DEVELOPMENT OF THE TRUNK DURING STAGES G TO K.
By the stage when the external gills have become conspicuous objects, the
rudiments of the greater number of the important organs of the body are
definitely established.
Owing to this fact the first appearance of the external gills forms a very
convenient break in the Elasmobranch development; and in the present
chapter the history is carried on to the period of this occurrence.
While the last chapter dealt for the most part with the formation of the
main organic systems from the three embryonic layers, the present one has
for its subject the gradual differentiation of these systems into
individual organs. In treating of the development of the separate organs a
divergence from the plan of the last chapter becomes necessary, and the
following arrangement has been substituted for it. First of all an account
is given of the development of the external epiblast, which is followed by
a description of the organs derived from the mesoblast and of the
notochord.
_External Epiblast._
During stages G to I the epiblast[192] is formed of a single layer of
flattened cells; and in this, as in the earlier stages, it deserves to be
especially noticed that the epiblast is never more than _one cell deep_,
and is therefore incapable of presenting any differentiation into nervous
and epidermic layers. (Pl. 11, figs. 1-5.)
Footnote 192: Unless the contrary is stated, the facts
recorded in this chapter apply only to the genera Scyllium and
Pristiurus.
The cells which compose it are flattened and polygonal in outline, but more
or less spindle-shaped in section. They present a strong contrast to the
remaining embryonic cells of the body in possessing a considerable quantity
of clear protoplasm, which in most other cells is almost entirely absent.
Their granular nucleus is rounded or oval, and typically contains a single
nucleolus. Frequently, however, two nucleoli are present, and when this is
the case an area free from granules is to be seen around each nucleolus,
and a dark line, which could probably be resolved into granules by the use
of a sufficiently high magnifying power, divides the nucleus into two
halves. These appearances probably indicate that nuclei, in which two
nucleoli are present, are about to divide.
The epiblast cells vary in diameter from .022 to .026 Mm. and their nuclei
from .014 to .018 Mm. They present a fairly uniform character over the
greater part of the body. In Torpedo they present nearly the same
characters as in Pristiurus and Scyllium, but are somewhat more columnar.
(Pl. 11, fig. 7.)
Along the summit of the back from the end of the tail to the level of the
anus, or slightly beyond this, epiblast cells form a fold--the rudiment of
the embryonically undivided dorsal fin--and the cells forming this, unlike
the general epiblast cells, are markedly columnar; they nevertheless, here
as elsewhere, form but a single layer. (Pl. 11, fig. 3 and 5, _df._)
Although at this stage the dorsal fin is not continued as a fold anteriorly
to the level of the anus, yet a columnar thickening or ridge of epiblast,
extending along the median dorsal line nearly to the level of the heart,
forms a true morphological prolongation of the fin.
On the ventral side of the tail is present a rudiment of the ventral
unpaired fin, which stops short of the level of the anus, but, though less
prominent, is otherwise quite similar to the dorsal fin and continuous with
it round the end of the tail. At this stage the mesoblast has no share in
forming either fin.
In many sections of the tail there may be seen on each side two folds of
skin, which are very regular, and strongly simulate the rudimentary fins
just described. The cells composing them are, however, not columnar, and
the folds themselves are merely artificial products due to shrinking.
At a stage slightly younger than K an important change takes place in the
epiblast.
From being composed of a single layer of cells it becomes two cells deep.
The two layers appear first of all anteriorly, and subsequently in the
remaining parts of the body. At first, both layers are formed of flattened
cells (Pl. 11, figs. 8, 9); but at a stage slightly subsequent to that
dealt with in the present chapter, the cells of the inner of the two layers
become columnar, and thus are established the two strata always present in
the epidermis of adult vertebrates, viz. an outer layer of flattened cells
and an inner one of columnar cells[193].
Footnote 193: The layers are known as epidermic (horny) and
mucous layers by English writers, and as Hornschicht and
Schleimschicht by the Germans. For their existence in all
Vertebrates, vide Leydig _Ueber allgemeine Bedeckungen der
Amphibien_, p. 20. Bonn, 1876.
