FORMATION OF THE LAYERS.
In the last chapter the blastoderm was left as a solid lens-shaped mass of
cells, thicker at one end than at the other, its uppermost row of cells
forming a distinct layer. There very soon appears in it a cavity, the
well-known segmentation cavity, or cavity of von Baer, which arises as a
small space in the midst of the blastoderm, near its non-embryonic end
(Pl. 7, fig. 1.).
This condition of the segmentation cavity, though already[116] described,
has nevertheless been met with in one case only. The circumstance of my
having so rarely met with this condition is the more striking because I
have cut sections of a considerable number of blastoderms in the hope of
encountering specimens similar to the one figured, and it can only be
explained on one of the two following hypotheses. Either the stage is very
transitory, and has therefore escaped my notice except in the one instance;
or else the cavity present in this instance is not the true segmentation
cavity, but merely some abnormal structure. That this latter explanation is
a possible one, appears from the fact that such cavities do at times occur
in other parts of the blastoderm. Dr Schultz[117] does not mention having
found any stage of this kind.
Footnote 116: _Qy. Journal of Microsc. Science_, Oct. 1874.
[This Edition, No. V.]
Footnote 117: _Centr. f. Med. Wiss._ No. 38, 1875.
The position of the cavity in question, and its general appearance, incline
me to the view that it is the segmentation cavity[118]. If this is the true
view of its nature the fact should be noted that at first its floor is
formed by the lower layer cells and not by the yolk, and that its roof is
constituted by both the lower layer cells and the epiblast cells. The
relations of the floor undergo considerable modifications in the course of
development.
Footnote 118: Professor Bambeke ("Poissons Osseux," _Mém.
Acad. Belgique_ 1875) describes a cavity in the blastoderm of
Leuciscus rutilus, which he regards as the true segmentation
cavity, but not as identical with the segmentation cavity of
Osseous Fishes, usually so called. Its relations are the same
as those of my segmentation cavity at this stage. This paper
came into my hands at too late a period for me to be able to do
more than refer to it in this place.
The other features of the blastoderm at this stage are very much those of
the previous stage.
The embryonic swelling is very conspicuous. The cells of the blastoderm are
still disposed in two layers: an upper one of slightly columnar cells one
deep, which constitutes the epiblast, and a lower one consisting of the
remaining cells of the blastoderm.
An average cell of the lower layer has a diameter of about 1/900 inch, but
the cells at the periphery of the layer are in some cases considerably
larger than the more central ones. All the cells of the blastoderm are
still completely filled with yolk spherules. In the yolk outside the
peculiar nuclei, before spoken of, are present in considerable numbers.
They seem to have been mistaken by Dr Schultz[119] for cells: there can
however be no question that they are true nuclei.
Footnote 119: _Loc. cit._
In the next stage the relations of the segmentation cavity undergo
important modifications.
The cells which form its floor disappear almost completely from that
position, and the floor becomes formed by the yolk.
The stage, during which the yolk serves as the floor of the segmentation
cavity, extends over a considerable period of time, but during it I have
been unable to detect any important change in the constitution of the
blastoderm. It no doubt gradually extends over the yolk, but even this
growth is not nearly so rapid as in the succeeding stage. Although
therefore the stage I proceed to describe is of long continuance, a
blastoderm at the beginning of it exhibits, both in its external and in its
internal features, no important deviations from one at the end of it.
Viewed from the surface (Pl. 8, fig. A) the blastoderm at this stage
appears slightly oval, but the departure from the circular form is not very
considerable. The long axis of the oval corresponds with what eventually
becomes the long axis of the embryo. From the yolk the blastoderm is still
well distinguished by its darker colour; and it is surrounded by a
concentric ring of light-coloured yolk, the outer border of which shades
insensibly into the normal yolk.
At the embryonic portion of the blastoderm is a slight swelling, clearly
shewn in Plate 8, fig. A, which can easily be detected in fresh and in
hardened embryos. This swelling is to be looked upon as a local
exaggeration of a slightly raised rim present around the whole
circumference of the blastoderm. The roof of the segmentation cavity
(fig. A, _s.c._) forms a second swelling; and in the fresh embryo this
region appears of a darker colour than other parts of the blastoderm.
It is difficult to determine the exact shape of the blastoderm, on account
of the traction exercised upon it in opening the egg; and no reliance can
be placed on the forms assumed by hardened blastoderms. This remark also
applies to the sections of blastoderms of this stage. There can be no doubt
that the minor individual variations exhibited by almost every specimen are
produced in the course of manipulations while the objects are fresh. These
variations may affect even the relative length of a particular region and
certainly the curvature of it. The roof of the segmentation cavity is
especially apt to be raised into a dome-like form.
The main internal feature of this stage is the disappearance of the layer
of cells which, during the first stage, formed the floor of the
segmentation cavity. This disappearance is nevertheless not absolute, and
it is doubtful whether there is any period in which the floor of the cavity
is quite without cells.
Dr Schultz supposes[120] that the entire segmentation cavity is, in the
living animal, filled with a number of loose cells. Though it is not in my
power absolutely to deny this, the point being one which cannot be
satisfactorily investigated in sections, yet no evidence has come under my
notice which would lead to the conclusion that more cells are present in
the segmentation cavity than are represented on Pl. 13, fig. 1, of my
preliminary paper[121], an illustration which is repeated on Pl. 7, fig. 2.
Footnote 120: _Loc. cit._
Footnote 121: _Loc. cit._
The number of cells on the floor of the cavity differs considerably in
different cases, but these cases come under the category of individual
variations, and are not to be looked upon as indications of different
states of development.
In many cases especially large cells are to be seen on the floor of the
cavity (Pl. 7, fig. 2, _bd_). In my preliminary paper[122] the view was
expressed that these are probably cells formed around the nuclei of the
yolk. This view I am inclined to abandon, and to substitute for it the
suggestion made by Dr Schultz, that they are remnants of the larger
segmentation cells which were to be seen in the previous stages.
Footnote 122: _Qy. Journal of Micros. Science_, Oct. 1874.
[This Edition, No. V.]
Plate 7, figs. 2, 3, 4 (all sections of this stage) shew the different
appearances presented by the floor of the segmentation cavity. In only one
of these sections are there any large number of cells upon the floor; and
in no case have cells been observed imbedded in the yolk forming this
floor, as described by Dr Schultz[123], but in all cases the cells simply
rested upon it.
Footnote 123: _Loc. cit._ Probably Dr Schultz, here as in
other cases, has mistaken nuclei for cells.
Passing from the segmentation cavity to the blastoderm itself, the first
feature to be noticed is the more decided differentiation of the epiblast.
This now forms a distinct layer composed of a single row of columnar cells.
These are slightly more columnar in the region of the embryonic swelling
than elsewhere, and become less elongated at the edge of the blastoderm. In
my specimens this layer was never more than one cell deep, but Dr
Schultz[124] states that, in the Elasmobranch embryos investigated by him,
the epiblast was composed of more than a single row of cells.
Footnote 124: _Loc. cit._
Each epiblast cell is filled with yolk-spherules and contains a nucleus.
Very frequently the nuclei in the layer are arranged in a regular row (vide
Pl. 7, fig. 3). In the later blastoderms of this stage there is a tendency
in the cells to assume a wedge-like form with their thin ends pointing
alternately in opposite directions. This arrangement is, however, by no
means strictly adhered to, and the regularity of it is exaggerated in Plate
7, fig. 4.
The nuclei of the epiblast cells have the same characters as those of the
lower layer cells to be presently described, but their intimate structure
can only be successfully studied in certain exceptionally favourable
sections. In most cases the yolk-spherules around them render the finer
details invisible.
There is at this stage no such obvious continuity as in the succeeding
stage between the epiblast and the lower layer cells; and this statement
holds good more especially with the best conserved specimens which have
been hardened in osmic acid (Pl. 7, fig. 4). In these it is very easy to
see that the epiblast simply thins out at the edge of the blastoderm
without exhibiting the slightest tendency to become continuous with the
lower layer cells[125].
Footnote 125: Prof. Haeckel ("Die Gastrula u. die Eifurchung
d. Thiere," _Jenaische Zeitschrift_, Vol. IX.) has
unfortunately copied a figure from my preliminary paper (_loc.
cit._) (repeated now), which I had carefully avoided using for
the purpose of describing the formation of the layers on
account of the epiblast cells in the original having been much
altered by the chromic acid, as a result of which the whole
section gives a somewhat erroneous impression of the condition
of the blastoderm at this stage. I take this opportunity of
pointing out that the colouration employed by Professor Haeckel
to distinguish the layers in this section is not founded on my
statements, but is, on the contrary, in entire opposition to
them. From the section as represented by Professor Haeckel it
might be gathered that I considered the lower layer cells to be
divided into two parts, one derived from the epiblast, while
the other constituted the hypoblast. Not only is no such
division present at this period, but no part of the lower layer
cells, or the mesoblast cells into which they become converted,
can in any sense whatever be said to be derived from the
epiblast.
The lower layer cells form a mass rather than a layer, and constitute the
whole of the blastoderm not included in the epiblast. The shape of this
mass in a longitudinal section may be gathered from an examination of Plate
7, figs. 3 and 4.
It presents an especially thick portion forming the bulk of the embryonic
swelling, and frequently contains one or two cavities, which from their
constancy I regard as normal and not as artificial products.
In addition to the mass forming the embryonic swelling there is seen in
sections another mass of lower layer cells at the opposite extremity of the
blastoderm, connected with the former by a bridge of cells, which
constitutes the roof of the segmentation cavity. The lower layer cells may
thus be divided into three distinct parts:
(1) The embryo swelling.
(2) The thick rim of cells round the edge of the remainder of the
blastoderm.
(3) The cells which form the roof of the segmentation cavity.
These three parts form a continuous whole, but in addition to these there
exist the previously mentioned cells, which rest on the floor of the
segmentation cavity.
With the exception of these latter, the lower layer is composed of cells
having a fairly uniform size, and exhibits no trace of a division into two
layers.
