be considered as an aggregation of the cells of the single-layered
ovarian epithelium which have grown into the stroma, and of which one
cell has become the ovum and the others the follicular epithelium. The
follicle, however, draws in with it into the stroma a number of
additional epithelial cells in the form of a stalk connecting the
follicle with the superficial epithelium. At a later period the lower
part of the stalk at its junction with the follicle becomes
continuously narrowed, and at the same time a rupture takes place in
the cells which form it. In this manner the follicle becomes at last
constricted off from the stalk, and so from its place of origin in the
superficial epithelium, and subsequently lies freely in the stroma of
the ovary."
Footnote 399: _Loc. cit._
He further explains that the separation of the follicles from the
epithelium takes place much earlier in Acanthias than in Raja, and that the
sinkings of the epithelium into the stroma may have two or three branches
each with a follicle.
Semper gives very little information with reference to the post-embryonic
formation of ova. He expresses his agreement on the whole with Ludwig, but,
amongst points not mentioned by Ludwig, calls attention to peculiar
aggregations of primitive ova in the superficial epithelium, which he
regards as either rudimentary testicular follicles or as nests similar to
those in the embryo.
My observations on this subject do not agree very closely with those either
of Ludwig or Semper. The differences between us partly, though not
entirely, depend upon the fundamentally different views we hold about the
constitution of the ovary and the nature of the epithelium covering it
(vide pp. 555 and 556).
In very young ovaries (Pl. 24, fig. 8) nests of ova (in my sense of the
term) are very numerous, but though usually superficial in position are
also found in the deeper layers of the ovary. They are especially
concentrated in their old position, close to the dorsal edge of the organ.
In some instances they do not present quite the same appearance as in the
embryo, owing to the outlines of the ova composing them being distinct, and
to the presence between the ova of numerous interstitial cells derived from
the germinal epithelium, and destined to become follicular epithelium.
These latter cells at first form a much flatter follicular epithelium than
in the embryonic periods, so that the smaller adult ova have a much less
columnar investment than ova of the same size in the embryo. A few
primitive ova may still be found in a very superficial position, but
occasionally also in the deeper layers. I am inclined to agree with Semper
that some of these are freshly formed from the cells of the germinal
epithelium.
In the young female with ova of about 0.5 mm. nests of ova are still fairly
numerous. The nests are characteristic, and present the various remarkable
peculiarities already described in the embryo. In many instances they form
polynuclear masses, not divided into separate cells, generally, however,
the individual ova are distinct. The ova in these nests are on the average
rather smaller than during the embryonic periods. The nests are frequently
quite superficial and at times continuous with the pseudo-epithelium, and
individual ova also occasionally occupy a position in the superficial
epithelium. Some of the appearances presented by separate ova are not
unlike the figures of Ludwig, but a growth such as he describes has,
according to my observations, no existence. The columns which he believes
to have grown into the stroma are merely trabeculæ connecting the deeper
and more superficial parts of the germinal epithelium; and his whole view
about the formation of the follicular epithelium round separate ova
certainly does not apply, except in rare cases, to Scyllium. It is, indeed,
very easy to see that most freshly formed ova are derived from nests, as in
the embryo; and the formation of a follicular epithelium round these ova
takes place as they become separated from the nests. A few solitary ova,
which have never formed part of a nest, seem to be formed in this stage as
in the embryo; but they do not grow into the stroma surrounded by the cells
of the pseudo-epithelium, and only as they reach a not inconsiderable size
is a definite follicular epithelium formed around them. The follicular
epithelium, though not always formed from the pseudo-epithelium, is of
course always composed of cells derived from the germinal epithelium.
In all the ova formed at this stage the nucleus would seem to pass through
the same metamorphosis as in the embryo.
In the later stages, and even in the full-grown female of Scyllium, fresh
ova seemed to be formed and nests also to be present. In Raja I have not
found freshly formed ova or nests in the adult, and have had no opportunity
of studying the young forms.
_Summary of observations on the development of the ovary in Scyllium and
Raja._
(1) The ovary in the embryo is a ridge, triangular in section, attached
along the base. It is formed of a core of stroma and a covering of
epithelium. A special thickening of the epithelium on the outer side forms
the true germinal epithelium, to which the ova are confined (Pl. 24,
fig. 1). In the development of the ovary the stroma becomes differentiated
into an external vascular layer, especially developed in the neighbourhood
of the germinal epithelium, and an internal lymphatic portion, which forms
the main mass of the ovarian ridge (Pl. 24, figs. 2, 3, and 6).
(2) At first the thickened germinal epithelium is sharply separated by a
membrane from the subjacent stroma (Pl. 24, figs. 1, 2, and 3), but at
about the time when the follicular epithelium commences to be formed round
the ova, numerous strands of stroma grow into the epithelium, and form a
regular network of vascular channels throughout it, and partially isolate
individual ova (Pl. 24, figs. 7 and 8). At the same time the surface of the
epithelium turned towards the stroma becomes irregular (Pl. 24, fig. 9),
owing to the development of individual ova. In still later stages the
stroma ingrowths form a more or less definite tunic close to the surface of
the ovary. External to this tunic is the superficial layer of the germinal
epithelium, which forms what has been spoken of as the pseudo-epithelium.
In many instances the protoplasm of its cells is produced into peculiar
fibrous tails which pass into the tunic below.
(3) _Primitive ova._--Certain cells in the epithelium lining the dorsal
angle of the body-cavity become distinguished as primitive ova by their
abundant protoplasm and granular nuclei, at a very early period in
development, even before the formation of the genital ridges. Subsequently
on the formation of the genital ridges these ova become confined to the
thickened germinal epithelium on the outer aspect of the ridges (Pl. 24,
fig. 1).
(4) _Conversion of primitive ova into permanent ova._--Primitive ova may in
Scyllium become transformed into permanent ova in two ways--the difference
between the two ways being, however, of secondary importance.
(_a_) A nest of primitive ova makes its appearance, either by continued
division of a single primitive ovum or otherwise. The bodies of all the ova
of the nest fuse together, and a polynuclear mass is formed, which
increases in size concomitantly with the division of its nuclei. The
nuclei, moreover, pass through a series of transformations. They increase
in size and form delicate vesicles filled with a clear fluid, but contain
close to one side a granular mass which stains very deeply with colouring
reagents. The granular mass becomes somewhat stellate, and finally assumes
a reticulate form with one more highly refracting nucleoli at the nodal
points of the reticulum. When a nucleus has reached this condition the
protoplasm around it has become slightly granular, and with the enclosed
nucleus is segmented off from the nest as a special cell--a permanent ovum
(figs. 13, 14, 15, 16). Not all the nuclei in a nest undergo the whole of
the above changes; certain of them, on the contrary, stop short in their
development, atrophy, and become employed as a kind of pabulum for the
remainder. Thus it happens that out of a large nest perhaps only two or
three permanent ova become developed.
(_b_) In the second mode of development of ova the nuclei and protoplasm
undergo the same changes as in the first mode; but the ova either remain
isolated and never form part of a nest, or form part of a nest in which no
fusion of the protoplasm takes place, and all the primitive ova develop
into permanent ova. Both the above modes of the formation continue through
a great part of life.
(5) _The follicle._--The cells of the germinal epithelium arrange
themselves as a layer around each ovum, almost immediately after its
separation from a nest, and so constitute a follicle. They are at first
flat, but soon become more columnar. In Scyllium they remain for a long
time uniform, but in large eggs they become arranged in two or three
layers, while at the same time some of them become large and flask-shaped,
and others small and oval (fig. 29). The flask-shaped cells have probably
an important function in the nutrition of the egg, and are arranged in a
fairly regular order amongst the smaller cells. Before the egg is quite
ripe both kinds of follicle cells undergo retrogressive changes (Pl. 25,
fig. 23).
In Raja a great irregularity in the follicle cells is observable at an
early stage, but as the ovum grows larger the cells gradually assume a
regular arrangement more or less similar to that in Scyllium (Pl. 25, figs.
30-33).
(6) _The egg membranes._--Two membranes are probably always present in
Elasmobranchii during some period of their growth. The first formed and
outer of these arises in some instances before the formation of the
follicular epithelium, and would seem to be of the nature of a vitelline
membrane. The inner one is the zona radiata with a typical radiately
striated structure. It is formed from the vitellus at a much later period
than the proper vitelline membrane. It is more developed in Scyllium than
in Raja, but atrophies early in both genera. By the time the ovum is nearly
ripe both membranes are very much reduced, and when the egg (in Scyllium
and Pristiurus) is laid, no trace of any membrane is visible.
(7) _The vitellus._--The vitellus is at first faintly granular, but at a
later period exhibits a very distinct (protoplasmic) network of fibres,
which is still present after the ovum has been laid.
The yolk arises, in the manner described by Gegenbaur, in ova of about
0.06 mm. as a layer of fine granules, which stain deeply with osmic acid.
They are at first confined to a stratum of protoplasm slightly below the
surface of the ovum, and are most numerous at the pole furthest removed
from the germinal vesicle. They are not regularly distributed, but are
aggregated in small masses. They gradually grow into vesicles, in the
interior of which oval solid bodies are developed, which form the permanent
yolk-spheres. These oval bodies in the later stages exhibit a remarkable
segmentation into plates, which gives them a peculiar appearance of
transverse striation.
Certain bodies of unknown function are occasionally met with in the
vitellus, of which the most remarkable are those figured at _x_ on Pl. 25,
fig. 25A.
(8) _The germinal vesicle._--A reticulum is very conspicuous in the
germinal vesicle in the freshly formed ova, but becomes much less so in
older ova, and assumes, moreover, a granular appearance. At first one to
three nucleoli are present, but they gradually increase in number as the
germinal vesicle grows older, and are frequently situated in close
proximity to the membrane.
THE MAMMALIAN OVARY (Pl. 26).
The literature of the mammalian ovary has been so often dealt with that it
may be passed over with only a few words. The papers which especially call
for notice are those of Pflüger[400], Ed. van Beneden[401], and especially
Waldeyer[402], as inaugurating the newer view on the nature of the ovary,
and development of the ova; and of Foulis[403] and Kölliker[404], as
representing the most recent utterances on the subject. There are, of
course, many points in these papers which are touched on in the sequel, but
I may more especially here call attention to the fact that I have been able
to confirm van Beneden's statement as to the existence of polynuclear
protoplasmic masses. I have found them, however, by no means universal or
primitive; and I cannot agree in a general way with van Beneden's account
of their occurrence. I have found no trace of a germogene (Keimfache) in
the sense of Pflüger and Ed. van Beneden. My own results are most in
accordance with those of Waldeyer, with whom I agree in the fundamental
propositions that both ovum and follicular epithelium are derived from the
germinal epithelium, but I cannot accept his views of the relation of the
stroma to the germinal epithelium.
Footnote 400: _Die Eierstöcke d. Säugethiere u. d. Menschen_,
Leipzig, 1863.
Footnote 401: "Composition et Signification de l'oeuf," _Acad.
r. de Belgique_, 1868.
Footnote 402: _Eierstock u. Ei._ Leipzig, 1870.
Footnote 403: _Trans. of Royal Society, Edinburgh_, Vol. XXVII.
1875, and _Quarterly Journal of Microscopical Science_,
Vol. XVI.
Footnote 404: _Verhandlung d. Phys. Med. Gesellschaft_,
Würzburg, 1875, N. F. Bd. VIII.
In the very interesting paper of Foulis, the conclusion is arrived at, that
while the ova are derived from the germinal epithelium, the cells of the
follicle originate from the ordinary connective tissue cells of the stroma.
Foulis regards the zona pellucida as a product of the ovum and not of the
follicle. To both of these views I shall return, and hope to be able to
shew that Foulis has not traced back the formation of the follicle through
a sufficient number of the earlier stages. It thus comes about that though
I fully recognise the accuracy of his figures, I am unable to admit his
conclusions. Kölliker's statements are again very different from those of
Foulis. He finds certain cords of cells in the hilus of the ovary, which he
believes to be derived from the Wolffian body, and has satisfied himself
that they are continuous with Pflüger's egg-tubes, and that they supply the
follicular epithelium. To the general accuracy of Kölliker's statements
with reference to the relations of these cords in the hilus of the ovary I
can fully testify, but am of opinion that he is entirely mistaken as to
their giving rise to the follicular epithelium, or having anything to do
with the ova. I hope to be able to give a fuller account of their origin
than he or other observers have done.