The history of the epiblast in Elasmobranchii is interesting, from the
light which it throws upon the meaning of the nervous and epidermic layers
into which the epiblast of Amphibians and some other Vertebrates is
divided. The Amphibians and Elasmobranchii present the strongest contrast
in the development of their epiblast, and it is worth while shortly to
review and compare the history of the layer in the two groups.
In Amphibians the epiblast is from the first divided into an outer stratum
formed of a single row of flattened cells, and an inner stratum composed of
several rows of more rounded cells. These two strata were called by
Stricker the nervous and epidermic layers, and these names have been very
generally adopted.
Both strata have a share in forming the general epiblast, and though
eventually they partially fuse together, there can be but little doubt that
the horny layer of the adult epiblast, where such can be
distinguished[194], is derived from the epidermic layer of the embryo, and
the mucous layer of the epiblast from the embryonic nervous layer. Both
layers of the epiblast assist in the formation of the cerebro-spinal
nervous system, and there also at first fuse together[195], though the
epidermic layer probably separates itself again, as the central epithelium
of the spinal canal. The lens and auditory sac are derived exclusively from
the nervous layer of the epidermis, while this layer also has the greater
share in forming the olfactory sac.
Footnote 194: Vide Leydig, _loc. cit._
Footnote 195: Vide Götte, _Entwicklungsgeschichte der Unke_.
In Elasmobranchii the epiblast is at first uniformly composed of a single
row of cells. The part of the layer which will form the central nervous
system next becomes two or three cells deep, but presents no distinction
into two layers; the remaining portions of the layer remain, as before, one
cell deep. Although the epiblast at first presents this simple structure,
it eventually, as we have seen, becomes divided throughout into two layers,
homologous with the two layers which arise so early in Amphibians. The
outer one of the two forms the horny layer of the epidermis and the central
epithelium of the neural canal. The inner one, the mucous layer of the
epidermis and the nervous part of the brain and spinal cord. Both layers
apparently enter into the formation of the organs of sense.
While there is no great difficulty in determining the equivalent parts of
the epidermis in Elasmobranchii and Amphibians, it still remains an open
question in which of these groups the epiblast retains its primitive
condition.
Though it is not easy to bring conclusive proofs on the one side or the
other, the balance of argument appears to me to be decidedly in favour of
regarding the condition of the epiblast in Elasmobranchii, and most other
Vertebrates, as the primitive one, and its condition in Amphibians as a
secondary one, due to the throwing back of the differentiation of their
epiblast into two layers to a very early period in their development.
In favour of this view are the following points: (1) That a _primitive_
division of the epiblast into two layers is unknown in the animal kingdom,
except amongst Amphibians and (?) Osseous Fish. (2) That it appears more
likely for a particular feature of development to be thrown back to an
earlier period, than for such an important feature as a distinction between
two primary layers to be absolutely lost during an early period of
development, and then to reappear again in later stages.
The fact of the epiblast of the neural canal being divided, like the
remainder of the layer, into nervous and epidermic parts, cannot, I think,
be used as an argument in favour of the opposite view to that here
maintained.
It seems probable that the central canal of the nervous system arose as an
involution from the exterior, and therefore that the epidermis lining it is
in reality merely a part of the external epidermis, and as such is
naturally separated from the true nervous structures adjacent to it[196].
Footnote 196: Vide Self, "Development of Spinal Nerves in
Elasmobranchii." _Phil. Transact._ 1876. [This Edition, No.
VIII.]
Leaving the general features of the external skin, I pass to the special
organs derived from it during the stage just anterior to K.
_The unpaired Fins._ The unpaired fins have grown considerably, and the
epiblast composing them becomes, like the remainder of the layer, divided
into two strata, both however composed of more or less columnar cells. The
ventral fin has now become more prominent than the dorsal fin; but the
latter extends forward as a fold quite to the anterior part of the body.