The cells are for the most part irregularly polygonal from mutual pressure;
and in their shape and arrangement, exhibit a marked contrast to the
epiblast cells. A few of the lower layer cells, highly magnified, are
represented in Pl. 7, fig. 2_a_. An average cell measures about 1/800 to
1/900 of an inch, but some of the larger ones on the floor attain to the
1/475 of an inch.
Owing to my having had the good fortune to prepare some especially
favourable specimens of this stage, it has been possible for me to make
accurate observations both upon the nuclei of the cells of the blastoderm,
and upon the nuclei of the yolk.
The nuclei of the blastoderm cells, both of the epiblast and lower layer,
have a uniform structure. Those of the lower layer cells are about 1/1600
of an inch in diameter. Roughly speaking each consists of a spherical mass
of clear protoplasm refracting more highly than the protoplasm of its cell.
The nucleus appears in sections to be divided by deeply stained lines into
a number of separate areas, and in each of these a deeply stained granule
is placed. In some cases two or more of such granules may be seen in a
single area. The whole of the nucleus stains with the colouring reagents
more deeply than the protoplasm of the cells; but this is especially the
case with the granules and lines.
Though usually spherical the nuclei not infrequently have a somewhat lobate
form.
Very similar to these nuclei are the nuclei of the yolk.
One of the most important differences between the two is that of size. The
majority of the nuclei present in the yolk are as large or larger than an
ordinary blastoderm cell; while many of them reach a size very much greater
than this. The examples I have measured varied from 1/500 to 1/250 of an
inch in diameter.
Though they are divided, like the nuclei of the blastoderm, with more or
less distinctness into separate areas by a network of lines, their greater
size frequently causes them to present an aspect somewhat different from
the nuclei of the blastoderm. They are moreover much less regular in
outline than these, and very many of them have lobate projections (Pl. 7,
figs. 2_a_ and 2_c_ and 3), which vary from simple knobs to projections of
such a size as to cause the nucleus to present an appearance of commencing
constriction into halves. When there are several such projections the
nucleus acquires a peculiar knobbed figure. With bodies of this form it
becomes in many cases a matter of great difficulty to decide whether or no
a particular series of knobs, which appear separate in one plane, are
united in a lower plane, whether, in fact, there is present a single
knobbed nucleus or a number of nuclei in close apposition. A nucleus in
this condition is represented in Pl. 7, fig. 2_b_.
The existence of a protoplasmic network in the yolk has already been
mentioned. This in favourable cases may be observed to be in special
connection with the nuclei just described. Its meshes are finer in the
vicinity of the nuclei, and its fibres in some cases almost appear to start
from them (Pl. 7, fig. 12). For reasons which I am unable to explain the
nuclei of the yolk and the surrounding meshwork present appearances which
differ greatly according to the reagent employed. In most specimens
hardened in osmic acid the protoplasm of the nuclei is apparently prolonged
in the surrounding meshwork (Pl. 7, fig. 12). In other specimens hardened
in osmic acid (Pl. 7, fig. 11), and in all hardened in chromic acid (Pl. 7,
fig. 2_a_ and 2_c_), the appearances are far clearer than in the previous
case, and the protoplasmic meshwork merely surrounds the nuclei, without
shewing any signs of becoming continuous with them.
There is also around each nucleus a narrow space in which the spherules of
the yolk are either much smaller than elsewhere or completely absent, vide
Pl. 7, fig. 2_b_.
It has not been possible for me to satisfy myself as to the exact meaning
of the lines dividing these nuclei into a number of distinct areas. My
observations leave the question open as to whether they are to be looked
upon as lines of division, or as protoplasmic lines such as have been
described in nuclei by Flemming[126], Hertwig[127] and Van Beneden[128].
The latter view appears to me to be the more probable one.
Footnote 126: "Entwicklungsgeschichte der Najaden," _Sitz. d.
k. Akad. Wien_, 1875.
Footnote 127: _Morphologische Jahrbuch_, Vol. 1. Heft 3.
Footnote 128: "Développement des Mammifères," _Bul. de l'Acad.
de Belgique_, XL. No. 12, 1875.
Such are the chief structural features presented by these nuclei, which are
present during the whole of the earlier periods of development and retain
throughout the same appearance. There can be little doubt that their
knobbed condition implies that they are undergoing a rapid division. The
arguments for this view I have already insisted on, and, in spite of the
observations of Dr Kleinenberg shewing that similar nuclei of Nephelis do
not undergo division, the case for their doing so in the Elasmobranch eggs
is to my mind a very strong one.
During this stage the distribution of these nuclei in the yolk becomes
somewhat altered from that in the earlier stages. Although the nuclei are
still scattered generally throughout the finer yolk-matter around the
blastoderm, yet they are especially aggregated at one or two points. In the
first place a special collection of them may be noticed immediately below
the floor of the segmentation cavity. They here form a distinct row or even
layer. If the presence of this layer is coupled with the fact that at this
period cells are beginning to appear on the floor of the segmentation
cavity, a strong argument is obtained for the supposition that around these
nuclei cells are being produced, which pass into the blastoderm to form the
floor. Of the actual formation of cells at _this_ period I have not been
able to obtain any satisfactory example, so that it remains a matter of
deduction rather than of direct observation.
Another special aggregation of nuclei is generally present at the periphery
of the blastoderm, and the same amount of doubt hangs over the fate of
these as over that of the previously mentioned nuclei.
The next stage is the most important in the whole history of the formation
of the layers. Not only does it serve to shew, that the process by which
the layers are formed in Elasmobranchii can easily be derived from a simple
gastrula type like that of Amphioxus, but it also serves as the key by
which other meroblastic types of development may be explained. At the very
commencement of this stage the embryonic swelling becomes more
conspicuously visible than it was. It now projects above the level of the
yolk in the form of a rim. At one point, which eventually forms the
termination of the axis of the embryo, this projection is at its greatest;
while on either side of this it gradually diminishes and finally vanishes.
This projection I propose calling, as in my preliminary paper[129], the
embryonic rim.
Footnote 129: _Qy. Journal Microsc. Science_, Oct. 1874. [This
Edition, No. V.]
The segmentation cavity can still be seen from the surface, and a marked
increase in the size of the blastoderm may be noticed. During the stage
last described, the growth was but very slight; hence the rather sudden and
rapid growth which now takes place becomes striking.
Longitudinal sections at this stage, as at the earlier stages, are the most
instructive. Such a section on the same scale as Pl. 7, fig. 4, is
represented in Pl. 7, fig. 5. It passes parallel to the long axis of the
embryo, through the point of greatest development of the embryonic ring.
The three fresh features of the most striking kind are (1) the complete
envelopment of the segmentation cavity within the lower layer cells, (2)
the formation of the embryonic rim, (3) the increase in distance between
the posterior end of the blastoderm and the segmentation cavity. The
segmentation cavity has by no means relatively increased in size. The roof
has precisely its earlier constitution, being composed of an internal
lining of lower layer cells and an external one of epiblast. The thin
lining of lower layer cells is, in the course of mounting the sections,
very apt to fall off; but I am absolutely satisfied that it is never
absent.
The floor of the cavity has undergone an important change, being now formed
by a layer of cells instead of by the yolk. A precisely similar but more
partial change in the constitution of the floor takes place in Osseous
Fishes[130].
Footnote 130: Götte, "Der Keim d. Forelleneies," _Arch. f.
Mikr. Anat._ Vol. IX.; Haeckel, "Die Gastrula u. die Eifurchung
d. Thiere," _Jenaische Zeitschrift_, Bd. IX.
The mode in which the floor is formed is a question of some importance. The
nuclei, which during the last stage formed a row beneath it, probably, as
previously pointed out, take some share in its formation. An additional
argument to those already brought forward in favour of this view may be
derived from the fact that during this stage such a row of nuclei is no
longer present.
This argument may be stated as follows:
Before the floor of cells for the segmentation cavity is formed a number of
nuclei are present in a suitable situation to supply the cells for the
floor; as soon as the floor of cells makes its appearance these nuclei are
no longer to be seen. From this it may be concluded that their
disappearance arises from their having become the nuclei of the cells which
form the floor.
It appears to me most probable that there is a growth inwards from the
whole peripheral wall of the cavity, and that this ingrowth, as well as the
cells derived from the yolk, assist in forming the floor of the cavity. In
Osseous Fish there appears to be no doubt that the floor is largely formed
by an ingrowth of this kind.
A great increase is observable in the distance between the posterior end of
the segmentation cavity and the edge of the blastoderm. This is due to the
rapid growth of the latter combined with the stationary condition of the
former. The growth of the blastoderm at this period is not uniform, but is
more rapid in the non-embryonic than in the embryonic parts.
The main features of the epiblast remain the same as during the last
stages. It is still composed of a very distinct layer one cell deep. Over
the segmentation cavity, and over the whole embryonic end of the
blastoderm, the cells are very thin, columnar, and, roughly speaking,
wedge-shaped with the thin ends pointing alternately in different
directions. For this reason, the nuclei form two rows; but both the rows
are situated near the upper surface of the layer (vide Pl. 7, fig. 5).
Towards the posterior end of the blastoderm the cells are flatter and
broader; and the layer terminates at the non-embryonic end of the
blastoderm without exhibiting the slightest tendency to become continuous
with the lower layer cells. At the embryonic end of the blastoderm the
relations of the epiblast and lower layer cells are very different. At this
part, throughout the whole extent of the embryonic rim, the epiblast is
reflected and becomes continuous with the lower layer cells.
The lower layer cells form, for the most part, a uniform stratum in which
no distinction into mesoblast and hypoblast is to be seen.
Both the lower layer cells and the epiblast cells are still filled with
yolk-spherules.
The structures at the embryonic rim, and the changes which are there taking
place, unquestionably form the chief features of interest at this stage.
