(Figs. 31, 32) ctse/f elongating and undergoing constriction, It will
be noticed that the constricted daughter nucleus is considerably larger
than its mate.

I have found but two cells with move than three nuclei, and these both
contained four. This condition is brought about by the division of both
nuclei of a binucleate cell. On a@ priori grounds, one would reason that
quadrinucleate cells would be nearly as abundant as those with three
nuclei, for, apparently, it must often happen that a pair of danghter
nuclei, arising as they do by a symmetrical and accurate constriction,
are ready to divide at almost the same moment. Yet there are doubt-
less influences which operate to prevent the division of one of the
nuclei. Although it is of course impossible to generalize on the char-
acteristics of quadrinucleate cells, it may be of interest to mention the
peculiarities of the two found. They are both large cells, of nearly
equal width at the ends, and the breadth of both exceeds half the
length. In one, both pairs of nuclei lie transversely, showing that the
second divisional plane was at right angles to the first. In the other,
represented in Figure 33, the lower pair of nuclei lie in the longitudinal
axis, the upper pair almost transversely. One of the quadrinucleate
cells is considerably larger than any cell near it, while the other (Fig.
33) though by no means small, is of much less dimensions than the im-
mense bi- and uninucleate cells around it. I am unable to assign any
reason for the multinuclear condition of this cell. One fact, however,
is worthy of note. The united volume of its four nuclei does not
exceed the bulk of the single nucleus of a neighboring cell. One can-
not, of course, ascertain what the size of the primitive nucleus of the
multinucleate cell was, but it is very improbable that it exceeded in
volume the nucleus of the uninucleate cell in question, for the latter
cell is considerably the larger of the two, and throughout this serosa the
size of the nuclei bears a direct ratio to the size of the cells.

As regards the influence or influenceg impelling nuclei to divide
independently of the division of the cell, nothing very definite can be
stated. It is certain that the absolute or relative size of the cell has little
or no influence upon the division of the nucleus. There are cells of all
sizes, from the largest to the very smallest (Fig. 3), which are binucle-
ate; and it is usual to find, side by side with bi- or multinucleate cells,

136 BULLETIN OF THE

others with a single nucleus that are actually larger than the former
(compare the cells in Figure 14). In such cases, the single nucleus is
always larger than the daughter nucleus of the other cells. I am unable
to see that multiplication of nuclei in the cell leads to any immediate
increase of nuclear material. The more they divide, the smaller they
become. Probably the most important office of division is a@ more
extensive distribution of nuclei throughout the cytoplasm, with correspond-
ing increase of nuclear surface ; and this, considering the great superfi-
cial extent of the cells, and the comparatively small size of the nuclei
(at least in the older membranes) must be a matter of some importance
for the activities of the cell. It is especially so in the case of elongated
cells. If such cells have but a single nucleus, a large part of the
cytoplasm must be remote from it; and if the nucleus is at the centre
of the cell, the cytoplasm at the ends of the cell will be most remote.
So, to restore the equilibrium between cytoplasm and nuclei, the nucleus
must elongate in the longitudinal axis of the cell, and the daughter
nuclei move toward the ends of the cell.

As a matter of fact, nearly all elongated cells have two nuclei, and
these lie in the long axis of the cell, usually rather nearer its ends than
toeach other. It cannot be denied that many short or squarish cells
also contain two nuclei; and, conversely, a few much elongated cells
can be found that have but one. In the latter case, it is interesting
to observe that almost invariably the nucleus has begun to elongate
in the longitudinal axis of the cell, and is often far advanced towards
division. We can say almost with certainty, then, that such cells are
of recent formation, and that the equilibrium between cytoplasm and
nucleus is promptly restored by division of the latter. It is true that
cases like that represented in Figure 12, where nuclear division takes
place in the short axis of an elongated cell, cannot be explained in this
manner. Such instances are so rare that they might almost be con-
sidered as abnormal ; but the difficulty of the matter lies in the fact
that we get all gradations between nuclei ranged in the true longitudi-
nal axis, and those placed in the transverse axis. It is common to find
them lying more or less obliquely in the cell, though the obliquity is
seldom so great as to prevent them from practically fulfilling the con-
ditions of the hypothesis.

It is not supposable that all the agencies impelling nuclei to divide,
and controlling the direction in which division shall take place, reside
in the cytoplasm ; possibly the most potent of them exist in the nucleus
itself. That axial differentiation, with definite pole and antipole, is as

MUSEUM OF COMPARATIVE ZOOLOGY. 137

characteristic of the resting nucleus as of the mitotic nucleus, was
postulated by Rabl (’85, p. 323) from a careful study of the chromatic
network in the “skein stage” of mitosis. In a recent paper (’89, pp. 23,
24), the same writer states that the “polar depression,” usually visible
in young daughter nuclei, persists much longer than usual in the epi-
thelial nuclei of the Triton ; so that for these mitotically dividing nu-
clei it is highly probable that polar differentiation is always present in
the resting state. Carnoy (’85) has shown that, in the resting nuclei
of the testicular cells of certain Arachnids, the chromatic filaments are
distinctly arranged with reference to a definite axis (Planche V. Figs.
165-169), and Van Gehuchten (’89) has found the same in glandular
cells of a Dipterous insect, Ptycoptera contaminata.

It is obvious that the discovery of an “ organic axis,” as Van Gehuch-
ten calls it, in amitotically dividing nuclei is more difficult, for here there
is no polar depression or longitudinal arrangement of chromatic fila-
ments to indicate its direction in the resting nucleus. It is usual for
each division of the nuclei of the serosa to take place at right angles, or
nearly so, to the plaue of the previous division. This is well seen in
many multinuclear cells, where one or both pairs of nuclei lie trans-
versely in the cell, and therefore at right angles, or nearly so, to the
direction of the first division (see cells 2 and 3, Fig. 14). In other
cases, however, two consecutive divisions take place in the same direc-
tion (Fig. 14, cell 2). It occurred to me that possibly there was an
organic axis in the nuclei of the serosa which in some cases exerted a
controlling influence upon the direction in which division took place,
but which in most instances was counteracted by influences resident in
the cytoplasm. Transverse divisions of the nucleus (Fig. 12) could then
be accounted for by assuming that the influence of the organic axis is _
dominant in these cases, while oblique divisions would be explainable on
the ground that neither influence was predominant, but that both acted
with about equal force in directions at right angles to each other. A
question of interest in this connection is, whether, when the cytoplasmic
influence is dominant, and tends to make the nucleus divide in a plane
parallel to its organic axis, division actually does take place in that
direction. If such were the case, an organic axis would be a fact’ of
slight morphological importance, and the longitudinal arrangement of
chromatin, which takes place in the earlier stages of constriction (Figs.
4, 6, 7), might occur in any direction, without reference to an organic
axis. If, on the contrary, it were necessary that the longitudinal fila-
ments should be arranged parallel to the organic axis, in order that

138 BULLETIN OF THE

division might take place transversely to the axis, this result could still
be attained by a rotation of the nucleus, even when the tendency was for
the nucleus to divide at right angles to the previous division. It is
obvious that rotation would occasionally be apparent, provided it took
place soon after division, and previous to the absorption of the proximal
end of the connecting filament. I examined a large number of prepara-
tions to find evidence of rotation, but I must admit that the evidence
was slight, and hardly sufficient to establish the hypothesis which I had
formulated. It is therefore put forth provisionally, in the hope that it
may lead to further investigations in this line.

The most striking instance of rotation was found in one of the quadri-
nucleate cells (Fig. 33, nuclei a and )). It is evident that three nuclear
divisions have taken place without any division of the cell, producing
two, three, and four nuclei. The arrangement of nuclei makes it rea-
sonably certain that the dower pair arose by division of one, and the
upper pair by division of the other nucleus of the binuclear stage.
Only under this supposition could the daughter nuclei of that stage
have had the normal arrangement, to which all the neighboring cells
rigidly conform. We further find, that, while the upper pair of nuclei
has arisen by a division in the long axis of the cell, the lower pair has
been produced by division in the transverse axis, and therefore in con-
formity with the law previously stated (p. 136). One nucleus of each
pair (a and 6) retains a remnant of the connecting filament, which is
directed, not toward the sister nucleus, but to a point 90° distant from
it. This condition could have been brought about only by rotation of
the nuclei, which in both cases has been through an are of 90°.

In the serosze from older embryos, the daughter nuclei almost inva-
riably recede from each other in the course of division, The amount
of recession is governed by the length of the cell (Fig. 15). In the
younger membranes, as already stated, the constriction is deep and
narrow, so that the nuclei not infrequently lie very near together
(Fig. 13). In these young membranes, however, the nuclei are larger,
and the cells are usually smaller, than in the old membranes. Since,
moreover, the darge binucleate cells of young membranes almost always
have their nuclei symmetrically placed at the ends, it is probable that
the nuclei gradually move apart after division, as the cell increases in
size.

It will be seen that my interpretation of the primary cause of the
division of these nuclei agrees in part with the hypothesis advanced by
Chun (’90) for the explanation of amitotic division in general. This is,

MUSEUM OF COMPARATIVE ZOOLOGY. 139

in brief, that the object of amitotic division is the distribution of nuclear
material throughout the cytoplasm, with corresponding increase of nu-
clear surface. He considers it the final phase of a series of conditions
which begins with a simple lobed nucleus, and includes branched nuclei
of various degrees of complication. In support of this interpretation,
Chun lays stress on the statement that cell division, after an amitotic
division of the nucleus, has seldom or never been observed with cer-
tainty, thereby implying that amitosis cannot have in view the multi-
plication of cells. I do not consider this as essential to the hypothesis,
nor, in fact, do I believe him correct on this point. The evidence of
cell division after amitosis seems to me abundant and conclusive. It
was observed by F. E. Schulze (75) in Amba polypodia; by Ranvier
(75), Biitschli (76), Flemming (82), Arnold (’87), and others, in leu-
cocytes ; by Kiikenthal (’85), in the lymphoid cells of Annelids ; and by
Carnoy (’85), in various cells of Arthropods. As the foregoing shows,
there is abundant evidence that, in the serosa of the scorpion, division
of the cell sometimes, at least, follows amitotic division of the nucleus.
Furthermore, the extremely regular and well ordered manner in which
the nuclei divide, and the similarity as to size and shape of the daughter
nuclei, seem to me decidedly against the notion that the sole object of
the division is to disseminate nuclear substance in the cytoplasm ; for
in those cases where amitosis is not followed by division of the cell, and
assumably takes place simply for the purpose of dissemination, the
nuclear products are very variable as to number, size, and shape.

II. The Amnion.
Plate I. Figs. 1 and 2; Plate II. Figs. 16-20.

The amnion is much thinner than the serosa, and like it is composed
of a single layer of flat, polygonal cells (Fig. 1, am.). But, while both
the cells and nuclei of the serosa have become enormously larger than
the blastodermic cells from which they originated, those of the amnion
have changed little as regards size. The boundaries of the amniotic
cells are not always visible, and I find that preparations, even when
hardened and stained in the same manner, show the greatest variation
in this respect. As a rule, the cell walls in the amnion are sharply and
clearly defined only in preparations of membranes from advanced em-
bryos. The same is true of the cell walls of the serosa.

In general, the amniotic cell has but one nucleus, which usually occu-
pies the centre of the cell. _Blochmann makes the same statement as to

140 BULLETIN OF THE

the number of nuclei in each cell, and he found no evidence of division
among them. ‘The outline of the nuclei, which measure about 15 p in
diameter, is frequently somewhat irregular or lobed. Like the nuclei of
the serosa, they are flattened tangentially (Fig. 2, nl. am.); but not-
withstanding this, they cause an outward bulging of the cell upon the
serosa, as shown in Figure 2. They contain always one or more highly
refractive, deeply staining nucleoli. The rest of the scanty chromatic
substance is in the form of minute granules, occasionally arranged partly
in a very faint network (Fig. 18, 6 and c). As in the nuclei of the
serosa, chromatic threads frequently unite the nucleoli.

Division of the amniotic nuclei is of rare occurrence. In only one of
my preparations are dividing nuclei at all abundant. The division
takes place without mitosis, but is of a different type from that of the
nuclei of the serosa. The only alteration of the chromatin is possibly
a change in the position of the nucleoli ; I have not been able to detect
any modification of the reticulum. The first sign of approaching divis-
ion is elongation of the nucleus (Figs. 16 and 18, a). A deep narrow
constriction appears at the equator of the nucleus (Fig. 17). This is
followed by the formation of an equatorial septum, at once partition-
ing off the nucleus into two daughter nuclei (Fig. 18, 6). If there are
but two nucleoli, it is the rule to find one in each daughter nucleus ; but
where there are several, they are often unequally apportioned. After
the formation of the septum, the daughter nuclei still adhere to each
other, and division seems always to be attained by deepening of the
equatorial constriction in the plane of the septum (Figs. 18, 6, c, and 19).
I have not found any evidence of a recession of the nuclei before
division of the cell. Furthermore, the rarity of binucleate cells makes
it very probable that cell division follows nuclear division promptly.
As in the serosa, division of the cell takes place by the formation of a
cell wall without marked constriction (Fig 20). The position of the
nuclei in this figure, and the frequency with which nuclei are found
near the boundaries of the cells (Fig. 1, am.) is evidence of the prompt-
ness of cell division after the division of the nucleus.

It is clear that Chun’s hypothesis will not hold in this case, for there
is even less tendency than in the serosa to accumulate nuclei in the
cell. This may be owing in part to the shape of the cell, for it is sel-
dom elongated. It would seem that, in case the cell becomes elongated,
nuclear division takes place and the cell divides immediately after the
nucleus. The orientation of the nuclei with reference to the cytoplasm
of their respective cells would then be accomplished by their migration
to the centre of the cells.

MUSEUM OF COMPARATIVE ZOOLOGY. 141

III. The Ovarian Capsule.

Plate II. Figs. 21-26.

The epithelium of the ovarian capsule is not often easily made out in
ordinary stained preparations, for the nuclei of muscle fibres and con-
nective-tissue cells lie not only just external to the epithelial nuclei,
but frequently in the same plane with them. In most of my prepara-
tions the boundaries of the epithelial cells cannot be seen at all, and I
have therefore confined my attention mainly to those which show them
distinctly. In shape, the cells are more or less irregular, oblong hexa-
gons (Figures 24 and 25 represent typical shapes). The cell walls are
broad and fibrillated, like those of the serosa, though the cells them-
selves are smaller even than those of the amnion. The nuclei are not
only larger in proportion to the cells, but often larger absolutely, than
the amniotic nuclei. The amount and arrangement of the chromatin
in the capsular nuclei (except in a certain phase) is almost precisely
like that already described for the nuclei of the amnion, but there is usu-
ally only one conspicuous nucleolus. The small amount of chromatic
substance, aside from the nucleolus, has a granular appearance, but
sometimes shows indications of a filamentous or reticular arrangement
(see Figs. 21, 23, 24). Seen in face view, the nuclei are circular, and
have a distinct nuclear membrane. The section (Fig. 2, n/. fol.) shows
that they are less flattened than the amniotic nuclei.

Here, again, we have amitotic division, and of precisely the same
type as prevails in the amnion. Apparently, division is not of common
occurrence, for I have been able to find only a few instances, and have,
unfortunately, not seen its earliest stages. Figures 21, 22, and 23 show
the simple manner in which it is effected. As each daughter nucleus
contains a nucleolus, and the ordinary resting nucleus has but one,
division of the nucleolus must precede division of the nucleus. In one
important respect the division of these nuclei differs from that of the
amniotic nuclei. The cell does not divide immediately after the nucleus,
and consequently a great number of cells are binucleate. Some even
contain three nuclei. I have obtained no evidence whatever of cell
division.

142 BULLETIN OF THE

IV. Degenerative Changes.

Plate II. Figs. 14, 24-26; Plate III. Figs. 28, 34.

The striking difference in the appearance of cells and nuclei, and
the different manner of division of the nuclei, exhibited by serose of
different ages, have frequently been referred to. Such changes, in
part at least, I believe to be due to degeneration of the membranes,
which, with the exception of the ovarian capsule, are temporary struc-
tures, soon to be cast off by the embryo. Hence it is not surprising to
find them undergoing degeneration iz toto. The degenerative changes
are about equally well marked in all three membranes; but on account
of the great size of cells and nuclei, the changes are most conspicuous in
the serosa. If the membrane comes from a young embryo, the walls
of the cells are unstainable, and therefore often difficult to make out.
The nuclei have a vesicular appearance, with smooth, rounded contour,
abundant karyoplasm, and scanty chromatic substance. For this reason
the nuclei seldom stand out clearly from the cytoplasm in a stained
preparation, often being no darker than the rest of the cell.

Serose from somewhat older embryos, while giving no sure signs of
degeneration, have nuclei slightly different from those of the youngest
membranes. The amount of chromatic substance appears to be larger.
It is gathered into denser and more deeply staining masses, and the
nucleoli become larger and more stainable (compare Figures 4 and 5, the
former from an older membrane than the latter). Many nuclei at this
stage become irregular in outline, and are more or less shrunken in
appearance, changes which prepare the way for complete degeneration,
found in membranes from the oldest embryos. The nucleus here
becomes shrunken into a formless mass, which stains deeply and uni-
formly. This condition seems to be due almost wholly to loss of the
karyoplasm, for the nuclear membrane is seen to be drawn closely
over the much condensed chromatic substance. The uniformly staining
effect, however, is generally believed to be produced by the solution of
a part of the chromatin in the karyoplasm ; this is best seen in nuclei
that have not completely degenerated, where the deeply stainable solid
chromatin is immersed in the less stainable matrix. Not all the nuclei
in a membrane are affected to the same degree by the degenerative
change. This is shown in Figure 14, where the nuclei of cell a, and that
of the cell farthest to the left, are more affected than any others. But
in the oldest membranes almost every nucleus has undergone extreme
degeneration.

-?

MUSEUM OF COMPARATIVE ZOOLOGY. 143

It is an interesting fact, that even the most thoroughly degenerated
membranes have numerous nuclei in all stages of division. The divid-
ing nuclei have undergone the same degenerative alteration as the rest.
It is impossible to state whether these nuclei had begun to divide after
the regressive change, or had been overtaken by these changes while
undergoing division; and it is equally impossible to say whether
degeneration would have prevented the nuclei from completing their
division. The division is essentially like that of younger nuclei, but
often unsymmetrical.

Not all the degenerative changes are confined to the nuclei. The
cells also give evidence of modification. Their walls become more
distinct, not only because they are denser and thicker, but on account
of their stainability with hematoxylin. The cytoplasm frequently has
a reticulated structure, which is densest about the nucleus. In the
oldest membranes, certain large groups of cells have nuclei surrounded
by a narrow bright ring, and outside this a much broader halo of a
radiating structure, which takes a deeper stain than the rest of the
cytoplasm (see Fig. 34). The appearance of the whole is strikingly
like that of the “attraction spheres” of ovarian and other cells, but in
this case has certainly nothing to do with mitosis. If the cell contains
two nuclei, or a dividing nucleus, each daughter nucleus is surrounded
by a halo. In early stages of division, however, the elongated nucleus
has a single halo. I am unable to account for these appearances ; I do
not regard them as attraction spheres, but rather as a result of degener-
ation. The attraction sphere should radiate from a centrosome ; here
it radiates from the nucleus as a centre. I may state, in passing, that
my search for centrosomes in the serosa has been wholly unsuccessful.
The pale ring is very generally present around nuclei that have under-
gone degeneration. It seems to have no intimate connection with the
radiating zone, being frequently found where the latter is absent.

