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pee men pe te Pep che rtonggione am tae 
: A SE ~~ 


HANDBOUND 
AT THE 


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UNIVERSITY OF 
TORONTO PRESS 


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y 
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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"79. Die in Deutschland gefundenen Landplanarien Rhynchodemus terres- 
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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. 


e 
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5 
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f fi 4 
¥ ; ; i 7 wre 
(|) re 


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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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a® 


5 
a 


a or ere 


Wows 


io 


- 
- a 


ee 


s 


= 


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 


‘ x8. 


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mrp — y 
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TULSA = 
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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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'77-78. Recherches pour servir a Vhistoire du batonnet optique chez les 
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'59. Ueber das Auge der Sapphirinen und Pontellen. Arch. f. Anat., Phy- 
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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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a aes 

- he : 

nya 

Bias ¢ ig? 
ee: 


es + 


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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pp- 45-78, Taf. IV.—-VI. 

’°83. Ueber Entwickelung von Sipunculus nudus. Arb. aus d. zool. Inst. 

Wien, Tom. V. Heft 1, pp. 61-140, Taf. IV.-IX. 
Hoyer, H. 

"90. Ueber den Nachweis des Mucins in Geweben mittelst der Farbe- 
methode. Arch. f. mik. Anat., Bd. XXXVI. Heft 2, pp. 310-374. 27. 
Sept., 1890. 

Keferstein, W. 

’62. Untersuchungen iiber niedere Seethiere. Zeitschr. f. wiss. Zool., Bd. 
XII. Heft 1, pp. 1-148, Taf. I-XI. 16. Juni, 1862. 

’65. LBeitrage zur anatomischen und systematischen Kenntniss der Sipuncu- 
liden. Zeitschr. f. wiss. Zool., Bd. XV. Heft 4, pp. 404-445, Taf. XXXI.- 
XXXII. .25. Oct., 1865. 

Keferstein, W., und Ehlers, E 

‘61. Zoologische Beitrage gesammelt im Winter 1859-60 in Neapel und 
Messina. II. Untersuchungen iiber die Anatomie des Sipunculus nudus. 
pp- 35-52, Taf. VI-VII. Leipzig, 1861. 

Leydig, F 

61. Die Augen und ueue Sinnesorgane der Egel. Arch. f. Anat. u. Physiol., 

1861, pp. 588-605. 
Nansen, F. 

‘87. The Structure and Combination of the Histological Elements of the 
Central Nervous System. Bergens Museums Aarsberetning for 1886, 
pp. 29-215, Pl I.-XI. 

Rietsch, M. 

’86. Etude sur les Géphyriens armés ou Echiuriens. 1° et 2"° Partie. 
Recueil Zool. Suisse, Tom. III. pp. 3138-515, Pl. XVIL-XXII. Juin- 
Juillet, 1886. 

Rohde, E. 

’87. Histologische Untersuchungen iiber das Nervensystem der Chaeto- 
poden. Zool. Beitrage (Schneider’s), Bd. II. Heft 1, pp. 1-81, Taf. 
I.-VII. 

Selenka, E. 

’83. Die Sipunculiden, eine systematische Monographie. Semper’s Reisen 

in den Philippinen, II. Theil, Bd. IV., 131 pp., 14 Pls. Wiesbaden, 1883. 
Shipley, A. E. 

90. On Phymosoma varians. Quart. Jour. Mier. Sci., Vol. XX XI. pp. 1-27, 
Pl.I-IV. April, 1890. 

‘91. Ona New Species of Phymosoma, with a Synopsis of the Genus and 
some Account of its Geographical Distribution. Quart. Jour. Micr. Sci., 
Vol. XXXII. pp. 111-126, Pl. XI. March, 1891. 


182 BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY. 


Spengel, J. W. 

77. Anatomische Mittheilungen titber Gephyreen. Amtlicher Bericht der 
50. Versammlung deutscher Naturforscher und Aerzte in Minchen 
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- 


| 


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 


VOL. XxI.— NO. 3. 19 
\ 


290 BULLETIN OF THE 


view, however, affirms that the posterior end of the duct grows back- 
ward free from adjacent tissue, the cellular material being wholly de- 
rived from an anterior proliferation. For Selachii this method of origin 
has been maintained by Balfour (’78), and for Amniotes by a large num- 
ber of observers ; e. g. Weldon (’83) and Mihalkovics (’85) in Reptiles ; 
Gasser (’77), Sedgwick (81), Schmiegelow (81 and ’82), and 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. 

\ 


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. 