The general relations of these parts are very fairly shewn in Pl. 7,
fig. 5, which represents a section passing through the median line of the
embryonic region. They are however more accurately represented in Pl. 7,
fig. 5_a_, taken from the same embryo, but in a lateral part of the
embryonic rim; or in Pl. 7, fig. 6, from a slightly older embryo. In all of
these figures the epiblast cells are reflected at the edge of the embryonic
rim, and become perfectly continuous with the hypoblast cells. A few of the
cells, immediately beyond the line of this reflection, precisely resemble
in character the typical epiblast cells; but the remainder exhibit a
gradual transition into typical lower layer cells. Adjoining these
transitional cells, or partly enclosed in the corner formed between them
and the epiblast, are a few unaltered lower layer cells (_m_), which at
this stage are not distinctly separated from the transitional cells. The
transitional cells form the commencement of the hypoblast (_hy_); and the
cells (_m_) between them and the epiblast form the commencement of the
mesoblast. The gradual conversion of lower layer cells into columnar
hypoblast cells, is a very clear and observable phenomenon in the best
specimens. Where the embryonic rim projects most, a larger number of cells
have assumed a columnar form. Where it projects less clearly, a smaller
number have done so. But in all cases there may be observed a series of
gradations between the columnar cells and the typical rounded lower layer
cells[131].
Footnote 131: When writing my earlier paper I did not feel so
confident about the mode of formation of the hypoblast as I now
do, and even doubted the possibility of determining it from
sections. The facts now brought forward are I hope sufficient
to remove all scepticism on this point.
In the last described embryo, although the embryonic rim had attained to a
considerable development, no trace of the medullary groove had made its
appearance. In an embryo in the next stage of which I propose describing
sections, this structure has become visible.
A surface view of a blastoderm of this age, with the embryo, is represented
on Pl. 8, fig. B; and I shall, for the sake of convenience, in future speak
of embryos of this age as belonging to period B.
The blastoderm is nearly circular. The embryonic rim is represented by a
darker shading at the edge. At one point in this rim may be seen the
embryo, consisting of a somewhat raised area with an axial groove (_mg_).
The head end of the embryo is that which points towards the centre of the
blastoderm, and its free peripheral extremity is at the edge of the
blastoderm.
A longitudinal section of an embryo of the same age as the one figured[132]
is represented on Pl. 7, fig. 7. The general growth has been very
considerable, though as before explained, it is mainly confined to that
part of the blastoderm where the embryonic rim is absent.
Footnote 132: Owing to the small size of the plates this
section has been drawn on a considerably smaller scale than
that represented in fig. 5.
A fresh feature of great importance is the complete disappearance of the
segmentation cavity, the place which was previously occupied by it being
now filled up by an irregular network of cells. There can be little
question that the obliteration of the segmentation cavity is in part due to
the entrance into the blastoderm of fresh cells formed around the nuclei of
the yolk. The formation of these is now taking place with great rapidity
and can be very easily followed.
Since the segmentation cavity ceases to play any further part in the
history of the blastoderm, it will be well shortly to review the main
points in its history.
Its earliest appearance is involved in some obscurity, though it probably
arises as a simple cavity in the midst of the lower layer cells (Pl. 7,
fig. 1). In its second phase the floor ceases to be formed of lower layer
cells, and the place of these is taken by the yolk, on which however a few
scattered cells still remain (Pl. 7, figs. 2, 3, 4). During the third
period of its history, a distinct cellular floor is again formed for it, so
that it comes a second time into the same relations with the blastoderm as
at its earliest appearance. The floor of cells which it receives is in part
due to a growth inwards from the periphery of the blastoderm, and in part
to the formation of fresh cells from the yolk. Coincidently with the
commencing differentiation of hypoblast and mesoblast the segmentation
cavity grows smaller and vanishes.
One of the most important features of the segmentation cavity in the
Elasmobranchii which I have studied, is the fact that throughout its whole
existence its roof is formed of _lower layer cells_. There is not the
smallest question that the segmentation cavity of these fishes is the
homologue of that of Amphioxus, Batrachians, etc., yet in the case of all
of these animals, the roof of the segmentation cavity is formed of epiblast
only. How comes it then to be formed of lower layer cells in
Elasmobranchii?
To this question an answer was attempted in my paper, "Upon the Early
Stages of the Development of Vertebrates[133]." It was there pointed out,
that as the food material in the ovum increases, the bulk of the lower
layer cells necessarily also increases; since these, as far as the
blastoderm is concerned, are the chief recipients of food material. This
causes the lower layer cells to encroach upon the segmentation cavity, and
to close it in not only on the sides, but also above; from the same cause
it results that the lower layer cells assume, from the first, a position
around the spot where the future alimentary cavity will be formed, and that
this cavity becomes formed by a simple split in the midst of the lower
layer cells, and not by an involution.
Footnote 133: _Quart. Journ. of Microscop. Science_, July,
1875. [This Edition, No. VI.]
All the most recent observations[134] on Osseous Fishes tend to shew that
in them, the roof of the segmentation cavity is formed alone of epiblast;
but on account of the great difficulty which is experienced in
distinguishing the layers in the blastoderms of these animals, I still
hesitate to accept as conclusive the testimony on this point.
Footnote 134: Oellacher, _Zeit. f. Wiss. Zoologie_, Bd. XXIII.
Götte, _Archiv f. Mikr. Anat._ Vol. IX. Haeckel, _loc. cit._
In the formation a second time of a cellular floor for the segmentation
cavity in the third stage, the Elasmobranch embryo seems to resemble that
of the Osseous Fish[135]. Upon this feature great stress is laid both by Dr
Götte[136] and Prof. Haeckel[137]: but I am unable to agree with the
interpretation of it offered by them. Both Dr Götte and Prof. Haeckel
regard the formation of this floor as part of an involution to which the
lower layer cells owe their origin, and consider the involution an
equivalent to the alimentary involution of Batrachians, Amphioxus, &c. To
this question I hope to return, but it may be pointed out that my
observations prove that this view can only be true in a very modified
sense; since the invagination by which hypoblast and alimentary canal are
formed in Amphioxus is represented in Elasmobranchii by a structure quite
separate from the ingrowth of cells to form the floor of the segmentation
cavity.
Footnote 135: This floor appears in most Osseous Fish to be
only partially formed. Vide Götte, _loc. cit._
Footnote 136: _Loc. cit._
Footnote 137: _Loc. cit._
The eventual _obliteration_ of the segmentation cavity by cells derived
from the yolk is to be regarded as an inherited remnant of the involution
by which this obliteration was primitively effected. The passage upwards of
cells from the yolk, may possibly be a real survival of the tendency of the
hypoblast cells to grow inwards during the process of involution.
The last feature of the segmentation cavity which deserves notice is its
excentric position. It is from the first situated in much closer proximity
to the non-embryonic than to the embryonic end of the blastoderm. This
peculiarity in position is also characteristic of the segmentation cavity
of Osseous Fishes, as is shewn by the concordant observations of
Oellacher[138] and Götte[139]. Its meaning becomes at once intelligible by
referring to the diagrams in my paper[140] on the Early Stages in the
Development of Vertebrates. It in fact arises from the asymmetrical
character of the primitive alimentary involution in all anamniotic
vertebrates with the exception of Amphioxus.
Footnote 138: _Loc. cit._
Footnote 139: _Loc. cit._
Footnote 140: _Loc. cit._
Leaving the segmentation cavity I pass on to the other features of my
sections.
There is still to be seen a considerable aggregation of cells at the
non-embryonic end of the blastoderm. The position of this, and its
relations with the portion of the blastoderm which at an earlier period
contained the segmentation cavity, indicate that the growth of the
blastoderm is not confined to its edge, but that it proceeds at all points
causing the peripheral parts to glide over the yolk.
The main features of the cells of this blastoderm are the same as they were
in the one last described. In the non-embryonic region the epiblast has
thinned out, and is composed of a single row of cells, which, in the
succeeding stages, become much flattened.
The lower layer cells over the greater part of their extent, have not
undergone any histological changes of importance. Amongst them may
frequently be seen a few exceptionally large cells, which without doubt
have been derived directly from the yolk.
The embryonic rim is now a far more considerable structure than it was.
Vide Pl. 7, fig. 7. Its elongation is mainly effected by the continuous
conversion of rounded lower layer cells into columnar hypoblast cells at
its central or anterior extremity.
This conversion of the lower layer cells into hypoblast cells is still easy
to follow, and in every section cells intermediate between the two are to
be seen. The nature of the changes which are taking place requires for its
elucidation transverse as well as longitudinal sections. Transverse
sections of a slightly older embryo than B are represented on Pl. 7,
fig. 8_a_, 8_b_ and 8_c_.
Of these sections _a_ is the most peripheral or posterior, and _c_ the most
central or anterior. By a combination of transverse and longitudinal
sections, and by an inspection of a surface view, it is rendered clear
that, though the embryonic rim is a far more considerable structure in the
region of the embryo than elsewhere (compare fig. 6 and fig. 7 and 7_a_),
yet that this gain in size is not produced by an outgrowth of the embryo
beyond the rest of the germ, but by the conversion of the lower layer cells
into hypoblast having been carried far further towards the centre of the
germ in the axial line than in the lateral regions of the rim.
The most anterior of the series of transverse sections (Pl. 7, fig. 8_c_) I
have represented, is especially instructive with reference to this point.
Though the embryonic rim is cut through at the sides of the section, yet in
these parts the rim consists of hardly more than a continuity between
epiblast and lower layer cells, and the lower layer cells shew no trace of
a division into mesoblast and hypoblast. In the axis of the embryo,
however, the columnar hypoblast is quite distinct; and on it a small cap of
mesoblast is seen on each side of the medullary groove. Had the embryonic
rim resulted from a projecting growth of the blastoderm, such a condition
could not have existed. It might have been possible to find the hypoblast
formed at the sides of the section and not at the centre; but the reverse,
as in these sections, could not have occurred. Indeed it is scarcely
necessary to have recourse to sections to prove that the growth of the
embryonic rim is towards the centre of the blastoderm. The inspection of a
surface view of a blastoderm at this period demonstrates it beyond a doubt
(Pl. 8, fig. B). The embryo, close to which the embryonic rim is alone
largely developed, does not project outwards beyond the edge of the germ,
but inwards towards its centre.