_The paired Fins._ Along each side of the body there appears during this
stage a thickened line of epiblast, which from the first exhibits two
special developments: one of these just in front of the anus, and a second
and better marked one opposite the front end of the segmental duct. These
two special thickenings are the rudiments of the paired fins, which thus
arise as special developments of a continuous ridge on each side, precisely
like the ridges of epiblast which form the rudiments of the unpaired fins.
Similar thickenings to those in Elasmobranchii are found at the ends of the
limbs in the embryos of both Birds and Mammals, in the form of caps of
columnar epiblast[197].
Footnote 197: For Birds, vide _Elements of Embryology_, Foster
and Balfour, pp. 144, 145, and for Mammals, Kölliker,
_Entwicklungsgeschichte_, p. 283.
The ridge, of which the limbs are special developments, is situated on a
level slightly ventral to that of the dorsal aorta, and extends from just
behind the head to the level of the anus. It is not noticeable in surface
views, but appears in sections as a portion of the epiblast where the cells
are more columnar than elsewhere; precisely resembling in this respect the
forward continuation of the dorsal fin. At the present stage the posterior
thickenings of this ridge which form the abdominal fins are so slight as to
be barely visible, and their real nature can only be detected by a careful
comparison between sections of this and the succeeding stages. The
rudiments of the anterior pair of limbs are more visible than those of the
posterior, though the passage between them and the remainder of the ridges
is most gradual. Thus at first the rudiments of both the limbs are nothing
more than slight thickenings of the epiblast, where its cells are more
columnar than elsewhere. During stage K the rudiments of both pairs of
limbs, but especially of the anterior pair, grow considerably, while at the
same time the thickened ridge of epiblast which connects them together
rapidly disappears. The thoracic limbs develop into an elongated projecting
fold of epiblast, in every way like the folds forming the unpaired fins;
while at the same time the cells of the subjacent mesoblast become closely
packed, and form a slight projection, at the summit of which the fold of
the epiblast is situated (Pl. 11, fig. 9). The maximum projection of the
thoracic fin is slightly in advance of the front end of the segmental duct.
The abdominal fins do not, during stage K, develop quite so fast as the
thoracic, and at its close are merely elongated areas where the epiblast is
much thickened, and below which the mesoblast is slightly condensed. In the
succeeding stages they develop into projecting folds of skin, precisely as
do the thoracic fins.
The features of the development of the limbs just described, are especially
well shewn in Torpedo; in the embryos of which the passage from the general
linear thickening of epiblast into the but slightly better marked
thickening of the thoracic fin is very gradual, and the fact of the limb
being nothing else than a special development of the linear lateral
thickening is proved in a most conclusive manner.
If the account just given of the development of the limbs is an accurate
record of what really takes place, it is not possible to deny that some
light is thrown by it upon the first origin of the vertebrate limbs. The
facts can only bear one interpretation, _viz.: that the limbs are the
remnants of continuous lateral fins_.
The unpaired dorsal fin develops as a continuous thickening, which then
grows up into a projecting fold of columnar cells. The greater part of this
eventually atrophies, but three separate lobes are left which form the two
dorsal fins and the upper lobe of the caudal fin.
The development of the limbs is almost identically similar to that of the
dorsal fins. There appears a lateral linear thickening of epiblast, which
however does not, like the similar thickening of the fins, grow into a
distinct fold. Its development becomes confined to two special points, at
each of which is formed a continuous elongated fold of columnar cells
precisely like the fold of skin forming the dorsal fins. These two folds
form the paired fins. If it be taken into consideration that the continuous
lateral fin, of which the rudiment appears in Elasmobranchii, does not
exist in any adult Vertebrate, and also that a continuous dorsal fin exists
in many Fishes, the small differences in development between the paired
fins and the dorsal fins will be seen to be exactly those which might have
been anticipated beforehand. Whereas the continuous dorsal fin, which often
persists in adult fishes, attains a considerable development before
vanishing, the originally continuous lateral one has only a very ephemeral
existence.
