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http://www. archive.org/details/cu31924024759486

STUDIES

FROM THE

MORPHOLOGICAL LABORATORY

IN THE

From the Batrour Liprary, New Museums, CaMBripar.

The Balfour Library will be glad to receive publications in

exchange for the “Studies.”

London:

Cc. J. CLAY AND SONS,
CAMBRIDGE UNIVERSITY PRESS WAREHOUSE,
AVE MARIA LANE.

1886

On the Fate of the Blastopore and the Presence
of a Primitive Streak in the Newt (Triton
cristatus).

By

Alice Johnson,
Demonstrator of Biology, Newnham College, Cambridge.

With Plate XVIL.

THE coincidence of the blastopore with the anus in the Newt
has already been observed by Mr. Sedgwick.! His assertion
was, he says, based only upon surface views. He therefore
suggested to me that I should attempt to verify it by cutting
sections of the embryos, and my results confirm what he has
stated.?

I. Tae Fate or tHe Biastorore.

A.—At the close of segmentation the blastopore is placed in
the normal position at the hind end of the embryo. With the
greater growth of the dorsal surface, consequent on the appear-
ance of the medullary folds and formation of the medullary canal,
it comes to occupy a place on the ventral surface at some distance
from the hind end (vide fig. 11). Its distance from the hind end
increases as development goes on (vide figs. 12, 18). The tail

1 A. Sedgwick, “On the Origin of Metameric Segmentation and some other
Morphological Questions,” this Journal, January, 1884.

? Some of the main points of this paper have already appeared in a com-
munication made to the Royal Society in June, 1884.

12

166 ALICE JOHNSON.

begins to bud out behind it, at a time when about ten meso-
blastic somites have been formed, as a small conical knob whose
blunt apex points forwards, and the tail has become very dis-
tinct in the stage represented in fig. 18, when there are about
eighteen somites, and the rudiments of the sense organs, cere-
bral vesicles, visceral arches, &c., have appeared.

The blastopore leads into the hind gut, whose cavity is here
broad, but very shallow (vide fig. 8). At a greater distance
from the blastopore the cavity becomes much narrower and no
deeper (vide fig. 6), so that it is very difficult to follow it in
transverse sections. In the longitudinal sections, however, its
continuity is quite apparent (vide figs. 11,12, 18). Fig. 14
represents a transverse section, showing the open blastopore at
a time before the tail is formed, and figs. 15, 16, 17, a series of
transverse sections, showing the passage of the blastopore into
the hind gut at a considerably later stage with a very distinct
tail.

I find no stage at which the blastopore is closed.

B. Historical. — Scott and Osborn! describe a posterior
dilatation of the medullary canal, the sinus rhomboidalis,
which remains open for some time after the rest of the canal is
closed. They say that its folds enclose the blastopore, and,
therefore, when they come together, a neurenteric canal is
formed. Their account of the exact date of the closure of the
sinus rhomboidalis is a little obscure, but seems to indicate
that it takes place while the number of mesoblastic somites is
quite small, and before the rudiments of the visceral arches
and of the tail have appeared.

Hertwig? figures an open blastopore at a slightly later stage
than this, but he describes it as being situated at the end of a
small conical process, which, judging from his surface views of
the embryo, one would take to be the tail.

1 W. B. Scott and H. F. Osborn, “On the Early Development of the
Common Newt,” this Journal, October, 1879.

2 O. Hertwig, ‘ Die Entwicklung des Mittleren Keimblattes der Wirbel-
thiere,’ Jena, 1881.

ON THE FATE OF THE BLASTOPORE IN THE NEWT. 167

Bambeke! states that the blastopore disappears before the
formation of the medullary folds.

As to the fate of the blastopore in other Amphibia, I con-
clude from Clarke’s? account of the development of Ambly-
stoma that it becomes the anus in this form, though the fact
is not actually expressed in so many words. He says (p. 7),
“At the extreme anal end the (medullary) folds remain sepa-
rate over a small area, the space formerly occupied by the
vitelline plug (the mass of yolk-cells which projects into the
cavity of the blastopore and nearly fills it up at an earlier
stage), and form a rounded edge about this small cavity or
pit” (p. 8). “Inaventral view . . . are seen both the
optic vesicles . . . and the anus at the posterior end of
the neural tube” (p. 9). “The beginning of the tail also
shows distinctly, and its median ridge, at the end of which is
the dark cavity of the anus, is now much increased in size.”
No mention is made of the closure of the blastopore, and
Clarke’s figures (pl. ii, figs. 9, 10, 12, 14) confirm my deduc-
tion.

In Pelobates, Bambeke® states that the anus appears to
him to correspond to the place formerly occupied by the
‘pouchon de Hecker” (vitelline plug). He figures it at a
comparatively early stage (vide plate iv, fig. 5).

II. Toe Primitive Streak.

A.—The first structure to appear on the surface of the ovum
after the segmentation has been completed is a groove which
generally extends from the blastopore along the greater part
of the dorsal surface. This is the “ Rickenrinne” of the
German observers, the “ Sillon médian” or “Sillon primi-

1 Ch. van Bambeke, “ Nouvelles Recherches sur l’embryologie des Batra-
ciens,” ‘ Archives de Biologie,’ vol. i, 1880.

2 §, F. Clarke, “ Development of Amblystoma Punctatum,” part i, external,
‘Studies from the Biological Laboratory of the Johns Hopkins University,’
No. 2, 1880.

3 Ch. van Bambeke, “ Recherches sur le Développement du Pélobate
brun,” ‘ Mémoires Couronnés, &c., de l’Acad. Roy. de Belgique,’ 1868.

170 ALICE JOHNSON.

of the primitive groove at an earlier stage, and corresponds in
position more or less with the future mouth.

As the medullary folds approach one another the primitive
groove becomes gradually obliterated in the narrowing and
folding up of the medullary plate, and the primitive streak
remains only in the hind region. At the front end of this
reduced primitive streak the sides of the medullary plate come
together to form a solid mass instead of the thick-walled canal
that exists in front. This fact is illustrated in figs. 6, 7, 8,
and 9, which are taken from a series of transverse sections
through the hind end of an embryo. The medullary canal in
passing back round the hind end gradually loses its lumen
(vide fig. 6, where the medullary canal is seen above and the
solid mass of epiblast cells below). Further forwards on the
ventral surface this solid mass becomes fused with the under-
lying hypoblast cells and the lateral plates of mesoblast. The
primitive streak, thus constituted, forms a slightly pronounced
ridge on the surface of the embryo (vide fig. 7). Nearer the
blastopore the ridge is flatter (vide fig. 8). In fig. 9 the blasto-
pore itself is seen with the continuity of the layers at its lips.

Fig. 10 shows the primitive streak, as seen in transverse
section, of an embryo with a distinct tail, rudiments of the
visceral clefts, &c. In figs. 11, 12, and 13 the primitive streak
of different stages is shown in longitudinal section, but it can-
not then be distinguished so clearly.

I have been unable to find at any stage the neurenteric
canal mentioned by Scott and Osborn.

In the course of the development, the medullary canal is
gradually differentiated backwards out of the primitive streak,
and the hind gut, from being curved as seen in fig. 13, becomes
straight.

The arrangement of the layers in the primitive streak of the
Newt at the stage represented in figs. 6—9 resembles closely
that described by Professor Balfour in the tail of the embryo
Lepidosteus.}

1 F. M, Balfour and W. N. Parker, “On the Structure and Development
of Lepidosteus.” ‘Phil. Trans. of the Roy. Soc.,’ part ii, 1882.

ON THE FATE OF THE BLASTOPORE IN THE NEWT, 171

B. Historical.—The great breadth and flatness of the
medullary plate at its first appearance is a well-known
characteristic of Amphibian embryos. They are further dis-
tinguished at this period from the embryos of other Vertebrates
by the division of the medullary plate into two symmetrical
halves by means of the dorsal or primitive groove. This
feature, as well as the continuity of the primitive groove with
the blastopore, has been noticed by almost all observers of
Amphibian embryology. Hertwig! alone denies the continuity
of the two structures, but it has been described by Bambeke?®
in Triton and Axolotl; by Clarke? in Amblystoma; by
Ecker‘ in the Frog; by Gotte’ in Bombinator; and by Bam-
beke® in Pelobates. Prévost and Dumas’ figure the primitive
groove in the middle of the medullary plate of the Frog, but
do not mention its blastopore.

The only other Vertebrate, as far as I know, in which a
similar disproportionately broad medullary plate and a like
relation of the primitive groove to the medullary plate and
blastopore have been described, is the Sturgeon. Kowalevsky,
Owsjannikoff, and Wagner® describe in the embryo Sturgeon
a specially broad medullary plate, in the middle of which is an
opaque streak, which they call the “ Primitivstreif,’”’ though
they do not assert that any fusion of the layers exists there.
A “Primitivrinne” runs down the centre of the ‘ Primi-
tivstreif,”” ending in the blastopore (vide their figures on pp.

1 O. Hertwig, loc. cit.

2 Ch. van Bambeke, “ Nouvelles Recherches, &c.,” loc. cit.

3 §. F. Clarke, loc. cit.

4 A. Ecker, “Icones Physiolog.,” 1851—1859.

5 A. Gotte, “ Die Entwicklungsgeschichte der Unke,” Leipzig, 1875.

6 Ch. van Bambeke, “ Recherches sur le développement du Pélobate brun,”
loc. cit.

7 Prévost and Dumas, “‘ Deuxiéme mém. s. 1. génération. Développement de
*ceuf des Batraciens,” ‘Ann. Sci. Nat.,’ ii, 1824.

8 A, Kowalevsky, Ph. Owsjannikoff, and N. Wagner, “Die Entwicklung d.
Store,” ‘ Vorlauf. Mittheilung. Mélanges Biologiques tirés du Bulletin de
VAcad,,’ Imp., St. Pétersbourg, vol. vii, 1870.

172 ALICE JOHNSON.

175,176). In Salensky’s! account, however, no such structure
as a primitive groove is mentioned or figured.

The solid condition of the hind end of the medullary canal,
such as I find in the Newt, has been described by Strahl? for
the Lizard, and by Gasser’ for the Bird.

III. Summary or Facts anp GeneRAL CONSIDERATIONS.

In the Newt (1) the anus of Rusconi, or blastopore, becomes
the actual anus of the adult.

(2) A primitive streak exists on the dorsal surface in front
of the open blastopore.

(3) The primitive groove extends along the whole of the
dorsal surface from the open blastopore, and for a short dis-
tance in front of the medullary folds.

(4) The front end of the primitive groove deepens into a
distinct pit, at the apex of which there is, almost certainly, a
fusion between the hypoblast and epiblast.

The Newt affords another instance of the variability of
position of the last open part of the blastopore in different
groups of the Chordata.

In Amphioxus, the blastopore is posterior, and gives rise to
a neurenteric canal on the formation of the medullary folds
and closure of the medullary canal.

The same is the case with the Ascidians. In ine cats
the blastopore is converted into a neurenteric canal on the
closure of the medullary folds. Behind this, there is a yolk
blastopore, which closes without leaving a trace.

No neurenteric canal is known in Teleosteans, and an
invagination, giving rise to a blastopore, has not been de-
scribed.

1 W. Salensky, ‘‘ Recherches sur le développement du Sterlet,” ¢ Archives
de Biologie,’ vol. ii, 1881.

> H. Strahl, “ Beitrige zur Entwicklung von Lacerta agilis,” ‘ Arch. f.
Anat. u. Phys.,’ 1882.

3 Gasser, “ Der Primitivstreifen bei Vogelembryonen,” ‘ Schriften d. Gesell.
zur Beford. d. gesammten Naturwiss. zu Marburg,’ vol. ii, supplement i, 1879.

ON THE FATE OF THE BLASTOPORE IN THE NEWT. 173

In Petromyzon, an invagination takes place. The blasto-
pore remains open for a long time, though not permanently.
The medullary canal is formed first as a solid cord, which
becomes continuous with the hypoblast at the lip of the
blastopore, thus forming the rudiment of a neurenteric canal.

In Acipenser, the invagination blastopore is converted into a
neurenteric canal.

In Lepidosteus, there is no open blastopore of the ordinary
kind, formed by means of an invagination, but Professor
Balfour says: “ In the region of the tail, the axial part of the
hypoblast, the notochord and the neural cord fuse together,
and the fused part so formed is the homologue of the neuren-
teric canal of other types. Quite at the hinder end of the
embryo, the mesoblastic plates cease to be separable from the
axial structures between them” (‘ Comp. Embryology, vol. ii,
p- 93). This arrangement seems to be comparable with the
primitive streak and neurenteric canal at its front end, such as
is found in the higher Chordata.

In Amphibians generally, the invagination blastopore gives
rise to a neurenteric canal. In the Newt, however, the in-
vagination blastopore becomes the anus. A primitive streak
extends along the dorsal surface in front of the blastopore, and
I believe that there is no neurenteric canal. The primitive
groove, which extends in front of the medullary folds, has a
deep pit at its anterior end. In Amblystoma also, as men-
tioned above, the blastopore probably becomes the anus.

In Reptiles, there is an invagination blastopore, which
becomes a neurenteric canal. Behind this point there is a
primitive streak. The anus is formed along the line of the
primitive streak, which extends at least as far forwards as the
opening of the allantois into the alimentary canal in the
Lizard,! and probably in all types having an allantois. Strahl?
states that in the Lizard, the invagination begins in the middle
of the primitive streak, near, but not at, its front end. By the

1W. F. R. Weldon, “Note on the Early Development of Lacerta
muralis,’ this Journal, January, 1883.
2-H, Strahl, loc. cit.

174 ALICE JOHNSON.

time that the hypoblast has been perforated by the invagination ,
the differentiation of the layers has extended as far back as the
blastopore. Therefore, when a neurenteric canal is formed,
this exists at the front end of the now reduced primitive
streak.

In Birds, the invagination blastopore occurs comparatively
late in development, e.g. it is most fully developed in the
Duck with twenty-six mesoblastic somites and a medullary
canal closed except at the extreme hind end. A neurenteric
canal is found at a later stage. The primitive streak exists
only behind the invagination blastopore and its corresponding
structure, the neurenteric canal. At the latter, the hypoblast
is fused with the epiblast and mesoblast, but remains separate
from these two layers throughout the rest of the primitive
streak.

In Mammals, the invagination blastopore begins as a pit in
the epiblast at the front end of the primitive streak.1 It then
extends downwards and perforates the blastoderm completely.
When the medullary groove is formed, it constitutes a neuren-
teric canal, piercing the floor of the hind end of the groove,
but, before the medullary folds close, its ventral opening into
the archenteron has become obliterated, and its upper part
alone remains.

The view that the primitive streak represents part of the
original blastopore is now so generally accepted that it may be
assumed here for purposes of argument.

No reason has been suggested for the various behaviour of
the blastopore of the Chordata in these different cases. It is
sometimes a simple opening which gives rise to a neureuteric
canal and then vanishes altogether. In other instances, it
is elongated and composed of a primitive streak with an.
opening (which becomes a neurenteric canal) generally at its
front end, but in one form (the Lizard) in the middle, or the
opening may be (in the Newt) at the hind end of the primitive
streak and persist without having any connection with a neur-

1 'W. Heape, “On the Germinal Layers and Early Development of the
Mole,” ‘ Proc. Roy. Soc.,’ 1881.

ON THE FATE OF THE BLASTOPORE IN THE NEWT. 175

enteric canal. Sometimes the blastopore merely consists of a
primitive streak with no opening at all.

In all cases, except the Newt, the opening is restricted to
the embryonic stages, and may be described as an embryonic
structure. As to the variations, we can only say, for want of
a more definite reason, that they are for purposes of embryonic
convenience.

It is obvious, from all the facts adduced, that the original
Vertebrate blastopore was elongated, but its present condition
shows that great changes have taken place, since, even in the
embryo, part or parts of the opening have been obliterated,
these parts varying in different embryos. It is obvious, too,
that the anus, at any rate, is derived from the original blasto-
pore, and therefore is probably not an entirely new formation,
acquired within the group, but is homologous with the anus of
the primitive ancestral form.

My results show also that the primitive streak (i. e. blasto-
pore) extends much further forwards than was supposed. In
fact, the pit found at the front end of the primitive groove in
the Newt corresponds in position more or less with the future
mouth as has been remarked. This points to the probability
of a connection between the blastopore and mouth, and so
supports Mr. Sedgwick’s' view that the blastopore of the
Chordata was an elongated dorsal slit, the ends of which gave
rise to the mouth and anus.

In Peripatus,? too, it is known that the asstogane is an
elongated structure, the middle part of which closes, while the
ends become respectively the mouth and anus of the adult.

The fusion of the embryonic layers is most distinct at the
hind end of the embryo. I believe that it exists also at the
front end of the primitive groove in the Newt. In the middle
region of the body its existence is doubtful, but the fact that
the primitive groove extends along the dorsal surface from the

1 A. Sedgewick, loc. cit.
2 F.M. Balfour, “Anatomy and Development of Peripatus capensis,”
this Journal, April, 1883.

176 ALICE JOHNSON.

open part of the blastopore to the anterior pit seems to prove
that the blastopore as a whole is dorsal and not ventral.

A slight additional argument in favour of this view may
perhaps be found in the much greater nearness of the archen-
teron to the dorsal surface than to the ventral in early stages
of development before the yolk has been absorbed. It seems
natural that the cavity should exist near the surface from
which the involution to form it originally sprung.

It has already been mentioned that, in the Lizard, the primi-
tive streak extends in front of the anus on the ventral surface
as far as the opening of the allantois into the alimentary canal.
Scott and Osborn described, at a comparatively late stage (with
rudiments of external gills, &c.) in the Newt, a very distinct
fusion of the hypoblast and epiblast in the middle ventral line
behind the mouth. I have myself observed the fusion which
they say is connected with the early formation of the thyroid
body. Can this also be part of the primitive streak? If so,
neither the mouth nor anus represent the extreme ends of the
blastopore.

A possible connection between the two methods of formation
of the mesoblast in Vertebrates, viz. as outgrowths from the
primitive streak or lips of the blastopore, and as outgrowths
from the hypoblast, is suggested by the theory of an elongated
dorsal blastopore. We may suppose that, at a time when the
blastopore was a long narrow open slit, the archenteron was a
large cavity opening into it in the median line, and the meso-
blast consisted of a pair of pouches opening into it on each
side for its whole length. When the blastopore became closed
and a separation between the epiblast and hypoblast ensued,
the mesoblast naturally retained its connection with the latter,
since it was functionally from the beginning an appendage of
the archenteron. Of course, where the primitive streak existed
the mesoblast would keep as far as possible traces of its
original condition, but in regions where the primitive streak
was obliterated the mesoblast could only proceed from the
hypoblast.

In conclusion, I wish to express my very sincere thanks to

ON THE FATE OF THE BLASTOPORE IN THE NEWT. 177

Mr. Sedgwick for his kindness in helping me both in my work
and in the preparation of this paper.

EXPLANATION OF PLATE XVII.

Illustrating Miss Johnson’s paper on “ The Fate of the Blas-
topore and the Presence of a Primitive Streak in the
Newt.”

List of References.

a. Archenteron. au. Auditory vesicle. 6.c. Body-cavity. 6. Blastopore.
ch, Notochord. ch.’ Rudiment of notochord. ep. Epiblast. ep'. Thickened
epiblast of medullary area. 0. Fore-brain. fg. Fore-gut. 4.g. Hind-gut.
hy. hypoblast. m.c. Medullary canal. m.c.’ Solid medullary canal. mes.
Mesoblast. m.s. Mesoblastic somite. o.v. Optic vesicle. pr.g. Primitive
groove. pr.s. Primitive streak. s. Space between hypoblast and mesoblast.
v.c. Outgrowth of fore-gut to form visceral cleft.

Fie. 1—Transverse section through embryo before the formation of the
medullary folds. The section is taken through about the middle of the
embryo.

Fig. 2.—Transverse section through the same embryo, taken at some dis-
tance further forwards.

Figs. 3, 4, 5.—Transverse sections through embryo in which the formation
of the medullary folds has just taken place, and the medullary area is still
very broad. Fig. 3 passes through the blastopore, and is the most posterior
of the series. Fig. 4 is taken through the dorsal surface at some distance in
front of the blastopore. Fig. 5 is the most anterior of the series near the
front end of the posterior part of the primitive groove.

Fics. 6, 7, 8, 9.—Transverse sections through an embryo with several meso-
blast somites. Fig. 6 is the most posterior, and fig. 9 the most anterior of
the series.

Fic. 10.—Section through the primitive streak of an older embryo, with
rudiments of the visceral clefts, tail, &c. The section is oblique, i. e. between
the transverse and horizontal planes, consequently the primitive streak appears
deeper than usual.

Fie. 11.—Longitudinal section through the hind part of an embryo with
the medullary folds just closed. The section is slightly oblique.

18

178 ALICE JOHNSON.

Fig. 12.—Longitudinal section through the hind part of an embryo with
about twelve somites.

Fie. 13.—Longitudinal section through an embryo with about eighteen
somites.

Fie. 14.—Transverse section through an embryo before the formation of
the tail, showing the open blastopore.

Fies. 15, 16, 17.—Transverse sections through a considerably older embryo

than that of fig. 14, showing continuity of blastopore with hind.gut. Fig. 15
is the most posterior, and fig. 17 the most anterior of the series.

The light grey colour signifies the epiblast and organs derived from it, the
dark grey signifies mesoblast, and the yellow signifies hypoblast and organs
derived from it.

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On the Suprarenal Bodies of Vertebrata.
By

w.F. R. Weldon, B.A.,

Fellow of St. John’s College, Cambridge; Lecturer on Invertebrate
Morphology in the University.

With Plates XVIII and XIX.

THE suprarenal bodies of Vertebrates are, as is well known,
made up of two sets of elements, sharply distinguished from
one another, both by their adult structure, and by their mode
of origin in the embryo. The substance which from its posi-
tion in the mammalian suprarenal is known as “ medullary ” is
now almost universally admitted to consist of metamorphosed
nerve-cells, which arise from one or more of the ganglia of
the sympathetic system. As to the origin of the remainder,
however, the so-called ‘cortical ’”’ substance, little is certainly
known. In Elasmobranchs, Balfour! describes the homologue
of this substance as “ making its appearance . . . asarod-like
aggregate of mesoblast cells, rather more closely packed than
their neighbours, between the two kidneys near their hinder
ends;” but he leaves it an open question, whether these cells
arise from the general indifferent mesoblast surrounding them,
or whether they are derived from any of the adjacent organs
of the embryo.

These observations of Balfour were followed, in 1882, by two

1“ Hlasmobranch Fishes,” p. 246.

180 W. F. BR. WELDON.

important papers by Braun! and Mitsukuri,? the one dealing
with the development of the suprarenals in lizards, the other
in mammals.

In lizards, Braun describes the cortical substance as arising
‘as a thickening in the walls of the vena cava inferior.” In
the earliest stage figured by him, a large mass of cortical
blastema is already established, as seen in Pl. 1, fig. 4 of his
paper. In this figure, as in all the others given by Dr.
Braun, it is noticeable, as he himself says, that “ the flattened,
nucleated endothelium (of the blood-vessel} is easily to be
distinguished ” from the adjacent tissue, and that it shows no
sign of proliferation. It is therefore difficult to conclude from
this account that the suprarenals arise as appendages of the
blood-vessels themselves, Braun’s observations throwing little
more light upon the real origin of the cortical substance than
did the earlier ones of Balfour.

In the same way Mitsukuri, treating of mammals, finds the
first rudiment of the cortical substance in a little knot of
isolated mesoblast cells ‘on each side of and ventral to the
aorta, on the inner side of the Wolffian bodies, and dorsal to
the mesentery.”

Gottschau, in a later paper? has described in mammals
phenomena nearly in accordance with those observed in lizards
by Braun,—emphasising more than Mitsukuri the connection
between the cortical substance and the adjacent blood-vessels.

From none of these observations can we learn anything of
the mode of origin of the blastema described, each author
taking up its history at a point when the cells composing it
have already lost any connection which they may primitively
have possessed with another embryonic organ. Janosik* has
attempted to trace the earlier history in mammals, and has

| «Bau u. Entwick. d. Nebennieren bei Reptilien,” Semper’s ‘ Arbeiten,’
Bd. v.

7 “On the Developmeut of the Suprarenal Bodies in Mammalia,” ‘ Quart.
Journ. Mic. Sci.,’ 1882.

* © Archiv, fiir Anat. u. Phys.,’ 1883.

4 «Archiv fir Mikr, Anat.,’ 1883.