The space between the embryonic rim and the yolk (Pl. 7, fig. 7, _al._) is
the alimentary cavity. The roof of this is therefore primitively formed of
hypoblast and the floor of yolk. The external opening of this space at the
edge of the blastoderm is the exact morphological homologue of the anus of
Rusconi, or blastopore of Amphioxus, the Amphibians, &c. The importance of
the mode of growth in the embryonic rim depends upon the homology of the
cavity between it and the yolk, with the alimentary cavity of Amphioxus and
Amphibians. Since this homology exists, the direction of the growth of this
cavity ought to be, as it in fact is, the same as in Amphioxus, etc., viz.
towards the centre of the germ and original position of the segmentation
cavity. Thus though a true invagination is not present as in the other
cases, yet this is represented in Elasmobranchii by the continuous
conversion of lower layer cells into hypoblast along a line leading towards
the centre of the blastoderm.
In the parts of the rim adjoining the embryo, the lower layer cells, on
becoming continuous with the epiblast cells, assume a columnar form. At the
sides of the rim this is not strictly the case, and the lower layer cells
retain their rounded form, though quite continuous with the epiblast cells.
One curious feature of the layer of epiblast in these lateral parts of the
rim is the great thickness it acquires before being reflected and becoming
continuous with the hypoblast (Pl. 7, fig. 8_c_). In the vicinity of the
point of reflection there is often a rather large formation of cells around
the nuclei of the yolk. The cells formed here no doubt pass into the
blastoderm, and become converted into columnar hypoblast cells. In some
cases the formation of these cells is very rapid, and they produce quite a
projection on the under side of the hypoblast. Such a case is represented
in Pl. 7, fig. 8_b_, _n.al_. The cells constituting this mass eventually
become converted into the lateral and ventral walls of the alimentary
canal.
The formation of the mesoblast has progressed rapidly. While many of the
lower layer cells become columnar and form the hypoblast, others, between
these and the epiblast, remain spherical. The latter do not at once become
separated as a layer distinct from the hypoblast, and, at first, are only
to be distinguished from them through their different character, vide Plate
7, figs. 6 and 7. They nevertheless constitute the commencing mesoblast.
Thus much of the mode of formation of the mesoblast can be easily made out
in longitudinal sections, but transverse sections throw still further light
upon it.
From these it may at once be seen that the mesoblast is not formed in one
continuous sheet, but as two lateral masses, one on each side of the axial
line of the embryo[141]. In my preliminary account[142] it was stated that
this was a condition of the mesoblast at a very early period, and that it
was probably its condition from the beginning. Sections are now in my
possession which satisfy me that, from the very first, the mesoblast arises
as two distinct lateral masses, one on each side of the axial line.
Footnote 141: Professor Lieberkühn (_Gesellschaft zu Marburg_,
Jan. 1876) finds in Mammalia a bilateral arrangement of the
mesoblast, which he compares with that described by me in
Elasmobranchii. In Mammalia, however, he finds the two masses of
mesoblast connected by a very thin layer of cells, and is
apparently of opinion that a similar thin layer exists in
Elasmobranchii though overlooked by me. I can definitely state
that, whatever may be the condition of the mesoblast in
Mammalia, in Elasmobranchii at any rate no such layer exists.
Footnote 142: _Loc. cit._
In the embryo from which the sections Pl. 7, fig. 8_a_, 8_b_, 8_c_ were
taken, the mesoblast had, in most parts, not yet become separated from the
hypoblast. It still formed with this a continuous layer, though the
mesoblast cells were distinguishable by their shape from the hypoblast. In
only one section (_b_) was any part of the mesoblast quite separated from
the hypoblast.
In the hindermost part of the embryo the mesoblast is at its maximum, and
forms, on each side, a continuous sheet extending from the median line to
the periphery (fig. 8_a_). The rounder form of the mesoblast cells renders
the line of junction between the layer constituted by them and the
hypoblast fairly distinct; but towards the periphery, where the hypoblast
cells have the same rounded form as the mesoblast, the fusion between the
two layers is nearly complete.
In an anterior section the mesoblast is only present as a cap on both sides
of the medullary groove, and as a mass of cells at the periphery of the
section (fig. 8_b_); but no continuous layer of it is present. In the
foremost of the three sections (fig. 8_c_) the mesoblast can scarcely be
said to have become in any way separated from the hypoblast except at the
summit of the medullary folds (_m_).
From these and similar sections it may be certainly concluded, that the
mesoblast becomes first separated from the hypoblast as a distinct layer in
the posterior region of the embryo, and only at a later period in the
region of the head.
In an embryo but slightly more developed than B, the formation of the layer
is quite completed in the region of the embryo. To this embryo I now pass
on.
In the non-embryonic parts of the blastoderm no fresh features of interest
have appeared. It still consists of two layers. The epiblast is composed of
flattened cells, and the lower layer of a network of more rounded cells,
elongated in a lateral direction. The growth of the blastoderm has
continued to be very rapid.
In the region of the embryo (Pl. 7, fig. 9) more important changes have
occurred. The epiblast still remains as a single row of columnar cells. The
hypoblast is no longer fused with the mesoblast, and forms a distinct
dorsal wall for the alimentary cavity. Though along the axis of the embryo
the hypoblast is composed of a single row of columnar cells, yet in the
lateral part of the embryo its cells are less columnar and are one or two
deep.
Owing to the manner in which the mesoblast became split off from the
hypoblast, a continuity is maintained between the hypoblast and the lower
layer cells of the blastoderm (Pl. 7, fig. 9), while the two plates of
mesoblast are isolated and disconnected from any other masses of cells.
The alimentary cavity is best studied in transverse sections. (Vide Pl. 7,
fig. 10_a_, 10_b_ and 10_c_, three sections from the same embryo.) It is
closed in above and at the sides by the hypoblast, and below by the yolk.
In its anterior part a floor is commencing to be formed by a growth of
cells from the walls of the two sides. The cells for this growth are formed
around the nuclei of the yolk; a feature which recalls the fact that in
Amphibians the ventral wall of the alimentary cavity is similarly formed in
part from the so-called yolk cells.
We left the mesoblast as two masses not completely separated from the
hypoblast. During this stage the separation between the two becomes
complete, and there are formed two great lateral plates of mesoblast cells,
one on each side of the medullary groove. Each of these corresponds to a
united vertebral and lateral plate of the higher Vertebrates. The plates
are thickest in the middle and posterior regions (Pl. 7, fig. 10_a_ and
10_b_), but thin out and almost vanish in the region of the head. The
longitudinal section of this stage represented in Pl. 7, fig. 9, passes
through one of the lateral masses of mesoblast cells, and shews very
distinctly its complete independence of all the other cells in the
blastoderm.
From what has been stated with reference to the development of the
mesoblast, it is clear that in Elasmobranchii this layer is derived from
the same mass of cells as the hypoblast, and receives none of its elements
from the epiblast. In connection with its development, as two independent
lateral masses, I may observe, as I have previously done[143], that in this
respect it bears a close resemblance to mesoblast in Euaxes, as described
by Kowalevsky[144]. This resemblance is of some interest, as bearing on a
probable Annelid origin of Vertebrata. Kowalevsky has also shewn[145] that
the mesoblast in Ascidians is similarly formed as two independent masses,
one on each side of the middle line.
Footnote 143: _Quart. Journ. of Microsc. Science_, Oct., 1874.
[This Edition, No. V.]
Footnote 144: "Embryologische Studien an Würmern u.
Arthropoden." _Mémoires de l'Acad. S. Pétersbourg._ Vol. XIV.
1873.
Footnote 145: _Archiv für Mikr. Anat._ Vol. VII.
It ought, however, to be pointed out that a similar bilateral origin of the
mesoblast had been recently met with in Lymnæus by Carl Rabl[146]. A fact
which somewhat diminishes the genealogical value of this feature in the
mesoblast in Elasmobranchii.
Footnote 146: _Jenaische Zeitschrift_, Vol. IX. 1875. A
bilateral development of mesoblast, according to Professor
Haeckel (_loc. cit._), occurs in some Osseous Fish. Hensen,
_Zeit. für Anat. u. Entw._ Vol. 1., has recently described the
mesoblast in Mammalia as consisting of independent lateral
masses.
During the course of this stage the spherules of food-yolk immediately
beneath the embryo are used up very rapidly. As a result of this the
protoplasmic network, so often spoken of, comes very plainly into view.
Considerable areas may sometimes be seen without any yolk-spherule
whatever.
On Pl. 7, fig. 7_a_, and figs. 11 and 12, I have attempted to reproduce the
various appearances presented by this network: and these figures give a
better idea of it than any description. My observations tend to shew that
it extends through the whole yolk, and serves to hold it together. It has
not been possible for me to satisfy myself that it had any definite limits,
but on the other hand, in many parts all my efforts to demonstrate its
presence have failed. When the yolk-spherules are very thickly packed, it
is difficult to make out for certain whether it is present or absent, and I
have not succeeded in removing the yolk-spherules from the network in cases
of this kind. In medium-sized ovarian eggs this network is very easily
seen, and extends through the whole yolk. Part of such an egg is shewn in
Pl. 7, fig. 14. In full-sized ovarian eggs, according to Schultz[147], it
forms, as was mentioned in the first chapter, radiating striæ, extending
from the centre to the periphery of the egg. When examined with the highest
powers, the lines of this network appear to be composed of immeasurably
small granules arranged in a linear direction. These granules are more
distinct in chromic acid specimens than in those hardened in osmic acid,
but are to be seen in both. There can be little doubt that these granules
are imbedded in a thread or thin layer of protoplasm.
Footnote 147: _Archiv für Mikr. Anat._ Vol. XI.
I have already (p. 252) touched upon the relation of this network to the
nuclei of the yolk[148].
Footnote 148: A protoplasmic network resembling in its
essential features the one just described has been noticed by
many observers in other ova. Fol has figured and described a
network or sponge-like arrangement of the protoplasm in the
eggs of Geryonia. (_Jenaische Zeitschrift_, Vol. VII.)