While the facts of development strongly favour a view which would regard
the limbs as remnants of a primitively continuous lateral fin, there is
nothing in the structure of the limbs of adult Fishes which is opposed to
this view. Externally they closely resemble the unpaired fins, and both
their position and nervous supply appear clearly to indicate that they do
not belong to one special segment of the body. They appear rather to be
connected with a varying number of segments; a fact which would receive a
simple explanation on the hypothesis here adopted[198].
Footnote 198: For the nervous supply in fishes, vide Stannius,
_Peripher. Nerv. System d. Fische_. In Osseous Fishes he states
that the thoracic fin is supplied by branches from the first
three though sometimes from the first four spinal nerves. In
Acipenser there are branches from the first six nerves. In
Spinax the limb is supplied by the rami anteriores of the
fourth and succeeding ten spinal nerves. In the Rays not only
do the sixteen anterior spinal nerves unite to supply the fin,
but in all there are rami anteriores from thirty spinal nerves
which pass to the thoracic limb.
My researches throw no light on the nature of the skeletal parts of the
limb, but the suggestion which has been made by Günther[199] with reference
to the limb of Ceratodus (the most primitive known), that it is a
modification of a series of parallel rays, would very well suit the view
here proposed.
Footnote 199: _Philosophical Transactions_, 1871.
Dr Dohrn[200] in speaking of the limbs, points out the difficulties in the
way of supposing that they can have originated _de novo_, and not by the
modification of some pre-existing organ, and suggests that the limbs are
modified gill-arches; a view similar to which has been hinted at by
Professor Gegenbaur[201].
Footnote 200: _Ursprung d. Wirbelthiere and
Functionswechsels._
Footnote 201: _Grundriss d. Vergleichenden Anat._ p. 494.
Dr Dohrn has not as yet given the grounds for his determination, so that
any judgment on his views is premature.
None of my observations on Elasmobranchii lends any support to these views;
but perhaps, while regarding the limbs as the remains of a continuous fin,
it might be permissible to suppose that the pelvic and thoracic girdles are
altered remnants of the skeletal parts of some of the gill-arches which
have vanished in existing Vertebrates.
The absence of limbs in the Marsipobranchii and Amphioxus, for reasons
already insisted upon by Dr Dohrn[202], cannot be used as an argument
against limbs having existed in still more primitive Vertebrates.
Footnote 202: _Loc. cit._
Though it does not seem probable that a dorsal and ventral fin can have
existed contemporaneously with lateral fins (at least not as continuous
fins), yet, judging from such forms as the Rays, there is no reason why
small balancing dorsal and caudal fins should not have co-existed with
fully developed lateral fins.
_Mesoblast. G-K._
The mesoblast in stage F forms two independent lateral plates, each with a
splanchnic and somatic layer, and divided, as before explained, into a
vertebral portion and a parietal portion. At their peripheral edge these
plates are continuous with the general mesoblastic tissue of the
non-embryonic part of the blastoderm; except in the free parts of the
embryo, where they are necessarily separated from the mesoblast of the
yolk-sac, and form completely independent lateral masses of cells.
During the stages G and H, the two layers of which the mesoblast is
composed cease to be in contact, and leave between them a space which
constitutes the commencement of the body-cavity (Pl. 10, fig. 1). From the
very first this cavity is more or less clearly divided into two distinct
parts; one of them in the vertebral portion of the plates of mesoblast, the
other in the parietal. The cavity in the parietal part of the plates alone
becomes the true body-cavity. It extends uninterruptedly through the
anterior parts of the embryo, but does not appear in the caudal region,
being there indicated only by the presence of two layers in the mesoblast
plates. Though fairly wide below, it narrows dorsally before becoming
continuous with the cavity in the vertebral plates. The line of junction of
the vertebral and parietal plates is a little ventral to the dorsal summit
of the alimentary canal (Pl. 10, fig. 5). Owing to the fact that the
vertebral plates are split up into a series of segments (protovertebræ),
the section of the body-cavity they enclose is necessarily also divided
into a series of segments, one for each protovertebra.