The life history of the serosa cells corresponds closely with that of
certain cells in the Malpighian vessels of Aphrophora spumaria de-
scribed by Carnoy (’85, p. 219). The cells at the two extremities of
the tubes contain nuclei not greatly different from those of young
serose, but the nuclei of the middle portion are irregular, jagged,
and filled with amorphous chromatin. They therefore bear a strong
resemblance to the degenerated nuclei of the serosa. Furthermore, the
origin of the peculiar nuclei of the middle portion of the Malpighian
vessel agrees closely with that of the degenerated nuclei of an old
serosa. It is thus described by Carnoy (p. 220): ‘Sur les petites

144 BULLETIN OF THE

larves on rencontre tous les intermédiaires entre les noyaux des extrémi-
tés et ceux du milieu. Peu a peu le boyau s’efface, Je noyau lui-méme
se rétrécit et perd la regularite de ses contours a cause du plissement
de sa membrane ; a la fin la nucléine ne forme plus a l’intérieur qu’une
masse compacte et homogene, & peu prés comme cela se présente dans
la téte des spermatozoides.” In both cases the degenerated nuclei are
found in stages of division ; in both, the cytoplasmic reticulum is distinct
only in old cells, and where these cells are binucleate it is dicentric,
with filaments radiating from the nuclei. The dicentricity of the binu-
cleate cells is a point to which Carnoy calls special attention (p. 229).
He considers that here the radiating filaments of the cytoplasmic retic-
ulum answer to the polar asters of karyokinesis, and that the nucleus
has the function of a centrosome. The same reasoning would apply
to the degenerated cells of the scorpion’s serosa.

The regressive metamorphosis undergone by the epithelial cells of
the ovarian capsule (Figs. 24-26) is very peculiar. Here, again, the
cell walls are affected in the same way as in the serosa and amnion, for
they are not distinctly seen until after the nuclei have degenerated.
Nearly all of the epithelial cells of an old capsule have two nuclei, which
are dissimilar in size and appearance (Figs. 24 and 25). The smaller
takes a rather deep, uniform stain, almost as dark as that of the chro-
matin of the other. A nucleolus is always present, and frequently
minute granules of chromatic substance. The uniformly staining char-
acter of the nucleus is doubtless produced by chromatic substance held
in solution by the karyoplasm, a condition of common occurrence with
degenerating nuclei. The larger nucleus (Figs. 24 and 25) takes oniy a
slight stain, owing to the scantiness of its chromatic substance, which is
present in the usual form of isolated granules and an imperfect network.
By examination of a large number of cells, I found nuclear differentia-
tion of every degree, beginning with nuclei almost alike in size and
stainability (Fig. 24), then passing to examples of marked dissimilarity
(Fig. 25), where the pale nucleus has become almost invisible, and the
smaller deeply staining one has attained a very sharp, definite outline.
As the pale nucleus becomes more and more shadowy, its shape becomes
irregular. Near cells of this sort others can be found which contain only
a single deeply staining nucleus (Fig. 26), the other having disappeared
altogether. In case of trinucleate cells, I have invariably found two of
them to be of the pale sort.

I am unable to offer any other explanation of these changes than that
they are the result of degeneration or of decreased activity of the tissue.

MUSEUM OF COMPARATIVE ZOOLOGY. 145

But why one nucleus should become altered in one way, and the other
in an entirely different manner, is difficult to say. A very similar dif-
ferentiation of nuclei has been observed by Chun (’90) in the egg germs
of a Siphonophore (Stephanophys). He found only one nucleus in the
youngest germs, while the middle-sized and larger egg cells contained
two of different size, the larger being pale, and the smaller staining in-
tensely. The smaller nucleus moves to the periphery of the egg and is
no longer visible when the latter is ripe. ‘The larger nucleus persists
as the germinative vesicle. In only one instance did he see a stage
that showed that the smaller nucleus budded out of the larger. Chun
compares the small, deeply staining nucleus to the “ Stoffwechselkern”
(macronucleus), and the pale one to the “ Fortpflanzungskern ” (micro-
nucleus) of the ciliate Infusoria.

Summary.

1. The embryo of the scorpion is enveloped by three membranes, the
ovarian capsule, the serosa, and the amnion.

2. The ovarian capsule is an enlargement of the ovarian tube ; the
serosa and amnion arise from the blastoderm of the egg.

3. Serosa and amnion are at first distinct, and joined to each other
by minute fibres. These afterwards disappear, and the membranes
coalesce,

4, The serosa is composed of immense flat cells, very variable in size
and shape. The cell walls are fibrillated.

5. The majority of the serosa cells have two large nuclei of equal
size. ‘There are rarely more than two.

6. The nuclei are disk-shaped, have a distinct nuclear membrane, and
chromatin in the form of granules and filaments, the latter forming an
indistinct reticulum. There are usually several nucleoli.

7. The cytoplasm of the serosa has a distinct reticular structure.

8. Nuclear division in the serosa is amitotic, and takes place by con-
striction, preceded by elongation of the nucleus. It is followed or ac-
companied by recession of the daughter nuclei, which remain for some
time comnected by a fine strand.

9. Constriction of the nucleus is usually accompanied by a longitudi-
nal arrangement of some of the chromatic threads, radiating from the
constricted part. The nucleoli are distributed about equally to the
daughter nuclei. ;

VOL. XXII. — NO. 3. 10

146 BULLETIN OF THE

10. Nuclear division may be followed by division of the cell, but not
often immediately. The cell divides by the formation of a cell wall,
either with or without constriction.

11. The binucleate condition of cells is independent of their size ;
but, in general, the size of the nucleus, or nuclei, is proportional to the
size of the cell.

12. Elongated cells of the serosa are generally binucleate. The
nuclei almost invariably lie in the long axis of the cell, near the ends.

13. A binucleate cell becomes trinucleate by division of one of its
nuclei, and quadrinucleate by the division of both. Very rarely the
division is tripartite, and the three nuclei are produced simultaneously
from a single one.

14. Division of the amniotic nuclei is also amitotic, but the constric-
tion is supplemented by a septum at the equator of the elongated
nucleus.

15. ‘There is apparently no rearrangement of the chromatic substance.
Nucieoli are apportioned equally to the daughter nuclei.

16. Division of the nucleus is quickly followed by division of the cell,
so that binucleate cells are not common.

17. The epithelium of the ovarian capsule is composed of small
hexagonal or rectangular cells, which frequently contain two or more
nuclei.

18. The nuclei are very similar to those of the amnion, but usually
contain only one nucleolus.

19. Nuclear division is amitotic, and precisely like that of the amni-
otic nuclei. Each daughter nucleus contains one nucleolus.

20. No instance of cell division was observed.

21. All three membranes undergo degeneration as the embryos ap-
proach maturity.

22. In the serosa the cytoplasmic reticulum becomes more distinct,
and is seen to radiate from the nuclei. The cell-walls become stainable.

23. The chromatic substance of the nuclei becomes grouped into
dense masses; the reticulum and nucleoli become more distinct. The
outlines of the nuclei become irregular.

24. As degeneration proceeds, the cytoplasm frequently forms a halo
of radial structure around the nucleus.

MUSEUM OF COMPARATIVE ZOOLOGY. 147

25. The nuclei finally become reduced to uniformly staining, irregu-
lar masses of chromatin, which has partly entered into solution. Such
nuclei are found in all stages of division.

26. In binucleate cells of the ovarian epithelium the nuclei become
dimorphic.

27. The chromatic substance of one of the nuclei enters into solution
in the karyoplasm, and the nucleus becomes reduced in size.

28. The other nucleus loses its stainability, and increases in size. It
finally disappears.

V. Discussion of Amitosis.

As long as karyokinesis was supposed to be a uniform process, all the
complicated details of which were carried out with the greatest exact-
ness and in the same sequence, wherever it occurred, no one sought to
homologize it with the little known and far simpler “direct” division.
The latter had, apparently, so restricted a range, and had received so
little attention, that its very existence was denied ; and it was generally
anticipated that, in the few kinds of cells in which it was stated to oc-
cur, a better technique and more careful study would reveal mitotic
phenomena. This opinion seemed to receive confirmation by the dis-
covery of mitotic division in leucocytes and the Protozoa, thus carrying
mitosis back to the simplest types of cells and to the lowest forms
of life. The ascertainment of two facts has brought about a radical
change in our views regarding amitosis: (1) the variability of karyo-
kinesis, including, in some cases, the omission of apparently essential
steps ; and (2) the wide occurrence of amitosis, new instances of which
are constantly coming to light in various parts of the Animal Kingdom.
Inasmuch as it became necessary to recognize the existence of direct
division, efforts were naturally made to find links connecting it with
mitosis ; the variability of both mitosis and amitosis seemed to lend
strength to the theory which refers them to a single fundamental plan
of division. In this scheme, amitosis is considered either as a primitive
method from which mitosis was evolved, or else is looked upon as a
degenerate form of mitosis, occurring in nuclei which, from their patho-
logic or exhausted condition, have lost the power of dividing by the more
complicated process. By fixing epithelium of the salamander larva with
osmic acid, then treating it with Miiller’s fluid, and finally staining with
hematoxylin, Pfitzner (’86*) has shown conclusively that, even in cases of
very perfect mitosis, the karyoplasm maintains its integrity, and divides

148 BULLETIN OF THE

by a simple constriction, as in direct nuclear division. This fact has led
Waldeyer (’88) to the conclusion that karyokinesis in based upon the
simple scheme of division conceived by Remak. He says: “I would
interpret the facts in such a way that we have to regard as the funda-
mental form the simple amitotic division, which is now proved for many
cases ; it always takes place where the nucleus either is poor in chroma-
tin, or when it does not matter about strict bipartition of the chromatic
material. Should the latter be required, then we shall find mitosis,
since it is the most direct, most certain, and most simple manner in
which an exact bipartition of chromatic substance is brought about.”

It seems to me, however, that there are differences of so fundamental
a character between mitosis and amitosis, as at present understood,
that it is impossible to refer them to a single plan of division. Both,
indeed, achieve the same result, — division of the nucleus, including its
two constituents, chromatin and karyoplasm. In both cases, the karyo-
plasm divides by constriction. In amitosis, the chromatin undergoes
little if any change in preparation for division ; in mitosis it becomes con-
solidated into a limited number of thickened rods or loops (chromosomes),
which arrange themselves in the plane of division (“ mother star,”
“couronne équatoriale”?) and segment either longitudinally or trans-
versely, the halves moving to opposite poles (“ diaster ”), and undergo-
ing a reversed metamorphosis to form two daughter nuclei. If this
were all there is to karyokinesis, — and in some cases the process is
much simpler, — we might hope to find transitions between it and ami-
tosis ; for there are examples of amitosis in which the chromatic net-
work undergoes changes during division, and it would be conceivable
that the highly organized changes of the chromatic substance during
mitosis were either evolved from them, or that they were a simplifica-
tion of the more detailed changes. In mitosis, however, other struc-
tures besides chromosomes make their appearance, — the centrosomes,
attraction spheres, and spindle. These structures are not known to take
any part whatever in amitosis, and in this respect at least the two kinds
of division are fundamentally different. The most recent workers upon
karyokinesis agree in assigning to the spindle rays the function of
separating or dividing the chromosomes, and drawing (or pushing) the seg-
ments towards the poles. ‘The centrosomes are focal points towards which
the spindle rays converge, and lie entirely outside the nucleus. The for-
mation of the spindle has been carefully studied by many investigators
of karyokinesis, and, while there are very divergent views as to its ori-
gin and mode of action, the most recent workers in this field (of whom

MUSEUM OF COMPARATIVE ZOOLOGY. 149

E. van Beneden, Boveri, and Watase may be mentioned) are agreed that
the spindle arises from the cytoplasm. The same view with regard to
the spindle in the mitosis of vegetable cells was expressed by Stras-
burger, Guignard, and other botanists.

The centrosome, as a converging point for the spindle fibres and
polar rays, plays a most important part in karyokinesis, and, so far as
known, none at all in amitosis. The centrosome has indeed been found
by Flemming (’91) in leucocytes, which certainly divide amitotically ;
but there it is a single structure, and as Flemming’s figures show,
takes no part in the amitotic division of the nucleus. Whether it also
remains passive during the mztoézc division of leucocytes and in amitosis
followed by division of the cell, is not known. It has been supposed by
Carnoy (785) that spindle rays were present in certain nuclei which
divide amitotically, but this seems extremely doubtful, especially since
they have no perceptible action on the chromatic substance. I believe
it can be shown in every case of amitosis known, that the division of
the chromatin is accomplished ixdependently of chromosomes, spindle rays,
or any other visible influence outside of the nucleus.

The persistence of the nuclear membrane in amitosis, and its dis-
appearance in mitosis, were formerly considered points of distinction
between the two kinds of division; but, as is well known, more recent
studies have shown that the membrane persists in many cases of un-
doubted karyokinesis, especially among the Arthropods (Carnoy, ’85) and
Protozoa (Gruber, ’83, R. Hertwig, ’84, Pfitzner, ’86°, and Schewiakoff,
’88). Its presence seems to offer no obstacle to the karyokinetic
changes, and Watase (’91) has pointed out that it need not prevent
the formation of an extra-nuclear spindle, the rays of which may pene-
trate the membrane. In the nuclei of Opalina ranarum, and in the
micronuclei of Infusoria generally, where, according to all observers, the
nuclear membrane persists, the mitotic division is accompanied by con-
striction ; but the fact that constriction is here wszble may be considered
as in some measure a result of the persistence of the membrane, thereby
making evident the outline of the karyoplasm. Yet constriction does
not always take place when the membrane persists, for in the spermatic
cells of Pagurus striatus, figured by Carnoy (’85, Plate VII. Fig. 244),
the nuclear membrane is visible at all stages, and gives no evidence of
constriction.

The modification of the chromatic substance into chromosomes is
usually the most conspicuous feature of karyokinesis, and in most cases
serves to distinguish mitotic nuclei from any of the amitotic ones. The

150 BULLETIN OF THE

chromosomes invariably include all the stainable substance of the nu-
cleus, so that the presence of nucleoli in a nucleus undergoing constric-
tion may be taken as perhaps the strongest evidence of direct division.
The behavior of nucleoli in amitosis is of peculiar interest. Where
theré is a single nucleolus, it constricts previous to the constriction of
the nucleus, according thus with the Remakian scheme. The division
of the nucleolus, however, has rarely been observed. It was first de-
scribed, I believe, by F. E. Schulze (75), in the division of Ameba poly-
podia; has since been figured by Carnoy (’85, Plate I. Figs. 10, 12, 13)
for various amitotically dividing Arthropod cells, and by Hoyer (’90)
for the intestinal epithelium of Rhabdonema nigrovenosum. A peculiar
modification of the nucleolus, and its division into four segments pre-
vious to the constriction of the nucleus, was observed by Platner (’89,
pp. 145-149) in the Malpighian vessels of Dytiscus marginalis. It is
extremely probable that, whenever the nucleolus is a single and defi-
nitely organized structure, it always divides previously to or during con-
striction of the nucleus. Where there are several small nucleoli, they
may indeed arrange themselves so as to be equally apportioned to the
daughter nuclei ; but they are not known to divide, as the chromosomes
in mitosis do.

Amitotic division, even more than karyokinesis, is variable in its
phenomena. It takes place by constriction, by formation of division
planes, by gemmation, and by enlargement of one or more perforations
(Arnold, ’88, Flemming, ’89). It is either simple or multiple, and it
may or may not be accompanied by division of the cell. The resulting
nuclei may be equal or unequal. Amitosis occurs throughout both the
Animal and Vegetable Kingdoms ; but as far as animals are concerned,
it is far the most frequent among wnicellular organisms, ameebord cells
(leucocytes), and epithelial tissues. There seem to be no authentic instances
of it in connective tissues (except possibly the fat-cells of Arthropods,
described by Carnoy), none in nervous tissue, and but one or two in
muscle fibres (Carnoy, ’85, p. 221). Not only the nuclei of fixed tissues
divide by the direct method, but also those of nascent tissues, at least
among the Arthropods. Direct division is, however, of rare occurrence
in the embryo. I believe there are only two authentic instances of
it, — that discovered by Carnoy in the ventral plate of an embryo of
Hydrophilus piceus (’85, p. 224, Plate I. Fig. 11), and that found by
Wheeler (’89, p. 313) in the formation of the blastoderm of Blatta
germanica, where no instance of mitosis was detected. The embryonal
membranes of the scorpion I do not include under this head, because
they are temporary structures forming no vital part of the embryo.

MUSEUM OF COMPARATIVE ZOOLOGY. 151

Among the Metazoa, epithelial tissues offer by far the greatest num-
ber and the most interesting cases of amitosis. Furthermore, as Ziegler
(91) has very recently shown, epithelial cells of unusual size, with some
peculiar functional activity (generally secretion) are most apt to exhibit
this method of division. Cell division has seldom been observed to fol-
low amitosis in such large cells, which therefore become multinucleate.
Other epithelial cells which frequently furnish instances of amitosis are
those which are near the end of their functional activity. Cells of the outer
layer of a stratified epithelium sometimes divide amitotically, while
those of the deeper (and therefore younger) layers of the same epithelium
divide by mitosis. A good instance of this was recently described by
Dogiel (’90) in the epithelium of the bladder of Mammals. The nuclei
of the large epithelial cells lining the intestine of Arthropods very com-
monly divide by amitosis, as was found by Frenzel (’85) in the midgut of
Astacus and Maja; by Carnoy (’85) in the intestinal epithelium of Iso-
pods; and by Faussek (’87) in the digestive tract of a Cricket (Hremobia
muricata) and in the larva of #schna. The intestinal epithelium in all
Arthropods has an important secretory function. Cells whose function
is excretory likewise exhibit amitotic division of the nucleus, as in the
Malpighian vessels of Insects. The occurrence of amitosis in glandular
and excretory epithelium is readily explainable on Chun’s hypothesis,
for the functional activities of such cells are peculiarly intense, and it is
easy to see that a distribution of nuclear material in the cytoplasm is
of advantage to the cell. The occurrence of nuclei of unusual size (as
compared with the nuclei of other cells of the same animal) seems to
me likewise referable to the peculiar needs of the cytoplasm in these
cells.

Cases of amitosis peculiarly difficult of explanation are those pre-
sented by the germinal epithelium of the testis. So many observers
have reported direct division in sperm mother-cells, that there seems
no reasonable doubt of its occurrence. It has been suggested that the
cells which divide amitotically never produce spermatozoa, but merely
serve to secrete a fluid. This explanation, however, will not serve in
the case of certain Isopods (Oniscus asellus and Idotea sp.) in the testes
of which Carnoy (85, p. 222) found amitosis. the prevailing type of di-
vision, and mitosis of very rare occurrence. Direct division is found
more or less frequently in the testicular cells of many other Crustacea,
as the extensive work of Gilson (’84-87), and the investigations of
Sabatier (’85) show, and occasionally in the other groups of the Arthro-
pods. Among Vermes, it was found by Lee (’87) in Nemertians, and

152 BULLETIN OF THE

by Léwenthal (89) in a Nematode (Oxyuris ambigua). It need hardly
be said that amitosis in sexual cells is unexplained by any hypothesis
yet offered regarding the biological significance of this type of division,
and further investigations on this point are absolutely necessary before
we can form any general opinion in regard to it.