Metschnikoff (_Zeitschrift f. Wiss. Zoologie_, 1874) has
demonstrated its presence in the ova of many Siphonophoriæ and
Medusæ. Flemming ("Entwicklungsgeschichte der Najaden," _Sitz.
der k. Akad. Wien_, 1875) has found it in the ovarian ova of
fresh-water mussels (Anodonta and Unio), but regards it as due
to the action of reagents, since he fails to find it in the
fresh condition. Amongst vertebrates it has been carefully
described by Eimer (_Archiv für Mikr. Anat._, Vol. VIII.) in
the ovarian ova of Reptiles. Eimer moreover finds that it is
continuous with prolongations from cells of the epithelium of
the follicle in which the ovum is contained. According to him
remnants of this network are to be met with in the ripe ovum,
but are no longer present in the ovum when taken from the
oviduct.
During the stages which have just been described specially favourable views
are frequently to be obtained of the formation of cells in the yolk and
their entrance into the blastoderm. Two representations of these are given,
in Pl. 7, fig. 7_a_, and fig. 13. In both of these distinctly circumscribed
cells are to be seen in the yolk (_c_), and in all cases are situated near
to the typical nuclei of the yolk. The cells in the yolk have such a
relation to the surrounding parts, that it is quite certain that their
presence is not due to artificial manipulation, and in some cases it is
even difficult to decide whether or no a cell area is circumscribed round a
nucleus (Pl. 7, fig. 13). Although it would be possible for cells in the
living state to pass from the blastoderm into the yolk, yet the view that
they have done so in the cases under consideration has not much to
recommend it, if the following facts be taken into consideration. (1) That
the cells in the yolk are frequently larger than those in the blastoderm.
(2) That there are present a very large number of nuclei in the yolk which
precisely resemble the nuclei of the cells under discussion. (3) That in
some cases (Pl. 7, fig. 13) cells are seen indistinctly circumscribed as if
in the act of being formed.
Between the blastoderm and the yolk may frequently be seen a membrane-like
structure, which becomes stained with hæmatoxylin, osmic acid etc. It
appears to be a layer of coagulated albumen and not a distinct membrane.
SUMMARY.
At the close of segmentation, the blastoderm forms a somewhat lens-shaped
disc, thicker at one end than at the other; the thicker end being termed
the embryonic end.
It is divided into two layers--an upper one, the epiblast, formed by a
single row of columnar cells; and a lower one, consisting of the remaining
cells of the blastoderm.
A cavity next appears in the lower layer cells, near the non-embryonic end
of the blastoderm, but the cells soon disappear from the floor of this
cavity which then comes to be constituted by yolk alone.
The epiblast in the next stage is reflected for a small arc at the
embryonic end of the blastoderm, and becomes continuous with the lower
layer cells; at the same time some of the lower layer cells of the
embryonic end of the blastoderm assume a columnar form, and constitute the
commencing hypoblast. The portion of the blastoderm, where epiblast and
hypoblast are continuous, forms a projecting structure which I have called
the embryonic rim. This rim increases rapidly by growing inwards more and
more towards the centre of the blastoderm, through the continuous
conversion of lower layer cells into columnar hypoblast.
While the embryonic rim is being formed, the segmentation cavity undergoes
important changes. In the first place, it receives a floor of lower layer
cells, partly from an ingrowth from the two sides, and partly from the
formation of cells around the nuclei of the yolk.
Shortly after the floor of cells has appeared, the whole segmentation
cavity becomes obliterated.
When the embryonic rim has attained to some importance, the position of the
embryo becomes marked out by the appearance of the medullary groove at its
most projecting part. The embryo extends from the edge of the blastoderm
inwards towards the centre.
At about the time of the formation of the medullary groove, the mesoblast
becomes definitely constituted. It arises as two independent plates, one on
each side of the medullary groove, and is entirely derived from lower layer
cells.
The two plates of mesoblast are at first unconnected with any other cells
of the blastoderm, and, on their formation, the hypoblast remains in
connection with all the remaining lower layer cells. Between the embryonic
rim and the yolk is a cavity,--the primitive alimentary cavity. Its roof is
formed of hypoblast, and its floor of yolk. Its external opening is
homologous with the anus of Rusconi, of Amphioxus and the Amphibians. The
ventral wall of the alimentary cavity is eventually derived from cells
formed in the yolk around the nuclei which are there present.
* * * * *
Since the important researches of Gegenbaur[149] upon the meroblastic
vertebrate eggs, it has been generally admitted that the ovum of every
vertebrate, however complicated may be its apparent constitution, is
nevertheless to be regarded as a simple cell. This view is, indeed, opposed
by His[150] and to a very modified extent by Waldeyer[151], and has
recently been attacked from an entirely new standpoint by Götte[152]; but,
to my mind, the objections of these authors do not upset the well founded
conclusions of previous observations.
Footnote 149: "Wirbelthiereier mit partieller Dottertheilung."
Müller's _Arch._ 1861.
Footnote 150: _Erste Anlage des Wirbelthierleibes._
Footnote 151: _Eierstock u. Ei._
Footnote 152: _Entwicklungsgeschichte der Unke._ The important
researches of Götte on the development of the ovum, though
meriting the most careful attention, do not admit of discussion
in this place.
As soon as the fact is recognised that both meroblastic and holoblastic
eggs have the same fundamental constitution, the admission follows,
naturally, though not necessarily, that the eggs belonging to these two
classes differ solely in degree, not only as regards their constitution,
but also as regards the manner in which they become respectively converted
into the embryo. As might have been anticipated, this view has gained a
wide acceptance.
Amongst the observations, which have given a strong objective support to
this view, may be mentioned those of Professor Lankester upon the
development of Cephalopoda[153], and of Dr Götte[154] upon the development
of the Hen's egg. In Loligo Professor Lankester shewed that there appeared,
in the part of the egg usually considered as food-yolk, a number of bodies,
which eventually developed a nucleus and became cells, and that these cells
entered into the blastoderm. These observations demonstrate that in the
eggs of Loligo the so-called food-yolk is merely equivalent to a part of
the egg which in other cases undergoes segmentation.
Footnote 153: _Annals and Magaz. of Natural History_, Vol. XI.
1873, p. 81.
Footnote 154: _Archiv f. Mikr. Anat._ Vol. X.
The observations of Dr Götte have a similar bearing. He made out that in
the eggs of the Hen no sharp line is to be found separating the germinal
disc from the yolk, and that, independently of the normal segmentation, a
number of cells are derived from that part of the egg hitherto regarded as
exclusively food-yolk. This view of the nature of the food-yolk was also
advanced in my preliminary account of the development of
Elasmobranchii[155], and it is now my intention to put forward the positive
evidence in favour of this view, which is supplied from a knowledge of the
phenomena of the development of the Elasmobranch ovum; and then to discuss
how far the facts of the growth of the blastoderm in Elasmobranchii accord
with the view that their large food-yolk is exactly equivalent to part of
the ovum, which in Amphibians undergoes segmentation, rather than some
fresh addition, which has no equivalent in the Amphibian or other
holoblastic ovum.
Footnote 155: _Quart. Journ. of Micr. Science_, Oct. 1874.
Taking for granted that the ripe ovum is a single cell, the question arises
whether in the case of meroblastic ova the cell is not constituted of two
parts completely separated from one another.
Is the meroblastic ovum, before or after impregnation, composed of a
germinal disc in which _all_ the protoplasm of the cell is aggregated, and
of a food-yolk in which _no_ protoplasm is present? or is the protoplasm
present _throughout_, being simply _more concentrated_ at the germinal pole
than elsewhere? If the former alternative is accepted, we must suppose that
the mass of food-yolk is a something added which is not present in
holoblastic ova. If the latter alternative is accepted, it may then be
maintained that holoblastic and meroblastic ova are constituted in the same
way and differ only in the proportions of their constituents.
My own observations in conjunction with the specially interesting
observations of Dr Schultz[156] justify the view which regards the
protoplasm as present throughout the whole ovum, and not confined to the
germinal disc. Our observations shew that a fine protoplasmic network, with
ramifications extending throughout the whole yolk, is present both before
and after impregnation.
Footnote 156: _Archiv f. Mikr. Anat._ Vol. XXI.
The presence of this network is, in itself, only sufficient to prove that
the yolk _may_ be equivalent to part of a holoblastic ovum; to demonstrate
that it is so requires something more, and this link in the chain of
evidence is supplied by the nuclei of the yolk, which have been so often
referred to.
These nuclei arise independently in the yolk, and become the nuclei of
cells which enter the germ and the bodies of which are derived from the
protoplasm of the yolk. Not only so, but the cells formed around these
nuclei play the same part in the development of Elasmobranchii as do the
largest so-called yolk cells in the development of Amphibians. Like the
homologous cells in Amphibians, they mainly serve to form the ventral wall
of the alimentary canal and the blood-corpuscles. The identity in the fate
of the so-called yolk cells of Amphibians with the cells derived from the
yolk in Elasmobranchii, must be considered as a proof of the homology of
the yolk cells in the first case with the yolk in the second; the
difference between the yolk in the two cases arising from the fact that in
the Elasmobranch ovum the yolk-spherules bear a larger proportion to the
protoplasm than they do in the Amphibian ovum. As I have suggested
elsewhere[157], the segmentation or non-segmentation of a particular part
of the ovum depends solely upon the proportion borne by the protoplasm to
the yolk particles; so that, when the latter exceed the former in a certain
fixed proportion, segmentation is no longer possible; and, as this limit is
approached, segmentation becomes slower, and the resulting segments larger
and larger.
Footnote 157: "Comparison," &c., _Quart. Journ. Micr.
Science_, July, 1875. [This Edition, No. VI.]
The question how far the facts in the developmental history of the various
vertebrate blastoderms accord with the view of the nature of the yolk just
propounded is one of considerable interest. An answer to it has already
been attempted from a general point of view in my paper[158] entitled 'The
Comparison of the early stages of development in Vertebrates'; but the
subject may be conveniently treated here in a special manner for
Elasmobranch embryos.