My investigations on the mammalian ovary have been made almost entirely on
the rabbit--the type of which it is most easy to procure a continuous
series of successive stages; but in a general way my conclusions have been
controlled and confirmed by observations on the cat, the dog, and the
sheep. My observations commence with an embryo of eighteen days. A
transverse section, slightly magnified, through the ovary at this stage, is
represented on Pl. 26, fig. 35, and a more highly magnified portion of the
same in fig. 35A. The ovary is a cylindrical ridge on the inner side of the
Wolffian body, composed of a superficial epithelium, the germinal
epithelium (_g.e._), and of a tissue internal to this, which forms the main
mass of it. In the latter two constituents have to be distinguished--(1) an
epithelial-like tissue (_t_), coloured brown, which forms the most
important element, and (2) vascular and stroma elements in this.
The germinal epithelium is a layer about 0.03 - 0.04 mm. in thickness. It
is (vide fig. 35A, _g.e._) composed of two or three layers of cells, with
granular nuclei, of which the outermost layer is more columnar than the
remainder, and has elongated rather than rounded nuclei. Its cells, though
they vary slightly in size, are all provided with a fair amount of
protoplasm, and cannot be divided (as in the case of the germinal
epithelium of Birds, Elasmobranchii, &c.), into primitive ova, and normal
epithelial cells. Very occasionally, however, a specially large cell,
which, perhaps, deserves the appellation primitive ovum, may be seen. From
the subjacent tissue the germinal epithelium is in most parts separated by
a membrane-like structure (fluid coagulum); but this is sometimes absent,
and it is then very difficult to determine with exactness the inner border
of the epithelium. The tissue (_t_), which forms the greater mass of the
ovary at this stage, is formed of solid columns or trabeculæ of
epithelial-like cells, which present a very striking resemblance in size
and character to the cells of the germinal epithelium. The protoplasm of
these cells stains slightly more deeply with osmic acid than does that of
the cells of the germinal epithelium, so that it is rather easier to note a
difference between the two tissues in osmic acid than in picric acid
specimens. This tissue approaches very closely, and is in many parts in
actual contact with the germinal epithelium. Between the columns of it are
numerous vascular channels (shewn diagrammatically in my figures) and a few
normal stroma cells. This remarkable tissue continues visible through the
whole course of the development of the ovary, till comparatively late in
life, and during all the earlier stages might easily be supposed to be
about to play some part in the development of the ova, or even to be part
of the germinal epithelium. It really, however, has nothing to do with the
development of the ova, as is easily demonstrated when the true ova begin
to be formed. In the later stages, as will be mentioned in the description
of those stages, it is separated from the germinal epithelium by a layer of
stroma; though at the two sides of the ovary it is, even in later stages,
sometimes in contact with the germinal epithelium.
In most parts this tissue is definitely confined within the limits of the
ovary, and does not extend into the mesentery by which the ovary is
attached. It may, however, be traced _at the anterior end_ of the ovary
into connection with the walls of the Malpighian bodies, which lie on the
inner side of the Wolffian body (vide fig. 35B), and I have no doubt that
it grows out from the walls of these bodies into the ovary. In the male it
appears to me to assist in forming, together with cells derived from the
germinal epithelium, the seminiferous tubules, the development of which is
already fairly advanced by this stage. I shall speak of it in the sequel as
tubuliferous tissue. The points of interest in connection with it concern
the male sex, which I hope to deal with in a future paper, but I have no
hesitation in identifying it with the segmental cords (_segmentalstränge_)
discovered by Braun in Reptilia, and described at length in his valuable
memoir on their urogenital system[405]. According to Braun the segmental
cords in Reptilia are buds from the outer walls of the Malpighian bodies.
The bud from each Malpighian body grows into the genital ridge before the
period of sexual differentiation, and sends out processes backwards and
forwards, which unite with the buds from the other Malpighian bodies. There
is thus formed a kind of trabecular work of tissue in the stroma of the
ovary, which in the Lacertilia comes into connection with the germinal
epithelium in both sexes, but in Ophidia in the male only. In the female,
in all cases, it gradually atrophies and finally vanishes, but in the male
there pass into it the primitive ova, and it eventually forms, with the
enclosed primitive ova, the tubuli seminiferi. From my own observations in
Reptilia I can fully confirm Braun's statements as to the entrance of the
primitive ova into this tissue in the male, and the conversion of it into
the tubuli seminiferi. The chief difference between Reptilia and Mammalia,
in reference to this tissue, appears to be that in Mammalia it arises only
from a few of the Malpighian bodies at the anterior extremity of the ovary,
but in Reptilia from all the Malpighian bodies adjoining the genital ridge.
More extended observations on Mammalia will perhaps shew that even this
difference does not hold good.
Footnote 405: _Arbeiten a. d. Zool.-zoot. Institut Würzburg_,
Bd. IV.
It is hardly to be supposed that this tissue, which is so conspicuous in
all young ovaries, has not been noticed before; but the notices of it are
not so numerous as I should have anticipated. His[406] states that the
parenchyma of the sexual glands undoubtedly arises from the Wolffian
canals, and adds that while the cortical layer (Hulle) represents the
earlier covering of a part of the Wolffian body, the stroma of the hilus,
with its vessels, arises from a Malpighian body. In spite of these
statements of His, I still doubt very much whether he has really observed
either the tissue I allude to or its mode of development. In any case he
gives no recognisable description or figure of it.
Footnote 406: _Archiv f. mikros. Anat._ Vol. I. p. 160.
Waldeyer[407] notices this tissue in the dog, cat, and calf. The following
is a free translation of what he says, (p. 141):--"In a full grown but
young dog, with numerous ripe follicles, there were present in the vascular
zone of the ovary numerous branched elongated small columns (Schläuche) of
epithelial cells, between which ran blood-vessels. They were only separated
from the egg columns of the cortical layer by a row of large follicles.
There can be no doubt that we have here remains of the sexual part of the
Wolffian body--the canals of the parovarium--which in the female sex have
developed themselves to an extraordinary extent into the stroma of the
sexual gland, and perhaps are even to be regarded as _homologues of the
seminiferous tubules_ (the italics are my own). I have almost always found
the above condition in the dog, only in old animals these seminiferous
canals seem gradually to atrophy. Similar columns are present in the cat,
only they do not appear to grow so far into the stroma." Identical
structures are also described in the calf.
Footnote 407: _Loc. cit._
Romiti gives a very similar description to Waldeyer of these bodies in the
dog[408]. Born also describes this tissue in young and embryonic ovaries of
the horse as the _Keimlager_[409]. The columns described by Kölliker[410]
and believed by him to furnish the follicular epithelium, are undoubtedly
my tubuliferous tissue, and, as Kölliker himself points out, are formed of
the same tissue as that described by Waldeyer.
Footnote 408: _Archiv f. mikr. Anat._ Vol. X.
Footnote 409: _Archiv f. Anatomie, Physiologie, u. Wiss.
Medicin._ 1874.
Footnote 410: _Loc. cit._
Egli gives a very clear and accurate description of this tissue, though he
apparently denies its relation with the Wolffian body.
My own interpretation of the tissue accords with that of Waldeyer. In
addition to the rabbit, I have observed it in the dog, cat, and sheep. In
all these forms I find that close to the attachment of the ovary, and
sometimes well within it, a fair number of distinct canals with a large
lumen are present, which are probably to be distinguished from the solid
epithelial columns. Such large canals are not as a rule present in the
rabbit. In the dog solid columns are present in the embryo, but later they
appear frequently to acquire a tubular form, and a lumen. Probably there
are great variations in the development of the tissue, since in the cat
(not as Waldeyer did in the dog) I have found it most developed.
In the very young embryonic ovary of the cat the columns are very small and
much branched. In later embryonic stages they are frequently elongated,
sometimes convoluted, and are very similar to the embryonic tubuli
seminiferi. In the young stages these columns are so similar to the egg
tubes (which agree more closely with Pflüger's type in the cat than in
other forms I have worked at) that to any one who had not studied the
development of the tissue an embryo cat's ovary at certain stages would be
a very puzzling object. I have, however, met with nothing in the cat or any
other form which supports Kölliker's views.
My next stage is that of a twenty-two days' embryo. Of this stage I have
given two figures corresponding to those of the earlier stage (figs. 36 and
36A).
From these figures it is at once obvious that the germinal epithelium has
very much increased in bulk. It has a thickness 0.1 - 0.09 mm. as compared
to 0.03 mm. in the earlier stage. Its inner outline is somewhat irregular,
and it is imperfectly divided into lobes, which form the commencement of
structures nearly equivalent to the nests of the Elasmobranch ovary. The
lobes _are not_ separated from each other by connective tissue
prolongations; the epithelium being at this stage perfectly free from any
ingrowths of stroma. The cells constituting the germinal epithelium have
much the same character as in the previous stage. They form an outer row of
columnar cells internal to which the cells are more rounded. Amongst them a
few large cells with granular nuclei, which are clearly primitive ova, may
now be seen, but by far the majority of the cells are fairly uniform in
size, and measure from 0.01 - 0.02 mm. in diameter, and their nuclei from
0.004 - 0.006 mm. The nuclei of the columnar outer cells measure about
0.008 mm. They are what would ordinarily be called granular, though high
powers shew that they have the usual nuclear network. There is no special
nucleolus. The rapid growth of the germinal epithelium is due to the
division of its cells, and great masses of these may frequently be seen to
be undergoing division at the same time. Of the tissue of the ovary
internal to the germinal epithelium, it may be noticed that the
tubuliferous tissue derived from the Malpighian bodies is no longer in
contact with the germinal epithelium, but that a layer of vascular stroma
is to a great extent interposed between the two. The vascular stroma of the
hilus has, moreover, greatly increased in quantity.
My next stage is that of a twenty-six days' embryo, but the characters of
the ovary at this stage so closely correspond with those of the succeeding
one at twenty-eight days that, for the sake of brevity, I pass over this
stage in silence.
Figs. 37 and 37A are representative sections of the ovary of the
twenty-eighth day corresponding with those of the earlier stages.
Great changes have become apparent in the constitution of the germinal
epithelium. The vascular stroma of the ovary has grown into the germinal
epithelium precisely as in Elasmobranchii. It appears to me clear that the
change in the relations between the stroma and epithelium is not due to a
mutual growth, but entirely to the stroma, so that, as in the case of
Elasmobranchii, the result of the ingrowth is that the germinal epithelium
is honeycombed by vascular stroma. The vascular growths generally take the
paths of the lines which separated the nests in an earlier condition, and
cause these nests to become the egg tubes of Pflüger. It is obvious in
figure 37 that the vascular ingrowths are so arranged as imperfectly to
divide the germinal epithelium into two layers separated by a space with
connective tissue and blood-vessels. The outer part is relatively thin, and
formed of a superficial row of columnar cells, and one or two rows of more
rounded cells; the inner layer is much thicker, and formed of large masses
of rounded cells. The two layers are connected together by numerous
trabeculæ, the stroma between which eventually gives rise to the connective
tissue capsule, or tunica albuginea, of the adult ovary.
The germinal epithelium is now about 0.19 to 0.22 mm. in thickness. Its
cells have undergone considerable changes. A fair number of them (fig. 37A,
_p.o._), especially in the outer layer of the epithelium, have become
larger than the cells around them, from which they are distinguished, not
only by their size, but by their granular nucleus and abundant protoplasm.
They are in fact undoubted primitive ova with all the characters which
primitive ova present in Elasmobranchii, Aves, &c. In a fairly typical
primitive ovum of this stage the body measures 0.02 mm. and the nucleus
0.014 mm. In the inner part of the germinal epithelium there are very few
or no cells which can be distinguished by their size as primitive ova, and
the cells themselves are of a fairly uniform size, though in this respect
there is perhaps a greater variation than might be gathered from fig. 37A.