Thus the whole body-cavity consists of a continuous parietal space which
communicates by a series of apertures with a number of separate cavities
enclosed in the protovertebræ. The cavity in each of the protovertebræ is
formed of a narrowed dorsal and a dilated ventral segment, the latter on
the level of the dorsal aorta (Pl. 11, fig. 5). Cavities are present in all
the vertebral plates with the exception of a few far back in the tail; and
exist in part of the caudal region posterior to that in which a cavity in
the parietal plate is present.
_Protovertebræ._ Each protovertebra[203] or vertebral segment of the
mesoblast plate forms a flattened rectangular body, ventrally continuous
with the parietal plate of mesoblast. During stage G the dorsal edge of the
protovertebræ is throughout on about a level with the ventral third of the
spinal cord. Each vertebral plate is composed of two layers, a somatic and
a splanchnic, and encloses the already-mentioned section of the
body-cavity. The cells of both layers of the plate are columnar, and each
consists of a very large nucleus, invested by a delicate layer of
protoplasm.
Footnote 203: No attempt has been made to describe in detail
the different appearances presented by the protovertebræ in the
various parts of the body, but in each stage a protovertebra
from the dorsal region is taken as typical.
Before the end of stage H the inner or splanchnic wall of the protovertebra
loses its simple constitution, owing to the middle part of it, opposite the
dorsal two-thirds of the notochord, undergoing peculiar changes. These
changes are indicated in transverse sections (Pl. 11, figs. 5 and 6,
_mp´_), by the cells in the part we are speaking of acquiring a peculiar
angular appearance, and becoming one or two deep; and the meaning of the
changes is at once shewn by longitudinal horizontal sections. These prove
(Pl. 12, fig. 10) that the cells in this situation have become elongated in
a longitudinal direction, and, in fact, form typical spindle-shaped
embryonic muscle-cells, each with a large nucleus. Every muscle-cell
extends for the whole length of a protovertebra, and in the present stage,
or at any rate in stage I, acquires a peculiar granulation, which clearly
foreshadows transverse striation (Pl. 12, figs. 11-13).
Thus by stage H a small portion of the splanchnopleure which forms the
inner layer of each protovertebra, becomes differentiated into a distinct
band of longitudinal striated muscles; these almost at once become
functional, and produce the peculiar serpentine movements of the embryo,
spoken of in a previous chapter, p. 291.
It may be well to say at once that these muscles form but a very small part
of the muscles which eventually appear; which latter are developed at a
very much later period from the remaining cells of the protovertebræ. The
band developed at this stage appears to be a special formation, which has
arisen through the action of natural selection, to enable the embryo to
meet its respiratory requirements, by continually moving about, and so
subjecting its body to fresh oxydizing influences; and as such affords an
interesting example of an important structure acquired during and for
embryonic life.
Though the cavities in the protovertebræ are at first perfectly continuous
with the general body-cavity, of which indeed they merely form a
specialized part, yet by the close of stage H they begin to be constricted
off from the general body-cavity, and this process is continued rapidly,
and completed shortly after stage I, and considerably before the
commencement of stage K (Pl. 11, figs. 6 and 8). While this is taking
place, part of the splanchnic layer of each protovertebra, immediately
below the muscle-band just described, begins to proliferate, and produce a
number of cells, which at once grow in between the muscles and the
notochord. These cells are very easily seen both in transverse and
longitudinal sections, and form the commencing vertebral bodies (Pl. 11,
fig. 6, and Pl. 12, figs. 10 and 11, _Vr_).
At first the vertebral bodies have the same segmentation as the
protovertebræ from which they sprang; that is to say, they form masses of
embryonic cells separated from each other by narrow slits, continuous with
the slits separating the protovertebræ. They have therefore at their first
appearance a segmentation completely different from that which they
eventually acquire (Pl. 12, fig. 11).