ON THE SUPRARENAL BODIES OF VERTEBRATA. 181

been led to believe that the blastema of Gottschau, Mitsu-
kuri, and others arises as a series of (segmental?) outgrowths
from the peritoneum, in the angle between it and the root of the
mesentery and the peritoneum. As, however, very few figures
are given with this paper it is not easy to form an idea of the
exact nature of the events described.

This state of things led me to believe that it might be
worth while to examine carefully embryos younger than those
used by any previous observers, and so to trace the earlier
history of the cortical blastema. ‘his I have been able to do,
during the summer of the present year, in the chick, in
Lacerta muralis, and in Pristiurus. As my observations
are most complete in the case of Lacerta, I begin with an
account of the development in that type. In order fully to
understand the development of the suprarenal body, it will be
necessary to follow the development of the glomeruli of the
mesonephros, which has been described by Braun (loc. cit.)
After the formation of the segmental vesicles and Wolffian
duct each segmental vesicle gives off from its outer margin a
solid column of cells, which joins the Wolffian duct, and soon
acquires the yy) shape characteristic of the young segmental
tubes in so many Vertebrates. After this cord of cells has
united with the Wolffian duct, the lumen of the segmental
vesicle extends into it, and it takes on all the characters of
a segmental tubule. After this has happened, one wall of the
persisting segmental vesicle becomes pushed in by a plexus of
blood-vessels, and forms a glomerulus.

But while the wall of the glomerulus is being thus invagi-
nated, a proliferation of the cells composing it occurs at the
side opposite to the point of attachment of the segmental tube,
that is, on the inner margin of the glomerulus.

In fig. 1, I have attempted to represent the condition of things
in one of the anterior glomeruli of an embryo with about
twenty protovertebre. The section passes nearly through the
centre of the glomerulus, which is seen to be only partially
invaginated ; and I may here call attention to the manner in
which, in lizards at least, the invagination seems to take place

182 Ww. F. R. WELDON.

before the entrance of the blood-vessels, none of which are
to be seen in the section figured. The epithelium is much
more columnar than at a later stage, and is regularly one cell
thick on the outer side, while on the side undergoing invagina-
tion it is more or less regularly composed of two layers of cells ;
but at every point except one the whole glomerulus is bounded
by cells of a definitely epithelioid character, having no pro-
cesses, and showing no indication whatever of any tendency to
proliferation. At the inner margin, however, the case is
different; here the limiting cells are irregular in shape, and
can in no way be separated, by any sharp line of demarcation,
from the cells forming the mass (s.r. 6.), which is seen to
be attached to the inner wall of the glomerulus. This mass
gives rise both to the connecting tubules between testis and
epididymis and to the cortical substance of the suprarenals.
At present it is seen to extend for ashort distance dorsal-
wards, between the segmental tubule (s. ¢.) and the vena cava
(v.c.), and then to bend rather sharply ventralwards towards
the generative ridge, the anterior end of which (W.7r.) is seen
in the section. As a contrast to the continuity between the
cell mass in question and the cells bounding the cavity of the
glomerulus I would especially call attention to the distinctness
of the line of demarcation between it and the endothelium of
the vena cava, at the point where the two are in contact—a
distinctness which, persisting, as we shall see it to do, through
all stages of the development of the suprarenal blastema, ren-
ders it extremely difficult to believe that the endothelium is in
a state of proliferation, or that there is any real connection
between it and the suprarenal blastema.

The small blood-vessel (3. v.) which is seen in the figure
is also perfectly sharply separated from the adjacent tissues.

The: section represented in fig. 2, from an embryo about
4'5 mm. long, with twenty-four protovertebra, shows a further
advance in the development of the suprarenal blastema and its
associated glomerulus. The section, which passes through the
entrance of a segmental tube into the glomerulus, shows the
completion of the invagination, and the entrance of blood-

ON THE SUPRARENAL BODIES OF VERTEBRATA. 183

vessels (diagrammetrically indicated by shading). The epithe-
lium of the glomerulus is everywhere, except on its inner side,
formed of a single layer of cells, which are much flatter than
in the preceding stage, but on the inner side the cells pass,
as before, without any definite line of demarcation, into the
suprarenal blastema, which is still composed of a compact mass
of polygonal cells, without any distinction being visible between
the part which is going to form suprarenal body and that
which is going to form a seminiferous tubule. In this section
the distinction between the endothelial cells of the various
blood-vessels and the tissues surrounding them is even better
marked than in the one last described.

The appearances which I have attempted to describe are seen
first in the more anterior, then in the hinder glomeruli of all
that region of the mesonephros which is coextensive with the
generative ridge, and in one or two glomeruli in front of it.

The blastema which I have described grows, in the suc-
ceeding stages, in two directions: dorsalwards between the
cardinal vein (or vena cava) and the tubules of the mesone-
phros, and ventralwards into the prominence of the Wolffian
ridge. In such a section as that shown in fig. 3, for example,
which is taken from the posterior part of the mesonephros of
an embryo of 8 mm., two distinct regions may now be distin- -
guished, a region (s.r. 0.) dorsal to the point of origin from
the glomerulus, the cells composing which will go to form the
suprarenal, and a region (s. sér.) going from the glomerulus
ventralwards into the generative ridge, which is the rudiment
of the testicular network. No histological difference can as
yet be detected between the one region and the other, the
whole blastema being composed of a mass of polygonal cells
with rounded nuclei, the characters of which are everywhere
identical.

In an embryo of 10 mm. (figs. 4 and 5), a slight dis-
tinction between the two parts is for the first time apparent,
though the histological characters of adult suprarenal cells are
not acquired for some time. Of the two sections figured, that
shown in fig. 4 is taken in front of the Wolffian ridge; in it,

184 WwW. F. BR. WELDON.

therefore, the blastema attached to the glomerulus gives rise
only to suprarenal tissue. For this figure, I have purposely
chosen a section in which the contact between the suprarenal
rudiment and endothelium of the vena cava was as close and
as extensive as possible, in order to show the distinctness
which, in spite of their close apposition, exists between the two
structures, and to contrast once more this distinctness of the
vena cava endothelium with the irregular way in which the
cells of the glomerulus wall are merged in the blastema.
This section is also interesting from another point of view.
One of the arguments used by Dr. Braun, in order to disprove
the existence of any real connection between the rudiment of
the testicular network and that of the suprarenal, is that the
segmental rudiments of the former structures are well developed
before the appearance of any suprarenal tissue at all. Dr.
Braun believes that the whole of the outgrowth from each
glomerulus becomes converted into a seminiferous tubule.
But if this be so, what can be the function of such an outgrowth
in front of the testicular region ?

In fig. 5 is seen a section through the beginning of the
generative ridge: the suprarenal and seminiferous rudiments
are still continuous, but the one is a little more deeply stained,
and its component cells are a little smaller thanthe other. As
before, the endothelium of the surrounding blood-vessels forms
a distinct layer over the blastema, the cells of which are quite
sharply defined and clearly recognisable.

The upward growth of the suprarenal rudiment, already
well marked in fig. 5, is still better seen in fig. 6, from the
middle of the trunk of an embryo of 13 mm.—almost the
oldest in which a connection between suprarenal and semi-
niferous tubules can be seen. In an embryo of 18 mm.
(fig. 7), the separation has already taken place, and the
suprarenal is cut off by blood-vessels from all adjacent struc-
tures, though it remains now, as always before, perfectly
distinct from the endothelium of the vessels themselves.
This stage is only very slightly younger than the youngest
figured by Braun, as fig. 4, Pl. I. of his paper shows; the

ON THE SUPRARENAL BODIES OF VERTEBRATA. 185

chief difference between his figure and mine being that he has,
having overlooked the earlier stages, been led to an erroneous
form of opinion as to the mode of origin of the tissue which
he figures. From this point onwards, however, his observa-
tions as to the histological differentiation of the cortical
substance, and the entrance into it of the medullary ganglion
cells are so complete that it is needless to attempt to add
anything to his description.

In Pristiurus, as in other forms, the early history of the
suprarenals has only been traced from a point at which a meso-
blastic rudiment, distinct from all other organs, already existed.
This is the stage at which Balfour, in the passage already
quoted, begins his account of their development. I propose,
therefore, to trace the history of this blastema in Pristiurus,
which is the only Elasmobranch in which I have observed it.

In figs. 9 and 10 are shown two consecutive sections through
a Pristiurus embryo 8 mm. in length, at a stage corresponding
to Balfour’s Stage I—the stage immediately preceding that
in which he begins the history. Both these sections pass
through the opening into the body cavity of the same seg-
mental tube, which is seen to give off, just after the narrowing
of its funnel shaped opening into the body cavity, a small
process (s. 7.) ,which projects towards the root of the mesentery.
In fig. 9, which passes through the middle of this process, it
is seen to have a very considerable lumen. In fig. 10 it is
cut tangentially, and the lumen is therefore not apparent.

In figs. 11 and 12, from a slightly older embryo, this diver-
ticulum of the segmental tubule is seen to have obtained a
considerable size, and to project quite to the middle line over
the root of the mesentery. It is not seen in the figure to be
joined by a similar structure from the opposite side, because
the section copied was so oblique that the right hand side was
intervertebral. In the next following section, however (fig. 13),
the wall of the outgrowth of the other side is cut.

In an embryo of between 9 and 10 mm. the outgrowth has
become solid, and lies just over the root of the mesentery, as
shown in fig. 14; further, at this stage the outgrowths have

186 W. F. BR. WELDON.

so coalesced with those in front and behind that an interverte-
bral section, such as that shown in fig. 15, still passes through
them.

One feature of the sections of this age, which I do not
understand completely, is the sbifting of the position, with
regard to the segmental funnel, of the point of attachment of
the suprarenal outgrowth ; while in the preceding stage (see
fig. 12) the outgrowth was external to the primitive ova, open-
ing distinctly into the segmental funnel, it is now attached to
the peritoneal epithelium at the root of the mesentery internal
to the primitive ova. While I am unable to account for
this apparent change of position, I see no reason for doubt-
ing the identity of the structure I have called s. v. in figs. 14
and 15 with that similarly named in the preceding figures.

In the next stage, finally, which is a young embryo of
Balfour’s Stage IV, we find (fig. 16) the unpaired rod of meso-
blast described by him lying at the root of the mesentery,
but still attached segmentally (see the left hand side of the
figure) to the segmental funnel.

I have unfortunately no stage intermediate between this
and the stage last described, but it seems obvious that the
unpaired blastema existing at this stage must be produced by
the fusion of the paired outgrowths of the earlier stages.

An important point with regard to this blastema in Pris-
tiurus, which has apparently been overlooked by Balfour, is
that it extends throughout the whole length of the mesonephros.

It is well known that in an adult Elasmobranch there are
two sets of suprarenal bodies: one a series of paired, more or
less regularly segmental bodies, attached to the dorsal wall of
the cardinal vein on each side in the mesonephric region, and
the other one unpaired, median body, lying between the two
halves of the metanephros.

Balfour was of opinion that the bodies of the anterior set,
though they show in the adult a division into cortical and
nervous positions as distinct as that which exists in the supra-
renals of higher Vertebrates, were yet derived entirely from
sympathetic ganglia. The presence, in the anterior end of the

ON THE SUPRARENAL BODIES OF VERTEBRATA. 187

body, of a blastema such as I have described seems to throw
doubt on the correctness of such a view; though I have un-
fortunately been unable, owing to want of material, to prove
by examination of later stages the share which this blastema
takes in the formation of the paired anterior suprarenals.

In the chick, as might perhaps have been expected, from the
highly-modified development of the whole kidney, the mode
of origin of the suprarenal blastema differs in many important
points from that which has been described for the dogfish and
for the lizard.

Before the fourth day of incubation there is no trace of any
suprarenal rudiment whatever. By about the end of this day,
however, certain large cells, the rudiments of the cortical sub-
stance, make their appearance in the indifferent mesoblast at
the inner side of the mesonephros. The exact mode of origin
of these cells I have been unable to determine. At their first
appearance they lie, singly or in groups of two or three, in the
mesoblast between the aorta and the kidney, being distin-
guished from the surrounding cells by their rounded, un-
branched form, their larger size, and the clearness of their
protoplasm. During the end of the fourth day, and the early
part of the fifth, they increase in number, either by division or
by addition from the surrounding mesoblast, till in an embryo
of about the middle of the fifth day of incubation, they form
groups of a considerable size, which present in section the
appearances seen in fig. 17. The cells seen in this section,
though they are more numerous than at the time of their first
appearance, have not appreciably changed their relations to the
surrounding parts. They are seen to lie surrounded entirely
by branched mesoblast cells without any connection, either
with the epithelium of the adjacent glomeruli, or with the walls
of any blood-vessels. In this isolated condition the suprarenal
cells remain during the fifth and sixth days, travelling, how-
ever, gradually towards the mesonephric glomeruli, and at the
same time increasing in number, and tending to arrange them-
selves in irregular branched columns, having in section an
elliptical outline. During the seventh day they attach them-

188 Ww. F. B. WELDON.

selves to the epithelium of the glomeruli, so as to appear as in
fig. 18. In this figure the epithelium of the glomerulus is seen
to be distinct from the suprarenal for a short distance; but in
a part of the section I was unable, after a tolerably careful
examination, to convince myself of the existence of any distinct
layer of epithelial cells separating the cavity of the glomerulus
from the adjacent blastema.

Such a section as that shown in fig. 18 may be seen in almost
any glomerulus in the region of the suprarenal during the
seventh day. On the eighth day the appearance of the blas-
tema changes. While still retaining its connection with the
glomeruli (fig. 19) it has increased considerably in size, and its
component cells have acquired most of the histological charac-
ters which they present in the adult. The individual cells are
large, polygonal, and distinctly marked off one from the other ;
their protoplasm, which does not stain very readily with car-
mine or hematoxylin, is clear or very finely granular, and their
nuclei are clear, oval, or elliptical, with well-defined contours
and a number of coarse granules in their interior. The most
characteristic feature in the blastema of this age is, however,
the definite arrangement of the cells into columns, giving them,
more than at any earlier stage, the appearance of the cortical
substance of an adult suprarenal.

I have already said that the blastema during the eighth day
remains attached to the glomeruli; such appearances as those
seen at # in fig. 19, which are very frequent, tempt one
strongly to believe that at this time the number of the cells
composing it may be added to by proliferation from the glome-
rulus epithelium ; but I have not been able to satisfy myself
that this is the case.

From this time the changes in the cortical blastema, so far
as I have followed them, do not differ in any important parti-
culars from those described by Braun in Lacerta muralis.

A noticeable feature throughout the whole of the early
history of the organ under consideration in the chick, is the
very distinct separation between the cortical blastema and the
blood-vessels, the original blastema-cells being at a great

ON THE SUPRARENAL BODIES OF VERTEBRATA, 189

distance from any vessel, and the later tissue only approaching
one when it has so greatly increased in size as to have pushed
all the intervening mesoblast, so to speak, on one side. There
is no possibility of believing, in this case at least, that the
walls of the blood-vessels have the slightest share in the pro-
duction of the cortical blastema.

The great difference between the results of the investigations
of previous observers and those which have just been described,
is sufficiently obvious. If, however, the accuracy of my
observations be admitted, we have a much more rational expla-
nation of the phylogeny of the suprarenals than is possible if
we adopt the view of Braun, and others ;—an explanation which
receives support, both from the anatomical relations of the
adult organs, and with those of the corresponding organs in
Myxinoids and Teleosteans.

In Bdellostoma, I have already! attempted to show that
the head kidney has become modified so as to form an organ
functionally analogous to the suprarenals; while in Teleos-
teans, a2 most remarkable series of modifications, affecting
every region of the kidney, has been described by Balfour and
Emery; a series which seems to me to supply every stage
needful to complete our conception of the passage from such a
form as Bdellostoma to that of a higher vertebrate. Balfour
showed? that the head kidney of many adult Teleosteans con-
sisted, not of renal tissue, but of a mass of parenchymatous
“lymphatic” material, richly supplied with vessels, whose
function, whatever it might be, was certainly not that of a
normal kidney. He afterwards found the same kind of modi-
fication to exist in the head kidney of the Teleosteoid
Ganoids.®

Though the observations of Balfour left it highly probable
that the “lymphatic” tissue described by him was really a
result of the transformation of part of the embryonic kidney,

1 Quart. Journ. Mic. Sci., April, 1884.
2 Quart. Journ. Mic. Sci., 1882. ;
3 «On the Structure and Development of Lepidosteus,” ‘Phil. Trans.,

1882.

190 Ww. F. R. WELDON.

he did not investigate the details of its development. This
was afterwards done by Emery,! with the following re-
sults :—

In those Teleostei which he has studied, Professor Emery
finds that at an early stage the kidney consists entirely of a
single pronephric funnel, opening into the pericardium, and
connected with the segmental duct, which already opens to the
exterior. Behind this funnel, the segmental duct is surrounded
by a blastema, derived from the intermediate cell mass, which
afterwards arranges itself more or less completely into a series
of solid cords, attaching themselves to the duct (see fig. 8).
These develop a lumen, and become normal segmental tubules,
but it is, if I may be allowed the expression, a matter of
chance, how much of the blastema becomes so transformed
into kidney tubules, and how much is left as the “lym-
phatic” tissue of Balfour, this “lymphatic” tissue remain-
ing either in the pronephros only, or in both pro- and meso-
nephros.

We have here, as it seems to me, an explanation of the
reason why the suprarenals, while arising from the pronephros
in Myxinoids, are mesonephric in origin in the higher Verte-
brates. The same causes which led to the degeneration of the
original renal pronephros (causes among which the specialisa-
tion of the pericardium, and the development of the air-bladder
and lungs may have played a considerable part) —the same causes
which led to the establishment of the mesonephros as the chief
seat of renal secretion may, and indeed must, have rendered
advantageous the suppression of any glandular organ in the
pronephric region; and thus, when, in consequence of the change
of function of the Wolffian duct more and more of the meso-
nephros became useless as a kidney, it is easy'to understand
how some of its component parts underwent in their turn the
same change of function as had been undergone by the
anterior part of the renal organ at an earlier stage in its
evolution, stages in the completion of this process remaining

1 © Atti dell’ Accademia dei Lincei.,’ 1882.

ON THE SUPRARENAL BODIES OF VERTEBRATA. 191

both in the commencing modification of the Teleostean meso-
nephros on the one hand, and on the other in the suprarenal
of Amphibia, with its own “ portal” circulation, and its close
connection with the renal tissue.

192 w. I. R. WELDON.

EXPLANATION OF PLATES XVIII & XIX,

Illustrating Mr. W. F. R. Weldon’s Paper “‘On the Supra-
renal Bodies of Vertebrata.”’

Complete List of Reference Letters.

Al, Alimentary canal. do. Aorta. Bv. Blood-vessel. g/. Glomerulus.
g- ep. Glomerulus epithelium. pe.ep. Peritoneal epithelium. Mes. Mesentery.
v.c. Vena cava. s.¢. Segmental tubule. ss. s¢r. Testicular tubule. s. 7. 0.
Suprarenal blastema. W.7. Wolffian ridge.

The figures were in all cases drawn by the aid of a Zeiss’s camera lucida.

Fie. 1.—Transverse section through a glomerulus of an embryo of Lacerta
muralis with twenty-one protovertebre.

Fic. 2.—Similar section through an embryo with twenty-three protover-
tebree.

Fic. 3.—Similar section through an embryo 8 mm. long.

Fic. 4.—Similar section from an embryo 10 mm. long.

Fic. 5.—Similar section from an embryo of 11 mm.

Fie. 6.—Similar section from an embryo of 18 mm.

Fic. 7.—Similar section from an embryo of 18 mm.

Fic. 8.—Transforming blastema of teleostean kidney, copied from Emery.

Figs. 9 and 10.—Two consecutive sections through an embryo of Pris-
tiurus melanostomus of 8 mm,

Fics. 11—13.—Consecutive sections from an embryo of Pristiurus of 83 mm.
Fies. 14 and 15.—From an embryo of Pristiurus of 10 mm.

Fie. 16.—From an embryo of Pristiurus slightly older than that figured in
figs. 14 and 15. :

Fie. 17.—From a five-day chick.
Fie. 18.—From a seven-day chick.
Fic. 19.—From a nine-day chick.

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On a Peculiar Sense Organ in Scutigera
coleoptrata, one of the Myriapoda.

By

¥F. G. Heathcote, B.A.,
Trinity College, Cambridge.

With Plate XX,

——

In the spring of the year I was fortunate enough to get a
fair number of Scutigera in the South of Europe. In making
an examination of their anatomy I found a sense organ which
seemed to me to be of sufficient interest to render a more com-
plete examination desirable. This organ is placed on the
ventral surface of the head at a short distance behind the
mouth and near the base of the mandibles. Its external
appearance under a low power of the microscope (Zeiss’s
objective a a) is shown in fig. 1.

GENERAL FEATURES,

The organ which was first mentioned by Latzel, consists of
a chitinous sac with a slit-like opening (fig. 1, eo.). The
opening is placed between the base of the mandibles and the
‘maxilla. The sac has a somewhat complicated form which
will be best understood by reference to four diagrams (see
Plate XX, figs. a, B, c, D.

The first of these shows a rough outline of the appearance
of the organ from the ventral side; the second, third, and fourth
being diagrammatic sections through the dotted lines 4B, CD,
and EF. B is a transverse section through the anterior portion
of the organ. It shows the main sac communicating with the
Les ON aa

194 F. G@. HEATHCOTE.

exterior by a narrow neck and two lateral recesses opening into
the neck and placed ventrally to the main sac parallel with the
ventral surface of the head. The section C taken through the
median portion of the organ exhibits similar relations, excepting
that the median dorsal wall of the sac is projected into the
interior in two longitudinal folds which make a partial division ”
of the sac into three portions, two deep, wide lateral pouches,
and one deep narrow recess between them. This latter I shall
speak of as the median recess. A third section, D, is taken
through the hinder part of the organ. Here the lateral
recesses and the slit-like opening to the exterior are absent
and the two dorsal folds almost completely divide the main
sac in three portions.

It is worthy of note that the effect of the median and lateral
recesses is to produce a freely projecting lip or edge on the
dorsal and ventral aspects of each pouch. rH

The general shape of the interior of the organ is therefore
posteriorly that of two pouches projecting into the interior of
the head, while between them is a median dorsal recess formed
-by the folds above described, which constitute the inner walls
of the pouches where they approach one another. Anteriorly
the division into two pouches is not so perfect, but there are
two deep lateral ventral recesses, The slit-like opening to the
exterior begins at the anterior end and extends about a third
of the length of the whole organ.

The chitinous exoskeleton is continued into and lines the
whole organ. It is not, however, of uniform thickness. In the
median’ and lateral recesses and on the folds constituting the
lips of the pouches itis smooth, but in the pouches themselves
it is thrown into a number of folds and bears a large number
of chitinous hairs (fig. 2, 2.) which project into the lumen ot
the pouches. .The folds form alternate ridges and depressions,
so that when looked at from the surface through a microscope
the chitinous lining of the pouches has a reticulated appearance
(fig. 8). The hairs, whose length is about that of the diameter
of each pouch, are of peculiar form (fig. 9). Each consists of
a stout elliptical basal portion, the inner end of which is inserted

PECULIAR SENSE ORGAN IN SCUTIGERA COLEOPTRATA. 195

into the chitinous lining of the pouch and indeed projects for
a short distance on the inner side of the latter. The outer end
is prolonged into a long fine hair.

~GeneraL Features oF THE INTERNAL ANATOMY.

The hypodermic layer of cells or matrix lying beneath the
exoskeleton accompanies the chitin round the lateral recesses,
and at the edge of the folds which form the lateral lips of the
two pouches becomes continuous with a thick layer of sensory
epithelium which lines the greater part of the two pouches
(fig. 2, s.e.). On reaching the dorsal lips of the pouches, which
lips bound laterally the median recess, the epithelium loses its
sensory character and again becomes simple hypodermis. On
the dorsal part of the median recess the epithelium again
becomes sensory in character. The nerve supply is furnished
by two short thick nerves (fig. 2, N.) which arise from the front
portion. of the subcesophageal ganglion, The two nerves enter
‘the sensory epithelium, one to each pouch, near the posterior
part of the organ, and there breaks up into a number of fibres
‘which become lost in the epithelium. The form of the organ
as indicated by the division into two pouches (fig. 2, p.) and
‘the double nerve supply seem to me to show conclusively that
it is double, and that each of the two pouches with its other
parts is to be regarded as constituting a separate sense organ.

HistoLoey.