The cells are on the average about 0.016 mm. in diameter, and their nuclei
about 0.008 to 0.001 mm., considerably larger, in fact, than in the earlier
stage. The nuclei are moreover more granular, and make in this respect an
approach to the character of the nuclei of primitive ova.
The germinal epithelium is still rapidly increasing by the division of its
cells, and in fig. 37A there are shewn two or three nuclei in the act of
dividing. I have represented fairly accurately the appearance they present
when examined with a moderately high magnifying power. With reference to
the stroma of the ovary, internal to the germinal epithelium, it is only
necessary to refer to fig. 37 to observe that the tubuliferous tissue (_t_)
forms a relatively smaller part of the stroma than in the previous stage,
and is also further removed from the germinal epithelium.
My next stage is that of a young rabbit two days after birth, but to
economise space I pass on at once to the following stage five days after
birth. This stage is in many respects a critical one for the ovary, and
therefore of great interest. Figure 38 represents a transverse section
through the ovary (on rather a smaller scale than the previous figures) and
shews the general relations of the tissues.
The germinal epithelium is very much thicker than before--about 0.38 mm. as
compared with 0.22 mm. It is divided into three obvious layers: (1) an
outer epithelial layer which corresponds with the pseudo-epithelial layer
of the Elasmobranch ovary, average thickness 0.03 mm. (2) A middle layer of
small nests, which corresponds with the middle vascular layer of the
previous stage; average thickness 0.1 mm. (3) An inner layer of larger
nests; average thickness 0.23 mm.
The general appearance of the germinal epithelium at this stage certainly
appears to me to lend support to my view that the whole of it simply
constitutes a thickened epithelium interpenetrated with ingrowths of
stroma.
The cells of the germinal epithelium, which form the various layers, have
undergone important modifications. In the first place a large number of the
nuclei--at any rate of those cells which are about to become ova--have
undergone a change identical with that which takes place in the conversion
of the primitive into the permanent ova in Elasmobranchii. The greater part
of the contents of the nucleus becomes clear. The remaining contents
arrange themselves as a deeply staining granular mass on one side of the
membrane, and later on as a somewhat stellate figure: the two stages
forming what were spoken of as the granular and stellate varieties of
nucleus. To avoid further circumlocution I shall speak of the nucleus
undergoing the granular and the stellate modifications. At a still later
period the granular contents form a beautiful network in the nucleus.
The pseudo-epithelium (fig. 38A) is formed of several tiers of cells, the
outermost of which are very columnar and have less protoplasm than in an
earlier stage. In the lower tiers of cells there are many primitive ova
with granular nuclei, and others in which the nuclei have undergone the
granular modification. The primitive ova are almost all of the same size as
in the earlier stage. The pseudo-epithelium is separated from the middle
layer by a more or less complete stratum of connective tissue, which,
however, is traversed by trabeculæ connecting the two layers of the
epithelium. In the middle layer there are comparatively few modified
nuclei, and the cells still retain for the most part their earlier
characters. The diameter of the cells is about 0.012 mm., and that of the
nucleus about 0.008 mm. In the innermost layer (fig. 38B), which is not
sharply separated from the middle layer, the majority of the cells, which
in the previous stage were ordinary cells of the epithelium, have commenced
to acquire modified nuclei. This change, which first became apparent to a
small extent in the young two days after birth, is very conspicuous at this
stage. In some of the cells the nucleus is modified in the granular manner,
in others in the stellate, and in a certain number the nucleus has assumed
a reticular structure characteristic of the young permanent ovum.
In addition, however, to the cells which are becoming converted into ova, a
not inconsiderable number may be observed, if carefully looked for, which
are for the most part smaller than the others, generally somewhat oval, and
in which the nucleus retains its primitive characters. A fair number of
such cells are represented in fig. 38B. In the larger ones the nucleus will
perhaps eventually become modified; but the smaller cells clearly
correspond with the interstitial cells of the Elasmobranch germinal
epithelium, and are destined to become converted into the epithelium of the
Graafian follicle. In some few instances indeed (at this stage very few),
in the deeper part of the germinal epithelium, these cells commence to
arrange themselves round the just formed permanent ova as a follicular
epithelium. An instance of this kind is shewn in fig. 38B, _o_. The cells
with modified nuclei, which are becoming permanent ova, usually present one
point of contrast to the homologous cells in Elasmobranchii, in that they
are quite distinct from each other, and not fused into a polynuclear mass.
They have around them a dark contour line, which I can only interpret as
the commencement of the membrane (zona radiata?), which afterwards becomes
distinct, and which would thus seem, as Foulis has already insisted, to be
of the nature of a vitelline membrane.
In a certain number of instances the protoplasm of the cells which are
becoming permanent ova appears, however, actually to fuse, and polynuclear
masses identical with those in Elasmobranchii are thus formed (cf. E. van
Beneden[411]). These masses become slightly more numerous in the succeeding
stages. Indications of a fusion of this kind are shewn in fig. 38B. That
the polynuclear masses really arise from a fusion of primitively distinct
cells is clear from the description of the previous stages. The ova in the
deeper layers, with modified granular nuclei, measure about 0.016 -
0.02 mm., and their nuclei from 0.01 - 0.012 mm.
Footnote 411: _Loc. cit._
With reference to the tissue of the hilus of the ovary, it may be noticed
that the tubuliferous tissue (_t_) is relatively reduced in quantity. Its
cells retain precisely their previous characters.
The chief difference between the stage of five days and that of two days
after birth consists in the fact that during the earlier stage
comparatively few modified nuclei were present, but the nuclei then
presented the character of the nuclei of primitive ova.
I have ovaries both of the dog and cat of an equivalent stage, and in both
of these the cells of the nests or egg tubes may be divided into two
categories, destined respectively to become ova and follicle cells. Nothing
which has come under my notice tends to shew that the tubuliferous tissue
is in any way concerned in supplying the latter form of cell.
In a stage, seven days after birth, the same layers in the germinal
epithelium may be noticed as in the last described stage. The outermost
layer or pseudo-epithelium contains numerous developing ova, for the most
part with modified nuclei. It is separated by a well marked layer of
connective tissue from the middle layer of the germinal epithelium. The
outer part of the middle layer contains more connective tissue and smaller
nests than in the earlier stage, and most of the cells of this layer
contain modified nuclei. In a few nests the protoplasm of the developing
ova forms a continuous mass, not divided into distinct cells, but in the
majority of instances the outline of each ovum can be distinctly traced. In
addition to the cells destined to become ova, there are present in these
nests other cells, which will clearly form the follicular epithelium. A
typical nest from the middle layer is represented on Pl. 26, fig. 39A.
The nests or masses of ova in the innermost layer are for the most part
still very large, but, in addition to the nests, a few isolated ova,
enclosed in follicles, are to be seen.
A fairly typical nest, selected to shew the formation of the follicle, is
represented on Pl. 26, fig. 39B.
The nest contains (1) fully formed permanent ova, completely or wholly
enclosed in a follicle. (2) Smaller ova, not enclosed in a follicle. (3)
Smallish cells with modified nuclei of doubtful destination. (4) Small
cells obviously about to form follicular epithelium.
The inspection of a single such nest is to my mind a satisfactory proof
that the follicular epithelium takes its origin from the germinal
epithelium and not from the stroma or tubuliferous tissue. The several
categories of elements observable in such a nest deserve a careful
description.
(1) _The large ova in their follicles._--These ova have precisely the
character of the young ova in Elasmobranchii. They are provided with a
granular body invested by a delicate, though distinct membrane. Their
nucleus is large and clear, but traversed by the network so fully described
for Elasmobranchii. The cells of their follicular epithelium have obviously
the same character as many other small cells of the nest. Two points about
them deserve notice--(_a_) that many of them are fairly columnar. This is
characteristic only of the first formed follicles. In the later formed
follicles the cells are always flat and spindle-shaped in section. In this
difference between the early and late formed follicles Mammals agree with
Elasmobranchii. (_b_) The cells of the follicle are much more columnar
towards the inner side than towards the outer. This point also is common to
Mammals and Elasmobranchii.
Round the completed follicle a very delicate membrana propria folliculi
appears to be present[412].
Footnote 412: _Loc. cit._, Waldeyer, p. 23, denies the
existence of this membrane for Mammalia. It certainly is not so
conspicuous as in some other types, but appears to me
nevertheless to be always present.
The larger ova, with follicular epithelium, measure about 0.04 mm., and
their nucleus about 0.02 mm., the smaller ones about 0.022 mm., and their
nucleus about 0.014 mm.
(2) _Medium sized ova._--They are still without a trace of a follicular
epithelium, and present no special peculiarities.
(3) _The smaller cells with modified nuclei._--I have great doubt as to
what is the eventual fate of these cells. There appear to be three
possibilities.
(_a_) That they become cells of the follicular epithelium; (_b_) that they
develop into ova; (_c_) that they are absorbed as a kind of food by the
developing ova. I am inclined to think that some of these cells may have
each of the above-mentioned destinations.
(4) _The cells which form the follicle._--The only point to be noticed
about these is that they are smaller than the indifferent cells of the
germinal epithelium, from which they no doubt originate by division. This
fact has already been noticed by Waldeyer.
The isolated follicles at this stage are formed by ingrowths of connective
tissue cutting off fully formed follicles from a nest. They only occur at
the very innermost border of the germinal epithelium. This is in accordance
with what has so often been noticed about the mammalian ovary, viz. that
the more advanced ova are to be met with in passing from without inwards.
By the stage seven days after birth the ovary has reached a sufficiently
advanced stage to answer the more important question I set myself to solve,
nevertheless, partly to reconcile the apparent discrepancy between my
account and that of Dr Foulis, and partly to bring my description up to a
better known condition of the ovary, I shall make a few remarks about some
of the succeeding stages.
In a young rabbit about four weeks old the ovary is a very beautiful object
for the study of the nuclei, &c.
The pseudo-epithelium is now formed of a single layer of columnar cells,
with comparatively scanty protoplasm. In it there are present a not
inconsiderable number of developing ova.
A layer of connective tissue--the albuginea--is now present below the
pseudo-epithelium, which contains a few small nests with very young
permanent ova. The layer of medium sized nests internal to the albuginea
forms a very pretty object in well stained sections, hardened in
Kleinenberg's picric acid. The ova in it have all assumed the permanent
form, and are provided with beautiful reticulate nuclei, with, as a rule,
one more especially developed nucleolus, and smaller granular bodies. Their
diameter varies from about 0.028 to 0.04 mm. and that of their nucleus from
0.016 to 0.02 mm. The majority of these ova are not provided with a
follicular investment, but amongst them are numerous small cells, clearly
derived from the germinal epithelium, which are destined to form the
follicle (vide fig. 40Aand B). In a few cases the follicles are completed,
and are then formed of very flattened spindle-shaped (in section) cells. In
the majority of cases all the ova of each nest are quite distinct, and each
provided with a delicate vitelline membrane (fig. 40A) In other instances,
which, so far as I can judge, are more common than in the previous stages,
the protoplasm of two or more ova is fused together.
Examples of this are represented in Pl. 26, fig. 40A. In some of these the
nuclei in the undivided protoplasm are all of about the same size and
distinctness, and probably the protoplasm eventually becomes divided up
into as many ova as nuclei; in other cases, however, one or two nuclei
clearly preponderate over the others, and the smaller nuclei are indistinct
and hazy in outline. In these latter cases I have satisfied myself as
completely as in the case of Elasmobranchii, that only one or two ova
(according to the number of distinct nuclei) will develop out of the
polynuclear mass, and that the other nuclei atrophy, and the material of
which they were composed serves as the nutriment for the ova which complete
their development. This does not, of course, imply that the ova so formed
have a value other than that of a single cell, any more than the
development of a single embryo out of the many in one egg capsule implies
that the embryo so developing is a compound organism.
In the innermost layer of the germinal epithelium the outlines of the
original large nests are still visible, but many of the follicles have been
cut off by ingrowths of stroma. In the still intact nests the formation of
the follicles out of the cells of the germinal epithelium may be followed
with great advantage. The cells of the follicle, though less columnar than
was the case at an earlier period, are more so than in the case of
follicles formed in the succeeding stages. The previous inequality in the
cells of the follicles is no longer present.