Footnote 158: _Loc. cit._
In the woodcut, fig. 1, _A_, _B_, _C_[159], are represented three
diagrammatic longitudinal sections of an Elasmobranch embryo. _A_ nearly
corresponds with the longitudinal section represented on Pl. 7, fig. 4, and
_B_ with Pl. 7, fig. 7. In Pl. 7, fig. 7, the segmentation cavity has
however completely disappeared, while it is still represented as present in
the diagram of the same period. If these diagrams, or better still, the
woodcuts fig. 2 _A_, _B_, _C_ (which only differ from those of the
Elasmobranch fish in the smaller amount of food-yolk), be compared with the
corresponding ones of Bombinator, fig. 3, _A_, _B_, _C_, they will be found
to be in fundamental agreement with them. First let fig. 1, _A_, or fig. 2,
_A_, or Pl. 7, fig. 4, be compared with fig. 3, _A_. In all there is
present a segmentation cavity situated not centrally but near the surface
of the egg. The roof of the cavity is thin in all, being composed in the
Amphibian of epiblast alone, and in the Elasmobranch of epiblast and _lower
layer cells_. The floor of the cavity is, in all, formed of so-called yolk
(vide Pl. 7, fig. 4), which in all forms the main mass of the egg. In the
Amphibian the yolk is segmented, and, though it is not segmented in the
Elasmobranch, it contains in compensation the nuclei so often mentioned. In
all, the sides of the segmentation cavity are formed by lower layer cells.
In the Amphibian the sides are enclosed by smaller cells (in the diagram)
which correspond exactly in function and position with the lower layer
cells of the Elasmobranch blastoderm.
Footnote 159: This figure, together with figs. 2 and 3, are
reproduced from my paper upon the comparison of the early
stages of development in vertebrates.
[Illustration: FIG. 1.
Diagrammatic longitudinal sections of an Elasmobranch embryo.
_Epiblast_ without shading. _Mesoblast_ black with clear outlines to the
cells. _Lower layer cells_ and _hypoblast_ with simple shading.
_ep._ epiblast. _m._ mesoblast. _al._ alimentary cavity. _sg._ segmentation
cavity. _nc._ neural canal. _ch._ notochord. _x._ point where epiblast and
hypoblast become continuous at the posterior end of the embryo. _n._ nuclei
of yolk.
_A._ Section of young blastoderm, with segmentation cavity in the middle of
the lower layer cells.
_B._ Older blastoderm with embryo in which hypoblast and mesoblast are
distinctly formed, and in which the alimentary slit has appeared. The
segmentation cavity is still represented as being present, though by this
stage it has in reality disappeared.
_C._ Older blastoderm with embryo in which neural canal has become formed,
and is continuous posteriorly with alimentary canal. The notochord, though
shaded like mesoblast, belongs properly to the hypoblast.]
[Illustration: FIG. 2.
Diagrammatic longitudinal sections of embryo, which develops in the same
manner as the Elasmobranch embryo, but in which the ovum contains far less
food-yolk than is the case with the Elasmobranch ovum.
_Epiblast_ without shading. _Mesoblast_ black with clear outlines to the
cells. _Lower layer cells_ and _hypoblast_ with simple shading.
_ep._ epiblast. _m._ mesoblast. _hy._ hypoblast. _sg._ segmentation cavity.
_al._ alimentary cavity. _nc._ neural canal. _hf._ head fold. _n._ nuclei
of the yolk.
The stages _A_, _B_ and _C_ are the same as in figure .][Transcriber's
note: figure number is missing in the original.]
[Illustration: FIG. 3.
Diagrammatic longitudinal sections of Bombinator igneus. Reproduced with
modifications from Götte.
_Epiblast_ without shading. _Mesoblast_ black with clear outlines to the
cells. _Lower layer cells_ and _hypoblast_ with simple shading.
_ep._ epiblast. _l.l._ lower layer cells. _y._ smaller lower layer cells at
the sides of the segmentation cavity. _m._ mesoblast. _hy._ hypoblast.
_al._ alimentary cavity. _sg._ segmentation cavity. _nc._ neural cavity.
_yk._ yolk-cells.
_A_ is the youngest stage in which the alimentary involution has not yet
appeared. _x_ is the point from which the involution will start to form the
dorsal wall of the alimentary tract. The line on each side of the
segmentation cavity, which separates the smaller lower layer cells from the
epiblast cells, is not present in Götte's original figure. The two shadings
employed in the diagram render it necessary to have some line, but at this
stage it is in reality not possible to assert which cells belong to the
epiblast and which to the lower layer.
_B._ In this stage the alimentary cavity has become formed, but the
segmentation cavity is not yet obliterated.
_x._ point where epiblast and hypoblast become continuous.
_C._ The neural canal is already formed, and communicates posteriorly with
the alimentary.
_x._ point where epiblast and hypoblast become continuous.]
The relation of the yolk to the blastoderm in the Elasmobranch embryo at
this stage of development very well suits the view of its homology with the
large cells of the Amphibian ovum. The only essential difference between
the two ova arises from the roof of the segmentation cavity being in the
Elasmobranch embryo formed of lower layer cells, which are absent in the
Amphibian embryo. This difference no doubt depends upon the greater
quantity of yolk particles present in the Elasmobranch ovum. These increase
the bulk of the lower layer cells, which are thus compelled to creep up the
sides of the segmentation cavity till they close it in above.
In the next stage for the Elasmobranch, fig. 1 and 2 _B_ and Pl. 7, fig. 7,
and for the Amphibian, fig. 3, _B_, the agreement between the two types is
again very close. In both for a small portion (_x_) of the edge of the
blastoderm the epiblast and hypoblast become continuous, while at all other
parts the epiblast, accompanied by lower layer cells, grows round the yolk
or round the large cells which correspond to it. The yolk cells of the
Amphibian ovum form a comparatively small mass, and are therefore rapidly
enveloped; while in the case of the Elasmobranch ovum, owing to the greater
mass of the yolk, the same process occupies a long period. In both ova the
portion of the blastoderm, where epiblast and hypoblast become continuous,
forms the dorsal lip of an opening--the anus of Rusconi--which leads into
the alimentary cavity. This cavity has the same relation in both ova. It is
lined dorsally by lower layer cells, and ventrally by yolk or what
corresponds with yolk; the ventral epithelium of the alimentary canal being
in both cases eventually supplied by the yolk cells.
As in the earlier stage, so in the present one, the anatomical relations of
the yolk to the blastoderm in the one case (Elasmobranch) are nearly
identical with those of the yolk cells to the blastoderm in the other
(Amphibian). The main features in which the two embryos differ, during the
stage under consideration, arise from the same cause as the solitary point
of difference during the preceding stage.
In Amphibians, the alimentary cavity is formed coincidently with a true
ingrowth of cells from the point where epiblast and hypoblast become
continuous, and from this ingrowth the dorsal wall of the alimentary cavity
is formed. The same ingrowth causes the obliteration of the segmentation
cavity.
In the Elasmobranchii, owing to the larger bulk of the lower layer cells
caused by the food-yolk, these have been compelled to arrange themselves in
their final position during segmentation, and no room is left for a true
invagination; but instead of this there is formed a simple split between
the blastoderm and the yolk. The homology of this with the primitive
invagination is nevertheless proved by the survival of a number of features
belonging to the ancestral condition in which a true invagination was
present. Amongst the more important of these are the following:--(1) The
continuity of epiblast and hypoblast at the dorsal lip of the anus of
Rusconi. (2) The continuous conversion of indifferent lower layer cells
into hypoblast, which gradually extends backwards towards the segmentation
cavity, and exactly represents the course of the invagination whereby in
Amphibians the dorsal wall of the alimentary cavity is formed. (3) The
obliteration of the segmentation cavity during the period when the
pseudo-invagination is occurring.
The asymmetry of the gastrula or pseudo-gastrula in Cyclostomes,
Amphibians, Elasmobranchii and, I believe, Osseous Fishes, is to be
explained by the form of the vertebrate body. In Amphioxus, where the small
amount of food-yolk present is distributed uniformly, there is no reason
why the invagination and resulting gastrula should not be symmetrical. In
other vertebrates, where more food-yolk is present, the shape and structure
of the body render it necessary for the food-yolk to be stored away on the
ventral side of the alimentary canal. This, combined with the unsymmetrical
position of the anus, which primitively corresponds in position with the
blastopore or anus of Rusconi, causes the asymmetry of the gastrula
invagination, since it is not possible for the part of the ovum which will
become the ventral wall of the alimentary canal, and which is loaded with
food-yolk, to be invaginated in the same fashion as the dorsal wall. From
the asymmetry, so caused, follow a large number of features in vertebrate
development, which have been worked out in some detail in my paper already
quoted[160].
Footnote 160: _Quart. Journ. of Micr. Science_, July, 1875.
[This Edition, No. VI.]
Prof. Haeckel, in a paper recently published[161], appears to imply that
because I do not find absolute invagination in Elasmobranchii, I therefore
look upon Elasmobranchii as militating against his Gastræa theory. I cannot
help thinking that Prof. Haeckel must have somewhat misunderstood my
meaning. The importance of the Gastræa theory has always appeared to me to
consist not in the fact that an actual ingrowth of certain cells occurs--an
ingrowth which might have many different meanings[162]--but in the fact
that the types of early development of all animals can be easily derived
from that of the typical gastrula. I am perfectly in accordance with
Professor Haeckel in regarding the type of Elasmobranch development to be a
simple derivative from that of the gastrula, although believing it to be
without any true ingrowth or invagination of cells.
Footnote 161: "Die Gastrula u. Eifurchung d. Thiere,"
_Jenaische Zeitschrift_, Vol. IX.
Footnote 162: For instance, in Crustaceans it does not in some
cases appear certain whether an invagination is the typical
gastrula invagination, or only an invagination by which, at a
period subsequent to the gastrula invagination, the hind gut is
frequently formed.
Professor Haeckel[163] in the paper just referred to published his view
upon the mutual relationships of the various vertebrate blastoderms. In
this paper, which appeared but shortly after my own[164] on the same
subject, he has put forward views which differ from mine in several
important details. Some of these bear upon the nature of food-yolk; and it
appears to me that Professor Haeckel's scheme of development is
incompatible with the view that the food-yolk in meroblastic eggs is the
homologue of part of the hypoblast of the holoblastic eggs.
Footnote 163: _Loc. cit._
Footnote 164: _Loc. cit._
The following is Professor Haeckel's own statement of the scheme or type,
which he regards as characteristic of meroblastic eggs, pp. 98 and 99.