The histology of the cellular tissues demands a more detailed
account. The cells forming the matrix from which the
exoskeleton is renewed after. each moult are large, rather
columnar in their character, and have a well-defined nucleus,
They are closely applied to the chitin and accompany it up to
the end of the lateral recesses (refer to fig. 3, Ay.) where the
folds forming the ventral lips of the pouches begin. Here the
chitin is thrown into folds somewhat like. those which charac-
terise the surface of the pouches. The hypodermic cells here
lose their regularity of outline and follow the chitin into the

196 F. G. HEATHCOTE.

.folds and irregularities into which it is thrown (fig. 3, hy).
Their nucleus is larger and stains more deeply. At the lateral
‘ventral lip (fig. 2, dvi.) the cells are more elongated and more
closely packed together, and gradually take the character of
the sensory epithelium which forms the greater part of the
lining of the pouches. These sensory cells are long and
columnar and at their outer ends are prolonged into a blunt
projection of less diameter than the rest of the cell (fig. 7, oe.)
and about one third the length of the whole. At the folds
which bound the median recess the cells lose their sensory
character and take the form of the ordinary hypodermic cells.
The mass of sensory cells at the top of the median recess which
are continuous with the hypodermic cells are of a character
distinct from those described as lining the pouches. They are —
of irregular elongated shape and resemble ganglion-cells, the
inner end being sometimes bifurcated (fig. 6, Bi.). The sensory
epithelial layer is of considerable thickness (fig. 2).

_ Ihave hitherto spoken of the epithelial layers simply as in-
vesting the chitinots pouches with their hairs, but I will now
consider the means by which the hairs and cells come into
relation. There is no doubt that the terminal parts of the
sense cells project into the depressions (fig. 3) in the chitin,
caused by the folds spoken of above, and that each chitinous
hair is inserted into the chitin immediately outside this pro-
jecting part of a sense cell. I am also inclined to believe,
though, owing to the small amount of material at my command
my evidence on this point is not conclusive, that the bases of
the chitinous hairs, i.e. the part which projects on the inner
side of the chitinous lining, have a small cavity in their basal
parts, into which a threadlike prolongation of the sense cell
projects.

I have invariably found foreign bodies in the median and
lateral recesses, and as the latter are in communication with
the exterior they may possibly be grains of dirt or sand, but
I think that they may be concretions,

PECULIAR SENSE ORGAN IN SCUTIGERA COLEOPTRATA, 197

Concivusions.

_ The active predatory habits of this Myriapod and its power

of swift locomotion would seem to render well-developed sense
organs a necessity to it; in fact it has facetted eyes in place
of the simple eye-spots of most Myriapods.

The organ above described must, I think, be included among
the great number of widely dissimilar organs usually classed
together as auditory, and may be compared to the tympanic
organ of insects.

The auditory organs of insects have been investigated prin-
cipally by v. Siebold (‘Archiv fiir Naturg.” 1844), Leydig
(‘Milller’s Arch.,’? 1855 and 1860), v. Hensen (‘Zeitschr. f.
wiss. Zool.,’ tom. xvi, 1866), and v. Graber (‘ Denkschr. der K.
‘Akad. der Wissensch.,’ Wien., 1875). The tympanic organ of
the Acridiidz consists essentially of a tympanic membrane sup-
ported by a chitinous ring. Places in the tympanic membrane are
thickened, so as to form solid chitinous pieces of peculiar form,
the internal surface of which is covered with indentations in
which the extreme ends of the sensory apparatus end (Fr.
Leydig, ‘ Miiller’s Archiv,’ 1855, p. 401). The auditory nerve
spreads out on these chitinous pieces and forms a ganglion,
from which fibres ending in peculiar sense cells are given off.
A trachea lies close to the ganglion internally to it, and not
unfrequently swells to a vesicle.

On comparing the organ of Scutigera with such an organ
there is found to be a great similarity in the general plan.
Each pouch in Scutigera represents the insect tympanum.
In both cases we have a thick nerve breaking up into a number.
of sensory elements, which end in depréssed spaces in the
chitinous membrane. With regard to the chitin hairs which
project through the chitin in Scutigera, I think it will be
worth while to consider Hensen’s investigations on the auditory
rods of insects (Hérstifte, v. Hensen, 1. c.). He makes an
interesting comparison between these structures and the audi-
tory hairs of the crustacean auditory sac, and draws the con-

198. © ss. G..: HEATHOOTE.

clusion that the two structures present a very great morpho-
logical resemblance. If his arguments hold good it seems to
me permissible to compare the hairs of Scutigera to those in
‘the auditory sac in Crustacea, and also to the auditory rods
‘(Horstifte) in insects. a:
There is one point, however, in which the organ of Scutigera
differs greatly fromthe tympanic organ, viz. in the absence of
a tracheal vesicle. “I think it doubtful, however, whether this
tracheal vesicle is an essential part of the insect auditory
‘organ. . The swelling of the tracheal trunk seems not to take
place in all cases (Leydig, l.c.), and Hensen, in giving
‘what he considers the most probable hypothesis as to the
action of the tympanic organ, says: “Die tracheen schwin-
-gungen sind ohne Bedeutung.’ Balfour, in his short account
of the auditory organ of terrestrial insects (‘Comp. Emb.,’ il,
423), does not mention the tracheal vesicle. os
- I have examined this sense organ of Scutigera, both by dis-
‘section and by means of sections. I found that the tissues
-were best preserved by a mixture of corrosive sublimate and
acetic acid. The difficulty of cutting the chitin in sectioning
-was overcome by embedding in very hard paraffin.
My investigations were entirely carried on in the Cambridge
‘Morphological Laboratory.

° 1 Since forwarding this paper (November, 1884) to the editor of this
Journal my attention has been drawn to a paper by Dr. Haase in Schneider’s
‘Zool. Beitrage,’ 1884, upon “Schlundgerust und Maxillarorgan von
Scutigera.” ‘ : .
As I am on the point of leaving England on along voyage it is now too
late for me to make an extensive reference to this work, but I may add that
‘in my opinion Dr. Haase’s observations do not necessitate any alterations in
‘the foregoing paper. : fw : i ;

‘ are 4

i

PECULIAR SENSE ORGAN IN SCUTIGERA COLEOPTRATA, 199.

‘DESCRIPTION OF PLATE XX,

Illustrating Mr. F. G. Heathcote’s Paper “ On a Peculiar Sense
' Organ in Scutigera coleoptrata, one of the Myria-
poda.”

" Letters used in all the Figures.

---m. Mouth. f Furrow in chitin. ¢. 0. External opening of sense organ.
o. Sense organ. mal. 2ud maxilla. N.. Nerve.. .s, ¢.. Sense epithelium.
A. Chitinous hairs. Ay. Hypodermis. yp. Pouch. dr. Lateral recess. Mr.
Median recess. /v/. Lateral ventral lip. 2d? Median dorsal lip. cf.
Chitin. Bz. Bifurcated end of cell at top of median recess. oe. Outer
end of cell. 24. Base of chitinous hairs projecting into anberir bp.
Basal part of chitinous hair. 2%. Free end of hair.

Fig. 1 was drawn for me by Mr. Chapman ; all other figures were drawn by
myself with the aid of Zeiss’s camera lucida. Fig. 2 is combined from three
sections.

Fie. 1.—Ventral view of head of Scutigera coleoptrata. The sense
organ is seen through the chitin. mm. Mouth. A furrow in the chitinous
exoskeleton, marking out two irregular areas just in front of the organ. e o.
Opening of organ to the exterior. 0 on seen through the chitin. oc.
Eye. mal. 2nd maxilla.

Fig. 2.—Transverse section through the organ, showing the nerve (W.), sense
epithelium (se.), chitinous hairs (4.), hypodermis (Ay.), median recess (Mr.),
and lateral recesses (47.); also the two pouches (p.). (Zeiss’s c c objective.)
Owing to the action of the reagents used the sensory epithelium has shrunk
away from the chitinous lining of the sac. /v/, Lateral ventral lip. Md/.
Median dorsal lip. ,

Fic. 3.—Transverse section through the region of the lateral recess, show-
ing the transition from the hypodermis to the sensory epithelium. Ay. Hypo-.
dermic cells, se. Sense cells. 47. Lateral recess. cf. Chitin, 4. Chitin-
ous hairs projecting into the pouch. Jv/. Lateral ventral lip of pouch.
(Zeiss’s F objective.)

Fic. 4.—Transverse section, taken so as to show the hypodermic cells (Ay.)
joining the sense cells (se.) in the region of the median recess. (Zeiss’s F
objective.) Ay. Hypodermic cells. se. Sense cells. ch, Chitin.

Fig. 5.—Transverse section through the anterior part of the organ, show-
ing the hypodermic cells becoming continuous with the sense cells (se.). The _

900 F. G. HEATHCOTE,

section is in front of Fig. 4. (Zeiss’s p objective.) se. Sense epithelium.
ch. Chitinous lining of the organ. Ay. Hypodermis.

Fie. 6.—Tailed cell from sense epithelium at top of median recess. Bi,
Bifurcated end. oe. Outer end. (Zeiss’s F objective.)

Fic. 7.—Sense cells from epithelium (se.). oe. Outer end of cell,
(Zeiss’s ¥F objective.)

« Fig. 8.—Surface view of chitinous lining of pouch seen from the jatotual
sid showing the projecting bases of the chitinous hairs. 04, Base of hair
projecting through the chitin. (Zeiss’s water immersion L objective.)

Fic. 9.—Chitinous hair under high power. (Zeiss’s L water immersion.)
ép. Basal part of hair. BA. Part of the basal piece which projects through
the chitinous lining of the pouch towards the interior. 7%. The hair itself
projecting into the interior of the pouch, ;

oa
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Aye

F Huth, Lith® Bain

The Development of the Mole (Talpa Europea),
the Ovarian Ovum, and Segmentation of the
Ovum.

By

Walter Heape, M.A.,
Demonstrator of Animal Morphology in the University of Cambridge.

With Plate XXI.

Tue Ripe Ovarian Ovum.

Tne position of the ripe ovarian ovum in the ovary is
betrayed by the rounded semi-transparent Graafian follicle
in which it lies, projecting prominently on the surface of the
ovary.

If an ovary containing such a follicle be held firmly with a
pair of forceps on a slide, and the follicle be pricked with a
needle, or better still, sharply gashed with the point of a fine
scalpel, the ovum spirts out on to the slide together with a
not inconsiderable amount of clear transparent fluid, the
liquor folliculi.

In accordance with the degree of ripeness of the ovum thus
obtained it is more or less completely invested by a mass of
epithelial cells, in the midst of which it lay in the discus
proligerus within the follicle.

These epithelial cells are radially arranged round the ovum
(fig. 1). The cells of the innermost layer are more or less
elongated and their inner end, tapering somewhat, rests upon
a thick transparent membrane which surrounds the ovum,
the so-called zona radiata (the zona pellucida of the older

observers).
15

202 WALTER HEAPE.

The shape of the cells of this inner layer varies according
to age, as van Beneden has observed (No. 4), but they invari-
ably have the aspect of an epithelial investment. To this
layer of cells the misleading term of membrana granulosa has
been applied.

Tue Zona Raviata.

The zona radiata in fully ripe ova (vde figs. 1 and 2) is a
clear transparent membrane with a granular outer border upon
which the surrounding cells of the discus proligerus rest
(fig. 1).

_ The inner portion of this membrane is so transparent
that the outlines of the epithelial cells may clearly be seen
through it.

The origin of the granular outer portion has not been satis-
factorily traced ; it may possibly, according to Balfour (No. 1),
be due to the presence of the remains of the primary vitelline
membrane, within which the zona radiata has been subse-
quently produced. On the other hand, the appearance may be
due to the irregularity of the surface of the zona radiata itself,
this latter circumstance being in its turn occasioned partially
by the close adhesion of the surrounding cells of the discus
(fig. 6), partially by the open mouths of numerous canals
which pass radially through it, and to which I shall call atten-
tion directly (fig. 7).

I have not myself attempted in this paper to trace the
development of the ovarian ovum or its membranes, and must
therefore at present leave this question without further dis-
cussion.

The thickness of the zona varies in the two specimens
represented (figs. 1 and 2) between ‘008 and ‘001 mm. The
two ova themselves were both completely surrounded by the
cells of the discus proligerus, but in the one drawn in fig. 2
the greater portion of these cells has been carefully detached.

The radially striated appearance of the zona has long been
shown to be due to a vast number of fine canals passing radially
through it. These canals I find open on the inner side of the

THE DEVELOPMENT OF THE MOLE. 203

zona by a slightly dilated mouth, while on the outer side of
the zona they communicate with the exterior by a considerably
wider opening (fig. 7). Into the external openings of these
canals I have been able to trace prolongations of those cells
of the discus which are immediately in contact therewith
(fig. 7), and there appears to me no room for doubt that the
contents of these follicular cells are thus rendered available
for the nutriment and growth of the ovum.

Owing to the extreme minuteness of the canals it is quite
possible that they are only rendered visible by the protoplasm
of the follicular cells, which is less transparent than the zona
itself, passing through them, and the fact that careful observers
have not succeeded in detecting these pores would be accounted
for by the cessation of the nutrient process at the time of
observation. I may add I have observed the radial canals
through the zona in optical sections of various whole ova, as
well as in many actual sections of ova situated within the
Graafian follicle.

I have before mentioned that the close investment of the
ovum with follicular epithelium cells is in accordance with the
degree of ripeness of the ovum itself. When the latter is fully
mature only a very smal] number of, and in some instances no,
epithelium cells are carried out with it upon the rupture of
the follicle. Thus the attachment of the epithelium to the
zona ceases when the ovum becomes mature, and no further
nutriment is required, and this is of itself some further proof
of the nutrient function of the follicular epithelium cells.

I myself never detected any follicular cells within the zona,
such as has been described by Lindgren (No. 15), von Sehlen
(No. 21), and Virchow (No. 22); nor have I seen any trace of
a micropyle in the zona, such as M. Barry (No. 3) and others

held to exist.

Tur VirevuiIne MEMBRANE.

Within the zona radiata and enclosing the ovum itself in all
those ripe ovarian ova examined by me, is a second very thin

204 WALTER HEAPE.

membrane, the vitelline membrane (vide Reichert No. 18,
Meyer No. 17, and van Beneden No. 4). In the ovum
drawn in fig. 1, this membrane may be seen where a space
exists here and there between the zona and the ovum.

In fig. 2 no space was to be distinguished with the magnify-
ing power used (Zeiss p) for the drawing, but in fig. 7, which
is a drawing of a portion of the circumference of the same
ovum with a higher magnifying power (Zeiss, imm. 38), a
narrow space is clearly shown between the ovum and the zona,
and a very fine membrane is there discernible closely covering
the ovum. This membrane is, however, most clearly visible
in fig. 8, which is the drawing of an ovum in which maturation
has taken place ; in this specimen there is a considerable space
between the vitelline membrane and the zona, the former
being rendered still more evident on account of the contrac-
tion of the material of the ovum itself within the vitelline
membrane. The space between the vitelline membrane and
the zona radiata I propose to call the circum-vitelline
space.

The development of the membranes, about which there has
been considerable discussion, I propose to consider in a future

paper.
The Yolk.

The ripe ovarian ovum itself is composed of food-yolk of two
kinds—(1) homogeneous, partially transparent, vesicular bodies,
(2) minute highly refractive granules of various sizes,—and of
a network of protoplasm which divides the yolk into rounded
or cubical masses such as I have endeavoured to represent in
figs. 2 and 7. The two kinds of yolk are similar to those
described by most of the observers of Mammalian ovarian ova.
It is worthy of remark, however, that I found no globules in
the Mole’s ovum similar to those described by Beneden and
Julin (No. 6), and figured by those authors in their paper
(No. 7) on the ova in Cheiroptera.

The difference in the density of the yolk in various Mam-
malian ova is very remarkable and would, I suspect, if examined

THE DEVELOPMENT OF THE MOLE. 205

with regard to the early phases of development, throw some
light upon the curious differences which then occur.

Kolliker (No. 14, 2nd edit., p. 44) and Schulin (No. 20),
declare that the human ovum is markedly deficient in yolk
vesicles when compared with the ovum of the Cat or the Cow.
Bischoff (Nos. 8, 9, 10, 11), in his figures of the ova of
the Rabbit, Dog, Guinea-Pig, and Deer, shows that the
Deer’s ovum is not filled with such a dense mass of yolk as is
that either of the Dog or Rabbit, while the ovum of the
Guinea-Pig is remarkably transparent, a statement in the
latter case with which Reichert’s (No. 18) and my own obser-
vations fully coincide (vide fig. 21). The Mole’s ovum must
be classed in this particular with that of the Rabbit and Dog,
while the Bat’s ovum, it appears, is similar to that of the
Guinea-Pig.

The network, which has as far as I know hitherto only been
observed in Mammalian ova by Schafer (No. 19) in young
ovarian ova of the Rabbit, was very distinct in the ovum repre-
sented in fig. 2. A similar appearance was noted in other
ova, but in a considerable number no such network could be
detected. There appears to me, however, good reason to
believe that the appearance is due to a protoplasmic reticulum
in the meshes of which the food material lies.

Tue Nuccevs.

In all those ova in which the nucleus was observed it was
placed excentrically, the density of the yolk being so great
it could not be distinguished when lying in the centre of the
ovum. It was found to be either circular or oval in optical
section, and bounded by a distinct membrane. In the ovum
represented in fig. 2 the nucleus is indicated by acircular ring ;
its contents could not, however, be observed owing to the density
of the supervening yolk, the network before spoken of being
seen overlying the nucleus.

In figs. 3, 4,5, I have drawn the nuclei of three ova which
I obtained from the female from which the ovum drawn in

206 WALTER HEAPE.

fig. 2 was taken. I tore open these ova and isolated their
nuclei ; the one represented in fig. 3 was flattened by with-
drawing the fluid in which it was immersed from beneath the
coverslip, the other two are, as nearly as may be, not under the
influence of pressure. In all of them a homogeneous nuclear
substance bounds a central clear space in which lies the
nucleolus. Besides the nucleolus a small number of large and
small highly refractile irregular-shaped bodies are contained
within the nucleus.

In fig. 4 the nucleolus, which is not bounded by a membrane,
consists mainly of an aggregated mass of minute granules, a
single larger granule being embedded in the midst of these. A
ring of fourvery large irregular granules surrounds the nucleolus,
and a few fine granules are contained in the peripheral nuclear
substance.

In fig. 5 the boundary of the nucleolus is more distinct,
and the transparent space surrounding it is well marked. A
few small and medium-sized granules are contained within the
nucleolus, while a number of small particles are suspended in
the nuclear substance. —

Fig.3 shows still further differentiation. The nucleolus is free
from granules, is contained within a definite sharply-marked
outline, and within the nucleolus itself an appearance of radial
striation may be noticed. A ring of large granules (broken
by pressure) surrounds the nucleolus, and a number of smaller
particles are distributed peripherally.

It appears, therefore, from an examination of these three
nuclei, that a single nucleolus only is present, and that a
variable number of larger or smaller or of both-sized granules
are also contained within the nucleus. The nucleolus is
situated in a transparent central portion of the nucleus, while
in the peripheral homogeneous nuclear substance a number of
minute highly refractile granules are suspended. A few larger
irregular-shaped granules may be arranged close around, but
distinct from the nucieolus, while the latter may itself contain
smaller granules. Whether or not the isolated granules are to
be regarded as nucleolar material is a question I do not pretend

THE DEVELOPMENT OF THE MOLE. 207

to decide, but the appearance of the nucleoli in figs. 4 and 5,
considered in connection with the researches of Griiber (No.
12) on the nuclei of Protozoa, would suggest that such is
the case.

Mature Ovarian Ovum.

The phenomena of the maturation of the ovum I have not
had an opportunity of observing in all its phases, but I have
been fortunate enough to obtain a fully mature ovarian ovum
(or one almost in a mature condition) which has been repre-
sented in fig. 8.

In this latter the ovum lies freely within the zona radiata
and is separated from it by a considerable space, the circum-
vitelline space in which, according to v. Beneden, is a
fluid, the circum-vitelline fluid. The vitelline membrane
is here distinctly seen on account of the contraction of the
substance of the vitellus.

The ovum itself is very dense and contains a number of dark
granules not observed in less mature ova; it is separated from
the vitelline membrane by a narrow space excepting (1) at
certain points where pseudopodia-like processes of the vitellus
project across the space and are attached to the vitelline mem-
brane, and (2) at one spot where no contraction of the ovum
has occurred. At this latter point the vitellus is more trans-
parent than elsewhere, and the nucleus may there be seen in
close approximation to a dark oval body lying immediately
outside the vitelline membrane, while a second more trans-
parent oval body in which is a central dark mass may be seen
lying in the midst of the circum-vitelline space. These two
bodies are the polar bodies (p. 6.), the second of which has but
just been produced ; while the nucleus seen within the ovum
is the female pronucleus (f. p.).

It is possible to describe the vitellus as composed of a
cortical more clear, and a medullary granular portion such as
Beneden (No. 5) describes in the mature ovarian ovum of the
Rabbit, but the boundary of these layers is by no means easy
to define. The light-coloured space in which the nucleus is

208 WALTER HEAPE.

situated is continuous undoubtedly with the cortical portion
(vide Beneden, loc. cit.).

When fully mature the vitellus again swells out and there is
no space seen between the ovum and the vitelline membrane.
At the same time the distinction between cortical and medul-
lary portions ceases to be visible, and the female pronucleus
probably retires to the centre of the ovum, judging from its
behaviour in other types, and is no longer to be seen owing to
the density of the yolk. In this condition the ovum is fully
ripe and is ejected, by the bursting of the follicle, into the
funnel-shaped opening of the Fallopian tube.

Beneden (No. 5) describing the process of the formation of
polar bodies in the Rabbit’s ovarian ovum, concludes that the
germinal vesicle is ejected to form those bodies, and that the
ovum becomes therefore a non-nucleated cell, while Balfour
(No. 2, vol. i, p. 61) in criticising this statement suggests that
further observations “ will demonstrate that part of the ger-
minal vesicle remains in the ovum to form the female pro-
nucleus.”

The latter supposition, I would venture to think, is justified
by the observations above recorded, and I would suggest that
it is possible the supposed “ Monerula” condition of the ovum
described by van Beneden was due to the fact that the opacity
of the ovum and the retirement of the nucleus to its central
portion at the time the observation was made, prevented it
from being seen.

IMPREGNATION.

Impregnation takes place in the upper portion of the Fallo-
pian tube.

In fig. 10 an ovum is represented which was obtained from
the upper end of the oviduct ; it has not yet divided into seg-
ments, but spermatozoa have found their way within the zona
radiata and two nuclei (the male and female pronuclei) may be
seen approaching one another.

The vitellus is irregularly granular (for the sake of clearness
this condition has not been represented in the figure) and is

THE DEVELOPMENT OF THE MOLE. 209

closely surrounded by the vitelline membrane. The circum-
vitelline space is narrow, and within this space a number of
spermatozoa and also two polar bodies were observed. The
ovum appears to have expanded considerably since the matura-
tion stage when the circum-vitelline space was wide, for in the
ovum represented in the figure the polar bodies are greater in
diameter than is this space, and thus cause a depression on the
surface of the ovum.

As to the number of spermatozoa which actually enter the
substance of the ovum I have no more evidence than appears
in the drawing (fig. 10), in which if my interpretations are
correct, a single male pronucleus is present. No movement
was observed among the spermatozoa within the peri-vitelline
space ; they appear to be attached there, and indeed in the
case of a similarly-conditioned ovum when the zona was
removed, these spermatozoa remained fixed to the vitellus and
were not pulled away with the zona.

I have always failed to observe either the presence of cilia
or arotation of the ovum within the zona such as Bischoff
describes.

Tue SEGMENTATION.

The first segmentation furrow gives rise to two oval seg-
ments of which one is generally somewhat larger than the
other, although the difference in size may be quite inconsider-
able, or there may be no difference at all, as is practically the
case in the ovum figured (fig. 11), the one segment being
20:25 x 15°5, the other 19°75 x 16.

The vitellus in both segments is finely granular and presents
no difference in character in either segment. ;

The nuclei are distinct, numerous spermatozoa are contained
within the circum-vitelline space, and two polar bodies are
visible.

The zona radiata, with its rough granular outer border, is
distinctly striated.

The measurements of the segments of several other ova of
this stage are given in the table on p. 213.

210 WALTER HEAPE.

Four segments now make their appearance by the division
of the first two (fig. 12). Each of the segments is of different
size, and indeed in every ovum which I have examined of this
stage with one exception, such is the case. (For measure-
ments vide table p. 213.)

Spermatozoa and polar bodies are still to be seen in the
circum-vitelline space and have been found in ova as old as
fifteen segments, although the former in fewer numbers and
both considerably shrunk.