The tubuliferous tissue in the zona vasculosa appears to me to have rather
increased in quantity than the reverse; and is formed of numerous solid
columns or oval masses of cells, separated by strands of connective tissue,
with typical spindle nuclei.
It is partially intelligible to me how Dr Foulis might from an examination
of the stages similar to this, conclude that the follicle cells were
derived from the stroma; but even at this stage the position of the cells
which will form the follicular epithelium, their passage by a series of
gradations into obvious cells of the germinal epithelium and the
peculiarities of their nuclei, so different from those of the stroma cells,
supply a sufficient series of characters to remove all doubt as to the
derivation of the follicle cells. Apart from these more obvious points, an
examination of the follicle cells from the surface, and not in section,
demonstrates that the general resemblance in shape of follicle cells to the
stroma cells is quite delusory. They are in fact flat, circular, or oval,
plates not really spindle-shaped, but only apparently so in section. While
I thus fundamentally differ from Foulis as to the nature of the follicle
cells, I am on this point in complete accordance with Waldeyer, and my own
results with reference to the follicle cannot be better stated than in his
own words (pp. 43, 44).
At six weeks after birth the ovary of the rabbit corresponds very much more
with the stages in the development of the ovary, which Foulis has more
especially studied, for the formation of the follicular epithelium, than
during the earlier stages. His figure (_Quart. Journ. Mic. Sci._,
Vol. XVI., Pl. 17, fig. 6) of the ovary of a seven and a half months' human
foetus is about the corresponding age. Different animals vary greatly in
respect to the relative development of the ovary. For example, the ovary of
a lamb at birth about corresponds with that of a rabbit six weeks after
birth. The points which may be noticed about the ovary at this age are
first that the surface of the ovary begins to be somewhat folded. The
appearances of these folds in section have given rise, as has already been
pointed out by Foulis, to the erroneous view that the germinal epithelium
(pseudo-epithelium) became involuted in the form of tubular open pits. The
folds appear to me to have no connection with the formation of ova, but to
be of the same nature as the somewhat similar folds in Elasmobranchii. A
follicular epithelium is present around the majority of the ova of the
middle layer, and around all those of the inner layer of the germinal
epithelium. The nests are, moreover, much more cut up by connective tissue
ingrowths than in the previous stages.
The follicle cells of the middle layers are very flat, and spindle-shaped
in section, and though they stain more deeply than the stroma cells, and
have other not easily characterised peculiarities, they nevertheless do
undoubtedly closely resemble the stroma cells when viewed (as is ordinarily
the case) in optical section.
In the innermost layer many of the follicles with the enclosed ova have
advanced considerably in development and are formed of columnar cells. The
somewhat heterodox view of these cells propounded by Foulis I cannot quite
agree to. He says (_Quart. J. Mic. Sci._, Vol. XVI., p. 210): "The
protoplasm which surrounds the vesicular nuclei acts as a sort of cement
substance, holding them together in the form of a capsular membrane round
the young ovum. This capsular membrane is the first appearance of the
membrana granulosa." I must admit that I find nothing similar to this, nor
have I met with any special peculiarities (as Foulis would seem to
indicate) in the cells of the germinal epithelium or other cells of the
ovary.
Figure 41 is a representation of an advanced follicle of a six weeks'
rabbit, containing two ova, which is obviously in the act of dividing into
two. Follicles of this kind with more than one ovum are not very uncommon.
It appears to me probable that follicles, such as that I have figured, were
originally formed of a single mass of protoplasm with two nuclei; but that
instead of one of the nuclei atrophying, both of them eventually developed
and the protoplasm subsequently divided into two masses. In other cases it
is quite possible that follicles with two ova should rather be regarded as
two follicles not separated by a septum of stroma.
On the later stages of development of the ovary I have no complete series
of observations. The yolk spherules I find to be first developed in a
peripheral layer of the vitellus. I have not been able definitely to decide
the relation of the zona radiata to the first formed vitelline membrane.
Externally to the zona radiata there may generally be observed a somewhat
granular structure, against which the follicle cells abut, and I cannot
agree with Waldeyer (_loc cit._, p. 40) that this structure is continuous
with the cells of the discus, or with the zona radiata. Is it the remains
of the first formed vitelline membrane? I have obtained some evidence in
favour of this view, but have not been successful in making observations to
satisfy me on the point, and must leave open the question whether my
vitelline membrane becomes the zona radiata or whether the zona is not a
later and independent formation, but am inclined myself to adopt the latter
view. The first formed membrane, whether or no it becomes the zona radiata,
is very similar to the vitelline membrane of Elasmobranchii and arises at a
corresponding stage.
_Summary of observations on the mammalian ovary._--The general results of
my observations on the mammalian ovary are the following:--
(1) The ovary in an eighteen days' embryo consists of a cylindrical ridge
attached along the inner side of the Wolffian body, which is formed of two
parts; (_a_) an external epithelium--two or three cells deep (the germinal
epithelium); (_b_) a hilus or part forming in the adult the vascular zone,
at this stage composed of branched masses of epithelial tissue
(tubuliferous tissue) derived from the walls of the anterior Malpighian
bodies, and numerous blood-vessels, and some stroma cells.
(2) The germinal epithelium gradually becomes thicker, and after a certain
stage (twenty-three days) there grow into it numerous stroma ingrowths,
accompanied by blood-vessels. The germinal epithelium thus becomes
honeycombed by strands of stroma. Part of the stroma eventually forms a
layer close below the surface, which becomes in the adult the tunica
albuginea. The part of the germinal epithelium external to this layer
becomes reduced to a single row of cells, and forms what has been spoken of
in this paper as the pseudo-epithelium of the ovary. The greater part of
the germinal epithelium is situated internal to the tunica albuginea, and
this part is at first divided up by strands of stroma into smaller
divisions externally, and larger ones internally. These masses of germinal
epithelium (probably sections of branched trabeculæ) may be spoken of as
nests. In the course of the development of the ova they are broken up by
stroma ingrowths, and each follicle with its enclosed ovum is eventually
isolated by a layer of stroma.
(3) The cells of the germinal epithelium give rise both to the permanent
ova and to the cells of the follicular epithelium. For a long time,
however, the cells remain indifferent, so that the stages, like those in
Elasmobranchii, Osseous Fish, Birds, Reptiles, &c., with numerous primitive
ova embedded amongst the small cells of the germinal epithelium, are not
found.
(4) The conversion of the cells of the germinal epithelium into permanent
ova commences in an embryo of about twenty-two days. All the cells of the
germinal epithelium appear to be capable of becoming ova: the following are
the stages in the process, which are almost identical with those in
Elasmobranchii:--
(_a_) The nucleus of the cells loses its more or less distinct network, and
becomes very granular, with a few specially large granules (nucleoli). The
protoplasm around it becomes clear and abundant--primitive ovum stage. It
may be noted that the largest primitive ova are very often situated in the
pseudo-epithelium. (_b_) A segregation takes place in the contents of the
nucleus within the membrane, and the granular contents pass to one side,
where they form an irregular mass, while the remaining space within the
membrane is perfectly clear. The granular mass gradually develops itself
into a beautiful reticulum, with two or three highly refracting nucleoli,
one of which eventually becomes the largest and forms the germinal spot
_par excellence_. At the same time the body of the ovum becomes slightly
granular. While the above changes, more especially those in the nucleus,
have been taking place, the protoplasm of two or more ova may fuse
together, and polynuclear masses be so formed. In some cases the whole of
such a polynuclear mass gives rise to only a single ovum, owing to the
atrophy of all the nuclei but one, in others it gives rise by subsequent
division to two or more ova, each with a single germinal vesicle.
(5) All the cells of a nest do not undergo the above changes, but some of
them become smaller (by division) than the indifferent cells of the
germinal epithelium, arrange themselves round the ova, and form the
follicular epithelium.
(6) The first membrane formed round the ovum arises in some cases even
before the appearance of the follicular epithelium, and is of the nature of
a vitelline membrane. It seems probable, although not definitely
established by observation, that the zona radiata is formed internally to
the vitelline membrane, and that the latter remains as a membrane, somewhat
irregular on its outer border, against which the ends of the follicle cells
abut.
GENERAL OBSERVATIONS ON THE STRUCTURE AND DEVELOPMENT OF THE OVARY.
In selecting Mammalia and Elasmobranchii as my two types for investigation,
I had in view the consideration that what held good for such dissimilar
forms might probably be accepted as true for all Vertebrata with the
exception of Amphioxus.
_The structure of the ovary._--From my study of these two types, I have
been led to a view of the structure of the ovary, which differs to a not
inconsiderable extent from that usually entertained. For both types the
conclusion has been arrived at that the whole egg-containing part of the
ovary is really _the thickened germinal epithelium_, and that it differs
from the original thickened patch or layer of germinal epithelium, mainly
in the fact that it is broken up into a kind of meshwork by growths of
vascular stroma. If the above view be accepted for Elasmobranchii and
Mammalia, it will hardly be disputed for the ovaries of Reptilia and Aves.
In the case also of Osseous Fish and Amphibia, this view of the ovary
appears to be very tenable, but the central core of stroma present in the
other types is nearly or quite absent, and the ovary is entirely formed of
the germinal epithelium with the usual strands of vascular stroma[413]. It
is obvious that according to the above view Pflüger's egg-tubes are merely
trabeculæ of germinal epithelium, and have no such importance as has been
attributed to them. They are present in a more or less modified form in all
types of ovaries. Even in the adult Amphibian ovary, columns of cells of
the germinal epithelium, some indifferent, others already converted into
ova, are present, and, as has been pointed out by Hertwig[414], represent
Pflüger's egg-tubes.
Footnote 413: My view of the structure of the ovary would seem
to be that held by Götte, _Entwicklungsgeschichte d. Unke_, pp.
14 and 15.
Footnote 414: _Loc. cit._ 36.
_The formation of the permanent ova._--The passage of primitive ova into
permanent ova is the part of my investigation to which the greatest
attention was paid, and the results arrived at for Mammalia and
Elasmobranchii are almost identical. Although there are no investigations
as to the changes undergone by the nucleus in other types, still it appears
to me safe to conclude that the results arrived at hold good for
Vertebrates generally[415]. As has already been pointed out the
transformation which the so-called primitive ova undergo is sufficient to
shew that _they are not to be regarded as ova but merely as embryonic
sexual cells_. A feature in the transformation, which appears to be fairly
constant in Scyllium, and not uncommon in the rabbit, is the fusion of the
protoplasm of several ova into a syncytium, the subsequent increase in the
number of nuclei in the syncytium, the atrophy and absorption of a portion
of the nuclei, and the development of the remainder into the germinal
vesicles of ova; the vitellus of each ovum being formed by a portion of the
protoplasm of the syncytium.
Footnote 415: Since writing the above I have made out that in
the Reptilia the formation of the permanent ova takes place in
the same fashion as in Elasmobranchii and Mammalia.
As to the occurrence of similar phenomena in the Vertebrata generally, it
has already been pointed out that Ed. van Beneden has described the
polynuclear masses in Mammalia, though he does not appear to me to have
given a complete account of their history. Götte[416] describes a fusion of
primitive ova in Amphibia, but he believes that the nuclei fuse as well as
the bodies of the ova, so that one ovum (according to his view no longer a
cell) is formed by the fusion of several primitive ova with their nuclei. I
have observed nothing which tends to support Götte's view about the fusion
of the nuclei, and regard it as very improbable. As regards the
interpretation to be placed upon the nests formed of fused primitive ova,
Ed. van Beneden maintains that they are to be compared with the upper ends
of the egg tubes of Insects, Nematodes, Trematodes, &c. There is no doubt a
certain analogy between the two, in that in both cases certain nuclei of a
polynuclear mass increase in size, and with the protoplasm around them
become segmented off from the remainder of the mass as ova, but the analogy
cannot be pressed. The primitive ova, or even the general germinal
epithelium, rather than these nests, must be regarded as giving origin to
the ova, and the nests should be looked on, in my opinion, as connected
more with the nutrition than with the origin of the ova. In favour of this
view is the fact that as a rule comparatively few ova are developed from
the many nuclei of a nest; while against the comparison with the egg tubes
of the Invertebrata it is to be borne in mind that many ova appear to
develop independently of the nests.