Jetzt folgt der höchst wichtige und interessante Vorgang,
den ich als Einstülpung der Blastula auffasse und der zur
Bildung der Gastrula führt (Fig. 63, 64)[165]. Es schlägt
sich nämlich der verdickte Saum der Keimscheibe, der
"Randwulst" oder das _Properistom_, nach innen um und eine
dünne Zellenschicht wächst als directe Fortsetzung
desselben, wie ein immer enger werdendes Diaphragma, in die
Keimhöhle hinein. Diese Zellenschicht ist das entstehende
Entoderm (Fig. 64 _i_, 74 _i_). Die Zellen, welche dieselbe
zusammensetzen und aus dem innern Theile des Randwulstes
hervorwachsen, sind viel grösser aber flacher als die Zellen
der Keimhöhlendecke und zeigen ein dunkleres grobkörniges
Protoplasma. Auf dem Boden der Keimhöhle, d. h. also auf der
Eiweisskugel des Nahrungsdotters, liegen sie unmittelbar auf
und rücken hier durch centripetale Wanderung gegen dessen
Mitte vor, bis sie dieselbe zuletzt erreichen und nunmehr
eine zusammenhängende einschichtige Zellenlage auf dem
ganzen Keimhöhlenboden bilden. Diese ist die erste
vollständige Anlage des Darmblatts, Entoderms oder
"Hypoblasts", und von nun an können wir, im Gegensatz dazu
den gesammten übrigen Theil des Blastoderms, nämlich die
mehrschichtige Wand der Keimhöhlendecke als Hautblatt,
Exoderm oder "Epiblast" bezeichnen. Der verdickte Randwulst
(Fig. 64 _w_, 74 _w_), in welchem beide primäre Keimblätter
in einander übergehen, besteht in seinem oberen und äusseren
Theile aus Exodermzellen, in seinem unteren und inneren
Theile aus Entodermzellen.
In diesem Stadium entspricht unser Fischkeim einer
Amphiblastula, welche mitten in der Invagination begriffen
ist, und bei welcher die entstehende Urdarmhöhle eine grosse
Dotterkugel aufgenommen hat. Die Invagination wird nunmehr
dadurch vervollständigt und die Gastrulabildung dadurch
abgeschlossen, dass die Keimhöhle verschwindet. Das
wachsende Entoderm, dem die Dotterkugel innig anhängt, wölbt
sich in die letztere hinein und nähert sich so dem Exoderm.
Die klare Flüssigkeit in der Keimhöhle wird resorbirt und
schliesslich legt sich die obere convexe Fläche des
Entoderms an die untere concave des Exoderms eng an: die
Gastrula des discoblastischen Eies oder die "Discogastrula"
ist fertig (Fig. 65, 76; Meridiandurchschnitt Fig. 66, 75).
Die Discogastrula unsers Knochenfisches in diesem Stadium
der vollen Ausbildung stellt nunmehr eine kreisrunde Kappe
dar, welche wie ein gefüttertes Mützchen fast die ganze
obere Hemisphäre der hyalinen Dotterkugel eng anliegend
bedeckt (Fig. 65). Der Ueberzug des Mützchens entspricht dem
Exoderm (_e_), sein Futter dem Entoderm (_i_). Ersteres
besteht aus drei Schichten von kleineren Zellen, letzteres
aus einer einzigen Schicht von grösseren Zellen. Die
Exodermzellen (Fig. 77) messen 0.006 - 0.009 Mm., und haben
ein klares, sehr feinkörniges Protoplasma. Die
Entodermzellen (Fig. 78) messen 0.02 - 0.03 Mm. und ihr
Protoplasma ist mehr grobkörnig und trüber. Letztere bilden
auch den grössten Theil des Randwulstes, den wir nunmehr als
Urmundrand der Gastrula, als "_Properistoma_" oder auch als
"RUSCONI'schen After" bezeichnen können. Der letztere
umfasst die Dotterkugel, welche die ganze Urdarmhöhle
ausfüllt und weit aus der dadurch verstopften
Urmund-Oeffnung vorragt.
Footnote 165: The references in this quotation are to the
figures in the original.
My objections to the view so lucidly explained in the passage just quoted,
fall under two heads.
(1) That the facts of development of the meroblastic eggs of vertebrates,
are not in accordance with the views here advanced.
(2) That even if these views be accepted as representing the actual facts
of development, the explanation offered of these facts would not be
satisfactory.
* * * * *
Professor Haeckel's views are absolutely incompatible with the facts of
Elasmobranch development, if my investigations are correct.
The grounds of the incompatibility may be summed up under the following
heads:
(1) In Elasmobranchii the hypoblast cells occupy, even before the close of
segmentation, the position which, on Professor Haeckel's view, they ought
only eventually to take up after being involuted from the whole periphery
of the blastoderm.
(2) There is no sign at any period of an invagination of the periphery of
the blastoderm, and the only structure (the embryonic rim) which could be
mistaken for such an invagination is confined to a very limited arc.
(3) The growth of cells to form the floor of the segmentation cavity, which
ought to be part of this general invagination from the periphery, is mainly
due to a formation of cells from the yolk.
It is this ingrowth of cells for the floor of the segmentation cavity
which, I am inclined to think, Professor Haeckel has mistaken for a general
invagination in the Osseous Fish he has investigated.
(4) Professor Haeckel fails to give an account of the asymmetry of the
blastoderm; an asymmetry which is unquestionably also present in the
blastoderm of most Osseous Fishes, though not noticed by Professor Haeckel
in the investigations recorded in his paper.
The facts of development of Osseous Fishes, upon which Professor Haeckel
rests his views, are too much disputed, for their discussion in this place
to be profitable[166]. The eggs of Osseous Fishes appear to me
unsatisfactory objects for the study of this question, partly on account of
all the cells of the blastoderm being so much alike, that it is a very
difficult matter to distinguish between the various layers, and, partly,
because there can be little question that the eggs of existing Osseous
Fishes are very much modified, through having lost a great part of the
food-yolk possessed by the eggs of their ancestors[167]. This disappearance
of the food-yolk must, without doubt, have produced important changes in
development, which would be especially marked in a pelagic egg, like that
investigated by Professor Haeckel.
Footnote 166: A short statement by Kowalevsky on this subject
in a note to his account of the development of Ascidians, would
seem to indicate that the type of development of Osseous Fishes
is precisely the same as that of Elasmobranchii. Kowalevsky
says, _Arch. f. Mikr. Anat._ Vol. VII. p. 114, note 5,
"According to my observations on Osseous Fishes the germinal
wall consists of two layers, an upper and lower, which are
continuous with one another at the border. From the upper one
develops skin and nervous system, from the lower hypoblast and
mesoblast." This statement, which leaves unanswered a number of
important questions, is too short to serve as a basis for
supporting my views, but so far as it goes its agreement with
the facts of Elasmobranch development is undoubtedly striking.
Footnote 167: The eggs of the Osseous Fishes have, I believe,
undergone changes of the same character, but not to the same
extent, as those of Mammalia, which, according to the views
expressed both by Professor Haeckel and myself, are degenerated
from an ovum with a large food-yolk. The grounds on which I
regard the eggs of Osseous Fishes as having undergone an
analogous change, are too foreign to the subject to be stated
here.
The Avian egg has been a still more disputed object than even the egg of
the Osseous Fishes. The results of my own investigations on this subject do
not accord with those of Dr Götte, or the views of Professor Haeckel[168].
Footnote 168: I find myself unable without figures to
understand Dr Rauber's (_Centralblatt für Med. Wiss._ 1874, No.
50; 1875, Nos. 4 and 17) views with sufficient precision to
accord to them either my assent or dissent. It is quite in
accordance with the view propounded in my paper (_loc. cit._)
to regard, with Dr Rauber and Professor Haeckel, the thickened
edge of the blastoderm as the homologue of the lip of the
blastopore in Amphioxus; though an invagination, in the manner
imagined by Professor Haeckel, is no necessary consequence of
this view. If Dr Rauber regards the _whole_ egg of the bird as
the homologue of that of Amphioxus, and the inclosure of the
yolk by the blastoderm as the equivalent to the process of
invagination in Amphioxus, then his views are practically in
accordance with my own.
Apart from disputed points of development, it appears to me that a
comparative account of the development of the meroblastic vertebrate ova
ought to take into consideration the essential differences which exist
between the Avian and Piscian blastoderms, in that the embryo is situated
in the centre of the blastoderm in the first case and at the edge in the
second[169].
Footnote 169: I have suggested in a previous paper
("Comparison," &c., _Quart. Journal of Micr. Science_, July,
1875) that the position occupied by the embryo of Birds at the
centre, and not at the periphery, of the blastoderm may be due
to an abbreviation of the process by which the Elasmobranch
embryos cease to be situated at the edge of the blastoderm
(vide p. 296 and Pl. 9, fig. 1, 2). Assuming this to be the
real explanation of the position of the embryo in Birds, I feel
inclined to repeat a speculation which I made some time ago
with reference to the primitive streak in Birds (_Quart. Journ.
of Micr. Science_, 1873, p. 280). In Birds there is, as is well
known, a structure called the primitive streak, which has been
shewn by the observations of Dursy, corroborated by my
observations (_loc. cit._), to be situated behind the medullary
groove, and to take no part in the formation of the embryo. I
further shewed that the peculiar fusion of epiblast and
mesoblast, called by His the axis cord, was confined to this
structure and did not occur in other parts of the blastoderm.
Nearly similar results have been recently arrived at by Hensen
with reference to the primitive streak in Mammals. The position
of the primitive streak immediately behind the embryo suggests
the speculation that it may represent the line along which the
edges of the blastoderm coalesced, so as to give to the embryo
the central position which it has in the blastoderms of Birds
and Mammals, and that the peculiar fusion of epiblast and
mesoblast at this point may represent the primitive continuity
of epiblast and lower layer cells at the dorsal lip of the anus
of Rusconi in Elasmobranchii. I put this speculation forward as
a mere suggestion, in the hope of elucidating the peculiar
structure of the primitive streak, which not improbably may be
found to be the keystone to the nature of the blastoderm of the
higher vertebrates.