From this point the segmentation continues entirely
irregularly, and the segments formed are of various sizes.
Figs. 13 to 19 are sketches of ova with six, seven, eight,
nine, seventeen, and larger numbers of segments. A table of
the measurements of the segments of several of them will be
found on p. 218.

Throughout I have been unable to discover that the seg-
ments are arranged in any definite manner, and have not
found it possible to distinguish the slightest difference in the
contents or in the density of any segments during the process
of segmentation. In size the segments also appear to me to
bear no relation the one to the other.

Segmentation is carried on during the passage of the ovum
down the Fallopian tube, and is completed by the time the
former reaches the uterus.

After the close of segmentation, and when the ovum has
descended into the uterus, but not until then, the segments
are clearly divided into two layers. The arrangement is as
follows :—A single layer of cubical hyaline segments com-
pletely surrounds, except at one point, an inner mass of
rounded or polygonal densely granular segments. The gap in
the outer layer of hyaline segments is filled up by one of the
granular segments (fig. 20). The cause of this sudden change
is not absolutely clear, but I would suggest the enone hypo-
thesis as a probable explanation.

I have little hesitation in stating that not only have the
outer layer of segments become more hyaline than heretofore,
but the segments of the inner mass have become denser, and

THE DEVELOPMENT OF THE MOLE. 211

contain larger granules and more granules than they hitherto
have done; and I would suggest that the yolk material ori-
ginally contained in all the segments alike, has been trans-
mitted from those occupying the outermost layer to those lying
within, in order to allow the former segments to perform the
function, and exhibit such activity as is now required of them.

In order to make my meaning clear I will briefly state what
these changes are; for a detailed account of this subject, how-
ever, I must refer the reader to a former paper (No. 18).
Very shortly after the segmented ovum enters the uterus it
dilates into a vesicle—the ‘ blastodermic vesicle.” In the
early stages of this formation the change is due entirely to the
activity of the outer layer of segments; first by a flattening
out, and secondly by the multiplication of these cells; the
inner mass meanwhile remaining passively attached to one
point on the circumference of the vesicle.

Later the cells of the inner mass assist in the formation of
the vesical wall, and eventually the whole of the inner mass,
with the exception of a very small number of cells which form
hypoblast, become so disposed. The outer layer of segments
and the largest portion of the inner mass of segments, there-
fore, together form the epiblast of the blastodermic vesicle.

Eventually the epiblast of the embryo is formed from a
portion of the wall of the vesicle, the hypoblast of the embryo
from a small number of the inner mass-segments, while the
mesoblast has its origin from both epiblast and hypoblast
layers.

Primarily, therefore, the blastodermic vesicle is formed by
the energy of the outer layer of segments, and I would suggest
that the differentiation of the outer and inner segments, the
one from the other, after the ovum enters the uterus, is due to
the transmission of yolk contained in the outer segments to
the inner segments, this transmission being performed in order
that the changes about to take place in the constitution of the
ovum may more readily be performed.

Van Beneden, in his description of the Rabbit’s ovum in
1875 (No. 5) describes the first two segments formed as the

212 WALTER HEAPE.

one larger and hyaline, the other smaller and containing a
more dense vitelline material. The hyaline segment he calls
the epiblastic, the more opaque segment the hypoblastic
sphere. He then describes the order of the subsequent seg-
mentation phenomena, and declares that the segments derived
from the primary hyaline epiblastic sphere gradually grow
round those formed from the primary hypoblastic sphere, and
there results a structure precisely similar to that described
above (p. 210), which he calls the “ metagastrula” stage. This
metagastrula Beneden compares with the gastrula of lower
types, and he derives the epiblast of the blastodermic vesicle
and of the embryo from the outer “epiblastic” spheres, and
the hypoblast and a portion of the mesoblast from the inner
*‘hypoblastic spheres.”

There can be little doubt, however, that Beneden’s account
of the derivation of the layers is incorrect, and that the greater
portion of the inner segments, as well as the whole of the outer
segments, give rise to epiblast. When this is considered, and
when the probable homologies of the primitive streak are recol-
lected, any comparison of the so-called “ metagastrula”’ of the
Mammalian ovum with the gastrula of lower types is found to
be impossible, and the significance of whatever differences may
exist in the two primary segments is rendered unimportant.

In the absence of any figures in Beneden’s paper I have been
unable to compare the appearance of the segments he describes
in the Rabbit’s ovum with those I have examined in the Mole,
but I have myself examined segmenting ova of the Rabbit, and
have isolated the segments the one from the other, in order the
more clearly to compare them, and in no case have I been able
to distinguish the slightest difference in the density or con-
stitution of these segments.

If my observations are correct, then, the differentiation of
the segmentation spheres into two layers in the fully segmented
ovum is not a primary differentiation such as Beneden discerns,
but a secondary differentiation due to the peculiar circum-
stances of nutrition and development attending the formation
of the Mammalian embryo.

218

THE DEVELOPMENT OF THE MOLE.

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214 WALTER HEAPE.

SuMMARY.

The membranes surrounding the ripe ovarian ovum are two:
(1) a single outer, thick, zona radiata, with a granular peripheral
and a transparent inner portion, pierced radially by fine canals

‘through which nutriment is obtained by the ovum from the
follicular cells (of the discus proligerus) immediately in con-
tact with the zona: (2) an inner very delicate vitelline mem-
brane which closely covers the ovum itself; and between
these membranes is a space, the circum-vitelline space. The
confirmation of Reichert’s (No. 18), Meyer’s (No. 17), and
van Beneden’s (No. 4) observations as to the presence of the
inner delicate vitelline membrane appears of some interest as
many embryologists are still sceptical of its existence, while the
relation of the follicular cells with the radial canals of the
zona supports the view as to the source of the nutriment of
the ovarian ovum. On the other hand the fact that nothing
was seen comparable to a micropyle in the zona, such as M.
Barry (No. 3), and Meissner (No. 16), described, nor any
follicular cells within the zona such as Lindgren (No. 15),
von Sehlen (No. 21), and Virchow (No. 22), have observed, is
some further proof that the conditions of the material investi-
gated by these authors was abnormal.

The yolk contained within the ovum, which is of two kinds;
viz. (1) homogeneous vesicular bodies, (2) minute highly
refractile granules, is contained within the meshes of a proto-
plasmic reticulum ; it is dense and contains no large globules
such as Beneden (Nos. 6 and 7) describes in the Bat’s ova. The
rounded or oval nucleus contains a single centrally placed nu-
cleolus and a variable number of smaller or larger granules,
which may possibly be considered as nucleolar material.

During maturation the vitellus becomes divided into a
medullary granular, and a cortical non-granular portion, the
circum-vitelline space between the zona radiata and the vitelline
membrane is enlarged, while the vitellus itself contracts away
from the vitelline membrane excepting (1) here and there
where pseudopodia-like processes connect the two, and (2) at

THE DEVELOPMENT OF THE MOLE. 215

one spot where the polar bodies are formed. At this latter
place two polar bodies may be seen in the specimen figured,
outside the vitelline membrane, whilst the nucleus remains
as the female pronucleus lying in the peripheral portion of the
ovum. . Finally, the vitellus again expands and the nucleus
retires to the centre of the ovum and is no longer to be seen.
Assuming that these observations are correct, Beneden’s descrip-
tion of the ejection of the vesicle to form the polar bodies and
the subsequent non-nucleated condition of the ovum must be
considered erroneous.

Impregnation appears to be effected by a single spermatozoon,
although a considerable number of spermatozoa find their way
through the zona and may be seen lying passively in the circum-
vitelline space.

The segmentation occurs while the ovum travels down the
Fallopian tube. Two and then four segments are formed, after
which the course of segmentation is irregular. The segments
themselves are of irregular size and do not appear to be divi-
sible into two kinds (epiblastic and hypoblastic) as Beneden
describes. After its entrance into the uterus, a division of the
segments into an outer hyaline layer and inner deeply granular
mass takes place, and I would suggest the hypothesis that the
vitelline matter which was originally contained in all segments
alike has been transmitted from the outer segments to the
segments lying in the interior of the ovum, in order that the
former segments may the more readily and actively multiply
and flatten out to form the wall of the blastodermic vesicle.
The epiblast of the vesicle and of the embryo is derived from
the whole of the outer layer and by far the largest proportion
of the inner mass of segments. The hypoblast is derived from
the small remaining portion of the inner mass and the meso-
blast, subsequently, from both epiblast and hypoblast layers.
This being the case, the division of the segmentation spheres,
by Beneden, into epiblast and hypoblast spheres from the time
when the first two segments were formed, is incorrect; and at
the same time the theory of a comparison of the metagastrula
stage with the gastrula of other animals is likewise untenable.

21

ao

6 WALTER HEAPE.

Literature.

. FE. M. Batrour.— Structure and Development of the Vertebrate Ovary,”
“Quarterly Journal of Microscopical Science,’ xviii, 1878.

. F. M. Batrour.—‘ Comparative Embryology.’

. M. Barry.— Researches in Embryology,” Third Series, ‘ Philosophical
Transactions,’ pt. ii, 1840.

. Ep. van Bewepren.—“ Recherches sur la composition et la signification
de ceuf,” &c., ‘Mémoires de l’Académie Royale Belgique,” xxxiv,
1867—70.

. Ep. van Benzepen.—‘‘ La maturation de l’cuf, la fécondation et les
premigres phases du développement embryonaire des mammiféres,”
&c., ‘ Bulletins de Académie Royale des Sciences des Lettres, &c.,
de Belgique,’ 44 ann., 2nd series, xl, 1875.

. Ep. van Benepen anp Cu. Juziy.—‘ Recherches sur Ja structure de
Povaire, ovulation, &c., chez les Cheiroptéeres,” ‘ Bulletins de l’Acadé-
mie Royale de Belgique,’ 2nd series, xlix, No. 6, 1880.

. Ep. van BENEDEN aND Cu. JULIN.— Observations sur la maturation,
&c., de l’ceuf chez les Cheiroptéres,” ‘ Archives de Biologie,’ i, 1880.

8. Tu, L. W. Biscoorr.— Entwicklungsgeschichte des Kaninchens,’ 1842.
9. Tu. L. W. Biscnorr.— Entwicklungsgeschichte des Hunde-eies,’ 1845.

16.

17.

18

. Tu. L. W. Biscuorr.—‘ Entwicklungsgeschichte des Meerschweinschens,’
1852.

. Tu. L. W. Biscnorr.— Entwicklungsgeschichte des Rehes,’ 1854.

. A. Griiper.—* Ueber Kern und Kerntheilung beiden Protozoen,” ‘ Zeit-
schrift fiir Wissenschaftliche Zoologie,’ 1884 and 1885.

. W. Hearz.—* The Development of the Mole,” &., ‘Quarterly Journal
of Microscopical Science,’ xxiii, 1883.

. A. Kotiixer.— Entwicklungsgeschichte des Menschen und der Hoheren
Thiere.’

. H. Linperzn.— Ueber der Vorhandensein von wirklichen Porenkana-
cheu in der Zona pellucida des Saugethiere,” &c., ‘ Archiv fir Anatomie
und Physiologie,’ Anat. Abtheil., 1877.

G. Muissnzr.— Beobachtungen tiber das Hindringen den Samenelemente
in den Dotter,” ‘ Zeitschrift fir Wissenscha(tliche Zoologie,’ v and vi,
1854-5.

H, Mrrzrn.—* Ueber das Saugethierei,” ‘ Miiller’s Archiv,’ 1842.

. C. B. Retcuert.—‘ Beitrage zur Entwicklungsgeschichte des Meersch-

weinschens,’ 1862.

THE DEVELOPMENT OF THE MOLE. 217

19. E. A. Scuirer.—‘On the Structure of the Immature Ovarian Ovum in
the Fowl and Rabbit,” &c., ‘Proceedings of the Royal Society,’ xxx,
1880.

20. Karu Scuunin.—“ Zur Morphologie des Ovariums,” ‘ Archiv fiir Mikros-
kopische Anatomie,’ xix, 1881.

21. D. von SEHLEN.—“ Beitrage zur Frage nach d. Mikropyle d. Saugethiere,”
‘ Archiv fiir Anatomie und Physiologie,’ pt. i, 1882.

22. H. Vircnow.—-“ Durchtreten von Granulozen-Zellen durch die Zona-
pellucida des Saugethiereies,” ‘Archiv fiir Mikroskopische Anatomie,’
xxiv, 1884.

EXPLANATION OF PLATE XXI,

Illustrating Mr. W. Heape’s Paper on ‘“‘ The Development of
the Mole (Talpa Europea), the Ovarian Ovum, and
Segmentation of the Ovum.”

Reference Letters.

c.v. s. Circum-vitelline space. fe. Follicular epithelium. fp. Female
pronucleus. g. Granules within nucleus. m.c. Mucous coat. m. p. Male
pronucleus. . Nucleus. zc. Nucleolus. y. 4. Polar body. 7. ¢. Radial
canals. sp. Spermatozoa. v. m. Vitelline membrane. y. Yolk. z. Zona
radiata.

All the figures are drawings of the ova of the mole, except Fig. 21, which
represents a guinea-pig’s ovum. Figs. 13—19 have been copied for me by Mr.
H. A. Chapman.

Fic. 1.—Ovarian ovum not yet ripe, surrounded by follicular epithelial
cells, f. e. The outline of these cells is to be seen through the transparent
zona, z. The outer edge of the zona is granular. A vitelline membrane may
be distinguished here and there. (Zeiss D, occ. 2.)

Fie. 2.—A ripe ovarian ovum. A few follicular epithelial cells only remain
attached to the zona. Network of protoplasm permeating the vitellus
(Zeiss D, oce. 2.)

Fies. 3, 4, and 5.—Nuclei of three mature ovarian ova, similar to that
drawn in Fig. 2. Single nucleolus, ze., and large and small granules, g., in
each nucleus. Yolk vesicles, y., and granules surrounding nucleus in Fig. 4.
(Zeiss F, occ. 2.)

Fic. 6.—A portion of the circumference of ovum represented in Fig. 2,

16

218 WALTER HEAPE.

showing the uneven surface of the zona, z., and its granular outer border.
The radial canals, 7. ¢., passing through the zona, and the circum-vitelline
space between the vitellus and the zona, ¢c. v. s. (Zeiss, imm. No. 2.)

Fic. 7.—Small portion of the zona of the same ovum, more highly magni-
fied. The follicular epithelial cells, 7 ¢., are here seen to be prolonged into
processes which enter the radial canals, 7. ¢., passing through the zona. The
vitelline membrane, v. m., surrounding the ovum is here shown. (Zeiss,
imm. No. 3.)

Fie. 8.—Mature ovarian ovum. Vitellus has contracted, and a large
circum-vitelline space, ¢.v.s., left between vitelline membrane, v. m., and
zona, z. Vitellus has also contracted within the vitelline membrane, excepting
where amceboid-like processes connect the two, and at a spot where a polar
body, p. ., is seen lying against but outside the vitelline membrane. A second
polar body lies freely in the circum-vitelline space. The female pronucleus,
Jf. pis present within the ovum. (Zeiss D, occ. 2.)

Fic. 9 —More highly magnified portion of the same ovum, showing two
polar bodies, p. 4., outside, and female pronucleus, f p., within the vitelline
membrane.

Fic. 10.—Impregnated ovum. Male and female pronuclei, m. p. and / p.,
are visible within the ovum. Two polar bodies and numerous spermatozoa,
sp., in the circum-vitelline space. (Zeiss D, occ. 2.)

Fic. 11.—Ovum segmented into two.

Fie. 12.—Ovum segmented into four.

Fic. 13.—Ovum segmented into six.

Fic. 14.—Ovum segmented into seven.

Fie. 15.—Ovum segmented into eight.

Fic. 16.—Ovum segmented into nine.

Fic. 17.—Ovum segmented into fifteen.

Fics. 18 and 19.—Ova segmented into a number of segments.

The ova represented in Figs. 10—19 were all obtained from the Fallopian
tubes of moles.

Fic. 20.—Fully segmented ovum obtained from the anterior end of the
uterus of a mole. The segments are now divided into an outer layer of
hyaline segments, o. J., and an inner mass of densely granular segments, 7. m.
There is one spot on the circumference of the ovum where the hyaline seg-
ments are not continuous, and here one of the granular segments is inter-
posed. The layer of hyaline material m. c., outside the zona, is a coating of
mucous material which has collected there since the ovum entered the
uterus.

Fi. 21.—The ovum of a guinea-pig, segmented into four to show the large
yolk granules and the transparent appearance of the segments.

ra

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sce arcie cai

Studies M.L.Vol.II, Pl. XX1.

F Huth, Lith® Edin®

The Early Development of Julus Terrestris.!
By

F. G. Heathcote, M.A,,
Trinity College, Cambridge.

—_—

With Plates XXII & XXIII.

My investigations, the results of which are contained in the
following paper, were begun in June, 1882. I collected a
number of Chilognatha and kept them in glass jars, the bot-
toms of which were covered with damp earth. I soon found
that Julus terrestris* was the species best suited for my
purpose, as though the eggs presented some difficulties not
present in the eggs of other species, yet they were of a conve-
nient sizé and were easily to be procured in great numbers.

I fed the animals on sliced apples and occasionally on green
leaves, and this diet seemed to suit them well, for I never failed
to get several clumps of eggs in the breeding season, though
it is only this summer that I succeeded in getting them in any
number. The breeding season of these animals lasts from the
end of May till the end of August, though the weather has a
considerable influence on the time when they begin and leave
off breeding. I have observed copulation, which takes place
exactly as described by Cuvier (‘Régne animal, 8rd edit.,
1886, vol. ii, p. 330). I was unable to determine how long a
time elapses after copulation before the eggs are laid, but believe
it to be short. About four days before laying her eggs the

1 The numbers in brackets in the text refer to the list of papers at the end.

2 The species was kindly identified for me by Mr, T. D, Gibson Carmichael,
F.L.S., as Julus terrestris, Leach.

220 ¥. G. HEATHCOTE.

female constructs a sort of globular case of mud forthem. The
bottom of this was, in the case of my animals, formed by the
bottom of the glass, while at the top was a small round hole
which was closed up after the eggs were laid. Four days after
the case was begun the eggs were laid, each case containing a
clump of about a hundred eggs fastened together by a sticky
substance. By breaking away the top of the case I was able
to take out as many eggs as I wanted for examination, and
covering the remainder carefully with earth they proceeded
with their development without injury, though if exposed to
the air for about a quarter of an hour they shrivelled and were
destroyed.

Methods.

The principal difficulties with which I had to contend in the
preparation of the ova were, in the first place, the hard chitinous
chorion, and, secondly, the great amount of food-yolk.

With regard to the first of these difficulties, I tried to
remove the chorion by Bobretski’s method, but I failed com-
pletely in this. Lalso tried to burst the chorion by endosmosis
of various fluids. Perenny’s fluid burst the chorion quickly,
but as soon as the shell was burst in one place the: contents
rushed out, destroying the embryo. The state of preservation
of the tissues so preserved was not satisfactory, nevertheless I
gained some valuable series in this manner. I also tried
various strengths of nitric acid with unsatisfactory results. I
was therefore obliged to cut the ova with the chorion still on,
soaking them thoroughly in the hardest paraffine and cutting
rather thick sections. With regard to the preservation of the
tissues I tried a great variety of fluids and also the method of
preserving by heat described by Mr. Patten in his paper (12)
on the development of Phryganids ; but I found that I got the
best results from corrosive sublimate, osmic acid, and picric
acid. The last of these fluids, in some cases, burst the shell
after the contents were hardened and thus enabled me to gain
excellent series of sections.

The staining of my sections was a matter of much difficulty.

EARLY DEVELOPMENT OF JULUS TERRESTRIS. 221

Borax carmine stained well in the earlier stages, while the ovum
was still in the ovary, and also in later stages, when the embryo
was far advanced in development; but in the intermediate stages,
between about the tenth day and hatching, was wholly useless ;
staining the yolk-spherules equally with the nuclei. Hema-
toxylin was better, staining the nuclei deeply; but it also
stained the smaller yolk-spherules so as to make it a difficult
matter in some cases to distinguish between them and the
nucleoli. The best fluid was alum-carmine prepared after
Grenacher’s method. This fluid has the advantage of staining
the nuclei and nucleoli with a different tinge to that of the
yolk-spherules, and the result was most satisfactory. The diffi-
culties in the way of observing the course of development were
many and were only overcome by cutting a great number of
sections, only about one series in twenty being perfectly satis-
factory.

The warmth of the weather had a great influence on the rate
of development; one clump of eggs, for instance, was hatched
on the twelfth day after being laid, while another was not
hatched till the twenty-fifth. As the shorter period seemed to
be the most usual, I worked out a clump of eggs which hatched
on the twelfth day, and preserved a number each day, using
the results as a standard by which to estimate the progress of
development in other ova.

I propose in the present paper to begin with the ovum in
the ovary after it has attained a fair size and to trace its deve-
lopment up to the time of hatching, leaving for a future paper
its further development to the adult animal.

The Ovarian Ovum.

The ovum within the ovary is surrounded with a follicular
envelope derived from the cells of the ovary. It has a large
nucleus and a single large nucleolus, within which it is usually
possible to make out two or three vesicular spaces. The body
of the ovum stains slightly. The nucleus is large and distinct,
stains slightly, and.when viewed under a high power (;'; oil

222 F. G. HEATHCOTE.

immersion by Reichert) consists of a network of protoplasm,
chromatin granules, and more fluid protoplasm.

The nucleolus is round, very distinct, and stains very deeply.
At aslightly later stage a deeply stained mass appears in the
body of the ovum ; this is possibly equivalent to the yolk-nucleus
described by Carus in Spiders (4). It increases and finally
forms a very distinct ring within the body of the ovum, as
shown in fig. 1,7. It is a semi-fluid mass which stains deeply
but does not show any structure. I have not observed any
appearances like those described by Balbiani in his account of
the yolk-nucleus of Geophilus (10). This mass of deeply
staining, structureless material is the first food-yolk formed in
the course of development of the ovum. As the latter in-
creases in size, the ring of deeply staining material breaks up
and becomes more equally distributed throughout the ovum in
the form of small globules, which are more deeply stained
than the rest of the cell-substance, though not so deeply as the
ring before mentioned. These globules increase in size and
gradually take the appearance of yolk-spherules, such as are
present in all subsequent stages up to a very late period of
development. Yolk-spherules continue to be formed in the
protoplasm of the ovum up to a considerably later stage; such
spherules invariably stain deeply while quite small, though the
large spheres stain but slightly. I do not consider that the
process of formation of the first food-yolk differs in any
essential from that of the formation of the yolk-spherules at a
later stage. The fully developed ovum within the ovary is
shown in fig. 2; it is of an oval form with a thick milk-white
shell, which is formed from the follicular envelope of the earlier
stages. The body of the ovum consists of a great number of
yolk-spherules, which are embedded in and separated from one
another by strands of protoplasm which constitute a network
extending throughout the ovum. At the periphery is situate
the nucleus in which is a single large, deeply staining nucleolus.
Examination with a high power lens (;4, oil immersion, Reichert)
shows the nucleus to consist of a network of solid protoplasm,
enclosing a more fluid protoplasm in its meshes, and of chro-

EARLY DEVELOPMENT OF JULUS TERRESTRIS. 223

matin granules which are present in small numbers (fig. 15).
Within the deeply staining nucleolus, several vesicular spaces are
present. J am unfortunately unable to read Russian, but from
an examination of the figures of a Russian paper by Repiakoff,
published in 1883, on the development of Geophilus, I imagine
that the ovum of Geophilus at this stage is of similar structure.

I have been unable to observe anything of the impregnation
of the ova, which probably takes place immediately before
deposition.

My earliest stages occur late on the same day on which the
ova are laid; sections through such ova show (fig. 3) that the
protoplasmic network and yolk-spherules remain as before, but
the nucleus is no longer at the periphery, but is situated in a
mass of protoplasm in the centre of the ovum. This mass of
protoplasm is of irregular shape, but its long axis corresponds
with that of the ovum. From it amozba-like processes radiate
in all directions, forming a protoplasmic network throughout
the egg (fig. 17, a, 6). The nucleus is no longer a distinct
vesicle, but its position is marked by the chromatin granules
alone. There is no nucleolus.