After the separation of the vertebral bodies from the protovertebræ, the
remaining parts of the protovertebræ may be called muscle-plates; since
they become directly converted into the whole voluntary muscular system of
the trunk. At the time when the cavity of the muscle-plates has become
completely separate from the body-cavity, the muscle-plates themselves are
oblong structures, with two walls enclosing the cavity just mentioned, in
which the original ventral dilatation is still visible. The outer or
somatic wall of the plates retains its previous simple constitution. The
splanchnic wall has however a somewhat complicated structure. It is
composed dorsally and ventrally of a columnar epithelium, but in its middle
portion of the muscle-cells previously spoken of. Between these and the
central cavity of the plates the epithelium forming the remainder of the
layer commences to insert itself; so that between the first-formed muscle
and the cavity of the muscle-plate there appears a thin layer of cells, not
however continuous throughout.
At the end of the period K the muscle-plates have extended dorsally
two-thirds of the way up the sides of the spinal cord, and ventrally to the
level of the segmental duct. Their edges are not straight, but are bent
into an angular form, with the apex pointing forwards. Vide Pl. 12,
fig. 17, _mp_.
Before the end of the period a number of connective-tissue cells make their
appearance, and extend upwards from the dorsal summit of the muscle-plates
around the top of the spinal cord. These cells are at first rounded, but
become typical branched connective-tissue cells before the close of the
period (Pl. 11, figs. 7 and 8).
Between stages I and K the bodies of the vertebræ rapidly increase in size
and send prolongations downwards and inwards to meet below the notochord.
These soon become indistinguishably fused with other cells which appear in
the area between the alimentary cavity and the notochord, but probably
serve alone to form the vertebral bodies, while the cells adjoining them
form the basis for the connective tissue of the kidneys, &c.
The vertebral bodies also send prolongations dorsalwards between the sides
of the spinal cord and the muscle-plates. These grow round till they meet
above the spinal and enclose the dorsal nerve-roots. They soon however
become fused with the dorsal prolongations from the muscle-plates, at least
so far as my methods of investigation enable me to determine; but it
appears to me probable that they in reality remain distinct, and become
converted into the neural arches, while the connective-tissue cells from
the muscle-plates form the adjoining subcutaneous and inter-muscular
connective tissue.
All the cells of the vertebral rudiments become stellate and form typical
embryonic connective-tissue. The rudiments however still retain their
primitive segmentation, corresponding with that of the muscle-plates, and
do not during this period acquire their secondary segmentation. Their
segmentation is however less clear than it was at an earlier period, and in
the dorsal part of the vertebral rudiments is mainly indicated by the
dorsal nerve-roots, which always pass out in the interval between two
vertebral rudiments. Vide Pl. 12, fig. 12, _pr_.
_Intermediate Cell-mass._ At about the period when the muscle-plates become
completely free, a fusion takes place between the somatopleure and
splanchnopleure immediately above the dorsal extremity of the true
body-cavity (Pl. 11, fig. 6). The cells in the immediate neighbourhood of
this fusion form a special mass, which we may call the intermediate
cell-mass--a name originally used by Waldeyer for the homologous cells in
the Chick. Out of it are developed the urinogenital organs and the
adjoining tissues. At first it forms little more than a columnar
epithelium, but by the close of the period is divided into (1) An
epithelium on the free surface; from this are derived the glandular parts
of the kidneys and functional parts of the genital glands; and (2) a
subjacent stroma which forms the basis for the connective-tissue and
vascular parts of these organs.
To the history of these parts a special section is devoted; and I now pass
to the description of the mesoblast which lines the body-cavity and forms
the connective tissue of the body-wall, and the muscular and connective
tissue of the wall of the alimentary canal.
_Body-cavity and Parietal Plates._ By the close of stage H, as has been
already mentioned, a cavity is formed between the somatopleure and
splanchnopleure in the anterior part of the trunk, which rapidly widens
during the succeeding stages. Anteriorly, it invests the heart, which
arises during stage G, as a simple space between the ventral wall of the
throat and the splanchnopleure (Pl. 11, fig. 4). Posteriorly it ends
blindly.