In the maturation and segmentation of the ovum no instance of direct
division is known, and it is here that karyokinesis is exhibited in its
most complete form. The well known observations of Boveri (’87) on
the segmentation of the egg of Ascaris megalocephala are of special in-
terest on this point. He found a modification of the chromatic threads
as early as the two-blastomere stage, one of them (cell A) retaining the
four chromosomes characteristic of the nucleus after fertilization, the
other (cell B) undergoing a reduction of its chromosomes into the form
of granules. The two blastomeres arising by division of cell A undergo
the same differentiation, the nucleus of one (cell A’) retaining the
chromatic loops, the other (cell A?) undergoing reduction, so that in
the four-cell stage only one nucleus has retained its chromatic loops.
The systematic reduction of chromosomes was observed up to the 64-
cell stage. The important deduction Boveri makes from these facts
is, that the cells retaining their ancestral nuclear characters are the
Anlage of the sexual cells of the developing animal, and that the cells
whose nuclei undergo a modification of the chromosomes are all somatic
cells. In accordance with this hypothesis, the division of both male
and female sexual cells ought always to be karyokinetic, and of a
somewhat different type from the karyokinesis of the somatic cells of
the same animal. The latter statement, indeed, holds true for the
testicular cells of the salamander, as was discovered by Flemming (’87).
It also appears from the work of Carnoy, that in the post-embryonic
life of Arthropods mitotic division is of rare occurrence in the tissue
cells, but is of constant occurrence in the reproductive cells of the same
forms.

As has already been stated (p. 147), attempts have been made to
find a morphological connection between karyokinesis and direct divis-
ion, and thus to solve the puzzling question of the relations they bear
to each other. Carnoy (’85, p. 398) believes he has found transitions
between them in the division of the numerous nuclei of Opalina rana-
rum. Some of these show a distinct spindle, others none ; in both cases
the nuclear membrane persists, and division is accomplished by constric-
tion. Pfitzner (86°), however, found only mitosis in 0. ranarum. Car-
noy has also seen transitional forms of division in the spermatic cells of

MUSEUM OF COMPARATIVE ZOOLOGY. 153

Pagurus striatus, and P. callidus (Planche VII. Figs. 244, 245). A nu-
clear plate is here formed, both in perfect mitosis and in degenerated
mitosis; but in the former instance a spindle is formed, and the chromo-
somes segment individually, while in the latter the plate divides in t&to
by constriction, without the help of a spindle. This modified type of
mitosis, if we may so regard it, Carnoy considered as the result of
degradation (pp. 316, 317), inasmuch as it appeared only in old sperm
mother-cells after spermatozoa had become numerous in the testis.
This accords with the earlier view that direct division is concomitant
with senescence of the nuclei, based especially upon nuclear division in
plants (Schmitz, ’79, Johow, ’81). I have regarded this as a possible
explanation of the occurrence of amitotie division in the embryonal
envelopes of the scorpion, for these tissues are temporary structures
which obviously are near the end of their functional activity. This
explanation, however, will not fit all cases; for instance, the occurrence
of amitosis in embryonic cells, and its prevalence in the testicular cells
of some Isopods, already mentioned.

The hypothesis advanced by Chun seems to throw light upon many
of the cases of amitotic division which are referable to a sort of bud-
ding or branching of the nucleus, carried to such a point that the
buds or branches become constricted off as separate nuclear elements.
These cases are, of course, not to be confounded with a disintegration
of the nucleus, such as takes place in the macronucleus of Infusoria
after conjugation, and sometimes in the degeneration of tissues. The
distribution or extension of nuclear substance in the cytoplasm, whereby
the surface of the nucleus is increased, is an event of frequent occur-
rence. It is seen in the many forms of lobed nuclei, such as those of
the ovarian capsules of Amphibia (see Flemming, ’82), and in those of
leucocytes ; in hollow or perforated nuclei (giant cells); in branched
nuclei (spinning glands and Malpighian vessels of Lepidoptera) ; and in
the band-shaped and moniliform nuclei of many Infusoria. These pecn-
liar shapes are evidently produced by the activity of the nucleus itself,
probably correlated with a special function of the cytoplasm. From
the deeply incised lobation or band-shape of such nuclei it is an easy
step to the formation of separate smaller nuclei by the deepening of a
constriction already formed. »Such daughter nuclei will as a rule be
irregular in shape and unequal in size ; but if their production subserves
a definite and important function, we should expect that in some cases
their formation would become a regular process, governed by definite
laws. It is possible that the more symmetrical kinds of direct division

154 BULLETIN OF THE

are to be explained in this way, and such an explanation seems to apply
well, as suggested on a preceding page, in the case of the scorpion’s
serosa. Division of the cell does not follow as a rule, and upon this fact
Chun lays stress. But, so far as we know, there is nothing to exclude
the subsequent occurrence of cell division, and it is even probable that
cell division is induced by the presence of more than one nucleus. This
I take to be the case in the scorpion’s serosa, where I believe the division
of the cell is due in part to the dicentricity set up in the cytoplasm by
the division of the nucleus.

The study of nuclear division among the Protozoa seems likely to
throw much light upon the relations of amitosis to mitosis, for there can
be little doubt but that this group presents the most primitive types of
nuclear division. So far as known, the very lowest forms of animal cells
(Amebe) always divide by the direct method, as the study of Ameba
polypodia by F. E. Schulze ?75), and of Pelomyxa villosa, Amaba secunda,
and A. proteus by Gruber (’83 and 785), has shown. The division of the
nucleus of Ameba proteus takes place by a sharp equatorial cleft, passing
through the large, centrally placed nucleolus, and dividing that and the
peripheral zone of chromatin into two exactly equal halves, which after-
wards move apart. This is regarded by Gruber (’83, p. 385) as a simple
type of karyokinesis, because an exact division of the chromatin is accom-
plished. No kinetic change of the chromatic substance is necessary to
bring this about, hence none occurs. It seems to me that the absence
of centrosomes and a spindle effectually separates this type of division
from true karyokinesis, and until these are discovered, the nuclear di-
vision of Amaba proteus must be relegated to amitosis. The presence of
so perfect a type of karyokinesis as that found in Huglypha alveolata,
worked out so completely by Schewiakoff (’88), is strong evidence against
the hypothesis that karyokinesis was gradually evolved from direct di-
vision. For here, among the lowest forms of animal life, we have nuclei
dividing both by a simple constriction, and by the most highly developed
kinetic changes.

Nuclear division among the Infusoria is of special interest, for we
regularly find in the same individual nuclei very different in structure
and function, — macro- and micronuclei. The former divide directly,
the latter by karyokinesis. Apparent exceptions are seen in Spirochona
gemmipara, where, according to R. Hertwig (’77) the macronucleus
divides by karyokinesis ; and in Opalina ranarum, studied most carefully
by Pfitzner (’86>). As only one kind of nucleus is found in Opalina,
it is probable, as Biitschli suggests (788, p. 1500), that these are of

~

MUSEUM OF COMPARATIVE ZOOLOGY. 155

the micronuclear type, inasmuch as the division is in all essential re-
spects like that of micronuclei, and in the resting state the nuclei bear
no resemblance to macronuclei. The direct division of macronuclei is
often accompanied by a longitudinal arrangement of the chromatic fila-
ments, resembling that found in the scorpion’s serosa (see Figs. 6, 7, 8).
It seems to me that Carnoy is wrong in speaking of these longitudinal
filaments as a “spindle,” for it has never been shown that they converge
to the poles of the nucleus, and frequently they can be resolved into
granules, which is never the case with spindle fibres. Their resemblance
to the spindle of karyokinesis is deceptive. From their behavior with
stains, I regard them as consisting of chromatin, and Biitschli (’88,
p- 1526) speaks of this stage of the macronucleus as the ‘“ Kniuelsta-
dium,” implying that the parallel filaments are chromatic threads.

Among the Vertebrates, amitosis is unusual, and where it exists kary-
okinesis is generally found to occur in cells of the same kind. It is
almost confined to cells which do not form fixed tissues, as leucocytes of
all kinds, and “giant cells,” especially those of the red marrow. It also
occurs in testicular cells of Vertebrates. In leucocytes, according to
all observers, the nuclear division takes place by constriction, and is
frequently accompanied by division of the cytoplasm (Rauvier, 75;
Flemming, 782, p. 344; Arnold, ’87). But, as the recent work of
Fiemming (91) and others shows beyond a doubt, leucocytes also di-
vide by karyokinesis. It is difficult to say whether there is more than
a single kind of leucocyte, one dividing directly, the other indirectly,
or whether cells of the same kind divide in two different ways. In
ease of giant cells, it has been shown by Arnold (’84), Denys (’86),
Demarbaix (’89), and others, that division occurs both directly and by
multiple karyokinesis. Both kinds of division are followed by division
of the cytoplasm, leading to the formation of a brood of daughter cells
within the mother cell.

After going over the literature of amitosis, taking especial note of
the manner of its occurrence and distribution in the Animal Kingdom, I
have become convinced that it is not derived from mitosis, and, on the
other hand, is not the forerunner of the more complicated process. I con-
sider it another type of division altogether, which, along with karyoki-
nesis, has been transmitted from the simplest forms of life to the most
highly organized. While apparently every kind of nucleus may, at
some stage of its existence, divide by karyokinesis, many afterwards
exchange this type of division for the simpler process. The special
conditions which evoke the exchange are very imperfectly understood,

156 BULLETIN OF THE

and no hypothesis has yet been offered that will explain all the known
instances. Some of the hypotheses that have been suggested I have
already dwelt upon at length; others, as scantiness of chromatin, and
even its entire absence in the nucleus (Léwit, 90), seem to me still more
inadequate. 1

One fact in favor of the independence of the two types of division is
the sudden change from mitosis to amitosis, without any visible interme-
diate stages. Phylogenetically, this is seen in the abrupt transition from
the amitotic division of Amebe to the very perfect karyokinesis of the
nearly related Huglypha. Ontogenetically, of course, the exchange is far
more abrupt. In the conjugation of Infusoria, all divisions of the micro-
nucleus are undoubtedly mitotic, while the jirst (after conjugation) and
all subsequent divisions of the macronucleus, ztself formed from modified
micronuclei, are by direct division. Again, the amitosis of the blasto-
dermic nuclei of Blatta (Wheeler, ’89) is an abrupt change from the
perfect mitosis of segmentation. Other instances are the sudden change
from mitosis to amitosis in the layers of stratified epithelium, and in
the generations of spermatic cells.

Another fact in favor of my view is the almost universal distribution of
amitosis, and its occurrence in many kinds of cells with widely different
functions. It seems more reasonable to suppose that a process so widely
extended is inherited, and exists potentially in all cells, rather than to
look upon it as independently assumed in a multitude of special cases.
The latter supposition is opposed to all we know of the transmission of
fundamental characters.

While it is evident that both mitosis and amitosis appeared at a very
early period of organic life, it is impossible to say which appeared first.
But, on a priort grounds, we may conclude that the simpler type pre-
ceded the more complex.

CAMBRIDGE, September 28, 1891.

It was not until this paper had gone to press that I had access
to the recent communications on amitosis by Flemming (’91*), Léwit
(791), Verson (’91), Frenzel (’91), and O. vom Rath (91). Jn his review
of recent work on cell division, Flemming says (p. 139): “ Es ist also
nicht nur als feststehend anzusehen, dass Amitose vorkommt, sondern
auch, dass sie in normal lebenden Geweben vorkommt, und dass sie zur

MUSEUM OF COMPARATIVE ZOOLOGY. 157

Zellenvermehrung fiihren kann.” When, however, both mitosis and
amitosis occur in the same tissue, he considers it probable that only the
former is the zorma/ method of regeneration and of growth.

The brief papers by Lowit, Verson, and Frenzel are replies to Ziegler’s
(91) recent article on amitosis, and contain little that is new. Verson
describes briefly the early stages in the spermatogenesis of the silkworm
(Bombyx mori). He states that the spermatocytes originate from a
single large nucleus (‘“‘ Riesenkern”), which divides repeatedly and
unequally by amitosis. The small daughter nuclei thus produced divide
by mitosis, and at length form the spermatocytes. Frenzel adduces
instances of amitosis in the intestinal epithelium of Crustacea and
Insects which do not fall within Ziegler’s generalizations.

Vom Rath’s paper is a valuable contribution to our scanty knowledge
of the occurrence of amitosis in spermatogenesis. He shows very con-
clusively that, in the testis of the crayfish, amitosis does not occur in
the generations of sperm-forming cells, but only in abortive nuclei
(‘‘Randkerne”’), which soon degenerate into an amorphous mass. If
such a fate could be established for all amitotically dividing nuclei in
the testes of animals, it would be much easier to form a logical estimate
of amitosis.

158 BULLETIN OF THE

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MUSEUM OF COMPARATIVE ZOOLOGY. 159

Flemming, W.

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’89. Amitotische Kerntheilung im Blasenepithel des Salamanders. Arch. f.
mikr. Anat., Bd. XXXIV. p. 437.

"91. Ueber Theilung und Kernformen bei Leucocyten, und uber deren At-
tractionsspharen. Arch f. mikr. Anat., Bd. XXXVLII. p. 249.

91". Ueber Zellteilung. Verhandl. Anat. Gesellsch., 5. Versamm., p. 125.

Frenzel, J.

’°85. Ueber den Darmkanal der Crustaceen, nebst Bemerkungen zur Epithel-
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°91. Zur Bedeutung der amitotischen (direkten) Kernteilung. Biol. Cen-
tralb , Bd. XI. p. 558.

Gehuchten, A. van. ~
789. L’Axe organique du noyau. La Cellule, Tom. V. p. 177.
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'84~87. Etude comparée de la spermatogénése chez les Arthropodes. La

Cellule, Tom. I. p. 11; Tom. II. p. 83; Tom IV. p. 1.
Goppert, E.

"91. Kerntheilung durch indirekte Fragmentirung in der lymphatischen
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Gruber, A.

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’°85. Studien iber AmGben. Zeitschr. f. wiss. Zool., Bd. XLI. p. 186.

Hamann, O.

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’84. Ueber die Kerntheilung bei Actinosphaerium eichhorni. Jena. Zeitschr.,
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Hoyer, H.

90. Ueber ein fiir das Studium der direkten Kerntheilung vorziiglich

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Kikenthal, W.
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LV. p. 409.

Lowenthal, N.
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71. Embryologie des Skorpions. Zeitschr. f. wiss. Zool., Bd. XXI. p. 204.
Overlach, M.
85. Die pseudomenstruierende Mucosa uteri nach acuter Phosphorvergilt-
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MUSEUM OF COMPARATIVE ZOOLOGY. 161

Schmitz, F.
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EXPLANATION OF FIGURES.

All figures are from drawings made with the aid of an Abbé camera.

Jounson. — Nuclear Division.

PLATE I.

Fig. 1. Five cells of the serosa, two of them covered by the amnion, which is
omitted from the rest of the figure for the sake of clearness. am.,
amnion; sr., serosa. XX 180.

Fig. 2. Section through the embryonal membranes and ovarian capsule. The
fibrous appearance of the ovarian capsule is due to the presence of
muscle fibres and connective tissue. The boundary line between
amnion and serosa is visible only in the vicinity of the amniotic
nuclei. e’th. fol., epithelium of ovarian capsule (when the plates were
engraved [I still took this to be the follicular epithelium, hence the
error in the abbreviation) ; nl. fol., nucleus of capsular epithelium ;
nl. sr., nucleus of serosa; nl. am., nucleus of amnion. XX 680.

Figs. 3-15 are all from the serosa.

Fig. 3. Very small, binucleate cell. Xx 180.

Figs. 4-10. Nuclei at different stages of division. vac., vacuole; zx, new nuclear
wall within the old one. X 580.

Fig. 11. Two cells produced by division of a binucleate cell. X 130.

Fig. i2. Cell from the serosa of a young embryo, with dividing nucleus; the axis
of elongation corresponds with the short axis of the cell. X 130.

Fig. 13. Cell from serosa of a young embryo, with nucleus unequally divided and
daughter nuclei eccentric in position. X 130.

JOHNSON - NUCLEAR Division.

B Meisel Jith. Boston

Jounson. — Nuclear Division.

PLATE II.

Fig. 14. Piece of the serosa from an advanced embryo, with four adjacent tri-
nucleate cells (1, 2, 3, 4); nuclei of cell a and the large cell farthest
to left have undergone degeneration. X 90.

Fig. 15. Three cells of the serosa from an old embryo to show recession of daugh-
ter nuclei towards the ends of the cells. X 90.

Figs. 16-20 are from the amnion.

Figs. 16-19. Stages in the division of amniotic nuclei. In Figure 18 three stages ©
are shown, a, b,c. X 800.

Fig. 20. Two amniotic cells, apparently formed by recent division. X 3875.

Figs. 21-26 are from the capsular epithelium.

Figs. 21-23. Cells showing successive stages of nuclear division. X 800.

Figs. 24-26. Cells to show the degeneration of nuclei. In Figure 24 the nuclei
a.e but slightly differentiated; in Figure 25 the pale nucleus has
become much larger and very faint; in Figure 26 it has disappeared
altogether. x 800.

PL I

JOHNSON - NUCLEAR DIVISION

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Jounson. — Nuclear Division

PLATE III.

Figs. 27-34 are all from the serosa.

Fig. 27. A cell undergoing division by formation of a cell plate. The daughter
nuclei are still united by a connecting thread. The dotted line on the
left indicates the edge of the fragment of membrane in which this
cell occurs. From the serosa of an advanced embryo. X 804.

Fig. 28. A cell divided by constriction, without the formation of a cell plate. The
nuclei have undergone degeneration. From the serosa of an advanced
embryo. X 150.

Fig. 29. A cell, the nucleus of which has undergone tripartite division. From an
old serosa. XX 150.

Fig. 80. Nucleus of the same, more highly magnified. The chromatin is grouped
in granular masses. Two of the daughter nuclei are still united by
strands of the nuclear membrane. XX 6980.

Figs. 31-32: Constricted nuclei from a young serosa. One of the daughter nuclei
of each is larger than its mate, and has itself become elongated and
constricted. X 304.

Fig. 33. Quadrinucleate cell. The upper of the two original nuclei has divided
in a longitudinal, the lower in a transverse plane. Nucleus a still
shows a remnant of the connecting thread, and nucleus } retains the
conical form it had in division. Both nuclei have rotated 90° from
the plane of elongation. X 304.

Fig. 34. Cell from the serosa of a far advanced embryo. The nuclei have under-
gone extreme degeneration. Each nucleus is surrounded by a bright
ring, outside of which is a broad zone of a radiate structure, more
stainable than the rest of the cytoplasm. X 150.

soN NucLeAR Division.

No. 4.— A Fourth Supplement to the Fifth Volume of the Terres-
trial Air-breathing Mollusks of the United States and Adjacent
Territories. By W.G. Binney}

Tue following pages are believed to contain, all that has been added
to our knowledge of the subject prior to date.

Students are requested to note that in the Third Supplement, p. 214,
the figures of Arionta Diabloensis and Bridgesi are reversed. On p. 225,
Explanation of Plate VII., the references E and F are reversed: on
p- 226, Explanation of Plate XI., Figures D and G are reversed.

Buruineton, NEw Jersey, July 1, 1891.

Glandina decussata, Dress.

Plate I. Fig 4.

Under the name of decussata, specimens are found in most collections which
can hardly be referred to that species. I have figured one of them, and its
dentition has already been described and figured in my Third Supplement.
The shell is readily recognized by its more cylindrical form. Should it prove
distinct from decussata, I would suggest for it the specific name of Singleyana.
I received it from Bexar County, Texas, collected by Mr. Wetherby.

Selenites Vancouverensis, Lea, var Keepi, Hempnite.

Plate II. Fig. 5.

Shell umbilicated, greatly depressed, thin, smooth, shining, transparent, scarcely
marked by the delicate wrinkles; very light horn-color; whorls over four, some-
what flattened above and beneath, and scarcely descending at the aperture; spire

1 The Terrestrial Air-Breathing Mollusks of the United States and the Adjacent
Territories of North America, described and illustrated by Amos Binney. Edited
by A. A. Gould. Boston, Little and Brown, Vols. I., II., 1851; Vol. III., 1857.
Vol. IV., by W. G. Binney, New York, B. Westermann, 1859 (from Boston Journ.
Nat. Hist.). Vol. V., forming Bull. Mus. Comp. Zo6l., Vol. IV., 1878. Supplement
to same, in same, Vol. IX. No. 8, 1883. Second Supplement, in same, Vol. XIII.
No. 2, 1886. Third Supplement, in same, Vol. XIX. No. 4, May, 1890.