Early on the second day the nucleus and the central mass of
protoplasm divide into two parts. The division of the proto-
plasm is not, however, complete, the two resulting masses with
their nuclei remaining connected by a network of protoplasm.
This is shown in figs. 4 and 16. The two first segmentation
masses separate till they are some distance apart, though still
connected by strands of protoplasm ; they then divide, so that
we now have four segments all connected together. This pro-
cess is carried on until there are a considerable number of
these segmentation masses present, and early on the third day
the first formation of the blastoderm begins. At the close of
" segmentation the ovum consists of a number of these segmenta-
tion masses, resulting from the division of the original central
mass of protoplasm. Each of these masses has a dense central
portion, in which is situate the nucleus, while the outer portion
is broken up into innumerable processes, which connect the
masses together and permeate the yolk in every direction.

224 F. G. HEATHCOTE.

Ino fig. 17, a, 6, I have shown the protoplasmic network under
a high power. Early on the third day some of the segmenta-
tion masses make their appearance on the outside of the ovum
at different parts, and there undergo rapid division, the re-
sulting cells spreading out to form the blastoderm in a manner
very similar to that which takes place in Amphipods (14). In
figs. 6, 18, I have shown this process taking place.

The large flat-shaped cells which form the first beginning of
the blastoderm differ considerably from the segmentation
masses from which they originate. Their outline is clear and
distinctly marked; their nucleus is very distinct, of an oval
shape, with its long axis pointing in the direction of the long
axis of the cell. A section through an ovum in this stage,
when seen through a low power, shows the blastoderm cells as
flat, pavement-like cells, with a long-shaped nucleus. An oil
immersion lens, however, shows further details. Each cell is
directly continuous with the neighbouring blastoderm cells, and
also with the cells which remain in the yolk, by means of fine
processes of protoplasm. There is also a difference observable’
in the cells within the yolk, which at this stage constitute the
endoderm. Their outline is far more distinct; their nucleus
is round, deeply stained, and rather smaller than at an earlier
stage.

Fig. 6 shows a single segmentation mass appearing at the
surface of the ovum, and about to divide to give rise to
blastoderm cells.

Fig. 18 is part of a transverse section through an ovum at a
slightly later stage seen, under a high power; it shows a seg-
mentation mass which has divided, giving rise to several blas-
toderm cells, while some of the cells arising from the original
segmentation mass remain behind in the yolk as endoderm,
but are still connected with the blastoderm cells by processes. _

At the stage represented in the last-mentioned section the
blastoderm is present in isolated patches on the surface of the
ovum. At the close of the blastoderm formation, then, the
ovum consists of an external layer of flat cells—the ectoderm—
with deeply stained nuclei, these cells being continuous on the

EARLY DEVELOPMENT OF JULUS TERRESTRIS. 225

one hand with one another, and on the other with the cells in
the interior of the yolk by means of fine processes of proto-
plasm. The cells in the interior of the yolk are the direct
descendants of the first segmentation masses. They constitute
the endoderm. Their fate is various. Some of them are em-
ployed in the formation of the keel, which I am about to describe
in the next section ; that is, in the formation of the splanchnic
and somatic layers of the mesoderm. Another part is employed
in the formation of the endodermal lining of the mesenteron,
while a third part remains in the yolk after the mesenteron
is formed, and gives rise to mesoderm cells, which are em-
ployed in the formation of various muscles and of the circula-
tory system. These cells will be mentioned again in the last
part of this paper.

The flat surface cells enclosing the yolk constitute, as already
stated, the ectoderm, and give rise to the usual ectodermal
derivatives.

With regard to the retention of the primitive connection of
the cells of the ovum until this stage, nothing of the sort has,
I believe, been described before, except by Sedgwick in Peri-
patus (17). The most important part is, it seems to me, not
the connection of cell to cell, but the connection of layer to
layer by means of processes of the cells.

Formation of the Mesoderm.

About the middle of the fourth day several of the stellate
endoderm cells approach the ectoderm, in the middle line of
what will eventually be the ventral surface of theembryo. Such
cells are shown in figs. 7, 8, 19. Fig. 7 is an earlier stage
than that shown in fig. 8. That the cells are really endodermal,
and are not divided off from the ectoderm, is, I think, con-
clusively proved by the shape of the cells which at this period
compose the ectoderm. They are flat and thin and the nucleus
is long and oval, and lies in the direction of the long axis of
the cell. I cannot believe that they would divide in the direc-

226 F. G. HEATHCOTE.

tion of their long axis; and, in fact, before they do begin to
take part in the formation of the mesodermic keel, they undergo
an alteration, which I shall describe. When first the endo-
derm cells just mentioned begin to come together in the middle
line near the ectoderm their appearance is somewhat peculiar ;
their nucleus is small, round, and deeply stained ; their form is
stellate and their outline very distinct.

Processes pass from them to the ectoderm cells. This is
shown in fig. 19, which is a transverse section through an ovum
on the fourth day, taken in a plane such as to cut through the
first beginning of the keel. When a fair number of these cells
are assembled in the middle ventral line a change takes place
in the cells of the ectoderm just outside them. The latter
become more rounded, while their nuclei, instead of being long
and oval, become round. In fact they undergo an alteration
which causes them to resemble the cells which I have described
as assembling immediately below them. This alteration is
shown in figs. 19 and 20, which are transverse sections through
the first beginning of the keel.

The ectoderm cells in the middle line, after altering their
shape as I have described, increase by division, and take a con-
siderable share in the formation of the keel. The cells in the
middle line, both ectoderm and endoderm, continue to increase,
and are joined by more cells from the hypoderm, and eventually
on the fifth day we find a keel in the middle ventral line,
something like that described by Balfour in his paper on the
development of Agelena labyrinthica (16). Both ecto-
derm and endoderm have taken part in the formation of the
keel.

When the keel is fully formed the cells of which it is com-
posed are large, somewhat irregular in shape, and have a large
nucleus. They are all directly connected together, though,
owing to their being closely packed together, it is difficult to
see anything of their connections, except where one cell has
been somewhat separated from the others. The keel is of con-
siderable thickness, being about six or more cells deep in its
thickest part.

EARLY DEVELOPMENT OF JULUS TERRESTRIS. 227

The keel is shown in transverse section in fig. 9a, and
fig. 20. At the end of the sixth day the keel is still present
but an alteration is taking place in the cells of which it is com-
posed. They are no longer round and thick, but are becoming
elongated in the direction parallel to the surface. At the same
time they continue to multiply and spread themselves out, so
as to form two definite layers within the ectoderm (fig. 10).
These are the splanchnic and somatic layers of the mesoderm.
The cells of the ectoderm and of the somatic mesoderm are
still connected, and also the cells of the splanchnic and somatic
mesoderm.

On the eighth day the mesoderm extends round a great part
of the embryo—rather more than half way round. The keel
has almost disappeared (fig. 11).

On the ventral surface the cells are no longer flat but have
assumed a columnar form. Their nuclei are now oval in shape,
their long axis pointing, as does that of the cells to which they
belong, towards the interior of the ovum. This is in fact the
first formation of the ventral plate and is shown in fig. 10.
While these changes are going on the remnants of the keel are
disappearing. The mesoderm now becomes thicker on each
side of the ventral line. This is shown in fig. 21. Both
layers are concerned in this thickening, and at these points the
two layers become indistinguishable. Outside the thickenings,
that is, farther away from the middle ventral line, the two
layers are closely applied to each other and to the epiblast as
before. The effect of these changes is that the greater part of
the mesoderm is now arranged in two parallel longitudinal
bands along the ventral surface of the embryo; these bands
being connected in the middle line by a thin portion consisting
of two layers (fig. 22). Fig. 21 is a transverse section through
the ventral half of an ovum at this stage.

The two longitudinal bands now begin. to be constricted off
into the mesodermal somites. The latter are formed from
before backwards and their position corresponds with that of
the future segments of the body. The number of somites thus
formed is eight, corresponding to the eight segments with

228 F. G. HEATHCOTE.

which the embryo is finally hatched. The somites are at first
solid, but a cavity appears in them at a later period.

The ectoderm of the ventral plate now alters its character, the
cells becoming more pointed and much more closely packed
together.

From the Formation of the Stomodeum and Proc-
todeum to the Hatching of the Embryo.

Early on the ninth day the stomodeum is formed as an in-
vagination of the ectoderm near one end of the ventral surface.
Shortly after the first formation of the stomodzeum the proc-
todeeum appears as a shallow, somewhat wide invagination near
the end of the ventral surface.

The body segments, already established by the segmentation
of the mesoderm, now become more apparent, each being
marked by a deep transverse furrow in the ectoderm (figs. 24,
25,28). Fig. 12 is a section taken longitudinally through the
embryo, and shews the stomodeum, the proctodeum, the eight
mesodermal segments, and a single ectodermal furrow close
behind the stomodeum. Fig. 24 shows this first furrow under
a higher power. (Zeiss c.)

The endoderm cells are still scattered within the yolk, but
they are gradually becoming collected in the median line just
below the mesoderm. The stomodeum and proctodeum
become more deeply invaginated, extending a considerable
distance into the yolk and at the same time the endoderm cells
begin to form the mesenteron, arranging themselves round a
central lumen. Fig. 27 shows the formation of the proc-
todeum and the hypoblast cells beginning to form the
mesenteron.

At the end of the ninth day, then, the embryo is of a long
oval shape, with a deeply invaginated stomodzeum at the
anterior end and a proctodeum not quite so deep at the other;
the mesoderm is divided into eight segments ; a deep furrow in
the ectoderm marks off the first segment which will eventually
become the head, and the mesenteron is almost formed.

EARLY DEVELOPMENT OF JULUS TERRESTRIS. 229

The changes which take place on the tenth day result in the
embryo assuming its definite shape. These changes consist of
the completion of the ectodermal segmention, the formation of
the nervous system, and the formation of the ventral flexure.
Eight segments, including the head, are marked off from one
another by ectodermal furrows, the last segment being a long
one, from which the anal segment will eventually be divided
off. Hach of these eight mesodermal somites has now acquired
a cavity. This is shown in fig. 28, which is a vertical longitu-
dinal section through the second segment on the tenth day.

The two layers are distinguishable, the somatic being chiefly
concerned in the formation of the muscles of the limbs.

The ventral flexure now begins to be formed between the
seventh and eighth segments. Its first appearance, shown in
figs. 29, 80, is seen quite clearly from the outside through the
chorion. Metschnikoff has described it as occurring on the
tenth day in Strongylosoma, which hatched on the seventeenth
day, in a more advanced stage than Julus terrestris is at
the time of hatching.

The ventral flexure is first formed by a deepening of the
transverse furrow which forms the division between the seventh
and eighth segments. It is therefore first formed nearer the
anal end of the embryo. As the furrow deepens and the
embryo increases in size, the last segment grows in length.
The furrow does not deepen in a direction at right angles to
the long axis of the embryo, but in a slanting direction, as
shown in fig. 14, The effect of this is that the end segment is
bent round against the head segment. The eighth segment
just referred to is considerably longer than any of the others
except the head, and the tissues show a considerable differ-
ence to the tissues in other parts of the body. Even on
the eleventh and twelfth days, when the nervous system is
far developed in all other parts of the body, in the eighth
segment the tissues are imperfectly differentiated, the nerve-
cord not showing any ganglia but lying on the ectoderm
as a thin cord not quite separated from it. At a later period
of development the anal segment is constricted off from this

230 F. G. HEATHCOTE.

segment, and from its anterior part the future segments
formed in later life are developed. Just before the first
appearance of the ventral flexure when the body segments are
fully formed, the embryo developes a cuticular envelope over
the whole surface of the body. This may be seen during the
first formation of the ventral flexure surrounding the body but
hanging loosely from it. This envelope is the so-called
amnion of Newport.

Just before the first trace of the transverse furrow which
marks the beginning of the ventral flexure has made its
appearance, the nervous system begins to be formed. The
first traces of this consist in a thickening of the ectoderm on
each side of the middle line. This is soon followed by the for-
mation of a shallow furrow between the thickened parts; this
longitudinal furrow corresponds with that described by Mets-
chnikoff in Strongylosoma. Fig. 31 shows the furrow and the
ectodermal thickenings. Fig. 32 shows a later stage where the
nerve-cords are almost separated from the ectoderm. The
bilobed cerebral hemispheres are formed first and the nerve-
cords are formed from before backwards, the posterior portion
not being complete till a considerably later stage of develop-
ment.

The nerve-cords are widely separated, but are connected by a
thin median portion. In later embryonic life they are closely
approached to one another and almost form one cord.

On the eleventh day the embryo has increased considerably
in size. The ventral flexure is complete and the animal lies
with the long end segment folded closely against the rest of
the body, the end of the tail being against the stomodeum.
The nervous system is now completely separated from the ecto-
derm, and the ectoderm has now assumed its adult appearance.
It now separates a second membrane like that which I have
already described as occurring on the tenth day.

These two membranes I regard as equivalent to two moults
of the animal. The nerve-cords have considerably altered its
appearance; it has sunk deeply into the interior of the body
except in the end segment and now lie closely beneath the

EARLY DEVELOPMENT OF JULUS TERRESTRIS. 231

mesenteron. They are divided into ganglia, one pair being
present for each segment of the body; from each ganglion a
nerve is given off to the corresponding body segment. The
sub- and supra-cesophageal ganglia are almost formed.

The splanchnic layer of mesoderm covers the mesenteron,
the stomodwum, and proctodeum. The median part of the
somatic mesoderm lies above the nerve-cord, between it and
the gut; from thence it passes downwards to the body wall.
This arrangement is shown in fig. 34, which is a transverse
section through an embryo of the twelfth day.

Within the yolk, which is still present in great quantity in
the body-cavity, there are present a number of cells remaining
over from the hypoderm after the formation of the mesodermic
keel, and the mesenteron. These cells eventually give rise to
the circulatory system, to the muscles of the segments, in part
at any rate, and to other muscles ; they are therefore mesoderm
cells. The lumen of the mesenteron is now continuous with
that of the stomodeum and of the proctodeum.

Fig. 14 shows a longitudinal vertical section through an
embryo of this age. ;

On the twelfth day the Malpighian tubes grow out of the
proctodeum. Their lumen is from the first continuous with
that of the proctodeum. They end blindly and are enveloped
by the splanchnic mesoderm.

Fig. 34 is a transverse section through an embryo on the
twelfth day. The section is taken through a ganglion in the
posterior part of the body. It shows the two ganglia united
by a narrow median part and each giving off a nerve to the
ventral part of the body, where the rudiments of a pair of
limbs can already be traced. The Malpighian tubes are also
shown. This section also shows the body cavity divided into
four compartments by means of thin layers of mesoderm.
Late on this day the animal is hatched with only the rudi-
ments of its appendages, and I propose to reserve a full
description of the stage till a future time.

232 IF. G. HEATHOOTE.

Literature.

But little work has been done on the early development of
Chiloguatha. According to Newport, De Geer was the first to
watch the development of Julide (6). He observed that
Julus and Polyxenus were hatched with three pairs of limbs
and a fewer number of body segments than is possessed by the
adult animal.

Savi was the next observer. In 1817, in a paper quoted by
Newport (11) and which I have not been able to obtain, he
said that Julus was hatched without limbs. The next observer
was Waga. In 1840, he, in a paper quoted by Newport (11),
states that the young Julide are completely apodal at the time
of hatching. Gervais (8), the next observer, in 1844, gives a
great deal of fresh information about the later development of
Chilognatha, but has little to say with regard to the earlier
stages before hatching. He tells us, however, that Glomeris
marginata has three pairs of limbs before hatching; that
Polydesmus complanatus has also three pairs when
hatched.

Fabre (7) in 1855, investigated the development of Poly-
desmus, and describes it as having three pairs of limbs and
eight body segments, including the head segment, at the time
of hatching. He also investigated Julus aterrimus, and
describes it as hatching on the fifteenth day, being then
apodous and without any organ or appendage, and the shape of
the body being reniform ; five days afterwards, he tells us, that
he observed the first traces of body segmentation, and that
seven days after hatching the animal consisted of eight body
segments and possessed three pairs of limbs.

Metschnikoff found that the young of Julus Morreletti
were hatched with three pairs of limbs (9), while Newport found
that in Julus terrestris the just hatched young only pos-
sessed the rudiments of three pairs of limbs, and faint traces
of the antenne. My own investigations, which were carried

EARLY DEVELOPMENT OF JULUS TERRESTRIS. 233

out on the same species as Newport’s, confirm his account.
In my opinion the conclusion to be drawn from these different
accounts is that in different species of Chilognatha, and even in
closely allied species of Julide, the hatching of the embryo
takes place at very different stages of development.

In 1841, Newport published his paper on the organs of
reproduction and the development of the Myriapoda (11).
This is the first paper containing any real information of the
early stages in the development. On the first three days he
describes the appearance of the yolk-spherules as seen through
the chorion, and describes the whole contents of the egg as
' becoming firmer. On the fourth day he saw “a little granular
mass on one side of the shell” which he was inclined to regard
as the future being. He made no further observations till the
nineteenth day, when he describes the ventral flexure of the
embryo within the shell. On the twenticth day he was able
to make out six body segments. On the twenty-fifth day the
embryo was hatched.

I am inclined to think that the little granular mass which he
describes on the fourth day was the first beginning of the blas-
toderm.

Nothing more was written on the early development of the
Myriapoda till 1874, when Metschnikoff published his paper
(9), which contains the greater part of what we know of
Chilognath development. His fullest observations were made
on Strongylosoma. He describes the segmentation, the forma-
tion of the blastoderm, the formation of the ventral plate,
the ventral flexure of the embryo, the segmentation of the
mesoblast, and of the body, and gives a full description of the
later stages. As I shall have to discuss his paper in detail I
will not attempt to give a fuller account of it here.

In 1877, Stecker published a paper (13) in which he describes
the development of Julus fasciatus and several other species
of Chilognatha. His account does not agree either with mine or
with that of Metschnikoff. As his account has been fully
criticised by Balfour (2), I will not refer to it here at greater

length.
17

234 F. G. HEATHOOTE.

The above is a short account of the early literature of
Chilognath development in the first stages of development, and
as with the exception of Metschnikofi’s paper the only bearing
they have on my own work is to show that Chilognatha, even
in very closely allied species, are hatched at different stages
of development, I shall not refer to them again, with the
exception of Metschnikoff’s paper, which I shall mention
further in the next section of my paper when discussing the
bearing of my own work.

Summary.

With regard to the segmentation I have described, it will be
seen that it differs considerably from that seen by Metschni-
koff (9), who describes it as total; the ovum being divided
into two, four, &c., segments. I saw nothing of such a divi-
sion, nor does Newport, who observed the eggs of the same
species as I did, record any such appearances. Newport’s
observations were made on the eggs of a species found in
Madeira; that is in a hot climate; and as regards segmen-
tation were not carried on by means of sections. As the
amount and distribution of the food-yolk has a great influence
on the segmentation, I think it probable that in my species
the segmentation differs slightly from that in the species
investigated by Metschnikoff. The difference, however, con-
sisting in the external segmentation of the ovum is not, I
think, a very important one. The segmentation of Julus
terrestris, as I have described it, shows a remarkable re-
semblance to that found in Amphipods by Ulianin (14). He
describes an external segmentation by means of shallow
furrows formed in the surface of the ovum, which is composed
in great part of food-yolk; in each space thus marked out, a
large ameeba-like mass of protoplasm provided with a nucleus
is present ; the division of these protoplasmic masses coincides
with the formation of the furrows. When the blastoderm is
just about to be formed the furrows disappear. At the close
of segmentation, then the ovum is exactly like the ovum of

EARLY DEVELOPMENT OF JULUS TERRESTRIS. 235

Julus terrestris inasmuch as the segments are repre-
sented by protoplasmic masses each of which is provided with
a nucleus.

The formation of the blastoderm, as I have described it,
agrees in the main with that given by Metschnikoff for Stron-
gylosoma. According to this author, on the fifth day isolated
masses of cells make their appearance on the surface of the
ovum and spread themselves round it to form the blastoderm.
He was unable to trace the origin of these masses of cells.
What he saw was precisely what I have described in the earlier
part of this paper.

The formation of the blastoderm in Julus is, then, such as is
generally found in tracheate development.

The cells which at the conclusion of the blastoderm forma-
tion in Julus remain within the yolk, represent the endoderm,
and have apparently been overlooked by Metschnikoff.

The mode of formation of the mesoderm almost exactly
resembles that described by Balfour (16) for Spiders. Accord-
ing, however, to this author the greater part of the cells of the
keel or ridge are derived from the ectoderm, whereas in Julus
the ectoderm furnishes the greater part of them. Balfour
states that the keel in Spiders is probably the homologue of
the mesoblastic groove of the insect blastoderm. Patten (12)
describes a median longitudinal furrow in the ventral plate of
Phryganids which gives rise to the mesoblast and to part of
the endoderm.

In Peripatus (17) the mesoblast originates from the primitive
streak, i. e. from the indifferent tissue behind the blastopore,
which can be called neither ectoderm nor endoderm. I think
that all these structures are homologous.

With regard to the cells which, as I have already mentioned,
are employed, neither in the formation of the keel nor at a
later period in the formation of the mesenteron, but remain in
the body cavity as mesoderm cells directly descended from endo-
derm—Balfour states that in Agelena, after the establishment
of the hypoblast the cells remaining in the yolk are not entirely
hypoblastic, since they continue for the greater part of the

236 F. G. HEATHCOTE.

development to give rise to fresh cells, which join the meso-
blast. This is exactly what happens in Julus.

Metschnikoff has described the formation of the bands of
mesoblast and their division into somites, but his figures are
difficult to understand, as he has not drawn either the cell
outlines or the nuclei.

The formation of the ventral flexure has been described by
Metschnikoff, and, as I have already mentioned, was first seen
by Newport. The flexure is, as I have before said, formed
between the sixth and seventh post-cephalic segments; that is,
it marks off from the rest of the body the long eighth segment
in which the tissues are very imperfectly differentiated, and
from which the anal segment has yet to be cut off. It is
from this imperfectly differentiated segment that the future
additional body segments are formed in the later stages of
development.

The mesenteron of the adult animal is, as was pointed out
to me by the late Professor Balfour, marked with a series of
constrictions corresponding with the external segmentation of
the body, but no trace of such constrictions has as yet
appeared.

The wide separation of the nerve-cords in the embryo has,
so far as I know, not been pointed out by any author.

I propose to reserve for a future paper a more full descrip-
tion of the development of the nervous system, the circulatory
system, and the segmentation of the embryo, as well as the
account of the appendages and other points connected with the
further development of the embryo.

Parers REFERRED TO.

1. Barprani.—‘ Géneération des Vertébres,” p. 258.

2. Batrour.— Comparative Embryology,’ 1881.

3. v. Beneprn.—‘ Fécondation de l’ceuf,’ Liége, 1883.

4. v. Canus.— Entwicklung der Spinneneies,” ‘ Zeitschr. fiir wiss. Zool.,’
vol. ii.

5. Cuvier.— Reégne animal,” 3rd ed., 1836, vol. ii, p. 830.

EARLY DEVELOPMENT OF JULUS TERRESTRIS. 237

for}

. De Gzzr.— Mémoires,’

. Fasre.— Ann. des Sci. Nat.,’ 4 ser., vol. iii, 1855.

‘ Gzrvais.— Etudes sur les Myriapodes,” ‘Ann. des Sci. Nat.,’ 3 ser.,

vol. ii, 1844,
9. Mretscunikorr.—* Embryologie der doppelfiissigen Myriapoden,” ‘ Zeit.

fir wiss. Zool.’ 1874.

10. Mztscunixorr.—* Embryologisches itber Geophilus,” ‘Zeit. fiir wiss.
Zool.,’ 1875.

ll. Newrort.—* Development of Myriapoda,” ¢ Phil. Trans.,’ 1841.

12. W. Parren.— Development of Phryganids,” ‘ Quart. Journ. Mier. Sci.,’
October, 1884.

13, Srecker.— Die Anlage der Blatter bei den Diplopoden,” ‘Arch. fiir
Mic. Anat.,’ vol. xiv, 1877. ~

14, Unianin.— Zur Entwickl. der Amphipoden,” ‘Zeit. fiir wiss. Zool.,’
vol, xxxv, 1881.

15. Waca.— Revue Zoologique,’ 1840.

16. F, M. Batrour.—* Notes on the Development of the Arancina,” ‘Quart.
Journ. Mier. Sci.,’ vol. xx, 1880.

17, A. Sepewicx.—* The Development of Peripatus Capensis,” ‘Quart.

Journ. Mier. Sci.,’ July, 1885.

con

EXPLANATION OF PLATES XXII & XXIII,

Illustrating Mr. F. G. Heathcote’s Paper on “The Early
Development of Julus terrestris.”