Footnote 416: _Entwicklungsgeschichte d. Unke._
In support of my view about the nests there may be cited many analogous
instances from the Invertebrata. In none of them, however, are the
phenomena exactly identical with those in Vertebrata. In the ovary of many
Hydrozoa (_e.g. Tubularia mesembryanthemum_), out of a large number of ova
which develop up to a certain point, a comparatively very small number
survive, and these regularly feed upon the other ova. During this process
the boundary between a large ovum and the smaller ova is indistinct: in the
outermost layer of a large ovum a number of small ova are embedded, the
outlines of the majority of which have become obscure, although they can
still be distinguished. Just beyond the edge of a large ovum the small ova
have begun to undergo retrogressive changes; while at a little distance
from the ovum they are quite normal. An analogous phenomenon has been very
fully described by Weismann[417] in the case of Leptodera, the ovary of
which consists of a germogene, in which the ova develop in groups of four.
Each group of four occupies a separate chamber of the ovary, but in summer
only one of the four eggs (the third from the germogene) develops into an
ovum, the other three are used as pabulum. In the case of the winter eggs
the process is carried still further, in that the contents of the alternate
chambers, instead of developing into ova, are entirely converted, by a
series of remarkable changes, into nutritive reservoirs. Fundamentally
similar occurrences to the above are also well known in Insects. Phenomena
of this nature are obviously in no way opposed to the view of the ovum
being a single cell.
Footnote 417: _Zeit. für wiss. Zool._ Bd. XXVII.
With reference to the origin of the primitive ova, it appears to me that
their mode of development in Mammals proves beyond a doubt that they are
modified cells of the germinal epithelium. In Elasmobranchii their very
early appearance, and the difficulty of finding transitional forms between
them and ordinary cells of the germinal epithelium, caused me at one time
to seek (unsuccessfully) for a different origin for them. Any such attempts
appear to me, however, out of the question in the case of Mammals.
_The egg membranes._--The homologies of the egg membranes in the Vertebrata
are still involved in some obscurity. In Elasmobranchii there are
undoubtedly two membranes present. (1) An outer and first formed
membrane--the albuminous membrane of Gegenbaur--which, in opposition to
previous observers, I have been led to regard as a vitelline membrane. (2)
An inner radiately striated membrane, formed as a differentiation of the
surface of the yolk at a later period. Both these membranes usually atrophy
before the ovum leaves the follicle. In Reptilia[418] precisely the same
arrangement is found as in Elasmobranchii, except that as a rule the zona
radiata is relatively more important. The vitelline membrane external to
this (or as it is usually named the chorion) is, as a rule, thin in
Reptilia; but in Crocodilia is thick (Gegenbaur), and approaches the
condition found in Scyllium and other Squalidæ. It appears, as in
Elasmobranchii, to be formed before the zona radiata. A special internal
differentiation of the zona radiata is apparently found (Eimer) in many
Reptilia. No satisfactory observations appear to be recorded with reference
to the behaviour of the two reptilian membranes as the egg approaches
maturity. In Birds[419] the same two membranes are again found. The first
formed and outer one is, according to Gegenbaur and E. van Beneden, a
vitelline membrane; and from the analogy of Elasmobranchii I feel inclined
to accept their view. The inner one is the zona radiata, which disappears
comparatively early, leaving the ovum enclosed only by the vitelline
membrane, when it leaves the follicle. All the large-yolked vertebrate ova
appear then to agree very well with Elasmobranchii in presenting during
some period of their development the two membranes above mentioned.
Footnote 418: Gegenbaur, _loc. cit._; Waldeyer, _loc. cit._;
Eimer, _loc. cit._; and Ludwig, _loc. cit._
Footnote 419: Gegenbaur, Waldeyer, E. van Beneden, Eimer.
Osseous fish have almost always a zona radiata, which it seems best to
assume to be equivalent to that in Elasmobranchii. Internal to this is a
thin membrane, the equivalent, according to Eimer, of the membrane found by
the same author within the zona in Reptilia. A membrane equivalent to the
thick vitelline membrane of Elasmobranchii would seem to be absent in most
instances, though a delicate membrane, external to the zona, has not
infrequently been described; Eimer more especially asserts that such a
membrane exists in the perch within the peculiar mucous covering of the egg
of that fish.
In Petromyzon, a zona radiata appears to be present[420], which is divided
in the adult into two layers, both of them perforated. The inner of the two
perhaps corresponds with the membrane internal to the zona radiata in other
types. In Amphibia the single late formed and radiately striated (Waldeyer)
membrane would appear to be a zona radiata. If the suggestion on page 605
turns out to be correct the ova of Mammalia possess both a vitelline
membrane and zona radiata. E. van Beneden[421] has, moreover, shewn that
they are also provided at a certain period with a delicate membrane within
the zona.
Footnote 420: Carlberla, _Zeit. f. wiss. Zool._ Bd. XXX.
Footnote 421: _Loc. cit._
_The reticulum of the germinal vesicle._--In the course of description of
the ovary it has been necessary for me to enter with some detail into the
structure of the nucleus, and I have had occasion to figure and describe a
reticulum identical with that recently described by so many observers. The
very interesting observations of Dr Klein in the last number of this
Journal[422] have induced me to say one or two words in defence of some
points in my description of the reticulum. Dr Klein says, on page 323, "I
have distinctly seen that when nucleoli are present--the instances are
fewer than is generally supposed; they are accumulations of the fibrils of
the network." I have no doubt that Klein is correct in asserting that
nucleoli are fewer than is generally supposed; and that in many of these
instances what are called nucleoli are accumulations, "natural or
artificial," of the fibrils of the network; but I cannot accept the
universality of the latter statement, which appears to me most certainly
not to hold good in the case of ova, in which nucleoli frequently exist in
the absence of the network.
Footnote 422: [_Quarterly Journal Microscopical Science_, July
1878.]
Again, I find that at the point of intersection of two or more fibrils
there is, as a rule, a distinct thickening of the matter of the fibrils,
and that many of the dots seen are not merely, as Dr Klein would maintain,
optical sections of fibrils.
It appears to me probable that both the network and the nucleoli are
composed of the same material--what Hertwig calls nuclear substance--and if
Dr Klein merely wishes to assert this identity in the passage above quoted,
I am at one with him.
Although a more or less distinct network is present in most nuclei (I have
found it in almost all embryonic nuclei) it is not universally so. In the
nuclei of primitive ova I have no doubt that it is absent, though present
in the unmodified nuclei of the germinal epithelium; and it is present only
in a very modified form in the nuclei of primitive ova undergoing a
transformation into permanent ova. The absence of the reticulum does not,
of course, mean that the substance capable of forming a reticulum is
absent, but merely that it does not assume a particular arrangement.
One of the most interesting points in Klein's paper, as well as in those of
Heitzmann and Eimer, is the demonstration of a connection between the
reticulum of the nucleus and fibres in the body of the cell. Such a
connection I have not found in ova, but may point out that it appears to
exist between the sub-germinal nuclei in Elasmobranchii and the
protoplasmic network in the yolk in which they lie. This point is called
attention to in my _Monograph on Elasmobranch Fishes_, page 39[423], where
it is stated that "the network in favourable cases may be observed to be in
connection with the nuclei just described. Its meshes are finer in the
vicinity of the nuclei, and the fibres in some cases appear almost to start
from them." The nuclei in the yolk are knobbed bodies divided by a sponge
work of septa into a number of areas each with a nucleolar body.
Footnote 423: [This Edition, p. 252.]
EXPLANATION OF PLATES 24, 25, 26.
PLATE 24.
LIST OF REFERENCE LETTERS.
_dn._. Modified nucleus of primitive ovum. _do._. Permanent ovum in the act
of being formed. _dv._. Developing blood-vessels. _dyk._ Developing yolk.
_ep._ Non-ovarian epithelium of ovarian ridge. _fe._ Follicular epithelium.
_gv._ Germinal vesicle. _lstr._ Lymphatic region of stroma. _nn._ Nests of
nuclei of ovarian region. _o._ Permanent ovum. _ovr._ Ovarian portion of
ovarian ridge. _po._ Primitive ovum. _pse._ Pseudo-epithelium of ovarian
ridge. _str._ Stroma ingrowths into ovarian epithelium. _v._ Blood-vessel.
_vstr._ Vascular region of stroma adjoining ovarian ridge. _vt._ Vitelline
membrane. _x._ Modified nucleus. _yk._ Yolk. _zn._ Zona radiata.
Fig. 1. Transverse section of the ovarian ridge of an embryo of _Scy.
canicula_, belonging to stage P, shewing the ovarian region with thickened
epithelium and numerous primitive ova. Zeiss C, ocul. 2. _Picric acid._
Fig. 2. Transverse section of the ovarian ridge of an embryo of _Scyllium
canicula_, considerably older than stage Q. Zeiss C, ocul. 2. _Picric
acid._ Several nests, some with distinct ova, and others with the ova fused
together, are present in the section (_nn_), and several examples of
modified nuclei in still distinct ova are also represented. One of these is
marked _x_. The stroma of the ovarian ridge is exceptionally scanty.
Fig. 3. Transverse section through part of the ovarian ridge, including the
ovarian region of an almost ripe embryo of _Scyllium canicula_. Zeiss C,
ocul. 2. _Picric acid._ Nuclear nests (_n.n._), developing ova (_d.o._),
and ova (_o._), with completely formed follicular epithelium, are now
present. The ovarian region is still well separated from the subjacent
stroma, and does not appear to contain any cells except those of the
original germinal epithelium.
Fig. 4. Section through ovarian ridge of the same embryo as fig. 3, to
illustrate the relation of the stroma (_str._) and ovarian region. Zeiss _a
a_, ocul. 2. _Picric acid._
Fig. 5. Section through the ovarian ridge of an embryo of _Scyllium
canicula_, 10 cm. long, in which the ovary was slightly less advanced than
in fig. 3. To illustrate the relation of the ovarian epithelium to the
subjacent vascular stroma. Zeiss A, ocul. 2. _Osmic acid._ _y_ points to a
small separated portion of the germinal epithelium.
Fig. 6. Section through the ovarian ridge of an embryo of _Scyllium
canicula_, slightly older than fig. 5. To illustrate the relation of the
ovarian epithelium to the subjacent vascular stroma. Zeiss A, ocul. 2.
_Osmic acid._
Fig. 7. More highly magnified portion of the same ovary as fig. 6. To
illustrate the same points. Zeiss C, ocul. 2. _Osmic acid._
Fig. 8. Section through the ovarian region (close to one extremity, where
it is very small) from a young female of _Scy. canicula_. Zeiss C, ocul. 2.
_Picric acid._ It shews the vascular ingrowths amongst the original
epithelial cells of the ovarian region.
Fig. 9. Section through the ovarian region of the same embryo as fig. 8, at
its point of maximum development. Zeiss A, ocul. 2. _Picric acid._
Fig. 10. Section through superficial part of the ovary of an embryo,
shewing the pseudo-epithelium; the cells of which are provided with tails
prolonged into the general tissue of the ovary. At _f.e._ is seen a surface
view of the follicular epithelium of an ovum. Zeiss C, ocul. 2. _Picric
acid._
Fig. 11. Section through part of an ovary of _Scyllium canicula_ of stage
Q, with three primitive ova, the most superficial one containing a modified
nucleus.
Fig. 12. Section through part of an ovary of an example of _Scyllium
canicula_, 8 cm. long. The section passes through a nest of ova with
modified nuclei, in which the outlines of the individual ova are quite
distinct. Zeiss E, ocul. 2. _Picric acid._
Fig. 13. Section through part of ovary of the same embryo as in fig. 5. The
section passes through a nest of nuclei, with at the least two developing
ova, and also through one already formed permanent ovum. Zeiss E, ocul. 2.
_Osmic acid._
Figs. 14, 15, 16, 17, 18 [Figs. 17 and 18 are on Pl. 25]. Sections through
parts of the ovary of the same embryo as fig. 3, with nests of nuclei and a
permanent ova in the act of formation. Fig. 14 is drawn with Zeiss D D,
ocul. 2. Figs. 15, 16, 17, with Zeiss E, ocul. 2. _Picric acid._
PLATE 25.