VOL. XXII — NO. 4. 2

164 BULLETIN OF THE

flat, not rising above the body whorl; suture well impressed; umbilicus moderately
large, exhibiting most of the volutions; aperture transversely subcircular, wider
than high; lip simple, thickened, sinuous above, very slightly reflected at the base,
ends scarcely approached. Width ;; inch, height 4; ich.

Hills near Oakland, California. One specimen only.

This rare and interesting little shell I collected some years ago. It is a perfect
miniature form, in every respect, of S. Vancouverensis. I regard it as an extremely
small variety of that so called species. It is about the size of the variety of
S. Duranti, lately described as S. cwlatus, Mazyck, but differs very materially in form,
sculpture, and the general texture of the shell. It differs from var. Catalinensis in
being more robust, larger, and has a smaller umbilicus. I dedicate this pretty little
shell to Prof. Josiah Keep, of Mills College, California, who has done so much
through his interesting little book to stimulate the study of West Coast shells.

The above is Mr. Hemphill’s description, from “The Nautilus,” Vol. IV.
p. 42, 1890. My figure is drawn from an authentic specimen,

Selenites Vancouverensis, var. hybridus, Hempuict.

Shell broadly umbilicated, depressed, slightly convex above, surface shining,
polished, of a dark yellowish green color, lines of growth coarse, rib-like and regu-
lar on the spire, finer and more irregular on the body whorl, crossed by fine revolv-
ing lines that become fainter on the last whorl, suture well impressed ; aperture
rounded, broader than high, greatly indented above ; lip simple, very little reflected
below at its junction with the columella, very sinuous above, its terminations joined
by a very thin callus. Height % inch, breadth 1 inch.

Astoria, Oregon.

In the strong rib-like sculpturing of the spire, depressed form, and sinuous lip, it
resembles sportellus. In its greater diameter, dark greenish color, and the absence
of the decussating sculpture on the last whorl, it approaches Vancouverensis.

All our American Selenites commence life with a finely granulated shell. When
they have attained about two whorls, the stria begin to appear, and increase in
strength as the shell increases in size.

It is well known that all shell-bearing mollusks construct their shells in obedi-
ence to the laws of their constitutional characteristics and the environment, among
which I include affinity of matter and mechanical skill, the latter a faculty pos-
sessed to a greater or less degree by all animals. Some individuals in a colony of
shells display greater mechanical skill than others, or possess stronger imitative
powers, and closely follow the lines and styles of their forefathers, strictly attend-
ing to the details of sculpturing, not omitting a rib or line. Other individuals of
the same colony, not having this imitative faculty so strongly developed, may
change or vary the form of the shell by constructing it with more convex whorls,
generally resulting in a narrower or more elevated shell; or they may flatten the
whorls, resulting in a broader and depressed form. Some modification of the um-
bilicus generally follows the change in the form of the shell. In both cases the
sculpturing may be what we call characteristic of the species, or may be more or
less modified by the omission of one, two, or more ribs, or the ribs may be more

MUSEUM OF COMPARATIVE ZOOLOGY. 165

irregular in shape. A few lines may also be dropped, perhaps some added, or the
entire surface may be modified in obedience to the laws of the mechanical skill
possessed by the individual, and the affinity of matter secreted by the animal, for
the purpose of constructing the shell. An examination of a large number of
Selenites concarus, and of our West Coast forms, convinces one that the entire group
of Americana Selenites is the offspring of a single common type.

The above is Mr. Hemphill’s description, from ‘“‘ The Nautilus,” Vol. IV.
p- 42, 1890.

Selenites Duranti, var. Catalinensis, Hempui tt.

Plate II. Fig. 3.

I figure an authentic specimen. See Third Suppl., p. 221.

Selenites Vancouverensis, var. transfuga, Hempuict.

Shell very much depressed, planulate, broadly umbilicated, of a dirty white
color; whorls 34 or 4, flattened above, more rounded beneath, with regular strong
rib-like striz; suture well impressed, becoming deeper and channel-like as it ap-
proaches the aperture; aperture hardly oblique, slightly flattened above, with a
tendency to a corresponding depression below ; lip simple, roundly thickened inter-
nally, its terminations approaching, forming in some specimens a short columellar
lip, joined by a heavy raised callus in very adult specimens. Height 3; inch,
greatest diameter ;°;, lesser 7% inch.

San Diego, California, to Todos Santos Bay, Lower California.

This is the small flat shell that has been distributed as a variety of sporte/la, and
also as a variety of Voyanus. I find, however, on comparing it with the typical
Voyanus collected by me last fall, that it is quite a different shell. The ribs are
closer and finer than either sportellus or Voyanus, the umbilicus is much larger,
and it is a very much more depressed shell. I consider it, however, a deserter from
the Northern forms, and name it accordingly. It is a much larger and a more
globose form than simplilabris of Ansey.

The above is Mr. Hemphill’s description.

Selenites Vancouverensis, Lra.

The only differences that I can detect between this shell and Se/enites concava,
Say, are these. The umbilicus in the California shells is a little more contracted, the
color is a shade darker, the strie are a little closer, stronger, and more regular, and
the body whorl is a little more flattened at the aperture. Height g inch, breadth
¢ inch.

Sonoma Co. to Santa Cruz Co., California.

The above is Mr. Hemphill’s description of what he calls S. concavus, var.
occidentalis.

166 BULLETIN OF THE

Selenites Vancouverenis, var. tenuis, Hempar.y.

Shell broadly umbilicated, depressed, nearly planulate ; of a dirty greenish brown
color; whorls 5, flattened above, more rounded beneath, the last expanding later-
ally as it approaches the aperture, and crowded with fine oblique striz ; suture well
impressed ; aperture rounded, slightly flattened above; peristome simple, hardly
reflected below. Height } inch, breadth 7% inch.

Napa Co., California.

The small size, nearly planulate form, and thin, lean body whorl as it emerges
from the aperture, will serve to distinguish this shell from the other forms of
concavus found on the West Coast.

The above is Mr. Hemphill’s description. He refers ali these varieties to
concavus, but I use the specific name Vancowverensis for all Pacific Region forms.

Limax Hemphilli.
Plate II. Fig. 1.

Length (contracted) 19mm. Mantle long, 9mm. End of mantle to end of
body 9mm. Foot wide 2mm. Median tract of foot gray, lateral tracts brown.
Median area of foot rather wider than either lateral area. Mantle free an-
teriorly as far as respiratory orifice. Body tapering posteriorly, not carinate.
Mantle somewhat granulose, not concentrically striate. Color dark brown,
obscurely marbled with gray ; sides anteriorly grayish and paler.

Limax Hemphilli, W. G Brxney, 3d Suppl. T. M. V., p. 205, Plate VIII. Fig. E;
Plate I. Fig. 13; Plate II. Fig. 3 (1890).

A species of the Pacific Province, having been found from British Columbia
to San Tomas River, Lower California, by Mr. Henry Hemphill, in whose
honor it is named. ‘

The general outward appearance of this species resembles that of campestris,
but every specimen examined by me from numerous localities had a peculiarity
in its lingual dentition which seems to me of specific value, — the presence of
an inner cutting point to the lateral teeth, very much the same as is found in
agrestis. "The anatomy of this species is specifically distinct from agrestis in
wanting the trifurcate penis sac of the latter, even did its distribution not
preclude its being a form of agrestis. I have ventured therefore on giving it a
specifie name.

The penis sac is large, long, gradually tapering to the apex ; the genital
bladder is globular, on a short, stout duct.

I figure on the plate a variety from San Tomas River, Lower California,
called pictus by Mr. Cockerell. Its body is pale, reticulated with gray spots;
mantle with black or gray spots. Resembling L. Berendti, Strebel, from
Guatemala.

For lingual dentition, ete., see Third Supplement.

~J

MUSEUM OF COMPARATIVE ZOOLOGY. 16

Zonites Shepardi, Hemeartt.

2

Shell umbilicated, very small, depressed ; whorls 3 or 34, shining, transparent,
smooth, somewhat flattened; spire scarcely elevated above tlie body whorl; aper-
ture oblique, oval; peristome simple, acute, its ends hardly approaching; suture
well impressed; umbilicus pervious, and moderately large for so small a shell.
Great diameter, 2 mm. Height, 1 mm.

Santa Catalina Island, California.

This little shell belongs to the planulate forms, and somewhat resembles a minute
Z. Whitneyi.

I dedicate it to Miss Ida Shepard in recognition of her active services among the
mollusks of Long Beach, Cal., where she resides.

The above is Mr. Hemphill’s description.

Zonites Lawe.

Shell small, umbilicated, globose, flatter below, shining, light horn-colored,
marked with coarse wrinkles of growth; spire rounded; whorls 8, gradually
increasing, slightly convex, the last excavated below around the umbilicus ;
aperture oblique, rounded; peristome simple, acute, thickened with callus
within. Greater diameter 9 mm., lesser 7 mm.; height 4 mm.

Zonites placentula, part, W. G. Brxney, formerly, Terr. Moll. U.S. V., p. 124, Fig.
-44; Plate III. Fig. L (dentition).

Zonites Lawi, W. G. Binxey, Suppl. to Vol. V. p. 142; Plate IIL. Fig. E (also,
Ann. N. Y. Ac. Sci., Vol. L, Plate XV. Fig. E, as undetermined).

Mountains of Tennessee (Miss Law); a species of the Cumberland Subregion.

Readily distinguished from placentula by its larger size, higher rounded spire,
greater number of whorls, and more widely excavated umbilical region.

Jaw as usual in the genus.

Lingual membrane (Vol. V. Plate III. Fig. L, as placentula) with 25-1-25
teeth; three laterals and one transition tooth.

Zonites Caroliniensis, Cockerett.
Plate II. Fig. 7.

Among the specimens of Zonites sculptilis collected in the mountains of
North Carolina are many which differ from the type widely enough to be
considered a distinct species. Mr. Cockerell suggests for it the name Caro-
linensis, thus describing it : —

This species differs from scu/ptil’s in its fewer whorls, straighter columellar
margin, less iunate aperture, fewer radiating strie, and other points. It is figured
as sculptilis in Manual of American Land Shells, Fig. 231.

168 BULLETIN OF THE

Zonites sculptilis.
Plate III. Fig. 9.

For the sake of comparison with the preceeding species, I have given other
figures here of the true Z. sculptilis.

Zonites Simpsoni, Pitssry.

Plate I. Fig. 8,

I give an enlarged figure of an authentic individual of this species. For
the description see Third Suppl., p. 218.

Zonites Diegoensis, HemPuitt.

Plate III. Fig. 2.

Shell minute, umbilicated, thin, light horn-colored, with delicate incremental
strie, globose; whorls 34, convex; base swollen; suture deep; umbilicus broad ;
aperture narrow, rounded ; peristome thin, acute, its ends approximated, the inner
one slightly reflected. Greater diameter 33 mm., lesser 1}; height 13 mm.

Near Julian City, San Diego Co., California. On Cuyamaca Mountain, 4,500
feet elevation.

The above is Hemphill’s description. My figure is drawn from an authentic
specimen.

Zonites cuspidatus, Lewis.
Vol. V., Fig. in text; Suppl., Plate II. Fig. C.

Shell imperforate, small, slightly convex above, flattened below ; light horn
color, shining; whorls 6, gradually increasing in size, with wrinkles of growth,
the last not descending at the aperture; peristome thin, acute; aperture
rounded, bearing within behind the peristome a white callus, on which is
one subcentral and a second basal, erect, recurved tooth-like process, sepa-
rated by a rounded sinus; base often blackish, showing the white callus
prominently. Greater diameter 8 mm., lesser 6; height 4 mm.

Zonites cerinoideus, var. cuspidatus, Lewis, Proc. Phila. Ac. Nat. Sci., 1875, p. 334.
Zonites cuspidatus, W. G. Binney, Ann. N. Y. Ac. Nat. Sci., Vol. I. p. 359, Plate
XV. Fig. C; Suppl. to Terr. Moll. V., Plate II. Fig. C.

Mountains of Tennessee and North Carolina: a species of the Cumberland
Subregion.

The tooth-like processes within the aperture, strongly curved towards each
other, form an arched space.

MUSEUM OF COMPARATIVE ZOOLOGY. 169

Miss Law thus wrote from Philadelphia, Tenn., of this species: “ Unlike
gularis, it seems to be a rare shell, and I find it only by scraping off the sur-
face of the ground in the vicinity of damp mossy rocks. Its habits are more
like placentula than gularis. I never mistake one for a gularis, even before
picking it up; the thickened yellow splotch near the lip, and the thinner spot
behind, showing the dark animal through it, as well as its more globular form,
particularly on the base, make it look very different when alive.”

Zonites macilentus, SxurTtt.
Plate III. Fig. 3.

The individuals of this group are very often difficult to identify, on account
of the blending of their specific characters. The typical macilentus is distin-
guished by a very wide umbilicus and a single revolving lamina starting from
near the basal termination of the peristome. The figure of macilentus in Vol-
ume V. shows a second revolving lamina and a much smaller umbilicus. I
give here another figure of what appears to me to be the shell described as
macilentus. How constant are the characters of the species can be shown only
by a large suite of individuals.

Tebennophorus Hemphilli.

Plate III. Fig. 4.

I give a figure of the jaw already described by me.

Patula strigosa, Goutp, var. jugalis, Hempurtt.

Shell umbilicated, depressed with numerous prominent oblique striz; spire very
moderately elevated or depressed ; whorls 53, somewhat flattened above, but more
convex beneath, the last falling in front, with two dark revolving bands, one at the
periphery and the other above; the body whorl subcarinated at its beginning, but
more rounded as it approaches the aperture; suture well impressed; color ashy
white, with occasional horn-colored stains; umbilicus large, pervious, showing the
volutions ; aperture oblique, ovate, but in very depressed specimens the aperture
is at right angles with the axis of the shell; peristome simple, thickened, its ter-
minations approaching and joined by a thick heavy callus, making the peristome
in very adult specimens continuous. Height of the largest specimens 4 inch,
breadth 1 inch. Height of the smallest specimens 3% inch, breadth 42 inch.

Patula strigosa, var. jugalis, HEMPHILL, The Nautilus, 1890, p. 134, in Binney’s
3d Suppl., p. 215, figure. ¢

Banks of Salmon River, Idaho.

This is another interesting form of the very variable strigosa. It inhabits stone
piles, and other places where it can find shelter and protection against the fatal
rays of the summer’s sun, close along the banks of the river. It is interesting on

170 BULLETIN OF THE

account of its very @epressed form and the ovate form of the aperture, the heavy
callus joining or “ yoking”’ together the extremities of the peristome.

The above is Hemphill’s description.
The figure in the Third Supplement is drawn from an authentic specimen.

Patula strigosa, GovuLp, var. intersum, HEmpPuHILt.

Shell umbilicated, sublenticular, depressed, thin, dark horn-color, more or less
stained with darker chestnut. Whorls 54 or 6, somewhat flattened above, more
convex beneath, obtusely carinated at the periphery, and bearing numerous coarse
oblique rib-like stria, and two dark revolving bands; suture well impressed; um-
biticus large, pervious; aperture oblique, subangulated ; peristome simple, thick-
ened, its terminations joined by a thick callus. Height of the largest specimen 4
inch, breadth inch. Height of the smallest specimen ; inch, breadth 7% inch.

Patula strigosa, var. intersum, HemMpHILt, The Nautilus, 1890, p. 135.

Bluffs along the banks of Little Salmon River, Idaho.

This shell inhabits stone piles at the foot of a steep bluff back some distance
from the river. It seems to be quite rare, as I found but few specimens during
the two or three days of my stay in its vicinity, and many of them were dead. I
regard it as one of the most interesting shells found by me during the season, for
it combines the depressed angulated or keeled forms of the Haydeni side of the
series with the sculpturing of /dahoensis, two shells representing opposite charac-
ters in every respect. It thus becomes the companion of Wahsatchensis, a beautiful
shell, combining the same characters, but much more developed, and connected
with the large elevated forms. Var. intersum fills the opposite office, by uniting
these characters with the small depressed forms. Taken as a whole, this series of
shells, as now completed, seems to me to offer the best guide or key to the study
of species that the student can have. Every known external character belonging
to the genus Helix is so gradually modified and blended with opposite characters,
that, if one had the moulding or making of the many and various intermediate
forms, he could scarcely make the series more complete than Nature has done
herself.

The above is Hemphill’s description.

Patula strigosa, Goutp, var. globulosa, CocKERELL.

Small, globose, dark above periphery, with two bands, transverse grooved striz
rather well marked. Diameter 113, alt. 83mm. Black Lake Creek, Summit Co.
The specimen seems immature, but is remarkable as being the only form I have
seen in Colorado that is nearer to strigosa than Cooperi. It is doubtless allied to
var. Gouldi, Hemphill. (Cockerell.)

Patula strigosa, var. globulosa, COCKERELL, The Nautilus, 1890, p. 102.

The above is Cockerell’s description.

The above varieties of Patula strigosa are transversely ribbed. The following
are smooth or striate

MUSEUM OF COMPARATIVE ZOOLOGY. 171

Patula strigosa, Govxp, var. Buttoni, Hempuitt.
Plate I. Figs. 2 and 10.

I figure the typical and the toothed forms. See 3d Suppl., p. 220.

Patula strigosa, Goutp, var. albofasciata, Hempui.t.

Plate IV. Fig. 9.

Shell globose, elevated or depressed; whorls six, convex, with a broad white
band at the periphery, which shows just above the suture on two or three whorls
of the spire as it passes towards the summit or apex, separating two variable
chestnut-colored zones; the upper one in some specimens is often very dark, in
others very light passing into horn-color, and broken into blotches, stains, or
irregular lines, which pass up a few whorls of the spire and blend with the
horn-colored summit; the lower zone spreads towards the umbilicus in irregular
stains, often beautifully clouding the base of the shell, or is often broken into
irregular revolving lines, and other varied patterns of coloring; strie rib-like,
quite coarse in some specimens, in others finer and closely set together; aperture
circular, ovate, and occasionally pupeform; peristome simple, thickened, sub-
reflected at its junction with the columella, and partially covering the umbilicus,
the ends approached and often joined by a callus, the peristome sometimes bearing
a tooth-like process; umbilicus deep, moderately large, narrower in elevated and
broader in depressed specimens; suture well defined. Greater diameter of the
largest specimen 17 mm., height, 12 mm.; greater diameter of the smallest 12
mm., height 7 mm.; with all the intermediate sizes.

Box Elder Co., Utah.

Among leaves, brush, and grass, on limestone rock. Altitude, about 4,600 feet
above the sea.

This variety of strigosa is so very variable in all its characters I find it quite
difficult to draw a description that will cover all the individuals which I include in
it. Ihave given the measurements of the largest and smallest specimens, but there
are all the intermediates between those figures.

The above is Mr. Hemphill’s description. An authentic individual is figured
on the plate.

Patula strigosa, Goup, var. subcarinata, HempPnite.

Among the shells recently collected by Mr. Hemphill at Old Mission, Coeur
d’Alene, Idaho, was a marked variety of this species, for which Mr. Hemphill
suggests the name subcarinata. The specimens vary greatly in elevation of
the spire, and in the number and disposition of the revolving bands, often
quite wanting, as in the specimen figured in the Third Supplement. All have
a very heavy shell, the body whorl of which has an obsolete carina which
is well marked at the aperture, modifying the peristome very decidedly. See
the figure.