Complete List of Reference Letters.

bl. Blastoderm. ¢, iz mes. Cavity in mesoderm. ch. Chorion. ceph. seg.
Cephalic segment. c. p. Central mass of protoplasm. dors. ec. Dorsal ecto-
derm. ec. Ectoderm. f. Follicular envelope. g/. Ganglion. ez. Endoderm.
lon. fur. Longitudinal furrow. m. Mesoderm. Maip. t. Malpighian tube.
mes. 6. Mesodermal bands. mem. ex. Membranous envelope. mesex. Mesen-
teron. mes. Mesoderm. mes. hy. Mesoderm cells directly derived from
endoderm. m.&. Mesodermickeel. zw. Nucleus. xzcl. Nucleolus. p. netw.
Protoplasmic network. pr. Proctodeum. proc. Process. 7. Ring. 1. ap.
Rudimentary appendage. rem. %. Remainder of keel. seg. Segment. 3. M.
Segmentation mass. som. m. Somatic mesoderm. sp. m. Splanchuic meso-

238 F. G. HEATHCOTE.

derm. stom. Stomodeum. sub. gl. Subcesophageal ganglion. suprae. gl.
Supracesophageal ganglion. y. 4. Yolk hypoblast cell. y. sp. Yolk-spherules.
v.é. Ventral ectoderm. vf Ventral flexure. v. p. Ventral plate.

Fig. 1.—Section through an ovum while still in the ovary. (Zeiss, c.) uel.
Nucleolus. xv. Nucleus. r. Deeply-stained ring of first food-yolk. f Folli-
cular envelope of ovum.

Fie. 2.—Section of ovarian ovum shortly before laying. (Beck 3 in.)
auc. Nucleolus. zw. Nucleus. y.s. Yolk-spherules.

Fie. 3.—Section through ovum on first day, shortly after laying. ch.
Chorion. y. sp. Yolk-spherules. ¢. p. Central mass of protoplasm. zz.
Nucleus.

Fic. 4.—The central mass of protoplasm has divided into two. s. m. Seg-
mentation mass, Ww. Nucleus.

Fie. 5.—Section through an embryo on the third day. 42, Blastoderm.
s. m. Segmentation masses.

Fie. 6.—Section through an embryo on the third day, rather earlier than
Fig. 5. A segmentation mass has just appeared at the surface.

Fic. 7.—Ear!y on the fourth day. ec. Ectoderm. ez. Endoderm.

Fie. 8.—Fifth day, showing first formation of mesodermal keel. ec. Ecto-
derm. ez, Endoderm.

Fie. 9@.—Sixth day, transverse section through keel. ec. Ectoderm.
m. &, Mesodermal keel.

Fic. 9 6.—Section through anterior end of same embryo.

Fic. 10.—Sixth day, keel spreading out into mesoderm. ec. Ectoderm.
en. Endoderm. m'. Somatic mesoderm. m. First beginning of splanchnic
mesoderm. rem. %. Remainder of keel.

Fie. 11.—Seventh day. ez. Endoderm. ec. Ectoderm vv. p. Ventra
plate. s.m. Somatic mesoderm. sp. Splanchnic mesoderm.

Fie. 12.—Vertical longitudinal section through embryo of ninth day. st.
Stomodeum. seg. 1. First body segment. mes. Mesuderm. v. ec. Ventral
ectoderm. pr. Proctodeum. dors. ec. Dorsal ectoderm.

Fic. 13.—Longitudinal vertical section on tenth day. s¢. Stomodeum.
pr. Proctodeum. mesent. Mesenteron. mem. ex. Membranous envelope.
seg. Segment.

Fie. 14.—Longitudinal vertical section on eleventh day, taken a little to
one side of middle line so as to pass through all the ganglia on one side.
supraw. gl. Supracesophageal ganglion. sf. Stomodeum. pr. Proctodeum.
mem. en. Membranous envelope. mes. Mesoderm. mesez. Mesenteron. 2. gl.
Ganglia of nerve-cord. x. Nerve.

The above fourteen figures were drawn under a Zeiss’s microscope with a

EARLY DEVELOPMENT OF JULUS TERRESTRIS. 239

3 in. object-glass by Beck, and a No. 2 eye-piece by Zeiss. They form a com-
plete rather diagrammatic series up to the time of hatching.

Fic, 15.—Nucleus of ovarian ovum just before hatching. Drawn under 3;
oilimm. Reichert. zuci. Nucleolus. xz. Nucleus. y. sp. Yolk-spherules.

Fig. 16.—Section through dividing segmentation mass on second day:

ts Reichert’s oil imm.) y. sp. Yolk-spherules. xa. Nucleus. 2u2 Nucleus
of second segmentation mass.

Fic. 17 ¢.—Part of a section through first day ovum, showing protoplasmic
network. (3; Reichert’s oil imm.) _y. sp. Yolk-spherules. p. netw. Proto-
plasmic network.

Fic. 17 4.—Part of a section through second day ovum, showing network.
p. netw, Protoplasmic network.

Fic. 18.—Part of a transverse section through third day embryo, showing
segmentation mass dividing to form blastoderm. 4/. Blastoderm cells. y. A.
Yolk hypoblast. 2. Nucleus. (2; Reichert’s oil imm.)

Fie. 19.—Part of transverse section on fourth day, to show formation of
mesodermal keel. ec. Hctoderm. ez. Endoderm. xz. Nucleus. (5 Rei-
chert’s oil imm.)

Fie. 20 ¢.—Part of transverse section through sixth day ovum, to show keel.
ec. Ectoderm. m.%, Mesodermal keel. y. sp. Yolk-spherules. (Zeiss, D.)

Fie. 20 4.—Isolated cells of the keel. (2; Reichert’s oil imm.)

Fic. 21.—Part of a transverse section through an ovum on the ninth day
early, to show thickened bands of mesoderm. ec. Ectoderm. mes. 6. Meso-
dermal bands. (Zeiss, c.)

Fie. 22.—Part of transverse section through ninth day embryo, to show
median portion between mesodermal bands. ec. Ectoderm. sp. m. Splanch-
nic mesoderm. som. m. Somatic mesoderm. (75 Reichert’s oil imm.)

Fic. 23.—Isolated cells from transverse section on ninth day, to show con-
nection between mesoderm and ectoderm. (; Reichert’s oil imm.)

Fic. 24.—Transverse section through part of an embryo on the ninth day
late, to show the mesodermal segments. stom. Stomodeum. seg.’ Furrow
marking off the head segment. mm. seg. 1, 2, 3, &. Mesodermal segments.
(Zeiss, C.)

Fie. 25.—Longitudinal section through cephalic section. s¢. Stomodzum.
ec. Ectoderm. mes. Mesoderm. 1 seg. First segment. (Zeiss, D.)

' Fie. 26.—Endoderm cell from an embryo of same date when the mesenteron
is being formed. (4; Reichert’s oil imm.)

Fic. 27.—Longitudinal section through the proctodzeum in same embryo as
Fig. 25. The mesenteron is just being formed. pr. Proctodeum. ez. Endo-
derm. m. seg. Mesodermal segment. (Zeiss, D.)

240 Fr. G@. HEATHOOTE.

Fic. 28.—Longitudinal vertical section through first post-cephalic segment
of a slightly later embryo than Fig. 27. som. m. Somatic mesoderm,
ec. Ectoderm. mem. ex. Membranous envelope. sp. m. Splanchnic meso-
derm. cav. im mes. Cavity in mesoderm. (Zeiss, D.)

Fie. 29.—Longitudinal vertical section through part of a tenth day embryo,
to show ventral flexure. sp. m. Splanchnic mesoderm. som. m,’ Somatic
mesoderm. v./. Ventral flexure. (Zeiss, F.)

Fie. 30.—Longitudinal vertical section through embryo rather later than
Fig. 29, to show ventral flexure. ec. Ectoderm. mes. Mesoderm. v./. Ven-
tral flexure.

Fic. 31.—Transverse section through late tenth day embryo, to show ner-
vous system. sp. m. Splanchnic mesoderm. som. m. Somatic mesoderm. ec.
Ectoderm. ec. ¢#. Ectodermal thickening. oz. fur. Longitudinal furrow
between nerve-cords. ez. Endoderm forming gut. (Zeiss, D.)

Fie. 32.—Ventral part of a transverse section through an embryo of the
eleventh day, to show the nerve-cord and the Malpighian tubes. This section
is taken in the posterior region, about the sixth segment. som. mes. Somatic
mesoderm. g/l. Ganglia. Malp. ¢. Malpighian tubes. pr. Proctodeum-
sp. mes. Splanchnic mesoderm. y.s. Yolk-spherules. v. ec. Ventral ectoderm.
b. ce. Part of body cavity between the nerve-cord and ventral ectoderm.
(Zeiss, D.)

Fig. 33.—Vertical longitudinal section through part of a twelfth day
embryo, to show the stomodeum. sud. gl. Subcesophageal ganglion. supraw.
Supracesophageal ganglion. sf. Stomodeum. In this section the supra- and
sub-cesophageal ganglia are not cut exactly in the middle line, and so appear
smaller than they really are. (Zeiss, D.)

Fic. 34.—Transverse section through a twelfth day embryo in posterior
region of body. g/. Ganglia of nerve-cord. 7. app. Rudiment of appendage.
mes. Mesoderm forming a partition to the body cavity. pr. Proctodeum.
Malp. t. Malpighian tubes. m. ez. Mesoderm cells in the body cavity derived
directly from the endoderm.

All the figures were drawn by myself under a Zeiss’s camera lucida.

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Notes on the Development of the Newt
(Triton Cristatus).

By
Alice Johnson,
Demonstrator of Biology, Newnham College, Cambridge.
And

Lilian Sheldon,
Bathurst Student, Newnham College, Cambridge.

With Plates XXIV, XXV, and XXVI.

Taz present paper is a continuation of some observations
made by one of us on the early development of the Newt (14).
It was then shown that the blastopore of the embryo becomes
the permanent anus.

The same discovery has since been made in the Frog by Mr.
Spencer (21), in Petromyzon by Mr. Shipley (20), and in
Ceratodus by Mr. Caldwell (6). Dr. Gasser stated the same
fact with regard to Alytes obstetricans in 1882 (8), in a
paper with which the present writers have only recently
become acquainted.

Tue Post-Anat Gut.

The existence of a post-anal gut in the embryos of many
Vertebrates appears at first sight an important argument
against the view of the identity of the blastopore with the
anus, because it would naturally be supposed that the blasto-
pore must be at the extreme hind end of the gut. We find,
however, that a post-anal gut is present in the Newt embryo,
and its relations there, as described below, explain this diffi-

18

242 ALIOE JOHNSON AND LILIAN SHELDON.

culty. In a transverse section taken a very short distance in
front of the blastopore (anus), a portion of the dorsal wall of
the gut is partially constricted off (fig. 1), and a little further
back becomes completely separate (figs. 2 and 3), and may be
traced back into the tail as a solid mass of cells, lying just
below the notochord. Near the posterior end of the tail this
mass dilates (fig. 5), forming a portion which is probably
homologous with the caudal vesicle of the post-anal gut in
Elasmobranchs (1), and then fuses with the other structures in
the tail at the extreme end (figs. 6, 7).

This solid diverticulum of the alimentary canal appears from
its relations to be the post-anal gut, and its point of fusion
with the notochord and neural canal no doubt represents the
neurenteric canal.

At earlier stages the neurenteric canal, which we believe to
be always solid in the Newt, though open for a short time in
the Frog, is represented by the point at which the fused layers
pass into the blastopore. The neurenteric canal is then,
roughly speaking, vertical in direction, since the blastopore is
situated at the hind end of the ventral surface. When the tail
grows out behind the blastopore, the middle point of the
vertical neurenteric canal grows out with it, remaining always
at its tip, so that the canal becomes, as it were, drawn out into
a loop with dorsal and ventral horizontal limbs. The tail is at
first composed of undifferentiated tissue, and the differentiation
proceeds as usual from before backwards, the dorsal limb of
the loop being the medullary canal, and the ventral the post-
anal gut. The two limbs are still connected at the posterior
end of the tail by the neurenteric canal.

This mode of development seems to us to show that the tail
with the post-anal gut is a secondary structure, developed
after the permanent anus. The function of the post-anal gut
seems to be to provide material for the growth of the tail
during embryonic stages before the blood-vessels have formed.
With the appearance of the latter, the post-anal gut gradually
atrophies, a remnant of it being attached to the rectum just in
front of the anus in a newly hatched larva, At this time it is

NOTES ON THE DEVELOPMENT OF THE NEWT. 243

seen to cccupy the normal position of the post-anal gut, being
situated between the dorsal aorta and caudal vein.

In the Frog we find a post-anal gut with a wide lumen
behind the blastopore. The lumen gradually narrows towards
the hind end, and loses itself in the indifferent tissues of the
tail. This stage is somewhat later than that recently described
by Mr. Durham (23), with a complete open neurenteric canal.
The canal must evidently be drawn out with the growth of the
tail, and two regions can then be distinguished in it, the post-
anal gut and the neurenteric canal proper. The lumen is
obliterated from behind forwards, the neurenteric canal becom-
ing closed first. This would account for the condition we find.
Later, the lumen of the post-anal gut is lost, and it becomes
a solid structure.

Dr. Gasser gives an account of a post-anal gut in Alytes (8)
like that of the Newt. The lumen of the alimentary canal is
continued a very short way into it, and the rest forms a solid cord
in the tail. There is no open neurenteric canal in Alytes.

A post-anal gut of the same kind has been described by Mr.
Shipley in Petromyzon (20).

Tur StomopzuM AND Prrurrary Bopy.

The stomodzum developes as a solid ingrowth of the inner
layer of the epiblast just in front of the anterior wall of the
fore-gut (fig. 12). The lower part of the ingrowth fuses with
the fore-gut (figs. 14, 8, 9) while the upper projects freely and
forms the pituitary body (fig. 14). In fig. 8, which represents
an oblique transverse section, the relations of the pituitary
body to the stomodum and fore-gut may be clearly seen. It
grows upwards and applies itself closely to the infundibulum,
curling round it (fig. 14) and forming an indentation in its
floor (figs. 38, 37,36). The extreme end of the pituitary body
is shown in fig. 38, where it is hardly distinguishable from the
infundibulum.

The stomodzeum fuses with the fore-gut at a very early
stage, but no actual perforation is formed until a short time
after hatching. The region of fusion takes on gradually the

244 ALICE JOHNSON AND LILIAN SHELDON.

shape of the adult mouth, becoming first elongated trans-
versely, and then horseshoe shaped, with the concavity of the
horseshoe directed backwards. The consequence is that, in
transverse sections of late stages, the mouth appears to consist
of two lateral parts, which are the limbs of the horse-shoe.
We find that the pituitary body and stomodeum develope in
exactly the same way in the Frog as in the Newt.

The pituitary body has been described as originating from a
solid ingrowth of epiblast in Teleosteans by Hoffmann (13), and
it seems to arise somewhat similarly in Lepidosteus (2). Gdtte
also describes the same method of development in Bombinator
(9). (See his figs. 127, 128, 250, 252, 292, 298, 298, and 805.)

Tur Tuyrom Bopy.

From the hind end of the stomodeum proceeds a solid cord
of cells continuous along its dorsal border with the fore-gut
(figs. 9,10, 11). This is the thyroid body. Later a groove
is continued into it from the fore-gut, and its hind part
becomes a tube by the folding over of the edges of the groove.
Subsequently the hind end is completely constricted off from
the gut. We have not followed its development further.
Scott and Osborn (19) described it as being formed from a
fusion of hypoblast and epiblast in the median ventral line.
We think that this fusion is the stomodzeum, with which the
thyroid is continuous at its front end, and that the thyroid
itself is developed in a perfectly normal manner.

DEVELOPMENT OF PERIPHERAL NeERvous System.

There is no trace of the peripheral nervous system until the
neural canal has completely closed and become separate from
the external epiblast. Fig. 15 represents a transverse section
through an embryo of a stage just before the closure, showing
the epiblast in close contact with the neural canal, with which
its two layers are of necessity continuous at this time.

The appearance of the peripheral nervous system is preceded
by the formation of a neural ridge. In an embryo in which
this is first seen, the neural canal has lost all connection with

NOTES ON THE DEVELOPMENT OF THE NEWT. 245

the epiblast in the region of the neural ridge, but remains
connected with it in the median dorsal line behind the ridge,
while still further back the closure of the neural canal is not
yet complete. The neural ridge now extends through the
head (fig. 16) and the anterior part of the trunk (fig. 18).

It may be here stated briefly that, as far as our observations
extend, the development of the spinal nerves is perfectly
normal, The neural ridge is prolonged at regular intervals
into nerves, which grow down between the medullary canal
and muscle-plates. The upper part of each nerve developes a
ganglion, and the ventral root is formed later, whether as an
outgrowth from the medullary canal or from the ganglion we
are unable to say.

After our discovery of the neural ridge, we found that we
had been so far anticipated by Bedot (5), who described in
detail the development of the spinal nerves in the Newt. Our
observations only confirm his on this point.

The Cranial nerves, like the spinal, arise as paired lateral
outgrowths of the neural ridge, being completely separate from
the epiblast. Figs. 17, 18, and 19 illustrate those outgrowths,
which give rise respectively to the 3rd, 5th, and 7th nerves.
The 7th and 8th nerves are at first fused, and the common
rudiment may be called, for convenience of description, the
Facio-auditory nerve.

The Trigeminal nerve (fig. 18) is an outgrowth from
the dorsal surface of the brain, and is directed outwards and
downwards towards a lateral thickening of the epiblast, which
is cut transversely on one side of the section, and more
obliquely, so as to appear longer, on the other side.

The Facio-auditory has the same relations to the brain
as the Trigeminal, and, like it, is directed outwards and
downwards towards a‘lateral epiblastic thickening. The 9th
nerve grows out similarly towards a corresponding epiblastic
thickening. ‘These thickenings are situated slightly above the
level of the notochord, and are destined to give rise to the
mucous canals of the head. It will be most convenient to
take the future history of the nerves separately. ‘

246 ALICE JOHNSON AND LILIAN SHELDON.

The 8rd nerve is seen at a later stage in fig. 20. Its
point of attachment has been shifted down the side of the
brain, and the nerve is directed forwards towards the eye.
We have not ascertained whether or no there is any sensory
thickening of the epiblast corresponding to it, but it seems
possible that the ciliary ganglion may be fused with the
Gasserian, as is stated by Mr. Beard (4) to be the case in the
Frog. It would thus not have a separate sense organ of
its own.

The Trigeminal nerve grows downwards from the brain
till it reaches the level of the sensory epiblastic thickening,
and then fuses with it (fig. 21). The point of fusion con-
stitutes the Gasserian ganglion together with the sensory
thickening. It is not possible to decide if the epiblast actually
takes part in the formation of the ganglion. The mere
presence of dividing nuclei in this region, as insisted on by
Mr. Beard, seems to us to prove nothing, since all the tissues
of the body are actively growing, and consequently contain
numbers of such nuclei. We are inclined, therefore, to think
that the fusion of the nerve with the epiblast is merely a case
of innervation of a sense organ, exactly comparable to what
occurs in the nose and ear, and that, in all such cases, the
nerve-elements are derived from the brain and the sense
elements from the epiblast. Professor Marshall has shown
how early this fusion occurs in the case of the ear in the
Chick (16).

The root of the 5th nerve is at tet attached to the dorsal
surface of the brain (fig. 18). Later, the surface of attach-
ment widens out and extends further down the side (fig. 22),
and then gradually becomes confined to a small area situated
about half way down the brain (fig. 23). The point of
attachment is thus shifted downwards, no secondary attach-
ment being formed in this case while the first is lost, as has
been described by Professor Marshall in the eS (16) and in
Scyllium (17).

The Gasserian ganglion is for a short time fused into one
mass with the sensory epiblast. Soon it begins to sink deeper

NOTES ON THE DEVELOPMENT OF THE NEWT. 247

into the body, but remains attached to the surface by a cord
of cells, which constitutes the dorsal branch (ophthalmic) of
the 5th nerve (fig. 24). At the same time a nerve grows
down from the ganglion, which soon divides into two branches,
a posterior, the inferior maxillary, shown in figs. 24 and 26,
and an anterior, the superior maxillary, shown in fig. 24.

The Facio-auditory nerve grows downwards towards
its corresponding sensory thickening, and fuses with it at two
points, one behind the other. The anterior of these we
interpret as the sense organ belonging to the 7th nerve, and
the posterior as the ear. There is only a very short distance
between them, along which the nerve is not fused. In a later
stage, shown in fig. 37, the ear is seen to be clearly distin-
guishable from the sense organ of the 7th nerve, the ganglion
of which is still fused with the skin, while the ear itself is
completely separate, forming a simple closed vesicle (fig. 36).
The main trunk of the 7th passes on downwards, and fuses
with the epiblast of the dorsal wall of the first visceral cleft
(figs. 87 and 36). Afterwards, this second connection with
the epiblast is lost, and the nerve divides into two branches,
one behind and one in front of the first cleft (figs. 26, 31, and
32). At the same time the ganglion on the upper part of the
trunk has sunk deeper into the body, remaining attached to
the sensory thickening by a cord of cells constituting the
dorsal branch (ophthalmic) of the 7th nerve (fig. 31).

The facio-auditory nerve is now attached to the brain by
two roots, one behind the other; the anterior is shown in
fig. 26, and its connection with the ganglion and pre- and
post-branchial branches shows it to be the 7th nerve-root ;
the posterior passes into the walls of the auditory vesicle
(fig. 81), and is therefore the 8th nerve.

The 9th nerve fuses with its corresponding sensory epithe-
lium soon after its origin (fig. 27). The main trunk then ~
passes on and fuses with the epiblast of the 2nd gill-cleft, as
shown at a later stage in fig. 28. The root by this time has
shifted downwards from the dorsal surface of the brain. The
subsequent course of events is exactly the same as in the case

248 ALICE JOHNSON AND LILIAN SHELDON.

of the 7th nerve. The ganglion retreats further from the
surface of the body, remaining attached by the dorsal nerve to
the sense organ (fig. 34), and the ventral portion of the main
trunk divides into two branches, the post-branchial (fig. 35)
behind the second gill-cleft, and the pre-branchial (fig. 33) in
front of it.

The Vagus arises from the brain in the same manner as
the other cranial nerves, but we have not traced its further
development.

DEVELOPMENT oF NERVES IN THE FRog.

We have made a few observations on the development of
the nerves in the Frog in some series of sections cut by Mr.
Durham, and very kindly lent to us. Our observations, as far
as they extend, confirm in every respect what we have described
in the Newt. A neural ridge is formed on the dorsal surface
of the medullary canal after it has separated from the epiblast,
as shown in fig. 30, representing part of a transverse section
through the hind region of the trunk of an embryo. In this
embryo the neural ridge extended through the trunk, but was
less distinct in the head, where the nerves had begun to form
as outgrowths from it. Fig. 29 shows the origin of the facio-
auditory nerve. Its small size shows that it must be at a very
early stage. It is growing on each side from the dorsal surface
of the brain towards the auditory vesicle, which is beginning
to develope from the inner layer of the epiblast. It seems to
us that the whole appearance is inconsistent with the view
that the nerve has split off from the epiblast, as Mr. Spencer
asserts (21).

Historica, anp Crittcan.

Our observations are, on the whole, consistent with the
account of the derivation of nerves first put forward by Pro-
fessor Balfour in 1876 (1), afterwards confirmed by Professor
Marshall in other types, and since generally accepted. They

do not lend any support to the peculiar view of His, as to the
presence of a “ Zwischenstrang” (11).

NOTES ON THE DEVELOPMENT OF THE NEW’. 249

Sagemehl (18) derives the spinal nerves in the Frog from a
neural ridge, but states that they become detached later from
the spinal canal, and subsequently joined to it by the dorsal
and ventral roots. Bedot (5) states that in the Newt the
connection is never broken, and our researches lead us to agree
with him on this point.

Hoffmann (12) describes the spinal nerves in Teleosteans as
growing from a neural ridge, but appears to think that the
cranial nerves, which arise before the neural canal is closed,
are, partially at least, derived from the adjacent epiblast.

O. Hertwig (10), in a few scattered observations on the
spinal nerves of the Frog, is inclined to support His’ view.
More recently the theory of the derivation of the whole or
greater part of the cranial nerves from the epiblast has been
supported by Mr. Spencer (21) and Mr. Beard (4). This view
is a revival of that held by Gétte (9). Mr. Spencer asserts
that the whole nerve, including root and ganglion, is, in the
Frog, split off from the nervous layer of the epiblast. If this
be so, all the branches must ultimately be derived from the
same source. Mr. Beard confirms him in this statement, and
figures one section showing a thickened mass of epiblast con-
tinuous dorsally with the still open neural canal, but there is
nothing to show that this thickening becomes a nerve. Such
a split, as is figured between it and the external layer of epi-
blast, very often occurs in imperfectly preserved specimens,
We find no such thickenings in Newt embryos of similar
stages, a typical section of which is shown in fig. 15, and our
observations on the Frog lead us to doubt the accuracy of Mr.
Spencer’s account. We have attempted to show that it is, at
all events, not universally true for Amphibia, as Mr. Beard
assumes.