LIST OF REFERENCE LETTERS.
_do._ Permanent ovum in the act of being formed. _dyk._ Developing yolk.
_fe._ Follicular epithelium. _fe´._ Secondary follicular epithelium. _gv._
Germinal vesicle. _nn._ Nests of nuclei of ovarian region. _o._ Permanent
ovum. _pse._ Pseudo-epithelium. _str._ Stroma ingrowths into ovarian
epithelium. _vt._ Vitelline membrane. _x._ Modified nucleus. _yk._ Yolk
(vitellus). _zn._ Zona radiata.
[Figs. 17 and 18. Vide description of Plate 24.].
Fig. 19. Two nuclei from a nest which appear to be in the act of division.
From ovary of the same embryo as fig. 3.
Fig. 20. Section through part of an ovary of the same embryo as fig. 6,
containing a nest of nuclei. Zeiss F, ocul. 2. _Osmic acid._
Fig. 21. Ovum from the ovary of a half-grown female, containing isolated
deeply stained patches of developing yolk granules. Zeiss B, ocul. 2.
_Picric acid._
Fig. 22. Section through a small part of the ovum of an immature female of
_Scyllium canicula_, to shew the constitution of the yolk, the follicular
epithelium, and the egg membranes. Zeiss E, ocul. 2. _Chromic acid._
Fig. 23. Section through part of the periphery of a nearly ripe ovum of
_Scy. canicula_. Zeiss C, ocul. 2. It shews the remnant of the vitelline
membrane (_v.t._) separating the columnar but delicate cells of the
follicular epithelium (_f.e._) from the yolk (_yk._). In the yolk are seen
yolk-spherules in a protoplasmic network. The transverse markings in the
yolk-spherules have been made oblique by the artist.
Fig. 24. Fully formed ovum containing a second nucleus (_x_), probably
about to be employed as pabulum; from the same ovary as fig. 5. The
follicular epithelium is much thicker on the side adjoining the stroma than
on the upper side of the ovum. Zeiss F, ocul. 2. _Osmic acid._
Fig. 25. A. Ovum from the same ovary as fig. 21, containing in the yolk
three peculiar bodies, similar in appearance to the two small bodies in the
germinal vesicle. B. Germinal vesicle of a large ovum from the same ovary,
containing a body of a strikingly similar appearance to those in the body
of the ovum in A. Zeiss E, ocul. 2. _Picric acid._
Fig. 26. Section of the ovary of a young female of _Scyllium stellare_
16-1/2 centimetres in length. The ovary is exceptional, on account of the
large size of the stroma ingrowths into the epithelium. Zeiss C, ocul. 2.
_Osmic acid._
Fig. 27. Ovum of _Scyllium canicula_, 5 mm. in diameter, treated with osmic
acid. The figure illustrates the development of the yolk and a peculiar
mode of proliferation of the germinal spots. Zeiss A, ocul. 2.
Fig. 28. Small part of the follicular epithelium and egg membranes of a
somewhat larger ovum of _Scyllium canicul_a than fig. 22. Zeiss D D, ocul.
2.
Fig. 29. The same parts as in fig. 28, from a still larger ovum. Zeiss D D,
ocul. 2.
Fig. 30. Ovum of Raja with follicular epithelium. Zeiss C, ocul. 2.
Fig. 31. Small portion of a larger ovum of Raja than fig. 30. Zeiss D D,
ocul. 2.
Fig. 32. Follicular epithelium, &c., from an ovum of Raja still larger than
fig. 31. Zeiss D D, ocul. 2.
Fig. 33. Surface view of follicular epithelium from an ovum of Raja of
about the same age as fig. 33.
Fig. 34. Vertical section through the superficial part of an ovary of an
adult Raja to shew the relation of the pseudo-epithelium to the subjacent
stroma. Zeiss D D, ocul. 2.
PLATE 26.
COMPLETE LIST OF REFERENCE LETTERS.
_do._ Developing ovum. _fc._ Cells which will form the follicular
epithelium, _fe._ Follicular epithelium. _ge._ Germinal epithelium. _mg._
Malpighian body. _n._ Nest of cells of the germinal epithelium. _nd._
Nuclei in the act of dividing. _o._ Permanent ovum. _ov._ Ovary. _po._
Primitive ovum. _t._ Tubuliferous tissue, derived from Malpighian bodies.
Fig. 35. Transverse section through the ovary of an embryo rabbit of
eighteen days, hardened in osmic acid. The colours employed are intended to
render clear the distinction between the germinal epithelium (_ge._) and
the tubuliferous tissue (_t._), which has grown in from the Wolffian body,
and which gives rise in the male to parts of the tubuli seminiferi. Zeiss
A, ocul. 2.
Fig. 35A . Transverse section through a small part of the ovary of an
embryo from the same female as fig. 35, hardened in picric acid, shewing
the relation of the germinal epithelium to the subjacent tissue. Zeiss D D,
ocul. 2.
Fig. 35B. Longitudinal section through part of the Wolffian body and the
anterior end of the ovary of an eighteen days' embryo, to shew the
derivation of tubuliferous tissue (_t._) from the Malpighian bodies, close
to the anterior extremity of the ovary. Zeiss A, ocul. 1.
Fig. 36. Transverse section through the ovary of an embryo rabbit of
twenty-two days, hardened in osmic acid. It is coloured in the same manner
as fig. 35. Zeiss A, ocul. 2.
Fig. 36A. Transverse section through a small part of the ovary of an
embryo, from the same female as fig. 36, hardened in picric acid, shewing
the relation of the germinal epithelium to the stroma of the ovary. Zeiss D
D, ocul. 2.
Figs. 37 and 37A. The same parts of an ovary of a twenty-eight days' embryo
as figs. 36 and 36A of a twenty-two days' embryo.
Fig. 38. Ovary of a rabbit five days after birth, coloured in the same
manner as figs. 35, 36 and 37, but represented on a somewhat smaller scale.
_Picric acid._
Fig. 38A. Vertical section through a small part of the surface of the same
ovary as fig. 38. Zeiss D D, ocul. 2.
Fig. 38B. Small portion of the deeper layer of the germinal epithelium of
the same ovary as fig. 38. The figure shews the commencing differentiation
of the cells of the germinal epithelium into true ova and follicle cells.
Zeiss D D, ocul. 2.
Fig. 39A. Section through a small part of the middle region of the germinal
epithelium of a rabbit seven days after birth. Zeiss D D, ocul. 2.
Fig. 39B. Section through a small part of the innermost layer of the
germinal epithelium of a rabbit seven days after birth, shewing the
formation of Graafian follicles. Zeiss D D, ocul. 2.
Figs. 40A and 40B. Small portions of the middle region of the germinal
epithelium of a rabbit four weeks after birth. Zeiss D D, ocul. 2.
Fig. 41. Graafian follicle with two ova, about to divide into two
follicles, from a rabbit six weeks after birth. Zeiss D D, ocul. 2.
XIII. ON THE EXISTENCE OF A HEAD-KIDNEY IN THE EMBRYO CHICK, AND ON
CERTAIN POINTS IN THE DEVELOPMENT OF THE MÜLLERIAN DUCT[424].
BY F. M. BALFOUR AND A. SEDGWICK.
Footnote 424: From the _Quarterly Journal of Microscopical
Science_, Vol. XIX. 1879.
(With Plates 27 and 28.)
The following paper is divided into three sections. The first of these
records the existence of certain structures in the embryo chick, which
eventually become in part the abdominal opening of the Müllerian duct, and
which, we believe, correspond with the head-kidney, or "Vorniere" of German
authors. The second deals with the growth and development of the Müllerian
duct. With reference to this we have come to the conclusion that the
Müllerian duct does not develop entirely independently of the Wolffian
duct. The third section of our paper is of a more general character, and
contains a discussion of the rectifications in the views of the homologies
of the parts of the excretory system in Aves, necessitated by the results
of our investigations.
We have, as far as possible, avoided entering into the extended literature
of the excretory system, since this has been very fully given in three
general papers which have recently appeared by Semper[425], Fürbinger[426],
and by one of us[427].
Footnote 425: "Das Urogenital-System der Plagiostomen,"
_Arbeiten a. d. zool.-zoot. Institut. Würzburg_.
Footnote 426: "Zur vergl. Anat. u. Entwick. d.
Excretionsorgane d. Vertebraten," _Morphologisches Jahrbuch_,
Vol. IV.
Footnote 427: "On the Origin and History of the Urinogenital
Organs of Vertebrates," _Journal of Anat. and Phys._, Vol. X.
[This Edition No. VII.]
All recent observers, including Braun[428] for Reptilia, and Egli[429] for
Mammalia, have stated that the Müllerian duct develops as a groove in the
peritoneal epithelium, which is continued backward as a primitively solid
rod in the space between the Wolffian duct and peritoneal epithelium.
Footnote 428: _Arbeiten a. d. zool.-zoot. Institut. Würzburg_,
Vol. IV.
Footnote 429: _Beitr. zur Anat. u. Entwick. d.
Geschlechtsorgane_, Inaug. Diss., Zürich, 1876.
In our preliminary account we stated[430], in accordance with the general
view, that the Müllerian duct was formed as a groove, or elongated
involution of the peritoneal epithelium adjoining the Wolffian duct. We
have now reason to believe that this is not the case. In the earliest
condition of the Müllerian duct which we have been able to observe, it
consists of three successive open involutions of the peritoneal epithelium,
connected together by more or less well-defined ridge-like thickenings of
the epithelium. We believe, on grounds hereafter to be stated, that the
whole of this formation is equivalent to the head-kidney of the
Ichthyopsida. The head-kidney, as we shall continue to call it, takes its
origin from the layer of thickened epithelium situated near the dorsal
angle of the body-cavity, close to the Wolffian duct, which has been known
since the publication of Waldeyer's important researches as the germinal
epithelium. The anterior of the three open involutions or grooves is
situated some little distance behind the front end of the Wolffian duct. It
is simply a shallow groove in the thickest part of the germinal epithelium,
and forms a corresponding projection into the adjacent stroma. In front the
projection is separated by a considerable interval from the Wolffian duct;
but near its hindermost part it almost comes into contact with the Wolffian
duct. The groove extends in all for about five of our sections, and then
terminates by its walls becoming gradually continued into a slight
ridge-like thickening of the germinal epithelium. The groove arises as a
simple depression in a linear area of thickened germinal epithelium. The
linear area is, however, continued very considerably further forward than
the groove, and sometimes exhibits a slight central depression, which might
be regarded as a forward continuation of the groove. The passage from the
groove to the ridge may best be conceived by supposing the groove to be
suddenly filled up, so as to form a solid ridge pointing inwards towards
the Wolffian duct.
Footnote 430: _Proceedings of Royal Society_, 1878.
The ridge succeeding the first groove is continued for about six sections,
and is considerably more prominent at its posterior extremity than in
front. It is replaced by groove number two, which appears as if formed by
the reverse process to that by which the ridge arose, viz., by a hollowing
out of the ridge on the side towards the body-cavity. The wall of the
second groove is, after a few sections, continued into a second ridge or
thickening of the germinal epithelium, which, however, is so faintly marked
as to be hardly visible in its middle part. In its turn this ridge is
replaced by the third and last groove. This vanishes after one or two
sections, and behind the point of its disappearance we have failed to find
any further traces of the head-kidney. The whole formation extends through
about twenty-four of our sections and one and a half segments
(muscle-plates).
We have represented (Plate 27, Series A, Nos. 1-10) a fairly complete
series of sections through part of the head-kidney of an embryo slightly
older than that last described, containing the second and third grooves and
accessory parts. The connection between the grooves and the ridges is very
well illustrated in Nos. 3, 4, and 5 of this series. In No. 3 we have a
prominent ridge, in the interior of which there appears in No. 4 a groove,
which becomes gradually wider in Nos. 5 and 6. Both the grooves and ridges
are better marked in this than in the younger stage; but the chief
difference between the two stages consists in the third groove no longer
forming the hindermost limit of the head-kidney. Instead of this, the last
groove (No. 7) terminates by the upper part of its walls becoming
constricted off as a separate rod, which appears at first to contain a
lumen continuous with the open groove. This rod (Nos. 7, 8, 9, 10) situated
between the germinal epithelium and Wolffian duct is continued backward for
some sections. It finally terminates by a pointed extremity, composed of
not more than two cells abreast (Nos. 8-10).