172 BULLETIN OF THE

In examining the genitalia I find the base of the duct of the genital bladder
ereatly swollen along a fifth of the total length of the duct.
Mr. Hemphill (The Nautilus, 1890, p. 133) thus describes it : —

The shell in general form resembles a large, coarse elevated or depressed Cooperi:
It has six whorls, well rounded above and beneath, and subcarinated at the periph-
ery. The body whorl has two revolving dark bands, one above and the other
below the periphery ; sometimes the upper band spreads over the shell to the su-
ture, forming a dark chestnut zone that fades out as it passes toward the apex.
The peristome is simple, thickened, its terminations joined by a callus; aperture
obliquely subangulate; the suture is well impressed. Height of the largest speci-
men 1 inch, breadth 14 inches; height of the smallest specimen ¢ inch, breadth
1 inch.

Rathdrum, Idaho.

An authentic specimen is figured in the Third Supplement.

Patula strigosa, Goutp, var. bicolor, HEMPuHILL.
Plate IV. Fig. 7.

This shell is a colored variety of the last. It may be characterized as being of
a general dark horn-color mingled with dirty white; there are occasional zones of
dark horn-color above and fine dark lines beneath, but no defined bands. In some
of the specimens the light color prevails, in others the horn-color spreads over
the shell in irregular patches. Height 7 inch, breadth 1% inches.

Rathdrum, Idaho. (Hemphill.)

Patula strigosa, var. bicolor, HEMPHILL, The Nautilus, 1890, p. 183.

An authentic specimen is figured.

Patula strigosa, Goutp, var. lactea, HemPuitt.

Plate {V. Fig. 8.

This is a beautiful clear milk-white shell, with 53 whorls, subcarinated at the
periphery. In the elevated forms the aperture is nearly circular, as broad as high;
but in the depressed forms the aperture is broader than high, obliquely suban-
gulate. The lip is simple, thickened, its terminations joined by a heavy callus, —
the thickening of the lip and callus is a shade darker than the body of the shell.
Height of the largest specimen 1 inch, breadth 1} inches.

Rathdrum, Idaho.

The above varieties represent a colony of the largest specimens of the strigosa
group that I have collected. They are an important and very interesting addition
to the series, and serve to confirm my previous views on the relationship of what I
call the strigosa group. This colony inhabits open places in the dense pine forests
of the mountains, overgrown with deciduous bushes. They hibernate among

MUSEUM OF COMPARATIVE ZOOLOGY. Liye

leaves, brush, and roots of trees, and in protected and secure places, generally
on the north slopes of the mountains. (Hemphill.)

Patula strigosa, var. lactea, Hempuitt, The Nautilus, 1890, p. 134.

An authentic specimen is figured.

Patula strigosa, var. Utahensis, Hempui.t.

For locality, see 2d Supplment, p. 30. This is a rough, coarse, carinated variety,
figured in Terr. Moll. V., p. 158, Fig. 66. The peristome is sometimes continuous
by a heavy raised callus connecting its terminations. It is sometimes smaller
and more elevated. (2d Suppl., p. 33.)

Patula strigosa, Goutn, var. depressa, CocKERELL.

Shell flattish, maximum diameter 213, altitude 123 mm. Specimens of this
variety were sent to me by Miss A. Eastwood, who found them in a cafon near
Durango, Colorado. The same variety is figured by Binney, Man. Amer. Land
Shells (1885), p. 166, Fig. 153. (Cockerell.)

Patula strigosa, var. depressa, COCKERELL, The Nautilus, 1890, p. 102.

Patula strigosa, var. albida, Hempuitt.

_ Shell broadly umbilicated, greatly depressed, white, tinged with horn-color; sur-
face covered with fine oblique strie and fine microscopic revolving lines; whorls
6, convex, the last falling in front; spire very little elevated, apex obtuse, aperture
oblique, nearly round; peristome simple, thickened, subreflected at the columella,
its terminations approaching, joined by a thin callus. Height 3 inch, greatest di-
ameter 1 inch, lesser ? inch.

Near Logan, Utah.

Patula strigosa, var. albida, Hemputitt, The Nautilus, IV. p. 17, June, 1890.
The above is Hemphill’s description,

Patula strigosa, var. parma, Hempuitt.

Shell broadly umbilicated, greatly depressed, of a dark dirty horn-color, surface
somewhat rough, covered with coarse irregular stria, and microscopic revolving
lines ; whorls 54 or 6, subcarinated throughout, somewhat flattened above, rounded
beneath, and striped with two chestnut-colored bands, one above and the other
just at the periphery; spire very little elevated, umbilicus moderately large and
deep; aperture ovately round, oblique; peristome simple, subreflected, its termi-
nations approaching and joined by a thin callus. Height + inch, breadth 1 inch.

Near Spokane Falls, Washington.

Patula strigosa, var. parma, Hempuitt, The Nautilus, IV. p. 17, June, 1890.

The above is Hemphill’s description.

174 BULLETIN OF THE

Patula strigosa, var. rugosa, HempuHict.

Shell umbilicated, elevated or globosely depressed, of a dull brown ash-color;
surface rough, covered with coarse irregular oblique striz, and microscopic re-
volving lines; whorls 5, convex, with or without one or two narrow faint revolv-
ing bands. In most of the specimens the bands are obsolete; spire elevated,
obtusely conical; suture well impressed; umbilicus large, deep; aperture nearly
round; peristome simple, thickened, its terminations approaching and joined by
a thin callus. Height of the largest specimen # inch, greatest diameter 1 inch.
Height of the smallest specimen 4 inch, greatest diameter % inch.

New Brigham City, Utah.

A large rough robust form, with very convex whorls. Some of the specimens
so closely resemble solitaria, Say, that one not well acquainted with both forms
would be easily deceived, and refer it to that species. In its adolescent state the
lip is very thin or easily broken, and on the surface of the adult shells these frac-
tures give it a rough and uneven appearance.

Patula strigosa, var. rugosa, HEmMpuitt, The Nautilus, 1890, Vol. IV. p. 16.
¢

The above is Hemphill’s description.

Patula strigosa, var. carnea, HEMPHILL.

Shell umbilicated, greatly depressed, dark horn-color, rather solid, shining, sur-
face somewhat uneven and covered with irregular oblique striz; whorls 54, con-
vex, the last faintly subcarinated in the depressed specimens, falling in front,
sometimes faintly banded, but most of the specimens are plain and without bands ;
spire subconical, apex obtuse; suture well impressed, umbilicus large; aperture
circular; peristome simple, thickened, its terminations well approached and joined
by acallus. Height § inch, greater diameter §, lesser 3 inch.

Near Salt Lake, Utah.

Patula strigosa, var. carnea, HEMPHILL, The Nautilus, Vol. IV. p. 15, June, 1890.

The above is Hemphill’s description.

*

Patula strigosa, var. fragilis, Hempnivy.

Shell umbilicated, elevated or globosely depressed, translucent, thin, fragile,
somewhat shining, of a dark horn-color, surface covered by fine oblique striz ;
whorls 5, convex, the last descending in front and striped by two dark chestnut
bands, one above and the other below the periphery ; suture well impressed ; aper-
ture oblique; peristome simple, thickened ; umbilicus moderate, deep, partially
covered by the reflected peristome at the columella. Height of the largest speci-
men ,*; inch, greatest diameter § inch, lesser $ inch.

Near Franklin, Idaho, among red sandstone.

A very thin and almost transparent variety of the very variable strigosa. By its

~

MUSEUM OF COMPARATIVE ZOOLOGY. 175

peculiar shade, it is very evident that the animal has drawn largely from the red
sandstone for the material to build its shell.

Patula strigosa, var. fragilis, HEMPHILL, The Nautilus, Vol. IV. p. 17, June, 1890.
The above is Hemphill’s description.

Patula strigosa, var. picta, Hempxttt.

Shell umbilicated, elevated or globosely depressed, of a dirty white color, stained
more or less with chestnut; surface somewhat rough and uneven, covered with
moderately coarse oblique striz, and fine revolving lines; whorls 6, convex, sub-
carinated, with a broad white band at the periphery, and a dark zone of chestnut
on the upper side, extending from the peripheral band to the suture, fading out as
it traverses the whorls of the spire; beneath, on the base of the shell, it is striped
with numerous bands that sometimes extend into the umbilicus, and also into the
aperture; spire elevated; apex obtuse; suture well impressed; umbilicus moder-
ately large and deep, broader in the depressed than in the elevated forms; aper-
ture nearly circular; lip simple, subreflected, its terminations approaching and
joined by athin callus. Height % inch, greatest diameter 1} inches, lesser 1 inch.

Rathdrum, Idaho.

Patula strigosa, var. picta, HEMPHILL, The Nautilus, Vol. IV. p. 16, June, 1890.

The above is Hemphill’s description.

Patula strigosa, var. hybrida, Hempnitt.

Shell umbilicated, depressed, white, spire horn-color, surface of the shell cov-
ered with fine oblique striw, and widely separated revolving raised lines; whorls 5,
flattened above, rounded beneath, the last falling in front, and striped with two
faint chestnut bands; suture well impressed ; umbilicus large, showing nearly all
the volutions; aperture nearly circular; peristome simple, thickened, its termina-
tions approaching and joined by a thin callus. Height % inch, diameter # inch,
lesser 2 inch.

Near Logan, Utah.

This is an interesting shell, as it is the beginning of the forms of strigosa that
finally develop the revolving lines into prominent ribs, as seen on the surface of
var. Haydent, Gabb.

Patula strigosa, var. hybrida, Hempuity, The Nautilus, Vol. IV. p. 17, June, 1890.

The above is Hemphill’s description.

Mr. Cockerell (The Nautilus, 1890, p. 102) mentions by name only the fol-
lowing Colorado forms: —

P. strigosa Cooperi, form trifasciata, Ckll. Mesa Co.

P. strigosa Cooperi, form confluens, Ckll. West Mountain Valley, Custer Co.;
Garfield Co.; Mesa Co.

176 BULLETIN OF THE

P. strigosa Cooperi, form elevata, Ckll. Delta Co. :

P. strigosa Cooperi, form major, nov. Shell with diam. 25mm, Near head
of North Mam Creek, Mesa Co., Sept. 14, 1887.

P. strigosa Cooperi, var. minor, Ckll. Near Egeria, Routt Co., abundant. It
is quite a distinct local race.

Pristiloma, ANceEy.

Animal as in Patula.

Shell small, imperforate, horn-color, shining, many whorled ; spire de-
pressed conic; aperture sometimes armed with radiating, rather crowded,
palatal lamellee.

Northern and Arctic North America.

Types: Zonites Stearnsi and Lansingi, BLAND.
Formerly Pristina, ANCEY, and Anceyia, PILSBRY, preoc.

Jaw low, wide, slightly arcuate, ends little attenuated, blunt, with numer-
ous crowded broad ribs, denticulating either margin.

Lingual membrane with tricuspid centrals, bicuspid laterals, aculeate mar-
ginals, as in Zonites.

Separated from Microphysa by the ribbed jaw combined with the lingual
membrane of Zonites: a very unusual occurrence.

Pristina Lansingi, Brann.

Piate III. Fig. 6.

I give a better figure of this species.

Pristiloma Stearnsi, Buanp.
Vol. V., figures in text. Suppl., Plate I. Figs. N (dentition) and O (jaw).

Shell minute, imperforate, globose conic, striate, shining, horn-colored ;
suture impressed ; whorls 7, regularly increasing, the last not descending, very
globose, swollen below, excavated closely around the imperforate umbilical
region; aperture rounded; peristome simple, acute. Greater diameter 4 mm.,
lesser 34; height 25 mm.

Zonites Stearnsi, BLAND, Ann. N. Y. Lyc., XI. 74, Figs. 1, 2 (1875).
Microphysa Stearnsi, W. G. Binney, Terr. Moll. V., figs. in text; Suppl., Plate II.
Figs. N (dentition) and O (jaw).

Astoria, Portland, Oregon ; Olympia, Washington; Alaska. A species of
the Oregonian region.

It is larger, more elevated, and more distinctly striated than Lansingi, with
wider, more rounded, unarmed aperture.

MUSEUM OF COMPARATIVE ZOOLOGY. 177

The jaw is of the same type as described under P. Lansingi, with over 19
ribs. (Suppl., Plate II. Fig. O.)

The peculiar lingual membrane also is the same as in that species, with four
laterals on each side of the central tooth. (Suppl., Plate I. Fig. N.)

Punctum, Morse.

Animal as in Patula.

Shell minute, umbilicated, thin, horn-colored, depressed globose; whorls 4,
the last not descending ; spire slightly elevated ; aperture rounded ; peristome
thin, acute.

Europe and North America.

Jaw slightly arcuate, ends blunt, not acuminated, composed of numerous
subequal, overlapping distinct plates.

Lingual membrane as usual in the Helicide; bases of attachment sub-
quadrate, reflection small, tricuspid in the centrals, bicuspid in the laterals,
marginals irregularly denticulated.

Distinguished by the peculiar free plates of the jaw.

There are two species of Punctwm, conspectum and pygmeum.

Helicodiscus fimbriatus, Wernerey, var. salmonaceus, Hempuitt.
Plate Iil. Fig. 8.

I give a figure of this variety from an authentic specimen. See 3d Suppl.,
p- 189.

Anadenus, Hernemann.

Animal limaciform, subcylindrical, tapering behind ; tentacles simple; man-
tle anterior, concealing an internal shell-plate ; no longitudinal furrows above
the margin of the foot, and no caudal mucus pore ; a distinct locomotive disk ;
external respiratory and anal orifices on the right posterior margin of the
mantle; orifice of combined genital system behind and below the light eye-
peduncle. (See Plate I. Fig. 1.)

Internal shell-plate small, oval, flat, with posterior nucleus and concentric
strie. (See Plate.) .

Jaw with numerous ribs. See Plate III. Fig. 5.

Lingual membrane with tricuspid centrals, bicuspid laterals, and quadrated
marginals. (See same.)

Differs from Prophysaon by its posterior respiratory orifice, by the position
of the genital orifice, and by its locomotive disk.

Himalaya Mountains ; recently found in San Diego County, California, by
Mr. Hemphill.

VOL. XXII, —NO. 4. 12

178 BULLETIN OF THE

It will be remembered that Fischer considers Prophysaon a subgenus of
Anadenus.

The geographical distribution of Anadenus would seem to preclude its being
found in California, but to that genus only can J refer the species whose de-
scription here follows.

Anadenus Cockerelli, Hemruit.

Plate I. Fig. 1; Plate III. Fig. 5.

Length (contracted) 133 mm. ; mantle, length 43, breadth 2 mm. End of
mantle to end of body, 8mm. Foot, breadth 2mm. Foot with the locomotive
disk, being distinctly differentiated into median and lateral tracts. Respira-
tory orifice slightly posterior on right side of mantle. Genital orifice below
right tentacle. No caudal mucus pore. Locomotive disk about half as wide
as either lateral area. Sides of foot wrinkled, but not differentiated from
lateral areas, nor specially marked, the wrinkles being a continuation of the
transverse grooves of the lateral areas. Mantle tuberculate-rugose, oval in
outline, bluntly rounded at either end; not grooved as in Amalia. Mantle
free in front as far as respiratory orifice. Back rather bluntly keeled its
whole length; rugz# rather flattened and obscure, consisting of grooves en-
closing mostly hexagonal lozenge-shaped spaces, which are themselves rugose.
Color uniform brown-black, without markings, except some dark marbling on
the lighter sides. The portion beneath and in front of the mantle is pale, and
the head and neck have a gray tinge. Foot brown. Shell internal, thinnish,
white, oval in outline. Stomach large, swollen, broad. Liver pale ochrey.

Anadenus Cockerelli, Hempnutty, The Nautilus, Vol. IV. No. 1, May, 1890, p. 2.
Anadenulus, CocKERELL, Ann. Mag. Nat. Hist., Oct., 1890, p. 279.

Cuyamaca Mountains of San Diego Co., California. Mr. Henry Hemphill.

Jaw low, wide, slightly arcuate, ends blunt, anterior surface with about
twenty wide, flat ribs, squarely denticulating either margin. (Plate III.
Fig. &.

Lingual membrane short and narrow. Teeth 20-1-20, of which eight only
on either side are laterals. Centrals tricuspid, laterals bicuspid, marginals
quadrate, bluntly bicuspid. (Same Plate.)

Prophysaon Hemphilli.

From Portland, Oregon, Mr. Hemphill brought seventy-seven individuals of
a slug which may prove a variety of P. Hemphilli. They have the tawny color
of flavum. The internal shell is so delicate, it is impossible to remove it
without breaking it. The penis sac is as in P. Hemphilli. The mantle is
sometimes smooth, sometimes tuberculate; its fuscous lateral bands are some-
times united by a transverse posterior band. Some of the individuals had the
tail constricted preparatory to excision. (See below, under Phenacarion.)

MUSEUM OF COMPARATIVE ZOOLOGY. 179

Prophysaon Andersoni, J. G. Coorrr.

3d Suppl., Plate III. Fig. 1? Plate VII. Fig. C; Plate I. Fig. 8 (dentition) ;
Plate IX. Figs. I, J (enlarged surface).

Shield strongly granular-rugose, the respiratory orifice nearly median on its
right margin ; tail acute, with small gland; reddish gray, the body somewhat
clouded with black, the shield paler, clouded, or more usually with a dark
band on each side above the respiratory orifice, converging in an elliptic form;
a pale dorsal streak ; head uniform pale brown, tentacles darker ; foot and
often the mantle tinged with olive. Length 2.5 inches (Cooper).

Arion Andersoni, J. G. CoorrR, Proc. Phila. Ac. Nat. Sci., Plate III. Fig. F.

Prophysaon Andersoni, J. G. Cooper, Pr. Amer. Phil. Soc., 1879, p. 288.

Prophysaon Andersoni, W. G. Binney, Terr. Moll. V., 3d Suppl., Plate III. Fig.
1? Pl. VIL. Fig. C; Plate I. Fig. 3 (dentition); Plate IX. Figs. I, J
(surface).

A species of the Pacific Province, Straits of De Fuca to Oakland, California.

The characteristic of this species is the light dorsal band, which is not
present in P. Hemphilli. It has the broad vagina, stout, short, cylindri-
cal penis sac, and genital bladder of P. Hemphilli, as well as the foliated
reticulations.

In the many living and alcoholic specimens which I have examined, I have
failed to detect any appearance of a caudal mucus pore, which Dr. Cooper is
confident of having observed, excepting in eight individuals out of thirty col-
lected by Mr. Hemphill on San Juan Island.

Many individuals examined by me are excided as described under Phena-
carton foliolatus.

Figure 1 of Plate III. of 3d Suppl. was drawn from a specimen received from
Dr. Cooper. It represents the true Andersoni, distinguished by a light dorsal
band, and by genitalia such as I have described for P. Hemphilli. The same
form, also received from Dr. Cooper, is drawn by Mr. Cockerell on Plate VII.
Fig. C. Mr. Cockerell has shown me that I have confounded with it another
species, which he proposes to call P. fasciatum. See next species.

Specimens collected by Mr. Hemphill at Old Mission, Coeur d’Alene, Idaho,
appear to agree with specimens of this species received from Dr. Cooper.
The jaw is low, wide, slightly arcuate, with over 12 broad, stout ribs, denticu-
lating either margin. The lingual membrane is given in Plate II. Fig. 2, of
3d Suppl. The central and lateral teeth are slender and graceful. The latter
have, apparently, a second inner cutting point, as is found in Limazx agrestis.
I have so figured it, hoping to draw attention to it, and thus settle the question
of its being there. On Plate IX. I have given enlarged views of the surface,
drawn by Mr. Arthur F. Gray. (See Explanation of Plate IX. Figs. I and J
of 3d Suppl.)