Mr. Beard has described in Elasmobranchs (4) a fusion of
the typical cranial nerve with the sense organ of its segment.

“This corresponds with the dorsal fusions found by us in the
Newt. The ventral fusion of the nerve with the gill-cleft, as
described above in the Newt, corresponds to the second fusion
found by van Wijhe in Elasmobranchs (22), and to the ventral

250 ALICE JOHNSON AND LILIAN SHELDON.

fusion found by Froriep in Mammals (7). Mr. Beard considers
that, in Elasmobranchs, all the main branches of the nerve
except the post-branchial and the part between the ganglion and
the brain are split off from the epiblast. Van Wijhe holds
that the epiblast takes some share in the formation of the
ganglion at least, while Froriep expresses doubt as to this
point, comparing the fusion to the similar fusion of nerve-cells
and epithelium cells in the ear. We are strongly inclined to
the last view. Professor Marshall (16) has shown how very
early the nerve-cells of the ear become indistinguishably fused
with it, and there seems no reason why this should not be the
case with other sense organs. As to the splitting off of the
nerve-trunks from the skin, Mr. Beard’s observations and
deductions seem to us inconclusive.

In Elasmobranchs Professor Balfour mentioned and figured a
fusion between the mucous canals of the head and the nerves
supplying them, no line of demarcation existing between the
two structures (v. loc. cit., pp. 144, 145, plate xii, fig. 7). He
describes this as occurring first in his Stage P, but it is possible
that it may take place rather earlier in the Elasmobranchs, as
it certainly does in the Newt. Mr. Beard seems to have de-
tected the earlier fusion in Elasmobranchs, and to be unaware
that the fact of the fusion was described by Professor Balfour,
who found that the nerves were all derived from the brain out-
growths, as we believe to be the case in the Newt. It appears
to us that the epiblast in this animal takes no part in the
formation of the ganglion or nerve branches, and that the
special nerve to the sense organ is an outgrowth from the
ganglion, advancing pari passu with the withdrawal of the
latter from the surface, so that there is at no time any break
in the connection between the sense organ and its nerve supply.
The withdrawal of the ganglion and formation of the nerve
is only a result of the differentiation of the nerve supply into a
ganglionic and a fibrous part.

The disposition of these sense organs seems to us a very
insecure guide to the segmentation of the head. Mr. Beard
considers that the relations of the sense organs to the gill-

NOTES ON THE DEVELOPMENT OF THE NEWT. 25]

clefts shows them to be of segmental value, since they are in
some cases situated one above each gill-cleft. At the same
time he is obliged to assume the existence of more than one
now aborted gill-cleft, in order to account for the number of
the sense-organs. If the proof of the segmental value of the
sense organs is to depend on the number of the gill-clefts, and
the number of the gill-clefts is in turn to depend on the
segmental value of the sense organs, it is difficult to discern
which is the basis of the argument. Malbranc (15) shows that
even in the embryo multiplication of the sense organs by
division may occur, so that the number of them seems to be
indefinitely variable ; and Mr. Beard himself has described such
a division in the case of the sense organs of the facial nerve.
It seems, therefore, that there was primitively only one such
sense organ in this case, and that one cannot depend on the
number of the sense organs at any but the very earliest stages,
if even then, as indicating segmentation.

SuMMARY OF OBSERVATIONS.

1. Asolid post-anal gut is formed behind the blastopore (anus),
growing out into the tail, and fusing with the undifferentiated
tissues at its posterior end. The fusion of hypoblast and
epiblast in this region represents the neurenteric canal.

2. In the Frog the post-anal gut is at first hollow, but after-
wards becomes solid.

3. The stomodeum and pituitary body are derived from a
solid ingrowth of the inner layer of the epiblast. The hind
part of this ingrowth fuses with the front wall of the fore-gut,
but the perforation to form the actual mouth does not appear
till after hatching. The pituitary body grows upwards as a
solid cord, and applies itself to the infundibulum in the ordinary
manner.

4. From the hind border of the stomodeum proceeds a solid
rod of cells, which constitutes the thyroid body, and is de-
veloped from the cells of the middle ventral line of the fore-
gut.

168 ALICE JOHNSON.

tive” of Bambeke, and may obviously be called the primitive
groove. Hertwig! says that it is at all stages sharply marked
off from the blastopore by an intervening ridge of cells. In
my specimens this sometimes occurs, but it happens at least as
frequently that the groove is continuous with the blastopore at
its first appearance, and I always find them continuous after
the formation of the medullary folds.

Transverse sections through an embryo with a primitive
groove and before the medullary folds have been formed shows
that in the region of the groove the three embryonic layers are
continuous with one another (vide fig. 1, which represents
a section taken through about the middle of the embryo). It
happened in this embryo that the primitive groove was con-
tinuous with the blastopore. In the anterior part of the
embryo the groove flattened out and gradually disappeared.
Fig. 2 represents a section through the groove near its anterior
end, and shows that here the epiblast is distinct from the other
two layers, the mesoblast still retaining its connection with the
hypoblast. The mesoblast has generally been described (viz.
by Scott and Osborn, Hertwig, and Bambeke?) as being derived
exclusively from the hypoblast, except at the blastopore, from
the lips of which it grows. It appears to me, on the contrary,
that the greater part of it is derived from the primitive streak’
as in the higher Vertebrates, for it is seen in fig. 2 that the
mesoblast cells, where they are represented as derived from
the hypoblast, are much fewer in number than appears in
fig. 1, where they are shown growing out from the primitive
streak.

The primitive groove in another embryo of a slightly later
stage exhibits a deep pit at its anterior end. Iam unable at
present to state whether any fusion of the layers exists in the
region of this pit at this time.

The next step forwards in development consists in the for-

1 OQ. Hertwig, loc. cit.

> Ch. van Bambeke, “ Formation des feuillets enbryonnaires et de la Noto-
corde chez les Urodéles,” ‘ Bulletins de Acad. Roy. de Belgique,’ 2me série,
tome 1, 1880. .

ON THE FATE OF THE BLASTOPORE IN THE NEWT. 169

mation of the medullary folds. In fig. 2 the dorsal half of
the epiblast is seen to be thickened. This occurs first in the
anterior part of the body, where also the folds are first clearly
formed. They consist of a pair of sharply-marked ridges,
bounding a very wide, flat area. The medullary plate, which
includes the whole of the dorsal surface, is made up of narrow
deep columnar cells. The rest of the epiblast, which formerly
consisted of a single layer of columnar cells (vide figs. 1, 2),
now begins to divide into two layers of flatter cells. These
well-known peculiarities of the medullary plate and general
epiblast have already been sufficiently figured by previous
observers.

The primitive groove at this period extends from the blasto-
pore throughout the whole medullary plate. The consequent
division of the latter into two halves is especially conspicuous
in front. It occasionally happens that the groove is absent in
the middle region of the body. This was the case with the
embryo, transverse sections of which are represented in figs. 3,
4, 5, and in which the medullary folds existed anteriorly, but
diminished gradually and vanished behind. Fig. 3 shows the
open blastopore, with the three embryonic layers coalescing at
its edges in the ordinary manner. In fig. 4 the rounded pri-
mitive groove is seen indenting the primitive streak. In fig. 5
the groove is flatter, but the layers are still fused beneath it.
The blastopore itself at this stage is narrow and elongated.

In another specimen of the same stage as that just described
I find that the primitive groove extends for a short distance in
front of the medullary folds. Near its anterior end it becomes
rather suddenly considerably deeper and also loses its rounded
outline, being instead triangular in section and sharply pointed
at its apex. It presents in this region, in fact, an appearance
strikingly similar to that of the blastopore, although not com-
municating with the archenteron. I believe, however, that the
epiblast and hypoblast are fused at this point, and it can
hardly be doubted that this deep pit, with the fused layers at
its apex, represents the front end of the blastopore. It is
evidently the same structure as the pit found at the front end

12§

252 ALICE JOHNSON AND LILIAN SHELDON.

5. The development of the peripheral nervous system is
preceded by the appearance of a neural ridge, extending along
the whole length of the body.

6. The spinal nerves grow out from the neural ridge, and
pass downwards between the neural canal and muscle plates.

7. The cranial nerves also grow out from the neural ridge,
but are nearer to the surface than the spinal nerves, owing to
the absence of muscle plates in the head.

8. When each has attained a certain length it fuses with a
thickening of the epiblast, situated some distance above the
level of the notochord. (This is the case with the 5th, 7th,
and 9th nerves, and probably also with the vagus.)

9. At the point of fusion there is a thickening of the nerve-
trunk, forming a ganglion, which afterwards recedes from the
surface, remaining, however, attached to the sense organ by a
nerve.

10. The main truuk of the nerve passes on, and, in the cases
of the 7th and 9th nerves, fuses again with the epiblast of the
dorsal wall of the corresponding gill-cleft. Later, the nerve
becomes detached from the epiblast, and gives off two branches,
one behind and one in front of the gill-cleft.

11. The 5th nerve has no such second (ventral) fusion with
the epiblast, but divides below its first (dorsal) fusion into two
branches, the superior and inferior maxillary.

12. In the Frog a neural ridge is present at an early stage,
just after the closure of the neural canal. The facio-auditory
nerve grows out of the brain, and it is therefore probable that
the other cranial nerves have the same origin.

N.B.—Our figures are diagrammatic in so far that the outlines
of the cells were not perfectly apparent in all sections. This
appeared to us to be due to bad preservation, as the better the
specimens were preserved the more distinct and complete were
the cell outlines. It was generally possible to draw them accu-
rately with a camera and Zeiss obj. p, oc. 2. We have
therefore represented them throughout as distinct.

NOTES ON THE DEVELOPMENT OF THE NEWT. 253

List oF PAPERS REFERRED TO.

1. Baurour, F. M.—“ Elasmobranch Fishes.” ,
2. Batrour, F. M., and Parker, W. N.— On the Structure and Develop-

19.

20.

ment of Lepidosteus,” ‘ Phil. Trans. of the Royal Soc.,’ part ii, 1882.

. Bzarp, J.—-' Zoologischer Anzeiger,’ Nos. 161 and 162, 1884, and 192,

1885.

. Beard, J.—‘‘ The System of Branchial Sense Organs and their Asso-

ciated Ganglia in Ichthyopsida,” this Journal, November, 1885.

. Bepot, M.—“ Recherches sur le développement des nerfs spinaux chez

les Tritons,”’ ‘ Recueil Zoologique Suisse,’ tome i, 1884.

. CaLtpweLt, W. H.—“ Note on Ceratodus,”’ ‘ Nature,’ Jan. 8th, 1885.
. Froriepr, A.—‘ Ueber Anlagen von Sinnesorganen am Facialis, Glosso-

pharyngeus und Vagus, &.,” ‘Arch. f. Anat. u. Phys.,’ 1885, Heft i.

. Gasser, E.—“ Zur Entwicklung von Alytes Obstetricans,” ‘Sitzungs-

berichte der Marburger Naturgesell.,’ Oct., 1882.

. Gorts, A.— Die Entwicklungsgeschichte der Unke.”
. Herrwie, O.—“ Die Entwicklung des mittleren Keimblattes der Wir-

belthiere,” ‘Jen. Zeit.,’ vol. xvi, 1883.

. His, W.—“ Ueber die Anfange des peripherischen Nervensystems,”

‘Arch, f. Anat. u. Phys.,’ 1879,

. Horrmann, C. K.—“ Zur Ontogenie der Knochenfische,” ‘ Konigliche

Akad. v. Wissen. zu Amsterdam,’ 1882.

. Horrmann, C. K.— Zur Ontogenie der Knochenfische,” ‘Arch. f. mik.

Anat.,’ 1884.

. Jounsoy, A.—On the Fate of the Blastopore in the Newt,” this

Journal, Oct., 1884.

. Marpranc, M.— Von der Seitenlinie und ihren Sinnesorganen bei

Amphibien,” ‘Zeit. f. wiss. Zool.,’ Band xxvi, 1876.

. Marsuatt, A. M.—* On the Development of the Cranial Nerves in the

Chick,” this Journal, vol. xviii, 1878.

. Marswatt, A. M., and W. B. Spewcer.— Observations on the Cranial

Nerves of Scyllium,” this Journal, vol. xxi, 1881.

. SaGEMEHL, M.—‘ Untersuchungen iiber die Hutwicklung der Spinalner-

ven,” ‘Inaugural Dissertation Dorpat,’ 1882.

Scorr, W. B., and Ossorn, H. F.—‘On the Early Development of the
Common Newt,” this Journal, Oct., 1879.

Saipuey, A. E.—‘ On the Formation of the Mesoblast, and the Persist-
ence of the Blastopore in the Lamprey,” ‘ Proc. Roy. Soc.,’ 1885,

254 ALICE JOHNSON AND LILIAN SHELDON.

91. Spencer, W. B.—‘*Some Notes on the Early Development of Rana
temporaria,” this Journal, supplement, 1885.

92. Wisuz, T. W. van.—‘ Ueber die Mesodermsegmente u. d. Entwicklung
der Nerverf des Selachierkopfes,” ‘Kénigliche Akad. v. Wiss. zu
Amsterdam,’ 1882.

23. Dunnam, H. E.— Note on the Presence of a Neurenteric Canal in
Rana,” ‘Quart. Journ. Micr. Sci,’ June, 1886.

EXPLANATION OF PLATES XXIV, XXV, anp
XXVI,

Llustrating Alice Johnson’s and Lilian Sheldon’s Paper
“On the Development of the Newt (Triton cristatus).”

All the figures represent single sections. They were drawn with a Zeiss’s
camera lucida and Zeiss’s obj. a, oc. 2, except Figs. 1, 2, and 3, which were
drawn with obj. B, oc. 2; Figs. 6 and 7 with obj.c, oc.2; and Figs. 4, 5
and 30 with obj. cc, oc. 2. Fig. 12 was drawn with obj. 4, oc. 2, and after-
wards reduced by one half; and Figs. 18, 16, 17, 18, 36, 37 and 38 were
drawn with obj. a, oc. 2, and afterwards reduced by one third.

N.B.—Grey denotes epiblast, and organs derived from it; brown denotes
mesoblast ; and yellow denotes hypoblast, and organs derived from it.

Alphabetical List of Reference Letters.

Aud. Har. Bl. Blastopore. Ch. Notocbord. F. dud. r¢. Root of Facio-
auditory nerve. FF. B. Fore-brain. F. G. Fore-gut. Gass. Gasserian gang-
lion. G. VII. Ganglion of 7th nerve. G. ZX. Ganglion of 9th nerve.
H. B. Hind-brain. H. G. Hind-gut. Inf Infundibulum. Lat. V. Thicken-
ing of nervous layer of epiblast to form sense organ corresponding to 5th nerve.
Lat. VII. Thickening of nervous layer of epiblast to form sense organ corre-
sponding to 7th nerve. at. IX. Thickening of nervous layer of epiblast to
form sense organ corresponding to 9th nerve. M.B. Mid-brain. Mes. Meso-
blast. WV. C. Neural ridge. Od. Olfactory epithelium. O. V. Optic vesicle.
P.a.g. Post-anal gut. Pit. Pituitary body. Sp.c. Spinal cord. 84, Sto-
modeum. hal. Thalamencephalon. hy. Thyroid body. V.C. JZ. First
visceral cleft. V. C. ZZ. Second visceral cleft. JIZZ. Third nerve. V. Fifth
nerve. VJ, Seventh nerve. LY. Ninth nerve. JIZr¢, Root of 83rd nerve.
Vrt. Root of 5th nerve. ViI rt. Root of 7th nerve. VIII rt. Root of
8th nerve. ZX rt. Root of 9th nerve. Vd. Dorsal branch of 5th nerve.
VII d. Dorsal branch of 7th nerve. JIXd, Dorsal branch of 9th nerve,

NOTES ON THE DEVELOPMENT OF THE NEWT. 255

V sup. maz. Superior maxillary branch of 5th nerve. V inf. maz. Inferior
maxillary branch of 5th nerve. VIZ post-br. Post-branchial branch of 7th
nerve. IX post-br. Post-branchial branch of 9th nerve. VII pre-dr. Pree-
branchial branch of 7th nerve. IX pre-br. Pre-branchial branch of 9th
nerve. «. Fusion of 7th nerve with epiblast of gill-cleft.

Fics. 1—7.—Series of transverse sections through an embryo, to show
the relations of the post-anal gut to the hind-gut; Fig. 1 being the most
anterior, and Fig. 7 the most posterior.

Fig. 1. A little in front of the blastopore, to show the origin of the
post-anal gut from the hind-gut.

Fig. 2. Showing the post-anal gut completely separated from the hind-
gut.

Fig. 3. Through the blastopore.

Fig. 4. Behind the blastopore.

Fig. 5. Showing dilatation of the solid post-anal gut near the hind end
of the tail.

Fig. 6. Showing fusion of the post-anal gut with the notochord and the
neural canal.

Fig. 7. Showing fusion of the mesoblast with the other layers near the
extreme end of the tail.

Fies. 8—11 are taken from one series of transverse sections through the
anterior end of an embryo, to show the origin of the stomodeum, the pitui-
tary body, and thyroid body. Fig. 8 being the most anterior, and Fig. 11 the
most posterior.

Fig. 8. Showing the origin of the stomodseum and pituitary body, and
the fusion of the former with the anterior wall of the fore-gut. It
also shows the root of the facio-auditory nerve, and its ventral fusion
with the epiblast.

Fig. 9. Showing the hind end of the stomodeum.

Fig. 10. Showing the anterior end of the thyroid body as a solid rod of
cells attached to the ventral wall of the fore-gut.

Fig. 11. Showing the thyroid body near its posterior end.

Fic. 12.—Longitudinal vertical section through the head end of an embryo,
to show the origin of the stomodecum and pituitary body as a solid ingrowth
of epiblast in front of the fore-gut.

Fic. 13.—Transverse section through the trunk of an see shortly after
the closure of the medullary canal, to show the neural ridge.

Fie 14.—Longitudinal vertical section through the head end of a somewhat
older embryo than that from which Fig. 12 was taken, to show the relations
of the stomodeum and the pituitary body to the fore-gut, eae and
notochord.

Fig. 15.—Transverse section through the trunk of an embryo silly before the
closure of the medullary canal, showing the epiblast continuous dorsally with it.

256 ALICE JOHNSON AND LILIAN SHELDON.

Fic. 16.—Transverse section through the head end of an embryo at a stage
shortly after the closure of the medullary canal, to show the neural ridge in
the brain. Owing to the cranial flexure, all three divisions of the brain are
cut through.

Fic. 17,—Transverse section through an embryo slightly older than that
from which Fig. 16 was taken, showing the origin of the 3rd nerve as a paired
outgrowth from the neural ridge in the mid-brain.

Fic. 18.—Transverse section through the same embryo as that from which
Fig. 17 was taken, showing the origin of the 5th nerve from the neural ridge
in the hind-brain. The lateral thickening of epiblast on each side is
shown.

Fig. 19.—Transverse section through the hind-brain, to show the origin of
the 7th nerve as a paired lateral outgrowth of the neural ridge. The lateral
thickening of epiblast, which will give rise to the ear and sense-organ of the
7th nerve, is shown on each side.

Fic. 20.—Transverse section through a somewhat older embryo, showing
that the root of the 3rd nerve has shifted to the sides of the mid-brain.

Fic. 21.—Transverse section, showing the attachment of the Gasserian
ganglion to the epiblastic thickening forming the sense-organ corresponding
to the 5th nerve.

Fig. 22.—Slightly oblique transverse section, to show the shifting of the
root of the 5th nerve; its attachment is seen to extend continuously from
the summit of the brain to a point some way down its side.

Fie. 23.—Transverse section through an older embryo, to show the shifting
of the root of the 5th nerve. The nerve is now connected only with a small
area of the side-wall of the brain.

Fic. 24.—Transverse section through a still older embryo, showing on the
right side the superior maxillary and dorsal branches of the 5th nerve grow-
ing out from the Gasserian ganglion. On the left the Gasserian ganglion and
inferior maxillary are shown.

Fic. 25.—Transverse section through a young embryo, showing on the left
the root of the facio-auditory nerve and its fusion with the epiblast; on the
right the auditory epithelium and ventral continuation of the nerve.

Fic. 26.—Transverse section through the same embryo as that from which
Fig. 24 was taken, but slightly posterior to it. It shows on the right the
Gasserian ganglion and inferior maxillary branch of the 5th nerve; on the
left the root, ganglion, and pre-branchial branch of the 7th nerve.

Fie. 27.—Transverse section through a young embryo, showing the root of
the 9th nerve and its fusion with the lateral thickening of epiblast correspond-
ing to it. On the right the nerve is seen passing on to the 2nd visceral cleft.

Fic. 28.—Transverse section through a somewhat older embryo, It shows
on the right the root, ganglion, and main branch of the 9th nerve, the last
fusing with the epiblast of the dorsal wall of the 2nd visceral cleft. On the
left only the main branch and its fusion are seen,

NOTES ON THE DEVELOPMENT OF THE NEWT. 257

Fic, 29.—Transverse section through the head end of a Frog embryo,
showing the origin of the facio-auditory nerve as an outgrowth from the dorsal
surface of the hind-brain. The thickening of the nervous layer of epiblast to
form the ear is also shown.

Fie. 30.—Transverse section through the posterior part of the trunk of the
same Frog embryo shortly after the closure of the medullary canal, to show
the neural ridge. ;

Fies. 31—35.—Transverse sections through the same embryo as that from
which Figs. 24 and 26 were taken, but posterior to them,

Fig. 31. Showing on the right the ganglion and the dorsal and pre-
branchial branches of the 7th nerve; on the left the ear and the root
of the 8th nerve, and the 1st visceral cleft.

Fig. 32. Showing on the right the ganglion of the 7th nerve; on the left
the ear and the post-branchial branch of the 7th nerve.

Fig. 33. Showing on the right the ganglion and pre-branchial branch of
the 7th nerve ; on the left the ganglion and pre-branchial branch of the
9th nerve.

Fig. 34. Showing on the right the ganglion and pre-branchial branch of
the 7th nerve; on the left the root, ganglion, and dorsal branch of
the 9th nerve, and also the 2nd visceral cleft.

Fig. 35. Showing on the right the ear and post-branchial branch of the
7th nerve; on the left the ganglion and post-branchial branch of the
9th nerve.

Fies. 36—38.—Transverse sections through the head end of an embryo, to
show the relation of the pituitary body to the fore-gut and infundibulum.

Fig. 36. Showing the fusion of the posterior face of the pituitary body
with the wall of the fore-gut. It also shows the ear and the ventral
fusion of the 7th nerve with the epiblast of the dorsal wall of the 1st
visceral cleft.

Fig. 37. Slightly anterior to the preceding, showing the pituitary body in
close contact with the wall of the infundibulum. It also shows on the
left side the ear, the ganglion of the 7th nerve, and the ventral fusion
of the nerve with the epiblast.

Fig. 38. Showing the free tip of the pituitary body in close contact with
the wall of the infundibulum.

19

On Dinophilus Gigas.
By

W. F. R. Weldon, M.A.,
Fellow of St. John’s College, Cambridge; Lecturer on Invertebrate
Morphology to the University.

With Plate XXVII.

——

In the spring of last year Mr. Shipley brought to Cambridge
a few specimens of a Dinophilus, which he had found in
Mount’s Bay, near Penzance. These he was kind enough to
place at my disposal; and in April last I was able myself to
procure a larger number of specimens from the same locality.

The animals were found in considerable numbers on red
seaweeds, &c., in pools, near spring-tide low water mark, on
the rocks to the west of St. Michael’s Mount. The weeds were
placed in shallow white basins, with plenty of sea-water, for
from twelve to twenty-four hours, when the Dinophilus left the
weed, and were easily seen against the white wall of the vessel,
on the side turned towards the light.

The length of the body varied greatly, the smallest specimens
found being about 0°75 mm., while the largest were nearly two
millimetres in length. The colour was a brilliant orange, uni-
formly distributed in granules through the skin, and more
intensely developed in the stomach.

The body consists of a head or pre-oral lobe, seven post-
oral segments, and a ventral unsegmented tail.