This difference entails important modifications in development, and must
necessarily affect the particular points under discussion. As a result of
the different positions of the embryo in the two cases, there is present in
Elasmobranchii and Osseous Fishes a true anus of Rusconi, or primitive
opening into the alimentary canal, which is absent in Birds. Yet in neither
Elasmobranchii[170] nor Osseous Fishes does the anus of Rusconi correspond
in position with the point where the final closing in of the yolk takes
place, but in them this point corresponds rather with the blastopore of
Birds[171].
Footnote 170: Vide p. 296 and Plate 9, fig. 1 and 2, and Self,
"Comparison," &c., _loc. cit._
Footnote 171: The relation of the anus of Rusconi and
blastopore in Elasmobranchii was fully explained in the paper
above quoted. It was there clearly shewn that neither the one
nor the other exactly corresponds with the blastopore of
Amphioxus, but that the two together do so. Professor Haeckel
states that in the Osseous Fish investigated by him the anus of
Rusconi and the blastopore coincide. This is not the case in
the Salmon.
Owing also to the respective situations of the embryo in the blastoderm,
the alimentary and neural canals communicate posteriorly in Elasmobranchii
and Osseous Fishes, but _not_ in Birds. Of all these points Professor
Haeckel makes no mention.
The support of his views which Prof. Haeckel attempts to gain from Götte's
researches in Mammalia is completely cut away by the recent discoveries of
Van Beneden[172] and Hensen[173].
Footnote 172: "Développement Embryonnaire des Mammifères,"
_Bulletin de l'Acad. r. d. Belgique_, 1875.
Footnote 173: _Loc. cit._
It thus appears that Professor Haeckel's views but ill accord with the
facts of vertebrate development; but even if they were to do so completely
it would not in my opinion be easy to give a rational explanation of them.
Professor Haeckel states that no sharp and fast line can be drawn between
the types of 'unequal' and 'discoidal' segmentation[174]. In the cases of
unequal segmentation he admits, as is certainly the case, that the larger
yolk cells (hypoblast) are simply enclosed by a growth of the epiblast
around them; which is to be looked on as a modification of the typical
gastrula invagination, necessitated by the large size of the yolk cells
(vide Professor Haeckel's paper, Taf. II. fig. 30). In these instances
there is no commencement of an ingrowth in the _manner supposed for
meroblastic ova_.
Footnote 174: For an explanation of these terms, vide Prof.
Haeckel's original paper or the abstract in _Quart. Journ. of
Micr. Science_ for January, 1876.
When the food-yolk becomes more bulky, and the hypoblast does not
completely segment, it is not easy to understand why an ingrowth, which had
no existence in the former case, should occur; nor where it is to come
from. Such an ingrowth as is supposed to exist by Professor Haeckel would,
in fact, break the continuity of development between meroblastic and
holoblastic ova, and thus destroy one of the most important results of the
Gastræa theory.
It is quite easy to suppose, as I have done, that in the cases of discoidal
segmentation, the hypoblast (including the yolk) becomes enclosed by the
epiblast in precisely the same manner as in the cases of unequal
segmentation.
But even if Professor Haeckel supposes that in the unsegmented food-yolk a
fresh element is added to the ovum, it remains quite unintelligible to me
how an ingrowth of cells from a circumferential line, to form a layer which
had no previous existence, can be equivalent to, or derived from, the
invagination of a layer, which exists before the process of invagination
begins, and which remains continuous throughout it.
If Professor Haeckel's views should eventually turn out to be in accordance
with the facts of vertebrate development, it will, in my opinion, be very
difficult to reduce them into conformity with the Gastræa theory.
Although some space has been devoted to an attempt to refute the views of
Professor Haeckel on this question, I wish it to be clearly understood that
my disagreement from his opinions concerns matters of detail only, and that
I quite accept the Gastræa theory in its general bearings.
* * * * *
Observations upon the formation of the layers in Elasmobranchii have
hitherto been very few in number. Those published in my preliminary account
of these fishes are, I believe, the earliest[175].
Footnote 175: I omit all reference to a paper published in
Russian by Prof. Kowalevsky. Being unable to translate it, and
the illustrations being too meagre to be in themselves of much
assistance, it has not been possible for me to make any use of
it.
Since then there has been published a short notice on the subject by Dr
Alex. Schultz[176]. His observations in the main accord with my own. He
apparently speaks of the nuclei of the yolk as cells, and also of the
epiblast being more than one cell deep. In Torpedo alone, amongst the
genera investigated by me, is the layer of epiblast, at about the age of
the last described embryo, composed of more than a single row of cells.
Footnote 176: _Centralblatt f. Med. Wiss._ No. 33, 1875.
EXPLANATION OF PLATE 7.
COMPLETE LIST OF REFERENCE LETTERS.
_c._ Cells formed in the yolk around the nuclei of the yolk. _ep._
Epiblast. _er._ Embryonic ring. _es._ Embryo swelling. _hy._ Hypoblast.
_ll._ Lower layer cells. _ly._ Line separating the yolk from the
blastoderm. _m._ Mesoblast. _mg._ Medullary groove. _n´._ Nuclei of yolk.
_na._ Cells to form ventral wall of alimentary canal which have been
derived from the yolk. _nal._ Cells formed around the nuclei of the yolk
which have entered the hypoblast. _sc._ Segmentation cavity. _vp._ Combined
lateral and vertebral plate of mesoblast.
Fig. 1. Longitudinal section of a blastoderm at the first appearance of the
segmentation cavity.
Fig. 2. Longitudinal section through a blastoderm after the layer of cells
has disappeared from the floor of the segmentation cavity. _bd._ Large cell
resting on the yolk, probably remaining over from the later periods of
segmentation. Magnified 60 diameters. (Hardened in chromic acid.)
The section is intended to illustrate the fact that the nuclei form a layer
in the yolk under the floor of the segmentation cavity. The roof of the
segmentation cavity is broken.
Fig. 2_a_. Portion of same blastoderm highly magnified, to shew the
characters of the nuclei of the yolk _n´_ and the nuclei in the cells of
the blastoderm.
Fig. 2_b_. Large knobbed nucleus from the same blastoderm, very highly
magnified.
Fig. 2_c_. Nucleus of yolk from the same blastoderm.
Fig. 3. Longitudinal section of blastoderm of same stage as fig. 2.
(Hardened in chromic acid.)
Fig. 4. Longitudinal section of blastoderm slightly older than fig. 2.
Magnified 45 diameters. (Hardened in osmic acid.)
It illustrates (1) the characters of the epiblast; (2) the embryonic
swelling; (3) the segmentation cavity.
Fig. 5. Longitudinal section through a blastoderm at the time of the first
appearance of the embryonic rim, and before the formation of the medullary
groove. Magnified 45 diameters.
Fig. 5_a_. Section through the periphery of the embryonic rim of the
blastoderm of which fig. 5 represents a section.
Fig. 6. Section through the embryonic rim of a blastoderm somewhat younger
than that represented on Pl. 8, fig. B.
Fig. 7. Section through the most projecting portion of the embryonic rim of
a blastoderm of the same age as that represented on Pl. 8, fig. B. The
section is drawn on a very considerably smaller scale than that on fig. 5.
It is intended to illustrate the growth of the embryonic rim and the
disappearance of the segmentation cavity.
Fig. 7_a_. Section through peripheral portion of the embryonic rim of the
same blastoderm, highly magnified. It specially illustrates the formation
of a cell (_c_) around a nucleus in the yolk. The nuclei of the blastoderm
have been inaccurately rendered by the artist.
Figs. 8_a_, 8_b_, 8_c_. Three sections of the same embryo. Inserted mainly
to illustrate the formation of the mesoblast as two independent lateral
masses of cells; only half of each section is represented. 8_a_ is the most
posterior of the three sections. In it the mesoblast forms a large mass on
each side, imperfectly separated from the hypoblast. In 8_b_, from the
anterior part of the embryo, the main mass of mesoblast is far smaller, and
only forms a cap to the hypoblast at the highest point of the medullary
fold. In 8_c_ a cap of mesoblast is present, similar to that in 8_b_,
though much smaller. The sections of these embryos were somewhat oblique,
and it has unfortunately happened that while in 8_a_ one side is
represented, in 8_b_ and 8_c_ the other side is figured, had it not been
for this the sections 8_b_ and 8_c_ would have been considerably longer
than 8_a_.
Fig. 9. Longitudinal section of an embryo belonging to a slightly later
stage than B.
This section passes through one of the medullary folds. It illustrates the
continuity of the hypoblast with the remaining lower layer cells of the
blastoderm.
Figs. 10_a_, 10_b_, 10_c_. Three sections of the same embryo belonging to a
stage slightly later than B, Pl. 8. The space between the mesoblast and the
hypoblast has been made considerably too great in the figures of the three
sections.
10_a_. The most posterior of the three sections. It shews the posterior
flatness of the medullary groove and the two isolated vertebral plates.
10_b_. This section is taken from the anterior part of the same embryo and
shews the deep medullary groove and the commencing formation of the ventral
wall of the alimentary canal from the nuclei of the yolk.
10_c_ shews the disappearance of the medullary groove and the thinning out
of the mesoblast plates in the region of the head.
Fig. 11. Small portion of the blastoderm and the subjacent yolk of an
embryo at the time of the first appearance of the medullary groove × 300.
It shews two large nuclei of the yolk (_n_) and the protoplasmic network in
the yolk between them; the network is seen to be closer round the nuclei
than in the intervening space. There are no areas representing cells around
the nuclei.
Fig. 12. Nucleus of the yolk in connection with the protoplasmic network
hardened in osmic acid.
Fig. 13. Portion of posterior end of a blastoderm of stage B, shewing the
formation of cells around the nuclei of the yolk.
Fig. 14. Section through part of a young Scyllium egg, about 1/15th of an
inch in diameter.
_nl._ Protoplasmic network in yolk. _zp._ Zona pellucida. _ch._
Structureless chorion. _fep._ Follicular epithelium. _x._ Structureless
membrane external to this.