180 BULLETIN OF THE

Prophysaon fasciatum, CocKERELL.

Length (in alcohol) 19 mm. Mantle black, with indistinct pale subdorsal
bands, — an effect due to the excessive development of the three dark bands of
the mantle. Body with a blackish dorsal band, commencing broadly behind
mantle and tapering to tail, and blackish subdorsal bands. No pale dorsal
line. Reticulations on body squarer, smaller, more regular, and more sub-
divided than in P. Andersoni, Cooper. Penis sac tapering, slender. Testicle
large. Jaw ribbed. (Cockerell.)

_ Prophysaon fasciatum, COCKERELL, The Nautilus, 1890.
Prophysaon fasciatum, W. G. Brxney, 3d Suppl. to Terr. Moll. V., p. 209, Plate
VII. Fig. A.

Coeur d’Alene Mountains, Idaho; a species of the Central Region.

This species is described by Mr. Cockerell as distinct from Anderson, with
which I have formerly confounded it. (2d Suppl. to Vol. V., p. 42.) It hasa
dark band on each side of the body, running from the mouth to the foot, and a
central dorsal dark band. To this must be referred the descriptions of animal,
dentition, jaw, and genitalia formerly published by me as of Andersont.

I am indebted to Mr. Theo. D- A. Cockerell for a figure and description of
this species. The former is given on Plate VII. Fig. A, while the latter is
given here in the words of Mr. Cockerell, whose name must consequently be
associated with it as authority.

The animal extends itself into a long, cylindrical worm-like body with ob-
tuse ends; the mantle is covered with minute tubercles.

Jaw low, arcuate, ends blunt; with numerous (over 15) irregularly devel-
oped broad, stout ribs, denticulating either margin.

The lingual membrane has 30-1-0 teeth, with about 12 perfect laterals.
Centrals tricuspid ; laterals bicuspid; marginals with one long, stout, oblique
inner cutting point, and one outer short, blunt, sometimes bifid cutting point.
Resembling that of P. Hemphilli. Another membrane has 50-1-50 teeth.

Mr. Cockerell describes the penis sac as tapering; in specimens examined by
me it is cylindrical, as in Hemphill.

The internal shell is thick, easily extracted without breaking.

Phenacarion, Cockrret..}

Animal limaciform, cylindrical, blunt before, tapering behind; tentacles
simple; mantle large, anterior, pointed behind, concealing a delicate, thin,
subrudimentary calcareous shell-plate, easily fractured; no longitudinal fur-
rows along the margin of the foot; a caudal mucus pore; no distinct locomo-
tive disk; external respiratory and anal orifices on the right anterior margin

1 Phenax = an impostor, and Arion. Cockerell, The Nautilus, Vol. III. p. 126,
March, 1890.

MUSEUM OF COMPARATIVE ZOOLOGY. 181

of the mantle; orifices of the combined generative organs behind and below
the right eye-peduncle. (See 3d Suppl., Plate VIII. Fig. A.)

Jaw arcuate, with numerous ribs. (Plate IX. Fig. B of same.)

Lingual membrane with tricuspid centrals, bicuspid laterals, and quadrate
denticulated marginals. (Plate 1X. Fig. C of same.)

Northwestern parts of North America, in the Oregon Region.

Allied to Prophysaon, but distinguished by its more anterior respiratory
orifice, its rudimentary shell-plate, and decided caudal pore.

Phenacarion foliolatus, Goutp.

Color a reddish fawn, coarsely and obliquely reticulated with slate-colored
lines, forming areole, which are indented at the sides, when viewed by a mag-
nifier, so as to resemble leaflets; the mantle is concentrically mottled with
slate-color, and the projecting border of the foot is also obliquely lineated.
The body is rather depressed, nearly uniform throughout, and somewhat trun-
cated at the tip, exhibiting a conspicuous pit, which was probably occupied by
a mucus gland. The mantle is very long, smooth, and has the respiratory ori-
fice very small, situated a little in front of the middle. The eye-peduncles are
small and short. Length 85 mm.

Arion foliolatus, Goutp, Moil. U.S. Exp., page 2, Fig. 2, a, b (1852); Binney,
Terr. Moll., II. 380, Plate LX VI. Fig. 2 (1851); W. G. Binney, Terr.
Moll., 1V. 6; copied also by Tryon and W. G. Binney, L. & Fr. W.
Sh., I. 377.

Phenacarion foliolatus, CocKERELL, The Nautilus, 1890, III. 126; W. G. Bryney,
3d Suppl. to Terr. Moll. V., p. 206, Plate VIII. Fig. A; Fig. B (shell-
plate); Plate IX. Fig. B (jaw); Fig. C (dentition); Fig. D (genitalia).

Discovery Harbor, Puget Sound (Pickering) ; Olympia and Seattle, Wash-
ington (Hemphill).

Dr. Gould adds to the above description these words (Vol. II. p. 31): “ That
this animal belongs to the genus Arion there can be little doubt, from the
peculiar structure of the tail, as represented in Mr. Drayton’s figure, and from
the anterior position of the respiratory orifice. It is a well marked species,
characterized especially by the leaf-like areole by which the surface is
marked.”

It is with the greatest pleasure that I announce the rediscovery by Mr.
Henry Hemphill of this species, which has hitherto escaped all search by
recent collectors. It has till now been known to us only by the description
and figure of the specimen collected by the Wilkes Exploring Expedition,
almost fifty years ago, and given in Vols. II. and III. of Terrestrial Mollusks.
A single individual was found in December, 1889, at Olympia, Washington,
and sent to me living by Mr. Hemphill. It can thus be described. (See
Fig. A of Plate VIII. of 3d Suppl.)

Animal in motion fully extended over 100 millimeters. Color a reddish

182 BULLETIN OF THE

fawn, darkest on the upper surface of the body, mantle, top of head, and eye-
peduncles, gradually shaded off to a dirty white on the edge of the animal,
side of foot, back of neck, and lower edge of mantle, and with a similar light
line down the centre of back; foot dirty white, without any distinct locomo-
tive disk ; edge of foot with numerous perpendicular fuscous lines, alternating
broad and narrow; mantle minutely tuberculated, showing the form of the
internal aggregated particles of lime, the substitute of a shell-plate, reddish
fawn-color, with a central longitudinal interrupted darker band and a circular
marginal similar band, broken in front, where it is replaced by small, irregu-
larly disposed dots of same color; these dots occur also in the submarginal
band of light color. Body reticulated with darker colored lines, running
almost longitudinally, scarcely obliquely, toward the end of the tail, and con-
nected by obliquely transverse lines of similar color, the areas included in
the meshes of this network covered with crowded tubercles, as in Prophysaon
Andersoni, shown in Plate IX. Figs I, J. Tail cut off by the animal. (See
below.) Excepting its being of a deeper red, it agrees perfectly with Dr.
Gould’s description.

Mr. Hemphill writes of it: “I have to record a peculiar habit that is quite
remarkable for this class of animals. When I found the specimen, I noticed
a constriction about one third of the distance between the end of the tail and
the mantle. I placed the specimen in a box with wet moss and leaves, where
it remained for twenty-four hours. When I opened the box to examine the
specimen, I found I had two specimens instead of one. Upon examination of
both, I found my large slug had cut off his own tail at the place where I no-
ticed the constriction, and I was further surprised to find the severed tail piece
possessed as much vitality as the other part of the animal. The ends of both
parts at the point of separation were drawn in as if they were undergoing a
healing process. On account of the vitality of the tail piece, I felt greatly
interested to know if a head would be produced from it, and that thus it would
become a separate and distinct individual.” The animal on reaching me still
plainly showed the point of separation from its tail (see Fig. A). The tail
piece was in an advanced stage of decomposition. I have noticed the con-
striction towards the tail in many individuals. The edges of the cut were
drawn in like the fingers of a glove, after the excision.

The tail of the foliolatus having been cut off, I was unable to verify the
presence of a caudal pore from this individual. It was plainly visible in an-
other specimen from Seattle.

In the large Olympia individual, the irregularly disposed particles of lime
in the mantle, of unequal size, seemed attached to a transparent membranous
plate. With care I removed this entire, and figure it. It is suboctagonal in
shape (Plate VIII. Fig. B). Under the microscope it appears that the par-
ticles of lime do not cover the whole plate; at many points they are widely
separated. This aggregation of separate particles is the distinctive character of
the subgenus Prolepis, to which foliolatus would belong if retained in Arion.

MUSEUM OF COMPARATIVE ZOOLOGY. 183

The genitalia of the large individual from Olympia is figured on Plate IX,
Fig. D. The ovary is tongue-shaped, white, very long and narrow ; the ovi-
duct is greatly convoluted; the testicle is black in several groups of ceca ;
the vagina is very broad, square at the top with the terminus of the oviduct,
and the duct of the genital bladder entering it side by side; the genital blad-
der is small, oval, on a short narrow duct ; the penis sac is of a shining white
color, apparently without retractor muscle; it is short, very stout, blunt at the
upper end where the extremely long vas deferens enters, and gradually narrow-
ing to the lower end. There are no accessory organs. The external orifice of
the generative organs is behind the right tentacle. (See 3d Suppl., Plate IX.
Fig. D.)

The jaw is very low, wide, slightly arcuate, with ends attenuated and both
surfaces closely covered with stout, broad separated ribs, whose ends squarely
denticulate either margin. There are about 20 of these ribs. (See Plate IX.
Fig. B.)

The lingual membrane is long and narrow, composed of numerous longitu-
dinal rows of about 50-1-50 teeth, of which about 16 on each side (Plate IX.
Fig. C) may be called laterals. Centrals tricuspid, laterals bicuspid, marginals
with one long inner stout cutting point, and one outer short side cutting point.
The figure shows a central tooth with its adjacent first lateral, and four extreme
marginals.

Phenacarion Hemphilli.

This form is figured on Plate VIII. Fig. C of 3d Suppl. When extended
fuily, it is 70 mm. long. It is more slender and more pointed at the tail than
foliolatus. The body is a bright yellow, with bluish black reticulations. The
edge of the foot and the foot itself are almost black; shield irregularly
mottled with fuscous ; the body also is irregularly mottled with fuscous, and
has one broad fuscous band down the centre of the back, spreading as it joins
the mantle, with a narrower band on each side of the body. The other charac-
ters, external and internal, are given below. It loses its color on being placed
in spirits, becoming a uniform dull slate-color. Mantle lengthened oval.
Shell-plate represented by a group of calcareous grains concealed in the mantle;
it is impossible to remove it as one shell-plate. A decided caudal pore.

Phenacarion foliolatus, var. Hemphilli, W. G. Binney, 3d Suppl. to Terr. Moll. V.,
p- 208; Plate VIII. Fig. C; Plate X. Fig. H (genitalia).

Gray’s Harbor and Chehalis, Washington, and Portland, Oregon (Hemphill);
a species of the Oregon Region.

On the only living one of the lot from Gray’s Harbor, the pore was dis-
tinctly visible, and is figured on Plate VIII. Fig. C. Usually it seemed more
“a conspicuous pit” than a longitudinal slit, as in Zonites. At one time I
distinctly saw a bubble of mucus exuding from it. It opened and shut, and is

184 BULLETIN OF THE

still plainly visible on the same individual, which I have preserved in alcohol
and added to the Binney Collection of American Land Shells in the National
Museum at Washington.

Jaw low, wide, arcuate, ends attenuated, anterior surface with 16 ribs, den-
ticulating either margin.

Lingual membrane as in foliolatus ; teeth 50-1-50, with 19 laterals on each
side.

Genitalia (3d Suppl., Plate X. Fig. H) ; the form from Gray’s Harbor has
its generative system very much the same as described for foliolatus above. The
ovary is much shorter and tipped with brown, and is less tongue-shaped. The
penis sac tapers to its upper end. The vagina is not squarely truncated above.
The system much more nearly resembles that of Prophysaon Andersoni (see
Terr. Moll., V.) than that of the Olympia foliolatus.

Binneya notabilis, J. G. Cooper.

Plate I. Fig. 9.

A new figure is here given, drawn by Mr. Cockerell.

Triodopsis Mullani, Buanp, var. Blandi, Hempuitt.
Plate IL. Fig. 6.

Shell with the umbilicus partially closed, orbicularly depressed; dark horn-color,
obliquely striated; spire short, very slightly elevated, nearly planiform; aperture
semilunar, at a right angle with axis of the shell, with a very short nipple-like pari-
etal tooth; peristome thickened, white, plain, without teeth and roundly reflected.
Height 4 inch, breadth 3 inch.

Post Falls, and banks of Salmon River, Idaho.

Helix Mullani in form and size resembles very much the common tridentata of
the Eastern States. Among the various forms it assumes, nore are more marked
than the little depressed shell before me. It can be very readily separated from
the typical Helix Mullani, or its other varieties, by its very depressed form, small
size, and the absence of the teeth-like processes on the inner margin of the
peristome.

I cannot detect any microscopical revolving lines, or tubercles bearing hairs,
mentioned by Bland in his description of H. Mullani.

The above desciption is by Mr. Hemphill, who furnished me with the
specimen figured.

Polygyra septemvolva, var. Floridana, Hempuitt.

Shell deeply umbilicated, elevated, globose conic, light horn-color, with numerous
fine ribs above, but smooth beneath; whorls 5% or 6, the last subangular at the
periphery ; suture well impressed; spire greatly elevated with an obtuse apex;

MUSEUM OF COMPARATIVE ZOOLOGY. 185

aperture lunate, well rounded, and nearly circular; peristome reflected, rounded ir
front, the margins joined by a triangular tooth on the parietal wall. Greater diam-
eter 6 mm., altitude 5 mm.

Oyster Bay, Florida.

This is a small, very elevated form of the P. cereolus group.

The above is Mr. Hemphill’s description.

Mesodon ptychophorus, A. D. Browy, var. castaneus, Hemputtt.

Shell umbilicated, globosely depressed, of a dark chestnut color; surface covered
with coarse, irregular, widely separated lines of growth, and crowded, microscopical
revolving lines; whorls 54, convex, the last slightly descending in front, spire ele-
vated; suture well impressed, aperture subcircular; lip white, reflected and par-
tially covering the umbilicus, its terminations approaching; umbilicus small and
deep. Height § inch, diameter 1 inch.

Old Mission and Rathdrum, Idaho.

I regard H. ptychophorus as the progenitor of what I call the Townsendiana group
of West Coast land shells, and this colored variety seems to still further indicate
its relationship to Townsendiana, for the spire whorls of nearly all the specimens
of Townsendiana that I have collected are chestnut-colored. Townsendiana does not
begin to put on its wrinkles until it has made about four revolutions of the shell.
The wrinkles are probably due to its environment.

The above is Hemphill’s description, from The Nautilus, Vol. IV. p. 41,
1890.

Aglaja fidelis, var. flavus, Hempxttt.

Shell umbilicated, elevated, very faintly subcarinated, of a uniform light yellow
color throughout, without bands or other stains of coloring; whorls 64, convex, with
coarse oblique striz, and microscopic irregular revolving lines; peristome reflected
below, simple above; aperture roundly ovate; umbilicus moderate, and partially
covered by the reflected peristome; suture distinct. Greater diameter 34 mm., alti-
tude 23 mm.

Chehalis and San Juan Islands, Washington ; Port Orford, Oregon.

This is a rare and beautiful variety of this well known West Coast land snail.

The above is Mr. Hemphill’s description.

Aglaja fidelis, var. subcarinata, Hemputtt.

Shell orbicularly depressed ; umbilicated ; of a deep dark chestnut-color without
bands; whorls 64, convex or somewhat flattened, the last subcarinated at the
periphery ; striz coarse, oblique, crossed by numerous well defined wavy revolving
lines; peristome simple, thickened above, reflected below, and nearly covering the
umbilicus ; umbilicus moderate; aperture roundly ovate; suture well impressed.
Greater diameter 37 mm., altitude 20 mm.

Humboldt Co., California.

186 BULLETIN OF THE

This is a very dark, intermediate form of jidelis, which in its southern march
under changed conditions assumes a more carinated form, and is known to con-
chologists as infumata, Gould.

The above is Mr. Hemphill’s description.

Arionta Coloradoensis, STEaRns.

Shell orbicular, moderately depressed, whorls slightly elevated, apex obtuse,
number of whorls four to four and a half, rounded. Umbilicus narrow, showing the
penultimate whorl, though partially covered by the reflection of the lip at the point
of junction with the base of the shell. Aperture obliquely ovate, nearly circular,
and almost as broad as high. Lip slightly thickened and reflected, or simple, vary-
ing in this respect; more reflected and aperture more effuse at the columella.

Parietal wall in the heavier examples calloused, the callus connecting with the
inner edges of the outer lip above and below. Shell rather fragile, thin, translu-
cent ; surface smooth and shiny, and sculptured with fine incremental lines. Color
pale horn to white, and otherwise marked by a single narrow revolving reddish
brown band just above the periphery, which in some specimens is obscure or
absent. In some individuals certain faint scars upon the upper whorls imply an
occasionally hirsute character.

mm.
Maximum diameter of largest . . - . . - - + + 15.25
Minimum diameter of largest . . . - - +--+ + 18.26
Altitude oflargest . . . eer Wee ee 81 KN
Maximum diameter of siiallent sade La ape oe SB ab
Minimum diameter of smallest adult .. .. - - 12.00
Altitude of smallest adult . . .... =. +. =. 875

Grand Cafion of the Colorado, opposite the Kaibab plateau, at an elevation of
8,500 feet. (Mus. No. 104,100.)

The above, while exhibiting a facies or aspect of its own, its nevertheless sug-
gestive of H. Remondi, Gabb, Mazatlan, in the Mexican State of Sinaloa, and also
from the high mesas.or table lands in the neighborhood of Mulege, Lower Cali-
fornia. H. Carpenteri, Newcomb, which is a synonym of H. Remondi, is credited
by the author to “ Tulare Valley,” and has been found in other localities in Cali-

MUSEUM OF COMPARATIVE ZOOLOGY. 187

fornia. A glance at the map will show how widely separated geographically
H. Coloradoensis is from its nearest allies, and this discovery of Dr. Merriam’s
extends the distribution of the West Coast type of Helices farther to the eastward
than heretofore, and adds an area of great extent to that previously known.

The above description and figure were published by Stearns in Proce. U. S.
Nat. Mus., Vol. XIII. p. 206, Plate XV. Fig. 6, 7, 8, 1890, all copied above.

I have examiued the jaw and lingual dentition to find them similar to those
of the other species of Arionta.

Arionta Traski, var. proles, Hempuxi.t.

Shell umbilicated, very much depressed, thin, shining, of a dark horn-color ;
whorls 54, somewhat flattened above, convex beneath, the last slightly falling in
front, with a dark band above the periphery, and crowded with strong oblique
strie; suture well impressed; umbilicus moderately large and deep; aperture
hardly oblique ; peristome simple, thin, subreflected, its terminations approaching.
Height % inch, breadth { inch.

Tulare Co., California, near Fraser’s Mill.

A much flatter and more depressed form than any of the varieties of Traski that
Ihave seen. There are no revolving microscopical lines, as in Traski.

The above is Mr. Hemphill’s description.

Arionta tudiculata, var. Tularensis, Hemputit.

Shell umbilicated, very thin and frail, shining, of a light greenish horn-color,
globosely depressed; whorls 53, convex, the surface minutely granulated, and
crowded with fine oblique striz, with a single chestnut revolving band; suture
well impressed; umbilicus very small; aperture oblique, subcircular; peristome
simple, hardly thickened, its columellar portion expanding and nearly covering the
small umbilicus. Height $ inch, breadth inch.