304 BULLETIN OF THE 


similar differentiation of a part of the ccelom into a distinct excretory 
chamber. The condition in this region differs from that of the meso- 
nephros of this genus solely in the circumstance that the excretory cham- 
ber is not broken up into metameric portions ; this process takes place 
in the posterior region, and produces a typical mesonephros. 

It now remains for me to review the opinions of previous writers in 
respect to the nature of the pronephros. ‘The existence of a larval excre- 
tory system different from and earlier than the mesonephros appears to 
have been first suggested by Marcussen (’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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'74* Die Gastraea-Theorie, die phylogenetische Classification des Thierreichs 
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91. Etudes d’embryologie sur les Vertebrés. IV. Les Fentes Branchiales. 
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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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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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Kowalewsky, A. 
67. Kutwickelungsgeschichte des Amphioxus lanceolatus. Mém. Acad. 
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’85. Ueber die lymphoiden Zellen der Anneliden. Jena. Zeitschr. f. Natur- 

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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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'84. Die Polycladen (Seeplanarien) des Golfes von Neapel und der angren- 
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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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Lankester, E. Ray, and Arthur Willey. 

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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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°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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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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29. Ueber die Wolff’schen Korper bei den Embryonen der Frésche und 
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’°89. Ueber die Entwickelung des Urogenitalsystems des Menschen. Arch. 
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73. Beitrage zur Entwicklungsgeschichte der Knochenfische nach Beobacht- 
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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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'88>. Kencropin passntiaamepnms. Tpyaa OOmecrsa Ecrecrsoncnuraresett 
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Owen, Richard. 
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’67. Untersuchungen tiher die Entwicklung der Teleostier-Niere. Inaug. 

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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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’89. Zur Entwicklung des Excretionssystem der Selachier. Hine Erwiderung 
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’'89. Ueber die Entstehung des Vornierensystems bei Triton, Rana, und 
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"78, ’80. Vcropia passnria crepaagm (Acipenser ruthenus). TpyiH 
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81. Studier over Testis og Epididymis Udviklingshistorie. 96 Sid., 3 Taf. 
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Schultz, Alexander. 

°75. Zur Entwickelung des Selachiereies. Arch. f. mikr. Anat., Bd. XI. 

pp. 569-582, Taf. XXXIV. 1875. 
Schultze, Oscar. 

’°88. Die Entwickelung der Keimblatter und der Chorda dorsalis von Rana 
fusca. Zeitschr. f. wiss. Zool., Bd. XLVII. pp. 325-352, Taf. XXVIII, 
XXIX. 27 Nov., 1888. 


MUSEUM OF COMPARATIVE ZOOLOGY. 337 


Scott, W. B. 

81. Beitrage zur Entwickelung der Petromyzonten. Morph. Jahrb., Bd. 

VII. pp. 101-172, Taf. VIL-XI. 1881. 
Sedgwick, Adam. 

7802. Development of the Kidney in relation to the Wolffian Body in the 
Chick. Quart. Jour. Micr. Sci., Vol. XX. pp. 62-83, Pl. VI., VII. 
April, 1880. 

[ d/so in Stud. Morph. Lab. Univ. Cambridge, Vol. I.] 

*80°. On the Development of the Structure known as the Glomerulus of the 
Head-Kidney in the Chick. Quart. Jour. Micr. Sci., Vol. XX. pp. 372- 
374. July, 1880. 

*81. On the Early Development of the Anterior Part of the Wolffian Duct 
and Body in the Chick, together with some Remarks on the Excretory 
System of the Vertebrata. Quart. Jour. Micr. Sci., Vol. XXI. pp. 432- 
468, Pl. XXVI. July, 1881. 

Selenka, Emil. 

*82. Der embryonale Excretions apparat des kiemenlosen Hylodes martini- 
censis. Sitz.-Ber. d. k. Akad. Wiss. Berlin. No. 8, Sitz. v. 16. Feb., 
1882, pp. 117-124, Taf. II. 1882. 

Semon, Richard. 

°90. Ueber die morphologische Bedeutung der Urniere in ihrem Verhiltnis 
zur Vorniere und Nebenniere und tiber ihre Verbindung mit dem Genital- 
system. Anat. Anzeig., Bd. V. pp. 455-482, 8 Abbild. 22 Aug., 1890. 

[Im Auszuge mitgetheilt zz Veyhandl. d. X. internat. medic. Kongress- 
es, Berlin, 1890, Bd. II. Abt. 1, Anatomie, pp. 135, 136. Diskussion: 
Wiedersheim, Semon. April, 1891.] 