Our third stage, sections of which are represented in series B (Plate 27),
is considerably advanced beyond that last described. The most important
change which has been effected concerns the ridges connecting the
successive grooves. A lumen has appeared in each of these, which seems to
open at both ends into the adjacent grooves. At the same time the cells,
which previously constituted the ridge, have become (except where they are
continuous with the walls of the grooves) partially constricted off from
the germinal epithelium. The ridges, in fact, now form ducts situated in
the stroma of the ovarian ridge, in the space between the Wolffian duct and
the germinal epithelium. The duct continuous with the last groove is
somewhat longer than before. In a general way, the head-kidney may now be
described as a duct opening into the body-cavity by three groove-like
apertures, and continuous behind with the rudiment of the true Müllerian
duct. Although the general constitution of the head-kidney at this stage is
fairly simple, there are a few features in our sections which we do not
fully understand, and a few points about the organ which deserve a rather
fuller description than we have given in this general sketch.
The anterior groove (Nos. 1-3, series B, Pl. 27) is at first somewhat
separated from the Wolffian duct, but approaches close to it in No. 3. In
Nos. 2 and 3 there appears a rod-like body on the outer side of the walls
of the groove. In No. 2 this body is disconnected with the walls of the
groove, and even appears as if formed by a second invagination of the
germinal epithelium. In No. 3 this body becomes partially continuous with
the walls of the groove, and finally in No. 4 it becomes completely
continuous with the walls of the groove, and its lumen communicates freely
with the groove[431].
Footnote 431: A deep focus of the rather thick section
represented in No. 3 shewed the body much more nearly in the
position it occupies in No. 4.
The last trace of this body is seen on the upper wall of the groove in No.
5. We believe that the body (_r_1) represents the ridge between the first
and second grooves of the earlier stage; so that in passing from No. 3 to
No. 5 we pass from the first to the second groove. The meaning of the
features of the body _r_1 in No. 2 we do not fully understand, but cannot
regard them as purely accidental, since we have met with more or less
similar features in other series of sections. The second groove becomes
gradually narrower, and finally is continued into the second ridge (No. 8).
The ridge contains a lumen, and is only connected with the germinal
epithelium by a narrow wall of cells. A narrow passage from the body-cavity
leads into that wall for a short distance in No. 8, but it is probably
merely the hinder end of the groove of No. 7. The third groove appears in
No. 11, and opens into the lumen of the second ridge (_r_2) in No. 12. In
No. 13 the groove is closed, and there is present in its place a duct
(_r_3) connected with the germinal epithelium by a wall of cells. This duct
is the further development of the third ridge of the last stage; its lumen
opens into the body-cavity through the third and last groove (_gr_3). In
the next section this duct (_r_3) is entirely separated from the germinal
epithelium, and it may be traced backwards through several sections until
it terminates by a solid point, very much as in the last stage.
In the figures of this series (B) there may be noticed on the outer side of
the Müllerian duct a fold of the germinal epithelium (_x_) forming a second
groove. It is especially conspicuous in the first six sections of the
series. This fold sometimes becomes much deeper, and then forms a groove,
the upper end of which is close to the grooves of the head-kidney. It is
very often much deeper than these are, and without careful study might
easily be mistaken for one of these grooves. Fig. C, taken from a series
slightly younger than B, shews this groove (_x_) in its most exaggerated
form.
The stage we have just described is that of the fullest development of the
head-kidney. In it, as in all the previous stages, there appear to be only
three main openings into the body-cavity; but we have met in some of our
sections with indications of the possible presence of one or two extra
rudimentary grooves.
In an embryo not very much older than the one last described the atrophy of
the head-kidney is nearly completed, and there is present but a single
groove opening into the body-cavity.
In series D (Pl. 28) are represented a number of sections from an embryo at
this stage. Nos. 1 and 2 are sections through the hind end of the single
groove now present. Its walls are widely separated from the Wolffian duct
in front, but approach close to it at the hinder termination of the groove
(No. 2). The features of the single groove present at this stage agree
closely with those of the anterior groove of the previous stages. The
groove is continued into a duct--the Müllerian duct (as it may now be
called, but in a previous stage the hollow ridge connecting the first and
second grooves of the head-kidney)--which, after becoming nearly separated
from the germinal epithelium, is again connected to it by a mass of cells
at two points (Nos. 5, 6, and 8). The germinal epithelium is slightly
grooved and is much reduced in thickness at these points of contact (_gr_2
and _gr_3), and we believe that they are the remnants of the posterior
grooves of the head-kidney present at an earlier stage.
The Müllerian duct has by this stage grown much further backwards, but the
peculiarities of this part of it are treated in a subsequent section.
We consider that, taking into account the rudiments we have just described,
as well as the fact that the features of the single groove at this stage
correspond with those of the anterior groove at an earlier stage, we are
fully justified in concluding that _the permanent abdominal opening of the
Müllerian duct corresponds with the anterior of our three grooves_.
Although we have, on account of their indefiniteness, avoided giving the
ages of the chicks in which the successive changes of the head-kidney may
be observed, we may, perhaps, state that all the changes we have described
are usually completed between the 90th and 120th hour of incubation.
_The Glomerulus of the Head-Kidney._
In connection with the head-kidney in Amphibians there is present, as is
well known, a peculiar vascular body usually described as the glomerulus of
the head-kidney. We have found in the chick a body so completely answering
to this glomerulus that we have hardly any hesitation in identifying it as
such.
In the chick the glomerulus is paired, and consists of a vascular outgrowth
or ridge projecting into the body-cavity on each side at the root of the
mesentery. It extends from the anterior end of the Wolffian body to the
point where the foremost opening of the head-kidney commences. We have
found it at a period slightly earlier than that of the first development of
the head-kidney. It is represented in figs. E and F, Pl. 28, _gl_, and is
seen to form a somewhat irregular projection into the body-cavity, covered
by a continuation of the peritoneal epithelium, and attached by a narrow
stalk to the insertion of the embryonic mesentery (_me_).
In the interior of this body is seen a stroma with numerous vascular
channels and blood corpuscles, and a vascular connection is apparently
becoming established, if it is not so already, between the glomerulus and
the aorta. We have reason to think that the corpuscles and vascular
channels in the glomerulus are developed _in situ_. The stalk connecting
the glomerulus with the attachment of the mesentery varies in thickness in
different sections, but we believe that the glomerulus is continued
unbroken throughout the very considerable region through which it extends.
This point is, however, difficult to make sure of owing to the facility
with which the glomerulus breaks away.
At the stage we are describing, no true Malpighian bodies are present in
the part of the Wolffian body on the same level with the anterior end of
the glomerulus, but the Wolffian body merely consists of the Wolffian duct.
At the level of the posterior part of the glomerulus this is no longer the
case, but here a regular series of primary Malpighian bodies is present
(using the term "primary" to denote the Malpighian bodies developed
directly out of part of the primary segmental tubes), and the glomerulus of
the head-kidney may frequently be seen in the same section as a Malpighian
body. In most sections the two bodies appear quite disconnected, but in
those sections in which the _glomerulus_ of the Malpighian body comes into
view it is seen to be derived from the same formation as the glomerulus of
the head-kidney (Pl. 28, fig. F). It would seem, in fact, that the vascular
tissue of the glomerulus of the head-kidney grows into the concavity of the
Malpighian bodies. Owing to the stage we are now describing, in which we
have found the glomerulus most fully developed, being prior to that in
which the head-kidney appears, it is not possible to determine with
certainty the position of the glomerulus in relation to the head-kidney.
After the development of the head-kidney it is found, however, as we have
already stated, that the glomerulus terminates at a point just in front of
the anterior opening of the head-kidney. It is less developed than before,
but is still present up to the period of the atrophy of the head-kidney. It
does not apparently alter in constitution, and we have not thought it worth
while giving any further representations of it during the later stages of
its existence.
_Summary of the development of the head-kidney and glomerulus._--The first
rudiment of the head-kidney arises as three successive grooves in the
thickened germinal epithelium, connected by ridges, and situated some way
behind the front end of the Wolffian duct. In the next stage the three
ridges connecting the grooves have become more marked, and in each of them
a lumen has appeared, opening at both extremities into the adjoining
grooves. Still later the ridges become more or less completely detached
from the peritoneal epithelium, and the whole head-kidney then consists of
a slightly convoluted duct, with, at the least, three peritoneal openings,
which is posteriorly continued into the Müllerian duct. Still later the
head-kidney atrophies, its two posterior openings vanishing, and its
anterior opening remaining as the permanent opening of the Müllerian duct.
The glomerulus arises as a vascular prominence at the root of the
mesentery, slightly prior in point of time to the head-kidney, and slightly
more forward than it in position. We have not traced its atrophy.
We stated in our preliminary paper that the peculiar structures we had
interpreted as the head-kidney had completely escaped the attention of
previous observers, though we called attention to a well-known figure of
Waldeyer's (copied in the _Elements of Embryology,_ fig. 51). In this
figure a connection between the germinal epithelium and the Müllerian duct
is drawn, which is probably part of the head-kidney, and may be compared
with our figures (Series B, No. 8, and Series D, No. 4). Since we made the
above statement, Dr Gasser has called our attention to a passage in his
valuable memoir on "The Development of the Allantois[432]," in which
certain structures are described which are, perhaps, identical with our
head-kidney. The following is a translation of the passage:--
"In the upper region of Müller's duct I have often observed small
canals, especially in the later stages of development, which appear as
a kind of doubling of the duct, and run for a short distance close to
Müller's duct and in the same direction, opening, however, into the
body-cavity posterior to the main duct. Further, one may often observe
diverticula from the extreme anterior end of the oviduct of the bird,
which form blind pouches and give one the impression of being
receptacula seminis. Both these appearances can quite well be
accounted for on the supposition that an abnormal communication is
effected between the germinal epithelium and Müller's duct at unusual
places; or else that an attempt at such a communication is made,
resulting, however, only in the formation of a diverticulum of the
wall of the oviduct."
Footnote 432: _Beiträge zur Entwicklungsgeschichte d.
Allantois der Müller'schen Gange u. des Afters._ Frankfurt,
1874.
The statement that these accessory canals are late in developing, prevents
us from feeling quite confident that they really correspond with our
head-kidney.
Before passing on to the other parts of this paper it is necessary to say a
few words in justification of the comparison we have made between the
modified abdominal extremity of the Müllerian duct in the chick and the
head-kidney of the Ichthyopsida.
For the fullest statement of what is known with reference to the anatomy
and development of the head-kidney in the lower types we may refer to
Spengel and Fürbringer[433]. We propose ourselves merely giving a
sufficient account of the head-kidney in Amphibia (which appears to be the
type in which the head-kidney can be most advantageously compared with that
in the bird) to bring out the grounds for our determination of the
homologies.
Footnote 433: _Loc. cit._
The development of the head-kidney in Amphibia has been fully elucidated by
the researches of W. Müller[434], Götte[435], and Fürbringer[436], while to
the latter we are indebted for a knowledge of the development of the
Müllerian duct in Amphibians. The first part of the urinogenital system to
develop is the segmental duct (_Vornieregang_ of Fürbringer), which is
formed by a groove-like invagination of the peritoneal epithelium. It
becomes constricted into a duct first of all in the middle, but soon in the
posterior part also. It then forms a duct, ending in front by a groove in
free communication with the body-cavity, and terminating blindly behind.
The open groove in front at first deepens, and then becomes partially
constricted into a duct, which elongates and becomes convoluted, but
remains in communication with the body-cavity by from two to four
(according to the species) separate openings. The manner in which the
primitive single opening is related to the secondary openings is not fully
understood. By these changes there is formed out of the primitive groove an
anterior glandular body, communicating with the body-cavity by several
apertures, and a posterior duct, which carries off the secretion of the
gland, and which, though blind at first, eventually opens into the cloaca.