Tulare Co., California.

This is one of those puzzling intermediate forms uniting two species that can be
with equal propriety placed in one or the other. It has the exact form of the
typical Traski found at Los Angeles, and along the coast, though much smaller
and thinner, and it has the sculpturing of tudiculata much modified. It seems to
fill the gap quite completely between those two species.

The above is Mr. Hemphill’s description.

Arionta tudiculata, Bryey.
Plate II. Fig. 7, 8.

New figures are here given of the form cypreophila.
In The Nautilus, Vol. IV. p. 41, 1890, Mr. Hemphill also describes a
var. subdolus thus: —

188 BULLETIN OF THE

Shell narrowly umbilicated ; globosely depressed, of a dark yellowish color, sur-
face somewhat shining, covered with oblique striz, interrupted by numerous wavy
lines and oblong blister-like wrinkles, hardly perceptible to the naked eye ; whorls
54, convex, striped by a single chestnut band, double margined by lighter ones;
spire very little elevated, suture well impressed; lip simple, reflected, and nearly
covering the umbilicus, its terminations approaching and joined by a thin callus;
umbilicus narrow and small. Height 3 inch, greatest diameter 1 inch, lesser 7 inch.

San Jacinto Valley, San Diego Co., California.

A very depressed form, quite variable in size, some of the specimens not being
more than half the size of the measurements given. It is lighter colored than any
of the southern varieties of tudiculata except var. Binneyi.

Arionta Ayresiana, Newcoms.
Plate I. Fig. 7.

I give a new figure of this species.

Arionta intercisa, W. G. Bryyey.

In “ Zoe,” Vol. I. No. 11, January, 1891, p. 330, Mr. Hemphill describes
these varieties of A. intercisa : —

Var. minor. Smallest specimen, greatest diameter 18 mm., altitude 11 mm.
Uniform light yellowish chestnut-color, with and without a band, and varies
very much in form and elevation or depression of spire.

Var. elegans. Uniform ashy buff-color, faintly banded, and variable in form.

Var. nepos. Uniform ashy white ; spire horn-color, variable in form and
sculpturing.

Var. albida. Uniform milk-white, sometimes with a faint band at the
periphery; sculpture nearly obsolete.

In the same journal (p. 434) Mr. Hemphill thus describes several varieties
of redimita, which species he refers, however, to Kelletti: —

Var. castaneus. Uniform, polished, chestnut-color, darker band at the periph-
ery, spire sprinkled with fine ashen specks.

Var. hybrida. Uniform ash-white color, and a dark band at the periphery,
flecked with transverse markings and specks of dark brown and light chestnut.

Arionta ruficincta, Gass.
Plate I. Fig. 3.

A new figure is given of this species.

Arionta Kelletti, Forsss.

Mr. Hemphill, in Terr. Moll. V., 3d Suppl., has thus described several
varieties. I figure authentic specimens of each.
Var. albida (Plate IV. Fig. 3). This is a beautiful clear white translucent

MUSEUM OF COMPARATIVE ZOOLOGY. 189

variety, with no markings or stains of any kind. It is quite thin and frail,
and a trifle smaller than the average size of Kelletti.

Santa Catalina Island, California. Two specimens only found by me.

Var. castanea (Plate IV. Fig. 4). Among the numerous patterns of coloring
assumed by H. Kelletti, none are more conspicuous than this well marked va-
riety. The body whorl is of a deep shiny chestnut-color above the periphery,
and becomes lighter as it follows the whorls of the spire to the apex. The
band at the periphery is quite variable in the different specimens; it is gener-
ally light and well defined above, but below it is irregular, and spreads over
the base of the shell more or less.

Santa Catalina Island, California. This variety is not rare.

In “Zoe,” Vol. I. No. 11, pp. 333, 334, Mr. Hemphill has also thus described
several other forms.

Var. nitida (Plate IV. Fig. 2). Uniform, translucent, shining, dark horn-
color, with a poorly defined dark band, coalescing with a poorly defined whit-
ish band below it, at the periphery; spire faintly flecked with ashen gray.

Catalina Island.

Var. multilineata (Plate 1V. Fig. 1). Shell marked by alternate shades of
ashen white, chestnut, or brown, arranged in an irregular series of revolving
and sometimes wavy lines, with a broader and poorly defined band at the
periphery; markings finer beneath than above.

Var. frater. Shell of a beautiful, uniform, horn-buff color, sometimes fad-
ing into lighter horn-color, with a darker band at the periphery, and numerous
faint, alternate revolving lines of ashen or dark horn-color above and below;
generally, not always, lighter colored beneath, and sometimes with a whitish
zone beneath the band at the periphery.

Var. Californica. The shell is colored with a darker shade of uniform buff
than the above, dark band at the periphery, generally uniform in color above
and below; sometimes flecked with squarish dots.

Var. Forbest. Ground coloring whitish buff, with a revolving series of poorly
defined and coalescing lines, bands, and blotches.

Var. bicolor. Color very dark horn or brownish, flecked with numerous re-
volving very fine dots or irregular lines, with or without a very faint band at
the periphery.

Var. tricolor. Irregularly painted with numerous revolving whitish, brown-
ish, and chestnut flecks, blotches, and stains, with or without a band at the
periphery.

Var. albida. (See below.)

Var. albida, a. Milk white ground, very faintly stained with light horn,
and with poorly defined and fading lines.

Mr. Hemphill considers redimita as a form of Kellettt. (See that species.)

190 BULLETIN OF THE

Euparypha Tryoni, Newc.

Mr. Hemphill has thus described several varieties. (See Zoe, Vol. I. pp. 331,
332.)

Var. varius. The upper or dark zone is of a lighter shade of bluish brown
or chestnut than the type, and is flecked and sprinkled with ashen white;
band at the periphery dirty white beneath.

Var. nebulosa (Plate IV. Fig. 5). Lighter colored above than var. varius,
marbled and clouded with various patterns of dark brown and dirty white ;
dirty white beneath.

Var. fasciata (Plate IV. Fig. 6). Uniform light chocolate above and be-
neath, with a dark band at the periphery.

Var. Californica. Creamy buff-color, darker above than below the periph-
ery, very faintly banded.

Var. albida. Uniform creamy, and sometimes milk-white above and be-
neath, and without band.

Var. subcarinata. Among the subfossils that occur on Santa Barbara Island
we find a form of H. Tryoni which adds an interesting link to its history and
to its present form. It may be characterized as follows. Shell depressed glo-
bose, consisting of about 53 whorls, the last subcarinated at the periphery; in
other respects closely resembling the recent form. Greater diameter 23.15 and
20.11 mm., largest and smallest specimens.

Pomatia Humboldtiana, Vat.

Texas, at Altuda, at an elevation of 5,000 feet, where it, a single specimen in fair
condition, had been thrown out with soil by a prairie dog. (Mus., No. 118,366.)
William Lloyd.

This species has not before been reported from any locality within the territory
of the United States. It was described from Mexico, where it is found in the
neighborhood of the city of Mexico, and in other localities. The national collec-
tion contains several examples from the Real del Monte. It has a pretty close
resemblance to some of the varieties of the European H. (Pomatia) pomatia, and it
may possibly be an introduced form. 7. pomatia has for centuries been esteemed
as an article of food in various parts of Europe, and was regarded as a dainty by
the ancient Romans. It was propagated and raised in large quantities for their
use, and specially fed on certain plants to give the flesh a particular flavor.

Unmistakable specimens of another favorite edible snail common to Europe,
HI, (Pomatia) aspersa, is found in Mexico, and examples from Puebla, in the prov-
ince of Puebla, Mexico, were presented to the National Museum by the Mexican
Geographical Commission a few years ago. The presence of these two forms most
certainly suggests the question as to whether they were not introduced by the
Spaniards many years, centuries, ago, either for food purposes or incidentally in
the routine and accidents of commercial intercourse.

The above was published by Stearns in Proc. U. 8. National Museum, Vol.
XIV. p. 96,1891. It will be remembered that Helix Buffoniana was figured
as aspersa by Dr. Binney in Volume III.

MUSEUM OF COMPARATIVE ZOOLOGY. 191

Bulimulus Ragsdalei, Pitssry.

Plate II. Fig. 9.

Tt is about the size and form of B. Mooreanus, but rather more slender and
elevated. he surface is not smooth, as in the other American Bulimuli, but
strongly ribbed-striate longitudinally. The apex is blunt; peristome thick-
ened within ; columella reflexed over the narrow but open umbilicus. The
aperture is less than half the length of the shell; color brownish, corneus
somewhat translucent, the riblets opaque white. Height 22 mm., diam.
10 mm. ; height of aperture 10$ mm., diameter 7 mm.

Bulimulus Ragsdalei, Pitspry, The Nautilus, Vol. III. p. 122, March, 1890.
Proc. Acad. Nat. Sci. Phila., 1890, p. 296, Plate V. Fig. 3.

St. Jo, and at Warren’s Bend, twenty-five miles from Gainesville, and in
Cook and Montague Counties, Texas (Ragsdale).

A figure of an authentic specimen is given 14 the natural size. The descrip-
tion is a copy of the original.

‘

Bulimulis Dormani.

Plate I. Fig. 6.
A new figure is given.

Rhodea Californica.

This extralimital species has actually been received by Dr. Cooper from
Lower California. (Proc. Cal. Acad. Nat. Sci., 1891, p. 102.) It had been
quoted as an Achatina from Monterey. (See Vol. V.)

Pupa Californica.

Dr. Sterki in Nautilus, Vol. IV. page 7, mentions a variety, elongata, from
San Clemente Island ; on page 18, varieties trinotata, Diegoensis, and cyclops.

Pupa Coloradensis, Cockerett.

Shell brown, shiny, thinnish, striate, especially on penultimate whorl ; out-
line oblong-oval, barrel-shaped ; apex blunt; whorls 4; aperture pyriform;
peristome brown, thick, continuous by a well marked callus on parietal wall ;
outer lip not constricted. The teeth within the aperture are brown, one long,
one on parietal wall, one on columella, and two (the lower one largest) on outer
wall. Long. 13, lat.1 mm. Allied to P. corpulenta, but decidedly smaller,
more striate, and slightly narrower. (Cockerell.)

192 BULLETIN OF THE

Pupa Pilsbryana, Srerx1.

Shell minute, narrowly perforate, cylindrical-oblong to cylindrical, somewhat
attenuated towards the rather blunt apex, colorless (when fresh glassy) with a
very delicate bluish tint, smooth and polished, with few, irregular microscopic
strie which are more marked near the aperture. Whorls 44-54, moderately
rounded with a rather deep suture, especially in the upper half, regularly and
slowly increasing, the embryonal being relatively large, the last somewhat
ascending toward the aperture; the latter of moderate size, lateral, subovate,
margins approached, peristome somewhat expanded, without a thickened lip or
a callus in the palatal wall; outside is a barely perceptible trace of a crest near
the margin, and behind that a slight impression most* marked upon the inferior
palatal fold. Lamelle 4 or 5; one apertural, rather high, of moderate length,
simple; one columellar, horizontal, of moderate size, simple; basal very small or
wanting; palatals the typical, inferior deeper seated, of moderate size, superior
small or very small. Alt. 1.5-1.7, diam. 0.8-0.9 mm.

Pupa Pilsbryana, SterKx1, The Nautilus, Vol. III. p- 123, March, 1890.

There is a slight variation; the example from New Mexico being of lesser diam-
eter, and having no trace of a basal lamella.

The soft parts have not been seen so far, but will be of high interest, since, to
judge from the shell, our species seems to be an intermediate form between the
hordeacella, etc. group, and P. curvidens, especially its var. gracilis.

P. Pilsbryana has much resemblance in shape and size to small albino examples
of P. hordeacella, Pilsb., but under a glass is at once distinguished by the shorter
simple apertural lamella not ending at or very near the upper termination of the
palatal margin, as it does in hordeacella, and by the smooth surface. The fine bluish
hue may also be a distinguishing character if it prove constant.

The above is Sterki’s original description.

Pupa calamitosa.
Plate II. Fig. 1.

See 3d Suppl., p. 219. A reduced copy of one of the original figures is given
here.

Pupa Hemphilli, Srerx.

In examining a lot of about forty-five specimens of Pupa calamitosa from the
banks of San Tomas River, Lower California, I found there were two distinct
forms in them. The author says, in his description of P. calamitosa: ‘“ Several
specimens have only one lamella on the outer lip, and are rather larger than the
typical form described,” represented in Plate XII. Fig. 16 (loc. cit., No.7). Probably
I had a greater number of examples at disposition than Mr. Pilsbry. The two
forms proved to be distinct by an entirely different formation of the lamella, as

MUSEUM OF COMPARATIVE ZOOLOGY. 193

well as of the basal part of the shell. And among the whole number I found not
one intermediate or doubtful specimen. There is no doubt but that we have to
consider them as being specifically distinct, the more so since they live together in
the same locality. For the new species I would propose the name P. Hemphilli, in
honor of the man to whom we owe so many valuable additions to our malaco-
logical fauna.

As in shape and general appearance the two species are almost alike, it may be
the best way to characterize the one in question by comparing it with P. calamitosa,
Pilsb. P. Hemphilli averages a trifle larger than its companion, but either is some-
what variable in size. While ca/amitosa has a
minute perforation, Hemphilli is umbilicated in
quite a peculiar way. There is a nodule-like pro- Ba
jection on the umbilical part of the last whorl,
producing a rima beside the umbilicus ; in calam-
itosa there is nothing of this formation. On the
other hand, the latter has a small but distinct
groove-like impression just at the base, near the
aperture appearing as a slight projection inside. y
This feature is wanting in Hemphilli. Lamelle:
in the latter species, when looking from front, only one is generally seen in the
palatal wall, corresponding to the superior one in calamitosa, but longer; i. e. be-
ginning deeper in the throat, and fairly seen on the outside; also marked there by
a corresponding impression, ascending in a curve from near the base. A little dis-
tant from its inner end, just above the projection mentioned, there is another
lamella beginning, directed toward the base and ending there, also seen on the
outside. Quite generally there is a very small, thin, but well formed lamella in
the palatal wall, near the projecting auricle. The columellar fold is quite short
and small in Hemphilli, yet consisting of a vertical and a horizontal part. The
(main) apertural lamella is decidedly longer in our species, and the supra-
apertural higher and entire, while in ca/amitosa it is evidently composed of two
parts marked by an indentation in the middle, or even entirely separated, in quite
mature specimens.

About twenty examples, collected at San Diego, Cal., by Mr. Hemphill, are all
P. Hemphilli, no calamitosa among them. They are little different from the San
Tomas River specimens, except by a somewhat shorter palatal lamella.

The above is Sterki’s description (The Nautilus, July, 1870, Vol. IV. p. 27).
My figure was drawn by him from the type.

Pupa hordeacella, Pitspryr.
Plate Il, Fig. 2.

The shell is of a long-ovoid shape, smaller and slenderer than P. servilis, Gould,
translucent, waxen white, finely striate; the aperture is rounded, with a thin, ex-
panded peristome. Within, there is, on the parietal wall, an entering fold arising
near the termination of the outer lip, its edge a trifle sinuous or nearly straight;
the columella has a fold about in the middle. There is a tiny deep-seated fold on

VOL. XXII. — NO. 4. 13

194 BULLETIN OF THE

the base of aperture, near the columella, an entering fold within the outer lip,
equidistant from the above described parietal and columellar folds, and a tiny
denticle above it. ‘The columellar fold is not situated so high on the pillar as in
P. servilis. ‘The latter half of the body whorl is flattened on the outer lower por-
tion, as the Figure J shows. There is a low wave-like ridge or “crest” also, but
scarcely visible in many specimens. Alt. 1.8, diam. 8 mm. ”

Pupa hordeacella, Pruspry, Proc. Acad. N. Sci. Phila., 1890, p. 44, Plate I. Figs.
Gree TK

Arizona to Florida.

The figures were drawn with the aid of the camera lucida. They should be com-
pared with Gould’s excellent figures of P. servilis in the Boston Journal of Natural
History, Vol. [V., Plate 16, Fig. 14, and those of P. pellucida, in Strebel’s Beitrag
zur Kenntniss der Fauna mexikanischer Land- und Siisswasser-Conchylien, Theil
IV. Plate XV. Fig. 10. The latter are the more valuable in this connection, as
they are not only faithful drawings on a sufficiently large scale, but are the only
ones drawn from continental specimens (Vera Cruz, Mexico). The measurements
given by Strebel and Pfeffer are, alt. 24, diam. of last whorl fully 1 mm., alt. of
aperture mm. Gould’s P. servilis and Pfeffer’s P. pellucida were both described
from Cuba. I see no reason for not following W. G. Binney in considering them
synonymous, pellucidus having precedence. (Pilsbry.)

The above is Pilsbry’s description. I give also a reduced view of one of his
figures.

Pupa Clementina, Srerx1.

Shell very minute, narrowly perforate, cylindrical, pale horn-colored, transpar-
ent, with rather obtuse apex ; whorls 54, regularly increasing, moderately rounded,
with rather deep suture, smooth, with few microscopic striz, somewhat
shining; last whorl occupying rather more than two fifths of altitude,
somewhat ascending to the aperture, with a slight, revolving impression
on the middle of its last one third, ending at the auricle; a very slight,
flat crest elevation near the margin, only in the lower part; aperture lat-
eral, scarcely oblique, subovate with the palatal margin slightly flattened,
upper part of same somewhat sinuous, peristome a little expanded with
a slightly thickened lip just at the margin; lamelle 6, white, two on
the apertural wall, the apertural typical, and a rather long supra-aper-
tural, ending in a callus at the upper termination of the palatal margin; columel-
lar one typical, horizontal; basal very small, nodule-like, deep-seated ; palatals
two, typical, the inferior a little longer. Alt. 1.9, diam. 0.8 mm. ; apert., alt. 6,
diam. 0.5 mm.

Three examples of this species were collected by Mr. H. Hemphill on San Cle-
mente Island, California, among numerous P. Californica; Row. All were exactly
alike, well formed and fully mature. They cannot be referred to any one of our
species published, and doubtless represent a form of their own, although so far it
was not possible to examine the soft parts.

Pupa Cle-
mentina.

MUSEUM OF COMPARATIVE ZOOLOGY. 195

In size, shape, and general appearance it somewhat resembles /sthmia, yet lacks
the rib-like striation; the lamella would be typical for Vertigo and some of the
smaller Pupe but for the presence of the well developed supra-apertural which
P. Clementina has in common with P. calamitosa, Pilsbry, and Hemphilli, Sterki;
but, on the other hand, there is nothing of the characteristic palatal or gular folds
of these two species. Thus, in several regards, our form is an intermediate and
connecting one between different groups, and consequently deserves our special
interest.

Pupa Clementina, SterK1, The Nautilus, Vol. IV. No. 4, Plate I. Fig. 4, August,
1890,

The above is a copy of Sterki’s original description and figure.

Pupa Dalliana, STERKI.

Shell conic or ovate-conic, of greenish horn-color, transparent, finely irregularly
striate in the lines of growth, polished; whorls 43, well rounded, with deep suture
rather rapidly increasing, the last occupying about 2 of altitude
towards the aperture, somewhat ascending on the penultimate.
Aperture lateral, somewhat oblique, subovate, with just percepti-
bly flattened palatal margin; margins approximate, the ends pro-
tracted ; peristome shortly but decidedly expanded, with a very fine
thread-like lip near the margin, the same continuing as a very fine
callus on the apertural wall inside of the line connecting the ends of
the margins; palatal wall quite simple; no lamella. Alt. 1.2, diam.
1.3 mm.