This cavity forms in the region of the heart the rudiment of the
pericardial cavity. The remainder of the cavity forms the true body-cavity.
Immediately behind the heart the alimentary canal is still open to the
yolk-sac, and here naturally the two lateral halves of the body-cavity are
separated from each other. In the tail of the embryo no body-cavity has
appeared by stage I, although the parietal plates of mesoblast are
distinctly divided into somatic and splanchnic layers. In the caudal region
the lateral plates of mesoblast of the two sides do not unite ventrally,
but are, on the contrary, quite disconnected. Their ventral edge is
moreover much swollen (Pl. 11, fig. 1). At the caudal swelling the
mesoblast plates cease to be distinctly divided into somatopleure and
splanchnopleure, and more or less fuse with the hypoblast of the caudal
vesicle (Pl. 11, fig. 2).
Between stages I and K the body-cavity extends backwards behind the point
where the anus is about to appear, though it never reaches quite to the
extreme end of the tail. The backward extension of the body-cavity, as is
primitively the case everywhere, is formed of two independent lateral
halves (Pl. 11, fig. 9_a_). Anteriorly, opposite the hind end of the small
intestine, these two lateral halves unite ventrally to form a single cavity
in which hangs the small intestine (Pl. 11, fig. 8) suspended by a very
short mesentery.
The most important change which takes place in the body-cavity during this
period is the formation of a septum which separates off a pericardial
cavity from the true body-cavity.
Immediately in front of the liver the splanchnic and somatic walls of the
body come into very close contact, and I believe unite over the greater
part of their extent. The septum so formed divides the original body-cavity
into an anterior section or pericardial cavity, and a posterior section or
true body-cavity. There is left, however, on each side dorsally a rather
narrow passage which serves to unite the pericardial cavity in front with
the true body-cavity behind.
In Pl. 11, fig. 8_a_, there is seen on one side a section through this
passage, while on the other side the passage is seen to be connected with
the pericardial cavity.
It is not possible from transverse sections to determine for certain
whether the septum spoken of is complete. An examination of longitudinal
horizontal sections from an embryo belonging to the close of the stage K
has however satisfied me that this septum, by that stage at any rate, is
fully formed.
The two lateral passages spoken of above probably unite in the adult to
form the passage connecting the pericardial with the peritoneal cavity,
which, though provided with but a single orifice into the pericardial
cavity, divides into two limbs before opening into the peritoneal cavity.
The body-cavity undergoes no further changes of importance till the close
of the period.
_Somatopleure and Splanchnopleure._ Both the somatic and splanchnic walls
of the body-cavity during stage I exhibit a simple uniform character
throughout their whole extent. They are formed of columnar cells where they
line the dorsal part of the body-cavity, but ventrally of more rounded and
irregular cells (Pl. 11, fig. 5).
In them may occasionally be seen aggregations of very peculiar and large
cells with numerous highly refracting spherules; the cells forming these
are not unlike the _primitive ova_ to be described subsequently, but are
probably large cells derived from the yolk.
It is during the stage intermediate between I and K that the first changes
become visible which indicate a distinction between an epithelium
(endothelium) lining the body-cavity and the connective tissue adjoining
this.
There are at first but very few connective-tissue cells between the
epithelium of the somatic layer of the mesoblast and the epiblast, but a
connection between them is established by peculiar protoplasmic processes
which pass from the one to the other (Pl. 11, fig. 8). Towards the end of
stage K, however, there appears between the two a network of mesoblastic
cells connecting them together. In the rudimentary outgrowth to form the
limbs the mesoblast cells of the somatic layer are crowded in an especially
dense manner.
From the first the connective-tissue cells around the hypoblastic
epithelium of the alimentary tract are fairly numerous (Pl. 11, fig. 8),
and by the close of this period become concentrically arranged round the
intestinal epithelium, though not divided into distinct layers. A special
aggregation of them is present in the hollow of the rudimentary spiral
valve.
Behind the anal region the two layers of the mesoblast (somatic and
splanchnic) completely fuse during stage K, and form a mass of stellate
cells in which no distinction into two layers can be detected (Pl. 11,
figs. 9_c_, 9_d_).