In addition to these parts there is also formed on each side of the
mesentery, opposite the peritoneal openings, a very vascular projection
into this part of the body-cavity, which is known as the glomerulus of the
head-kidney, and which very closely resembles in structure and position the
body to which we have assigned the same name in the chick.
Footnote 434: _Jenaische Zeitschrift_, Vol. IX. 1875.
Footnote 435: _Entwicklungsgeschichte d. Unke._
Footnote 436: _Loc. cit._
The primitive segmental duct is at first only the duct for the head-kidney,
but on the formation of the posterior parts of the kidney (Wolffian body,
&c.) it becomes the duct for these also.
After the Wolffian bodies have attained to a considerable development, the
head-kidney undergoes atrophy, and its peritoneal openings become
successively closed from before backwards. At this period the formation of
the Müllerian duct takes place. It is a solid constriction of the ventral
or lateral wall of the segmental duct, which subsequently becomes hollow,
and acquires an opening into the body-cavity _quite independent of the
openings of the head-kidney_.
The similarity in development and structure between the head-kidney in
Amphibia and the body we have identified as such in Aves, is to our minds
too striking to be denied. Both consist of two parts--(1) a somewhat
convoluted longitudinal canal, with a certain number of peritoneal
openings; (2) a vascular prominence at the root of the mesentery, which
forms a glomerulus. As to the identity in position of the two organs we
hope to deal with that more fully in speaking of the general structure of
the excretory system, but may say that one of us[437] has already, on other
grounds, attempted to shew that the abdominal opening of the Müllerian duct
in the bird is the homologue of the abdominal opening of the segmental duct
in Amphibia, Elasmobranchii, &c., and that we believe that this homology
will be admitted by most anatomists. If this homology is admitted, the
identity in position of this organ in Aves and Amphibia necessarily
follows. The most striking difference between Aves and Amphibia in relation
to these structures is the fact that in Aves the anterior pore of the
head-kidney remains as the permanent opening of the Müllerian duct, while
in Amphibia, the pores of the head-kidney atrophy, and an entirely fresh
abdominal opening is formed for the Müllerian duct.
Footnote 437: Balfour, "Origin and History of Urinogenital
Organs of Vertebrates," _Journal of Anat. and Phys._ Vol. X.,
and _Monograph on Elasmobranch Fishes._ [This edition Nos. VII.
and X.]
II.
_The Growth of the Müllerian Duct._
Although a great variety of views have been expressed by different
observers on the growth of the Müllerian duct, it is now fairly generally
admitted that it grows in the space between a portion of the thickened
germinal epithelium and the Wolffian duct, but quite independently of both
of them. Both Braun and Egli, who have specially directed their attention
to this point, have for Reptilia and Mammalia fully confirmed the views of
previous observers. We were, nevertheless, induced, partly on account of
the _à priori_ difficulties of this view, and partly by certain peculiar
appearances which we observed, to undertake the re-examination of this
point, and have found ourselves unable altogether to accept the general
account. We propose first describing, in as matter-of-fact a way as
possible, the actual observations we have made, and then stating what
conclusions we think may be drawn from these observations.
We have found it necessary to distinguish three stages in the growth of the
Müllerian duct. Our first stage embraces the period prior to the
disappearance of the head-kidney. At this stage the structure we have
already spoken of as the rudiment of the Müllerian duct consists of a solid
rod of cells, continuous with the third groove of the head-kidney. It
extends through a very few sections, and terminates by a fine point of
about two cells, wedged in between the Wolffian duct and germinal
epithelium (described above, Nos. 7-10, series A, Plate 27).
In an embryo slightly older than the above, such as that from which series
B was taken, but still belonging to our first stage, a definite lumen
appears in the anterior part of the Müllerian duct, which vanishes after a
few sections. The duct terminates in a point which lies in a concavity of
the wall of the Wolffian duct (Plate 27, Nos. 1 and 2, series G). The
limits of the Wolffian wall and the pointed termination of the Müllerian
duct are in many instances quite distinct; but the outline of the Wolffian
duct appears to be carried round the Müllerian duct, and in some instances
the terminal point of the Müllerian duct seems almost to form an integral
part of the wall of the Wolffian duct.
The second of our stages corresponds with that in which the atrophy of the
head-kidney is nearly complete (series D and H, Plate 28).
The Müllerian duct has by this stage made a very marked progress in its
growth towards the cloaca, and, in contradistinction to the earlier stage,
a lumen is now continued close up to the terminal point of the duct. In the
two or three sections before it ends it appears as a distinct oval mass of
cells (No. 10, series D, and No. 1, series H), without a lumen, lying
between and touching the external wall of the Wolffian duct on the one
hand, and the germinal epithelium on the other. It may either lie on the
ventral side of the Wolffian duct (series D), or on the outer side (series
H), but in either case is opposite the maximum thickening of that part of
the germinal epithelium which always accompanies the Müllerian duct in its
backward growth.
In the last section in which any trace of the Müllerian duct can be made
out (series D, No. 11, and series H, No. 2), it has no longer an oval,
well-defined contour, but appears to have completely fused with the wall of
the Wolffian duct, which is accordingly very thick, and occupies the space
which in the previous section was filled by its own wall and the Müllerian
duct. In the following section the thickening in the wall of the Wolffian
duct has disappeared (Plate 28, series H, No. 3), and every trace of the
Müllerian duct has vanished from view. The Wolffian duct is on one side in
contact with the germinal epithelium.
The stage during which the condition above described lasts is not of long
duration, but is soon succeeded by our third stage, in which a fresh mode
of termination of the Müllerian duct is found. (Plate 28, series I.) This
last stage remains up to about the close of the sixth day, beyond which our
investigations do not extend.
A typical series of sections through the terminal part of the Müllerian
duct at this stage presents the following features:
A few sections before its termination the Müllerian duct appears as a
well-defined oval duct lying in contact with the wall of the Wolffian duct
on the one hand and the germinal epithelium on the other (series I, No. 1).
Gradually, however, as we pass backwards, the Müllerian duct dilates; the
external wall of the Wolffian duct adjoining it becomes greatly thickened
and pushed in in its middle part, so as almost to touch the opposite wall
of the duct, and so form a bay in which the Müllerian duct lies (Plate 28,
series I, Nos. 2 and 3). As soon as the Müllerian duct has come to lie in
this bay its walls lose their previous distinctness of outline, and the
cells composing them assume a curious vacuolated appearance. No
well-defined line of separation can any longer be traced between the walls
of the Wolffian duct and those of the Müllerian, but between the two is a
narrow clear space traversed by an irregular network of fibres, in some of
the meshes of which nuclei are present.
The Müllerian duct may be traced in this condition for a considerable
number of sections, the peculiar features above described becoming more and
more marked as its termination is approached. It continues to dilate and
attains a maximum size in the section or so before it disappears. A lumen
may be observed in it up to its very end, but is usually irregular in
outline and frequently traversed by strands of protoplasm. The Müllerian
duct finally terminates quite suddenly (Plate 28, series I, No. 4), and in
the section immediately behind its termination the Wolffian duct assumes
its normal appearance, and the part of its outer wall on the level of the
Müllerian duct comes into contact with the germinal epithelium (Plate 28,
series I, No. 5).
We have traced the growing point of the Müllerian duct with the above
features till not far from the cloaca, but we have not followed the last
phases of its growth and its final opening into the cloaca.
In some of our embryos we have noticed certain rather peculiar structures,
an example of which is represented at _y_ in fig. K, taken from an embryo
of 123 hours, in which all traces of the head-kidney had disappeared. It
consists of a cord of cells, connecting the Wolffian duct and the hind end
of the abdominal opening of the Müllerian duct. At the least one similar
cord was met with in the same embryo, situated just behind the abdominal
opening of the Müllerian duct. We have found similar structures in other
embryos of about the same age, though never so well marked as in the embryo
from which fig. K is taken. We have quite failed to make out the meaning,
if any, of them.
Our interpretation of the appearances we have described in connection with
the growth of the Müllerian duct can be stated in a very few words. Our
second stage, where the solid point of the Müllerian duct terminates by
fusing with the walls of the Wolffian duct, we interpret as meaning that
the Müllerian is growing backwards as a solid rod of cells, split off from
the outer wall of the Wolffian duct; in the same manner, in fact, as in
Amphibia and Elasmobranchii. The condition of the terminal part of the
Müllerian duct during our third stage cannot, we think, be interpreted in
the same way, but the peculiarities of the cells of both Müllerian and
Wolffian ducts, and the indistinctness of the outlines between them, appear
to indicate that the Müllerian duct grows by cells passing from the
Wolffian duct to it. In fact, although in a certain sense the growth of the
two ducts is independent, yet the actual cells which assist in the growth
of the Müllerian duct are, we believe, derived from the walls of the
Wolffian duct.
III.
_General considerations._
The excretory system of a typical Vertebrate consists of the following
parts:--
1. A head-kidney with the characters already described.
2. A duct for the head-kidney--the segmental duct.
3. A posterior kidney--(Wolffian body, permanent kidney, &c. The nature and
relation of these parts we leave out of consideration, as they have no
bearing upon our present investigations). The primitive duct for the
Wolffian body is the segmental duct.
4. The segmental duct may become split into (_a_) a dorsal or inner duct,
which serves as ureter (in the widest sense of the word); and (_b_) a
ventral or outer duct, which has an opening into the body-cavity, and
serves as the generative duct for the female, or for both sexes.
These parts exhibit considerable variations both in their structure and
development, into some of which it is necessary for us to enter.
The head-kidney[438] attains to its highest development in the
Marsipobranchii (Myxine, Bdellostoma). It consists of a longitudinal canal,
from the ventral side of which numerous tubules pass. These tubules, after
considerable subdivision, open by a large number of apertures into the
pericardial cavity. From the longitudinal canal a few dorsal diverticula,
provided with glomeruli, are given off. In the young the longitudinal canal
is continued into the segmental duct; but this connection becomes lost in
the adult. The head-kidney remains, however, through life. In Teleostei and
Ganoidei (?) the head-kidney is generally believed to remain through life,
as the dilated cephalic portion of the kidneys when such is present. In
Petromyzon and Amphibia the head-kidney atrophies. In Elasmobranchii the
head-kidney, so far as is known, is absent.
Footnote 438: I am inclined to give up the view I formerly
expressed with reference to the head-kidney and segmental duct,
viz. "that they were to be regarded as the most anterior
segmental tube, the peritoneal opening of which had become
divided, and which had become prolonged backwards so as to
serve as the duct for the posterior segmental tubes," and
_provisionally_ to accept the Gegenbaur-Fürbringer view which
has been fully worked out and ably argued for by Fürbringer
(_loc. cit._ p. 96). According to this view the head-kidney and
its duct are to be looked on as the primitive and unsegmented
part of the excretory system, more or less similar to the
excretory system of many Trematodes and unsegmented Vermes. The
segmental tubes I regard as a truly segmental part of the
excretory system acquired subsequently.--F. M. B.
The development of the segmental duct and head-kidney (when present) is
still more important for our purpose than their adult structure.
In Myxine the development of these structures is not known. In Amphibia and
Teleostei it takes place upon the same type, viz. by the conversion of a
groove-like invagination of the peritoneal epithelium into a canal open in
front. The head-kidney is developed from the anterior end of this canal,
the opening of which remains in Teleostei single and closes early in
embryonic life, but becomes in Amphibia divided into two, three, or four
openings. In Elasmobranchii the development is very different.
"The first trace of the urinary system makes its appearance as a knob
springing from the intermediate cell-mass opposite the fifth
protovertebra. This knob is the rudiment of the abdominal opening of
the segmental duct, and from it there grows backwards to the level of
the anus a solid column of cells, which constitutes the rudiment of
the segmental duct itself. The knob projects towards the epiblast, and
the column connected with it lies between the mesoblast and epiblast.
The knob and column do not long remain solid, but the former acquires
an opening into the body-cavity continuous with a lumen, which makes
its appearance in the latter."
The difference in the development of the segmental duct in the two types
(Amphibia and Elasmobranchii) is very important. In the one case a
continuous groove of the peritoneal epithelium becomes constricted into a
canal, in the other a solid knob of cells is continued into a rod, at first
solid, which grows backwards without any apparent relation to the
peritoneal epithelium[439].