This form has been collected by Mr. Hemphill near Clear Lake,
Lake Co., Cal., and I propose to name it in honor of Mr. William H. Dall. The
specimens before me were fifteen, fresh, remarkably uniform in their whole appear-
ance; all were more or less covered with a dark brown hard crust of slime and
dirt, generally thickest around the aperture. Doubtless this coating is done
“purposely ” by the animals, as in many other species also. When cleaned, it
shows about the size and shape of a well grown Vertigo ovata, Say; but by a good
eye, or under a glass, is at once recognized as something else, by the rounded
aperture and the absence of lamelle. (Sterki.)

Pupa Dallia na.

Pupa Dalliana, SterK1, The Nautilus, Vol. IV. No. 2, p. 19, June, 1890.

Dr. Sterki’s description is copied above. My figure was drawn by him from
the type.

Pupa syngenes, Pitsprr.
Shell subcylindrical but wider above, composed of eight narrow, convex whorls,

sinistrally convoluted ; texture as in P. muscorum, but color rather lighter brown.
Last whorl ascending, imperforate, bearing a strong high crest just behind the

196 BULLETIN OF THE

outer lip. Aperture shaped as in muscorum, having a single small parietal denticle.
Altitude 3%, diameter 13 mm.

Pupa syngenes, Pitspry, The Nautilus, 1890, Vol. III. p. 296, Plate V. Figs. 1, 2.

Two specimens of this form are before me, and I am in doubt whether to give
them a new name, as they may be only. sinistral monstrosities of the common
P. muscorum. The shells are labelled “ Arizona” in the Academy collection, col-
lector not known.

(Since the above paragraphs were in type, I have received a communication from
my friend, Dr. V. Sterki, to whom I sent a specimen of P. syngenes, which I at first
described as a variety of muscorum. He says: —

“Tam satisfied that it is a species, and not a var. of muscorum; the shape of the
whole shell, the last whorl so considerably flattened, and ascending, the number of
whorls, seem to me to prove its specifical rank. . . . After washing out the aper-
ture of your specimen, I saw a rather strong lamella or tooth on the columella, and
a barely perceptible trace of an inter-palatal lamella, which, however, is validified
by the impression on the outside.”)

The above is Pilsbry’s description. An authentic specimen drawn by Dr.
Sterki is figured here.

Vertigo ovata, Say.

Of V. tridentata Sterki writes (The Nautilus, 1890, p. 135): “It has a
wide distribution in the northern part of the country ; originally found in
Illinois, it has been collected in different parts of Ohio and New York, as
well as in Minnesota and Colorado. In general it is remarkably constant in
its characters ; yet there are slight differences ; here I found a few examples
from low ground, together with V. ovata; they were a trifle larger, Bee a
thicker and deeper colored shell than-those from upland places.”

MUSEUM OF COMPARATIVE ZOOLOGY. 197

Vertigo Oscariana, STERKI.

This is the most peculiar of our species. It is of the size of milium, but oblong,
with either end nearly equally pointed, the last whorl being considerably narrowed
and flattened towards the subtriangular, small aperture; shell thin, delicate, of
pale horn-color, as is the palatal wall and margin; the latter simple and straight,
with a very slight, thin callus inside ; lamella 3, whitish, rather small; one aper-
tural, one columellar (longitudinal), and the inferior palatal; some-
times there is also a very small superior palatal. Length 1.5, diameter
0.8 mm.

This remarkable Vertigo has been detected in Eastern Florida, on the
coast at Mosquito Island, etc., by Mr. Oscar B. Webster and his father,
Mr. Geo. W. Webster, of Lake Helen, Florida. These gentlemen took
much pains to ascertain the range of distribution of this form and some V: Cone
others, and it is consequently only just to name the species in honor of 20.
Mr. Webster. The most striking character of it, besides the narrowed ;
last whorl, is the thin and straight palatal wall and margin, so that, indeed, the
shell appears to be immature. But when seen under a glass of sufficient power,
the margin is completed, and, as already mentioned, there is a thin callus at a
little distance from the margin. Moreover, Mr. Webster wrote me that, of more
than 150 examples he had seen, all were alike.

A few days ago, in a lot of P. corticaria, Say, from Ithaca, N. Y., sent from
Texas, there was one example of this species, the shell dead, but in fair condition,
a little larger and less fragile than the Florida examples, and with a well marked
callus corresponding to a slight but distinct crest. The specimen may have been
collected in New York, and from its appearance at least I would ascribe to it an
origin north of Florida. Since the above was written, I have found a few exam-
ples in drift from Guadalupe River, Texas, collected by Mr. J. A. Singley, sent by
Mr. Wm. A. Marsh.

By the kindess of Mr. Webster I was enabled to see a living example. The foot
and the lower parts of the head are nearly colorless; head, eye-tentacles, and neck
light gray. Jaw very tender, thin, pale yellow, consisting of about 14 longitudinal
plates, shorter and wider in the middle, longer and narrower toward either end; it
is much like that of V. tridentata, Wolf. Odontophore about 0.36 mm. long, 0.1 mm.
wide, about 110 square rows in each $+ $-+ 3 teeth; central very small; laterals
gradually passing into marginals; the latter serrate. Different from that of
V. tridentata.

In drift with numerous minute shells, from Guadalupe River, Texas, kindly sent
by Wm. A. Marsh, I found one specimen of this species, which consequently is not
confined to Eastern Florida, where it was detected by Messrs. Webster, but may be
widely spread over the southern part of our country.

Vertigo Oscariana, StERKI, Proc. Ac. Nat. Sci. Phila., 1890, p. 33; The Nautilus,
1890, p. 136.

The above is Sterki’s description, and the figure is drawn by him from the
type.

198 BULLETIN OF THE

Vertigo Binneyana, STERKI.

They are of the size and general appearance of V. callosa, very narrowly per-
forate, cylindrical oblong, light chestnut-colored; whorls 5, moderately rounded,
nearly smooth; aperture relatively small, peristome little expanded; outer wall
with a well formed crest interrupted by a rather long revolving groove ;
corresponding to the crest there is a callus of lighter color; lamellz 6;
on the apertural wall a small supra-apertural and a well developed
apertural; columellar appearing rather massive; at the base, one
rather small but well formed, appearing tooth-like; palatals 2, long,
especially the inferior. Length 2.0 mm., diameter 1.0 mm.

Last year, Mr. W. G. Binney kindly presented me with two exam-
ples of a Vertigo collected at Helena, Montana, by Mr. H. Hemphill,
which seemed to be of a new species; but yet I did not like to publish
a description founded upon only these two specimens. Lately among
a number of small Pupide from different parts of British America sent by Mr. Geo.
W. Taylor of Ottawa, there were a few examples of this same species, from Win-
nipeg, Manitoba, dead and weathered, but good enough to be identified.

Probably there are other examples of this species in collections, and more will
be found in the Northwest. It is named in honor of Mr. W. G. Binney, to whom I
owe the two beautiful specimens in my collection.

ana.

Vertigo Binneyana, StERKI, Proc. Ac. Nat. Sci. Phila., 1890, p. 33.

The above is Sterki’s description. I am also indebted to him for the figure.

Vertigo callosa, Srerx.

There are in collections two different species under the name of V. Gouldii, Binn.
Their size and coloration is nearly the same, at least in most variations, as are also
the apertural lamelle as to number and position. Yet they are decidedly and con-
stantly distinct, especially by the formation of the outer wall at the aperture.
Judging from the descriptions and more especially from the figures, the true V.
Gouldii is the one characterized as follows: the last whorl is somewhat predomi-
nating, thus rendering the whole shell more ovate or conic ovate ; the palatal wall
near the aperture is decidedly flattened, or impressed, the impression comprising
also the crest and being especially well marked at the “auricle” (as I name the
more or less projecting part about the middle of the outer margin, to have a con-
cise expression), forming a roundish groove outside and a decidedly prejecting
angle inside, thus producing the “two curves meeting in the centre of the peri-
stome.” A feature not striking, but only seen by careful examination, is the posi-
tion of the short tooth-like lamella at the base, somewhat nearer the margin than
the end of the columella, the base perceptibly widened at that place; the said
lamella is probably an equivalent of the inferior columellar lamella, which in most
Vertigos stands very low, in many exactly at the base.

The other species, V. callosa, has the last whorl relatively less wide, so that the
whole shell is of a more oblong shape. In the palatal wall, only the part behind

MUSEUM OF COMPARATIVE ZOOLOGY. 199

the crest is somewhat flattened, while the latter itself forms one unbroken curve
from the base up to the suture, and at the moderately projecting auricle there is
only a slight flattening. The inferior columellar lamella is at the end of the col-
umella, sometimes wanting or a mere trace. Well worthy of notice is a pecuiiar
formation of the surface, the epiconch showing microscopic wrinkles or foliations
in the direction of the lines of growth producing a peculiar silky gloss, especially
on quite fresh examples, and more in some forms than in others.

The first two examples of this species I obtained in 1855 from Mr. Henry Moores,
of Columbus, Ohio, and in 1889 I saw a few more in his collection. In 1887, Mr.
E. W. Roper sent me some others from Massachusetts. Last year in different
collections I saw quite.a number of specimens from different places in New York
near the metropolis, under various names: V. Gouldw, milium, ovata, and also mixed
with Bollesiana. Of the Ohio examples the color is somewhat lighter, the callus and
the lamelle are strong and white, while in the Eastern examples they are somewhat
thinner and more of the color of the shell. The name cal/osa was thus mainly
derived from the Ohio form (which, however, may be regarded as a variety).

It is with some hesitation, however, that I now bring it under this head ; it is the
equivalent of the European V. pygmea, Drap., of which I have examples for com-
parison from different countries of the Old Continent, which I have partly col-
lected myself there during a number of years. The two may even be identical ;
at least it would be absolutely impossible to distinguish New York examples
from most Europeans. Both forms agree also in certain variations of the aper-
tural lamelle; the inferior columellar lamella may be absent in either, or there
may be present a small supra-palatal fold, thus rendering the number variable
from 4 to 6, the typical, however, being 5. An examination of the soft parts will
probably decide the question; so far I have not had an opportunity to make it.

On our continent, the range of distribution of the two species —V. Gouldii and
callosa —seems to be somewhat different, the former having been found in New
York, Ohio, Illinois, and Colorado, the latter from Massachusetts to Ohio.

Vertigo callosa, Srerxt, Proc. Ac. Nat. Sci. Phila., 1890, p. 31.

The above is Sterki’s description.

Vertigo parvula, Srer«r.

Among several hundred small Pupide collected in Northeastern Ohio (Summit
and Lake Counties) by Mr. A. Pettingell, there were two examples of a doubtless
new species, which I in the same way named V. parvula. It is about of the size,
shape, and appearance of V. (Angustula) milium, Gould; but ranges in quite another
group, having a quite simple palatal wall and margin, and only three lamellz.

In Texas, Vertigos seem to be decidedly rare. In many hundreds of Pupide
from that State which Mr. J. A. Singley and Mr. Wm. A. Marsh kindly forwarded
me there were only about half a dozen such; a few milium, one rugosula, one
Oscariana, as mentioned above, and one specimen of a form which probably will
prove to be a new species of quite peculiar formation.

Vertigo parvula, StERKI, The Nautilus, 1890, p. 136.

The above is Sterki’s description.

200 BULLETIN OF THE

Vertigo approximans, SrTerK1.

In 1887, Mr. A. A. Hinkley, of Dubois, Ill., sent me, with other Pupide, one
specimen of a Vertigo, probably new, and in 1889 another of the same. The said
gentleman and Mr. William A. Marsh kindly forwarded me all their Pupide for
examination, but so far I have found no other example, yet I am satisfied such will
be found. The form is related to Vertigo ovata and Gouldii, but different, and is
characterized by the two palatal lamella being close together, for which reason I
gave it the manuscript name V. approximans.

Vertigo approximans, STERK1, The Nautilus, 1890, p. 186.

The above is Sterki’s description.

Vertigo rugosula, STERKI.

Related to V. ovata and Gould; in shape more elongated than the latter, more
cylindrical, and somewhat larger. Apertural parts and lamellae much like those of
ovata; but the columella is decidedly longer and straighter, and the inferior colu-
mellar lamella is distinctly placed on it. Length 1.8-2.0, diameter 1.1mm. Of a
peculiar formation is the surface. Of the five well-rounded whorls, about one and
a half of the upper are nearly smooth; the following, with exception of
the last, are distinctively and regularly striated; the last is very finely
but distinctly rugose in the sense of the lines of growth, near the aper-
ture again striated. Color, dark chestnut.

This is a beautiful species, of which I saw the first example in the
collection of Mr. Bryant Walker, who had found it in April last at
Pass Christian, Mississippi. Last September, Mr. W. G. Mazyck col-
lected a number of them on Sullivan’s Island, 8. C. In either place
they were in company of Pupa rupicola, Say. Quite lately I have seen
one example from Lee County, Texas, sent by Mr. J. A. Singley; it was a dead
shell, and not fully mature, but recognizable. The species consequently seems to
be widely distributed along the South Atlantic and Gulf coasts. Two specimens
were sent in by Mr. H. Hemphill, who collected them at Fish Camp, Fresno Co., Cal.

In Eastern Florida, Volusia County, etc., a form has been found to be quite com-
mon which I refer to this species, but as a distinct variety which may be called
ovulum. It is somewhat smaller, ovate; the striation and rugosity of the surface
are less marked, and the inferior apertural lamella is wanting. In turn it has in
most examples a lamella at the base (between inferior columellar and inferior
palatal), and the callus in the palatal wall is rather strong. The coloration of part
of them is somewhat lighter. It cannot be confounded with V. ovata, Say, its rela-
tions to the type of rugosula being evident, and, in addition, ovata has been found
with it. Nor can it be referred to ventricosa. It is larger and stronger, of much
darker color, its surface is not so smooth and polished, it has three or even four
lamellz more, and the columella is longer.

Vertigo rugulosa, SrERK1, Proc. Acad. N. Sci. Phila., 1890, p. 34.

~~

\'

V. rugosula.

The above is Sterki’s description. The figure was drawn by him.

MUSEUM OF COMPARATIVE ZOOLOGY. 201

Liguus fasciatus, Mutt.

Plate I. Fig. 5.

The Vaccas Key variety, noticed in page 435 of the Manual of American
Land Shells, is figured in the plate.

Orthalicus undatus, Bruce.
Plate Il, Fig, 4,

I give a new figure of the variety of this species.

Holospira Arizonensis, STEARNS.

Shell dextral, elongately cylindrical, pupiform, dingy white to pale horn-color,
translucent. Number of whorls, twelve to thirteen. Slightly convex, the su-
tures distinctly defined. The upper six or seven
whorls rather abruptly tapering towards the obtuse
apex, which has a slightly twisted and rather a
papillose aspect. The last whorl is curved under
and constricted back of the mouth, forming an
umbilical notch. The apex and following whorl
are smooth; the three or four succeeding whorls
sharply and somewhat obliquely plicated longitu-
tudinally, the median and following whorls be-
coming somewhat obscurely sculptured other than
by distinct growth lines. The basal whorl is
strongly sculptured below, and back of the mouth,
and obtusely angulated underneath. Aperture
ovate, slightly angulated anteriorly, somewhat
effuse, rimmed and projecting. The dimensions of two examples are as
follows : —

mm.
WIDE SEE > f) 2) 5 eer eran b> '
ManeEnHUGPEEEEE eee. sss Gs eae ee OD
ie AGEIMICLCDE Tce =) dS eke fea ss «ee et ts wl
ROueRIGIReLCr: arse. Of so acdes ic «02 5 oa. . 4

Dos Cabezas, Arizona, where the above two specimens and numerous fragments
were found in a cave in November, 1889, by V. Bailey, and contributed to the
United States National Museum (No. 104,392) by Dr. C. Hart Merriam.

Among the species of this group that are geographically related is H. Remondi,
Gabb, described from Arivechi, Province of Sonora, Mexico, a form sharply sculp-
tured throughout, and in minor features also different; H. Pfeifferi, Menke, col-
lected by Remond at Hermosillo, in the same province, with the previously named

202 BULLETIN OF THE

species; and H. (Celocentrum) irregulare of Gabb, from the high table-lands back of
Mulege, in the peninsula of Lower California. All of these are separable at a
glance from Arizonensis.

The above is Stearns’s description and figure from Proc. U.S. National Mus.,
Vol. XIII. p. 208, Plate XV. Figs. 2, 3, 1890.

Onchidella borealis, Datt.

Coos Bay, Oregon.

It is gregarious in its habits. Fifty specimens were taken in a small crevice of
clay shale, near high tide. Single individuals, or several clustering together, were
taken afterwards lower down on the tide under loose stones. When in motion, the
animal moves off quite rapidly for so small a creature, with two short, stout pedun-
cles protruding in front of the mantle, bearing keen, sharp black eyes. The color is
dark slate, splashed with blotches and streaks of ashen white. The body when in
motion is 4 inch long, ;3; wide, $ high, and oblong-oval in form, a little broader
behind than before. It is covered with small tubercles, which are larger around ~
the edge of the mantle than those higher up on the body, giving the edge of the
mantle a serrated or tooth-like appearance when the animal is at rest. When
it is at rest on a smooth surface, the base of the animal is nearly circular,
or a little longer than wide, the centre of the body is elevated to quite a sharp
apex, which together with its color resembles some varieties of a very young
Acmea pelta, aad would be very readily taken for such by an inexperienced col-
lector. The foot is white, and works in rapid undulations when the animal is in
motion.

The above remarks are made by Mr. Hemphill in a recent letter.

Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.

Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Figs.
Fig.

Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.

SLMPABMAP wre

—

1
2
3
4.
5.
6
vi
9

Se Oe ee Co Ox aac NO

MUSEUM OF COMPARATIVE ZOOLOGY.

EXPLANATION OF PLATES.

PLATE I.

Anadenus Cockerelli. Animal and internal shell.
Patula strigosa, var. Buttoni.

Arionta ruficincta.

Glandina decussata, var. Singleyana.

Liguus fasciatus, var. from Key Vaccas.
Bulimulus Dormani.

Arionta Ayersiana.

Zonites Simpsoni, enlarged.

Binneya notabilis, enlarged.

Same as Figure 2, toothed variety.

PLATE II.

Pupa calamitosa, reduced from original figure.
Pupa hordeacella, from original figure.
Selenites Duranti, var. Catalinensis, enlarged.
Orthalicus undatus, variety.

Selenites Vancouverensis, var. Keepi, enlarged.
Triodopsis Mullani, var. Blandi.

, 8. Arionta tudiculata, var. cypreophila.

Bulimulus Ragsdalei, enlarged one half.

PLATE III.

Limax Hemphilli, var. pictus. Animal and internal shell.

Zonites Diegoensis, enlarged.

Zonites macilentus, enlarged.

Tebennophorus Hemphilli, jaw.

Anadenus Cockerelli, jaw and tongue.

Pristiloma Lansingi, enlarged.

Zonites Caroliniensis, enlarged.

Helicodiscus fimbriatus, var. salmonaceus, enlarged.
Zonites sculptilis, enlarged.

bo

we

204 BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY.

PLATE IV.
Fig. 1. Arionta Kelletti, var. multilineata.
Fig. 2. Arionta Kelletti, var. nitida.
Fig. 3. Arionta Kelletti, var. albida.
Fig. 4. Arionta Kelletti, var. castanea.
Fig. 5. Euparypha Tryoni, var. nebulosa.
Fig. 6. Euparypha Tryoni, var. fasciata.
Fig. 7. Patula strigosa, var. bicolor.
Fig. 8. Patula strigosa, var. lactea.
Fig. 9. Patula strigosa, var. albofasciata.

PLATE I.

MOLL.

BINNEY : 4TH SUPPL. TO TERR.

BINNEY: 4TH SUPPL. TO TERR. MOLL. PLATE It

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