The head is somewhat broader than the segment immediately
behind it ; its form is that of a truncated cone, and it is covered

ON DINOPHILUS GIGAS. ~ 259

with fine cilia, and with stiff sense hairs, the latter being
especially prominent in a pair of patches at the anterior end
(fig. 1, s. 2.). On the dorsal aspect of the head are two bright
red, kidney-shaped eyes, A small pair of ciliated pits, such as
are described by Korschelt, M‘Intosh, and Hallez was observed
(fig. 1, ¢. p.).

The second segment bears on its ventral surface the mouth,
which is an elongated slit, bounded by a number of slight folds,
which are richly ciliated.

The six following segments are tolerably uniform in diameter,
. each in the extended condition being slightly dilated in the
centre, and separated from its neighbour by an exceedingly
shallow constriction.

Behind the last segment the body narrows suddenly, forming
the tail.

The “ segmentation” of the body is only conspicuous in the
fully extended condition. By contraction the whole of the
dorsal and ventral surfaces become uniform, and the very
slightest indication on the sides alone remains to indicate the
series of swellings and constrictions referred to. Fig. 2, drawn
from a specimen which had contracted under the influence of
corrosive sublimate, but which was not in any way otherwise
distorted, shows this.!

The pre-oral lobe, the ventral surface of the body, and the
tail are uniformly covered with short vibratile cilia, and in each
segment the cilia are continued into a band which surrounds
the animal, while behind the cilia of each segmental ring is a
circlet of fine sense hairs (fig. 1, s. #.). Sensory hairs were
also specially conspicuous on the tail.

The pigment granules and numerous oil-globules in the skin
rendered the creature so opaque that little could be made out
in the living state, except the outline of the highly-coloured
stomach (fig. 1, s¢.) and the mouth (™.).

The three species of Dinophilus, possessing a brilliant yellow
pigment, which have hitherto been described, are D. vorti-

1 Only six post-oral ciliated rings are visible in this figure. I have noticed
the absence of the seventh in one or two preserved specimens.

260 WwW. FE. BR. WELDON.

coides (= D. capitata), D. metameroides, and D. cau-
data. From each of these the Cornish species differs in some
respect. From D. vorticoides it is distinguished by the
absence of a general coating of cilia on the dorsal surface, and
by the presence of definite “‘ segmental” ciliated bands ; from
D. metameroides it differs in the entirely superficial nature
of the apparent “segmentation,” adjacent segments not being
separated by infoldings of the body wall.

T have not been able to consult Levinsen’s recent description
of D. caudata,! but, so far as I can gather, it resembles D.
vorticoides rather than the present form.

It will be seen from what follows that a further character is
presented by the present species, which has not been recognised
in others—the possession of a well-marked nervous system. In
the absence of any detailed information as to the structure of
other forms it would be premature to regard this as a specific
character, but even without it there seems to be sufficient
warrant for establishing a new species, which I propose to
call D. gigas, from the large size of the sexually mature
individuals.

II.—Anaromy.

In its general structure D. gigas agrees closely with the
D. apatris of Korschelt, differing from it chiefly in the pre-
sence of a nervous system and in the histological structure
of the ectoderm. The paper of Korschelt? is so complete, and
contains so full an account of the previous observations on the
genus, that it is unnecessary to do more than refer the reader
to it before passing on to a detailed description of the present
species.

The ectoderm, as has already been seen, varies in charac-
ter in different parts of the body. In the head a transverse
section (fig. 3) shows a well-marked difference between the
dorsal and ventral portions. On the ventral side are seen cells

1 «Viddensk. Meddel. fra den naturh. Foren. in Kjébenhavn,’ 1879-80,
2 « Zeitschr. f. w. Zoologie,’ Bd. xxxvii, Hft. 3.

ON DINOPHILUS GIGAS. 261

of three kinds ; the most numerous (fig. 3, gr.) are columnar,
staining moderately deeply, and crowded with granules;
wedged in between these are certain cells, the peripheral ex-
tremities of which are conical (m. ep.), but which send inwards
fine processes, some of which are probably muscular, while
others are nervous. The cells of the third kind (fig. 3, x) are
pale, with deeply staining nuclei. Immediately below the
ectoderm, on the ventral side, is a delicate layer of transverse
muscles (7. m.), the fibres of which are, I believe, continuous
with many of the processes of the cells marked m. ep., though
this connection is not so easily seen in the head as it is in the
trunk (cf. fig. 10).

The dorsal ectoderm of the head is composed of an indiffer-
ent epithelium several cells thick (cf. fig. 3, where, however,
the curvature of the head has caused this portion to be cut
tangentially, so that the thickness of the ectoderm appears too
great).

Passing backwards, the dorsal and lateral surfaces of the
body are uniformly covered, between the head and the anus,
with a more or less cylindrical epithelium, one cell thick (ef.
figs. 4, 5, 6, 8), which is ciliated only in the region of the trans-
verse rings already referred to.

The ventral ectoderm, in the region of the mouth and lips,
is a simple columnar epithelium with narrow, elongated cells
(figs. 4—6), but behind the mouth, on the whole ventral sur-
face of the trunk, it has much the same structure as op the
corresponding side of the head. The myo-epithelial cells, with
their processes, are, however, much better marked (figs. 8 and
10, m. ep.), and their connection with the circular muscles is
more easily seen (fig. 10), while the cells lying between them
are all of one kind—large, finely granular, and paler, with
rounded nuclei (figs. 8 and 10, gr.). In the figures the whole
of the conical ectoderm elements with processes are labelled
m. ep.; it is, however, obviously probable that many are
nervous in nature.

In the tail the ectoderm is throughout of the same charac-
ter as that on the ventral side of the trunk, except that the

262 W. F. R. WELDON.

granular interstitial cells are replaced by elements secreting a
more or less sticky mucus. By means of this secretion the
animal can attach itself with some degree of firmness to foreign
objects.

Closely attached to the ectoderm is the central nervous
system, which consists of a brain and a pair of lateral ventral
nerve-cords,

The brain (fig. 3, 2. f. +n. ¢.) entirely fills the pre-oral
lobe. It consists of a central mass of nerve-fibres (x. f.) sur-
rounded by ganglion cells (n. c.). Embedded in its substance
are the two eyes (x), each consisting of one or two cells loaded
with granules of deep red pigment, surmounted by a small
cuticular lens.

The lateral nerve-cords (figs. 4, 5, 6, 8) are everywhere
in close contact with the skin. Large anteriorly, they grow
gradually smaller in passing backwards (cf. figs. 4 and 8) till
in the last segment they altogether disappear. Hach cord con-
sists of a mass of fibres (fig. 4, ». f.), which is in the anterior
region more or less completely separated from the skin by
nerve-cells (n. ¢.); in passing backwards, however, the nerve-
cells almost entirely disappear, and it is to this that the dimi-
nution in size of the cord is chiefly due.

No trace of commissures between the cords, nor of any
branches, could be found, though the presence of well-deve-
loped regions of sense hairs, already referred to, makes it cer-
tain that some kind of peripheral nervous plexus exists.

Just above the nerve-cords, throughout the whole length of
the trunk, runs a small bundle of longitudinal muscle-fibres
(1. m.). These, and the ventral circular fibres already men-
tioned, are the only traces of a muscular system which could
be found. The walls of the alimentary canal, except a small
part of the pharynx, and apparently the whole dorsal region
of the body, are entirely destitute of muscles.

The space between the body wall and the alimentary canal is
everywhere traversed by strands of connective tissue, which
forms a network with large spaces between the meshes. There
is no trace of an epithelial boundary to the spaces thus formed,

ON DINOPHILUS GIGAS, 263

neither is there any sign of a division of the cavity by trans-
verse septa.

In certain of the connective-tissue cells which thus traverse
the body cavity are “ flame cells” belonging to an excretory
system of the ordinary platyelminth type. The granular and
opaque character of the ectoderm made it extremely difficult to
observe these organs in the living animal, and I did not succeed
in finding them in sections. I can only say that there is cer-
tainly a group of “flame cells” at the points marked ne. in
fig. 1.

The alimentary canal presents all the well-known
characters distinctive of the genus. The mouth (fig. 1, m) is
an elongated slit bounded by curved, ciliated lips. It leads
into an upwardly-directed pharynx, which communicates ante-
riorly by a narrow opening with the cesophagus. The ceso-
phagus itself passes horizontally backwards. The section
represented in fig. 4 is taken immediately behind the point of
communication between these two structures, so that the ceso-
phagus (@.) is here entirely shut off from the pharynx (v. ph.).
The pharynx itself is seen to be a bounded vertical wall, com-
posed of pale, columnar, ciliated cells ; outside these lie masses
of gland-cells (m. g.), which are in places closely attached to
the pharyngeal epithelium; other similar gland-cells (e. gi.
lie at the base of the ectoderm of the lip.

A section or two further backwards (fig. 5) the pharynx is
seen to be composed of two portions—a main vertical portion,
the same as that seen in front, and a horizontal portion (h. ph.),
in the form of a lateral pouch on each side. In this, as in the
preceding section, groups of gland-cells are seen, attached both
to the pharynx and to the cesophagus.

Passing on to the region behind the mouth, the epithelium
of the vertical portion of the pharynx becomes darker and
streaked with bands of mucus thrown into it by the glands,
which still surround it (fig. 6). The ventral pouches have now
united to form a horizontal limb below the main body of the
organ, so that its lumen becomes L-shaped. Finally, still
further backwards, the vertical portion ends in a large muscular

264 WwW. F. R. WELDON.

bulb (fig. 7, m. ph.), lying ventral to the commencing stomach,
while the horizontal portion closes and in section disappears.

From a consideration of these sections, and from the dia-
grammatic longitudinal section given in fig. 11, it is obvious.
that the pharynx of this Dinophilus has the same structure as
that described by Korschelt, Hallez, and others, in the better
known species of the genus.

I have, however, been unable to make the animal evert its
pharynx, as some species are said to do. Irritation with fresh
water, acetic acid, &c., or stimulation by pressing the cover-
slip, were equally useless in this respect. Further, in no case
did my preserved specimens evert the pharynx in dying.

The cesophagus has already been seen; it is a narrow tube
lined by a ciliated epithelium (figs. 4—6), which opens, at
about the beginning of the second segment, into the large,
wide stomach (figs. 1, 2, and 8, s¢.), distinguished by its wide
lumen and its granular, brilliantly pigmented epithelium. The
cilia of the stomach are very long, and during life their action
produces a most violent agitation of the contents of the organ.

In the sixth segment the stomach bears on its ventral side
a small pyloric opening (fig. 9), leading into an intestine,
which is also ciliated. The stomach is prolonged, as a kind of
cecum, for a short distance behind the pylorus. The intestine
passes backwards through the seventh segment, diminishing
gradually in diameter, till at last it narrows suddenly and opens
to the exterior in the dorsal middle line.

The reproductive organs are in both sexes similar to those
described by Korschelt! in the female of D. apatris; that is,
they each consist of a Y-shaped mass of cells, the anterior
limbs of which lie under the posterior half of the stomach
(fig. 8, T,), while the posterior unpaired limb lies under the
intestine, or else, as is more generally the case (fig. 9, me.),
pushes this latter organ to oneside. The two sexes are similar
externally, until the ripening of the reproductive cells renders
the ova or spermatozoa distinguishable through the skin. At
the time of sexual maturity the gonads enlarge, so as to com-

* Loe. cit.

ON DINOPHILUS GIGAS. 265

pletely fill the body cavity, the alimentary canal becomes much
reduced in size, and it and the ectoderm appear to undergo a
kind of fatty degeneration. I could find no ducts of any kind
for the generative products, and from the condition of the
tissues of ripe individuals, I have no doubt that, when the
generative products are mature, the animals rupture their body
wall and die. Ifthis be true, it explains the sudden disap-
pearance of Dinophilus at the end of spring, which has been
noticed by Hallez! and others. In the case of D. gigas,
all the individuals collected at Mount’s Bay on April 22nd had
undergone so much degeneration that they were quite useless
for histological purposes, while the absolute number of indi-
viduals collected between the 16th and 28rd of April was so
small compared with the number obtained in the same time a
fortnight earlier, as to show that the process of disappearance
was beginning.

III.—On tue Systematic Posrtion or DINoPHILUS.

It is hardly necessary to indicate the points of resemblance
between Dinophilus and a fairly late Chetopod larva. The
ciliated rings and the ventral plate of ciliated ectoderm, asso-
ciated with a pair of unsegmented lateral nerve-cords; the ciliated
alimentary canal, with its large stomach, its narrow cesophagus
with a muscular pharynx, and its intestine; these are features
in which all species agree with a late Polygordius larva, while
in D. gyrociliatus Ed. Meyer finds that the excretory sys-
tem is “almost identical with that of a Nereis larva.”’* The only
point of difference between Dinophilus and the Archiannelids
is the absence of an epithelial body cavity, and this character,
in spite of the importance given to it by many observers,
seems to be, in this case at least, of secondary importance.
For in the first place the body cavity of Saccocirrus seems to
be devoid of any definite epithelium ;? while in the second place

1 ‘Histoire naturelle des Turbellariés,’ Lille, 1879.

2 Quoted by Lang, ‘Monographie der Polycladen,’ p. 679.

3 Compare the figures given by Fraipont, ‘ Archives de Biologie,’ Tome v,
Pl. xiv, which are confirmed by sections in the Cambridge Laboratory.

266 WwW. F. 8. WELDON.

the head cavity of Criodrilus and of many Polychets is, at
an early stage,! exactly in the condition which is permanent in
Dinophilus; it is a cavity, not bounded by any definite
“coelomic” epithelium, but traversed by mesodermic fibres,
which form a plexus running through it.

From these considerations it may plausibly be argued that
we have in Dinophilus a form representing in its main features
a stage in the evolution of Chetopods which is in the existing
members of that group repeated only in the larval condition—
a form in which the only archiannelid character which is not
developed is the epithelial and segmented character of the
body cavity.

That the epithelial character of the body cavity may be
acquired within the limits of a group, Saccocirrus, as already
pointed out, seems to prove; while the acquisition of segmen-
tation is well seen in the various species of Dinophilus itself.
Thus, in D. vorticoides? we find the whole body unseg-
mented, with a uniform covering of cilia; in D. apatris® we
have an external segmentation which is not shared by the ex-
cretory system; while in D. gyrociliatus we find the ne-
phridia composed of “simple, intracellular, segmental organs,
terminating in flame cells;’* and lastly, in D. metame-
roides we have the appearance of a commencing segmenta-
tation of the body cavity.®

But the anatomy of Dinophilus seems to show that from its
near connection with the Trochozoon ° it is related to other
forms besides Cheetopods. The pharynx seems especially to
show this. Comparing the longitudinal section (fig. 11) with
a similar section through the pharynx of Histriobdella
(fig. 18) we see that the pharyngeal apparatus is obviously

1 Cf. Hatschek, “Stud. tb. Entw. d. Anneliden,” ‘Arb. a. d. Zool. Inst.
Wien,’ 1878, and others.

2 KE. van Beneden, ‘ Bull. Acad. Roy. Belg.,’ Tome xviii.

3 Korschelf, loc. cit.

4 Hd. Meyer, quoted by Lang, loc. cit.

5 Hrallez, loc. cit.

6 L use this term to imply simply the type, whatever that may have been,
which is now ontogenetically represented by the trochospheres.

ON DINOPHILUS GIGAS. 267

homologous in the two cases. But the pharyngeal appendix
of Histriobdella carries three chitinous teeth, showing that this
organ may in some cases develope skeletal structures; and
when once this is ascertained the resemblance to the Molluscan
odontophore becomes obvious. Further, in Terebella, and
other Polychcets, the pharyngeal armature is developed from a
ventral and posterior diverticulum of the stomodeum
(fig. 14), which is apparently homologous with the correspond-
ing diverticulum of the Archiannelid pharynx. The wide dis-
tribution which some organ of this kind had among the Tro-
chozoa is evident from its persistence in the larve of such
creatures as Sipunculus and many others.

It sems, therefore, legitimate to conclude that in the pha-
ryngeal appendix of Dinophilus and the Archiannelids we have
a persistent record of some ancestral organ from which deve-
loped the stomodeal armature of least the Molluscs and
Cheetopods, and probably also of Rotifers and Crustacea,

As for the derivation of Dinophilus and the forms which it
represents from simpler types, there are, as Korschelt has
already pointed out, many features which connect it with the
Rhabdoccel Turbellarians. The body cavity and excretory
system especially are in exactly the same condition as those of
a Rhabdocel with well-developed coelomic spaces, such, for
example, as Mesostoma.

It is commonly stated that myo-epithelial cells are absent
from the ectoderm of Rhabdoceels, and that the muscle-fibres
are in this group devoid of nuclei. I hope, however, shortly
to show that, in Convoluta at least, certain of the ectoderm
cells have a structure practically identical with that just
described in Dinophilus.

The only characters of importance which separate Dino-
philus from the Rhabdoccels are, the possession of an anus,
and the metameric repetition of ciliated bands. Of these, the
second may very possibly have arisen within the limits of the
genus, since D. vorticoides is uniformly ciliated; but in
any case we havein Allostoma’ a precisely similar formation

) Graff, ‘Monographie der Turbellarien,’ Bd. i, Taf. 19.

268 WwW. F. BR. WELDON.

of a single ciliated ring in an undoubted Rhabdocel. The
assumption of a pelagic life might easily cause in any Rhab-
doccel a hypertrophy of the cilia in certain definite regions and
the consequent appearance of ciliated bands; and it seems safe
to predict that a more thorough investigation of the pelagic
inhabitants of those warm seas which are most favorable to
the development of surface faunas will reveal the existence of
genera in which this character has been developed.

The researches of Lang on Oligocladus and Cyclo-
porus! have shown that at least in Polyclads there is no dif_i-
culty in the temporary establishment of an anus in any region
of the body, and when this is once recognised the passage from
a temporary to a permanent condition is easy.

The pharynx of Dinophilus and of the lower Chzetopods
offers another strong proof of Turbellarian affinities. On com-
paring the diagrams given in figs. 11 to 16 we see that the
stomodeum of Dinophilus, Polygordius, and Histriob-
della possesses a posterior muscular thickening lying in the
wall of a lateral outgrowth from the pharynx, which is in all
cases conceivably, and in Dinophilus certainly, eversible. In
the embryo Terebella (fig. 14) a similar posterior outgrowth
from the stomodzum exists, which subsequently? envelopes
the whole circumference of the pharynx, and constitutes the
rudiment of the pharyngeal armature. In Nais (fig. 15) we
have a similar muscular thickening on the anterior wall of
the stomodzeum.

These facts receive at least a plausible explanation, if we
suppose that the various forms of pharyngeal apparatus just
mentioned are derived from a structure which primitively sur-
rounded the whole organ, persistence in the posterior region
only being in such forms as Polygordius, perhaps associated
with the filling up of the pre-oral lobe by the brain, while the
existence of an elongated proboscidiform prostomium in Nais
renders it most convenient to preserve the musculature in
front. But such a circumcesophageal apparatus as is here in-

1 Lang, op. cit., pp. 155, et seq.
? Salensky, ‘ Archives de Biologie,’ t. iv.

ON DINOPHILUS GIGAS. 269

dicated is exactly furnished by the Rhabdoccl pharynx
(fig. 16).

We seem, therefore, to have in Dinophilus a form which,
related on the one hand to the Archiannelids, retains on the
other many features characteristic of the ancestor common to
those groups (especially Chztopods, Gephyreans, Mollusca,
Rotifers, and Crustacea) which possess a more or less modified
trochosphere larva ; and of these the relations of the body cavity,
of the excretory system, and of the pharynx, seem to point
unmistakeably to a Turbellarian origin.

270 W. F. R. WELDON.

EXPLANATION OF PLATE XXVII,

Illustrating Mr. W. F. R. Weldon’s Paper ona “Species of
Dinophilus Gigas.”

List of Reference Letters.

an, Anus. c.p. Cephalic ciliated pits. ci. Transverse ciliated bands.
E. Hye. ¢. gl. Gland cells of lips. gr. Granular cells of ectoderm. 4. ph.
Horizontal diverticulum of pharynx. Jz. Intestine. /.m. Longitudinal muscle-
fibres. J. Mouth. m. ph. Muscular appendix of pharynx. m. ep. Myo-
epithelial cells of ectoderm. Je. Median lobe of gonad. ze. Position of
observed nephridia. 2.7. Nerve-fibres. .g. Nerve-cells. 2.7. Lateral
nerve-cord. @ Cisophagus. +7. m. Circular muscles. s¢. Stomach. s. 4.
Cephalic sense hairs. s4'. Post-cephalic rings of sense hairs. 2. Deep cells
of cephalic ectoderm. Br. Brain. S¢. Stomodeal musculature.

Fies. 1—10.—Dinophilus gigas.

Fig. 1. The live animul extended, seen by transmitted light.

Fig. 2. A specimen contracted by treatment with corrosive sublimate
solution, but not otherwise distorted. This figure shows fairly well the
shape assumed on irritation by the live creature.

Fig. 3. A transverse section through the pre-oral lobe.

Figs. 4—6. Transverse sections through the pharyngeal region.

Fig. 7. The muscular bulb of the pharynx, in transverse section.

Fig. 8. Section through the middle of the trunk.

Fig. 9. Section through junction of stomach and intestine.

Fig. 10. Section of ventral ectoderm. Zeiss’s im., oc. 2.

Fires. 11—16.—Diagrams of various forms of pharyngeal apparatus, as
seen in longitudinal sections of the head.

Fig. 11. Dinophilus (original),

Fig. 12. Polygordius (schematised from Uhljanin).

Fig. 18. Histriobdella (schematised from Foettinger).

Fig. 14. Terebella larva (schematised from Saleusky).

Fig. 15. Navis (schematised from Vejdovsky).

Fig. 16, Vortex (schematised from von Graff ).

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STUDIES

FROM THE

MORPHOLOGICAL LABORATORY

IN THE

UNIVERSITY OF CAMBRIDGE.

EDITED BY

ADAM SEDGWICK, M.A., F.RS.

FELLOW AND LECTURER OF TRINITY COLLEGE, CAMBRIDGE.

Vol. Il.

London:

Cc. J. CLAY AND SONS,
CAMBRIDGE UNIVERSITY PRESS WAREHOUSE,
AVE MARIA LANE.

1886

9705
“tp

A. oats 12.

THE last series of Studies which issued from
the Morphological Laboratory of Cambridge was
published under the superintendence of the late
Professor Francis M. Balfour.

To his original impulse and to the inspiring
influence of his memory is due whatever merit the
present series may possess.

CONTENTS.

PAGE

W. F. R. Wexpon.—Note on the early development of Lacerta
wouralis. Plates I. II. and III. . - _ . : 7 ,

AxicE JoHNsoN.—On the development of the pelvic girdle and hind

limb in the Chick. Plates IV. and V. 13
W. Huapz.—The development of the Mole (Zalpa Europea). The
formation of the germinal layers, and early development of the
medullary groove and notochord. With Plates VI. VII. VIIL re
and IX. : ; e F ; : : : d . F §

A. Sepe@wick.—On the origin of metameric segmentation and-some
other morphological questions. With Plates X.and XI. . 3 ET,

W. F. R. We,pon.—On the head kidney of Bdellostoma, with a sug-
gestion as to the origin of the suprarenal bodies. With Plate XII]. 119

W. Bareson.—The early stages in the development of Balanoglossus

(sp. incert.). With Plates XIII. XIV. XV. and XVI. : . 131
+A. SEDGWwiIcK.—On the original function of the canal of the central
nervous system of Vertebrata. . . . ‘ é : . 160

Anice Jounson. On the fate of the blastopore and the presence of
a primitive streak in the Newt (Triton cristatus). Plate XVII. 165

W. F. R. Weipon. On the suprarenal bodies of vertebrata. Plates

XVIII. and XIX. 179
F. G. Hearacore. On a peculiar sense organ in Scutigera coleo-

ptrata. Plate XX. . : ‘ r 193
Water Hearn. The development of the Mole (Talpa Huropea),

the ovarum ovum and the segmentation of the ovum. Plate

XXI. . 201
I. G. Hearncorz. The early development of Julus terrestris.

Plates XXII. and XXIII. . : ; F , , ; . 219
Atice Jounson anp Litian SHELpon. Notes on the development

of the Newt. Plates XXIV. XXV. XXVL ‘ ; : . 241
W. F.R. Wenpoyn. On Dinophilus gigas. Plate XXVIT. : . 258

+ Reprinted from the Proceedings of the Cambridge Philosophical Societ Y.

_ All the above are reprints from the Quarterly Journal of Microscopical Science
with the exception indicated.

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