Semper, Carl. 

74. Die Stammesverwandtschaft der Wirbelthiere und Wirbelosen. Arb. 
zool.-zoot. Inst. Wiirzburg, Bd. II. pp. 25-76, Taf. III-V., 1 Zylogr. 
1874. 

°75. Das Urogenitalsystem der Plagiostomen und seine Bedeutung fiir das 
der wibrigen Wirbelthiere. Arb. zool.-zoot. Inst. Wiirzburg, Bd. II. pp. 
195-509, Taf. X.-XXII. 1875. 

76. Die Verwandtschaftsbeziehungen der gegliederten Thiere. III. Stro- 
bilation und Segmentation (nebst einen Anhang). Arb. zool.-zoot. Inst. 
Wiirzburg, Bd. III. pp. 114-404, Taf. V-XV. 15 Oct., 1876. 

°78. Sind die Segmentalorgane der Anneliden homolog mit denen der Wir- 
belthiere? Eine Erwiderung an Herrn Dr. Firbringer. Morph. Jahrb., 
Bd. LV. pp. 322-327. 1878. 

Shipley, Arthur E. 

°87. On Some Points in the Development of Petromyzon fluviatilis. Quart. 
Jour. Mier. Sci., Vol. XXVII. pp. 325-370, Pl. XXVIL-XXIX. Jan, 
1887. 

[ 4/so in Stud. Morph. Lab. Univ. Cambridge, Vol. IIT. pp. 173-218. ] 

VOL. xXI.— NO. 4. 22 


338 BULLETIN OF THE 


Siemerling, Ernst. 
’'82. Beitrige zur Embryologie der Excretionsorgane des Vogels. Inaug. 
Diss. Marburg. 40 pp.,1 Taf. Marburg. 1882. 
Spee, Ferdinand Graf. 
'84. Ueber direkte Betheiligung, des Ektodermes an der Bildung der 
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Spengel, J. W. 
"76. Das Urogenitalsystem der Amphibien. Arb. zool.-zoot. Inst. Wiirz- 
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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. 
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'86. Ueber den Wolff’schen Gang und die Segmentalblaschen bei Lacerta. 
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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. 
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°83. Beobachtungen iiber Phreoryctis Menkeanus Hoffm. und Nais, ein 
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°89. Vaginalepithel und Vaginaldrusen. Virch. Arch. f. path. Anat., Bd. 
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MUSEUM OF COMPARATIVE ZOOLOGY. 339 


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°86 = ~= Lehrbuch der vergleichenden Anatomie der Wirbelthiere. 2te Auflage. 
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790%. Ueber die Entwicklung des Urogenitalapparates bei Krokodilen und 
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840 BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY. 


Ziegler, H. Ernst. 
°87. Die Entstehung des Blutes bei Knochenfischembryonen. Arch. f. mikr. 


Anat., Bd. XXX. pp. 596-665, Taf. XXXVI-XXXVIIT. 18 Nov., 1887. 
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Zschokke, F. 
°87. Studien iiber den anatomischen und histologischen Bau der Cestoden. 
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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Arch. ital. de Biol., XV. 1, p. 125. [Résumé of Zoja, ’90.] 


f ven 
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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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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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Davenport. — Budding in Bryozoa. 


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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DaveENPORT. — Budding in Bryozoa. 


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. 


a 


-_ DAVENPORT. - BUDDING IN BRYOZOA. 


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gm. 


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Fig. 52. 


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. 


er. 


DAVENPORT. — BuD 
L 


Ampilrm._ .s 


B Meisel, lith. Boston. 


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. 


5¢, Diagram showing arrangement of individuals in Figure 65. 


. 
; 


~~ 


ff 


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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; y= 


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. 


se 


cS 
i 


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@e ree 


BMeisel lith Boston. 


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t 


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. 


BMeisel lith. Boston. 


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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. 


—_ 


\Gay) 
oO. 


ms drm.+ end.) 


—— BMeisel,lith.Boston. 


NY 


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 


? 


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 
§ 


» ob. 


sston 


h.Bs 


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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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 


i Bostan 


inn, 


E Meisel 


HP. J. del. 


: 
; ; 
j + 3 
= ‘ ‘ an 
I “sy & 
" i. : 
/ 2 ae 
r tay " 
a = 
- } - 
| 
" 
- ui i) 
AY 
i) 
bs - 
, yt (Ay 
| Py 
i ih 
‘ 
. 


7 


a ris AL 
> os 


4 pea . 


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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