The alimentary canal, which at first lies close below the aorta, becomes
during this period gradually carried further and further from this,
remaining however attached to the roof of the body-cavity by a thin layer
of the mesoblast of the splanchnopleure formed of an epithelium on each
side, and a few interposed connective-tissue cells. This is the mesentery,
which by the close of stage K is of considerable length in the region of
the stomach, though shorter elsewhere.
* * * * *
The above account of the protovertebræ and body-cavity applies solely to
the genera Pristiurus and Scyllium. The changes of these parts in Torpedo
only differ from those of Pristiurus in unimportant though fairly
noticeable details. Without entering into any full description of these it
may be pointed out that both the true body-cavity and its continuations
into the protovertebræ appear later in Torpedo than in Pristiurus and
Scyllium. In some cases even the muscle-plates become definitely separated
and independent before the true body-cavity has appeared. As a result of
this the primitive continuity of the body-cavity and cavity of the
muscle-plates becomes to a certain extent masked, though its presence may
easily be detected by the obvious continuity which at first exists between
the somatic and splanchnic layers of mesoblast and the two layers of the
muscle-plate. In the muscle-plate itself the chief point to be noticed is
the fact that the earlier formed bands of muscles (_mp´_) arise very much
later, and are less conspicuous, in Torpedo than in the genera first
described. They are however present and functional.
The anatomical relations of the body-cavity itself are precisely the same
in Torpedo as in Pristiurus and Scyllium, and the pericardial cavity
becomes separated from the peritoneal in the same way in all the genera;
the two lateral canals connecting the two cavities being also present in
all the three genera. The two independent parietal plates of mesoblast of
the posterior parts of the body have ventrally a swollen edge, as in
Pristiurus, and in this a cavity appears which forms a posterior
continuation of the true body-cavity.
_Resumé._ The primitive independent mesoblast plates of the two sides of
the body become divided into two layers, a somatic and a splanchnic
(Hautfaserblatt and Darmfaserblatt). At the same time in the dorsal part of
the mesoblast plate a series of transverse splits appear which mark out the
limits of the protovertebræ and serve to distinguish a dorsal or vertebral
part of the plate from a ventral or parietal part.
Between the somatic and splanchnic layers of the mesoblast plate a cavity
arises which is continued quite to the summit of the vertebral part of the
plate. This is the primitive body-cavity; and at first the cavity is
divided into two lateral and independent halves.
The next change which takes place is the complete separation of the
vertebral portion of the plate from the parietal; thereby the upper
segmented part of the body-cavity becomes isolated and separated from the
lower and unsegmented part. In connection with this change in the
constitution of the body-cavity there are formed a series of rectangular
plates, each composed of two layers, a somatic and a splanchnic, between
which is the cavity originally continuous with the body-cavity. The
splanchnic layer of the plates buds off cells to form the rudiments of the
vertebral bodies which are originally segmented in the same planes as the
protovertebræ. The plates themselves remain as the muscle-plates and
develop a special layer of muscle (_mp´_) in their splanchnic layer.
In the meantime the parietal plates of the two sides unite ventrally
throughout the intestinal and cardiac regions of the body, and the two
primitively isolated cavities contained in them coalesce. Posteriorly
however the plates do not unite ventrally, and their contained cavities
remain distinct.
At first the pericardial cavity is quite continuous with the body-cavity;
but by the close of the period included in the present chapter it becomes
separated from the body-cavity by a septum in front of the liver, which is
however pierced dorsally by two narrow channels.
The parts derived from the two layers of the mesoblast (not including
special organs or the vascular system) are as follow:--
From the somatic layer are formed
(1) A considerable part of the voluntary muscular system of
the body.
(2) The dermis.
(3) A large part of the intermuscular connective tissue.
(4) Part of the peritoneal epithelium.
From the splanchnic layer are formed
(1) A great part of the voluntary muscular system.
(2) Part of the intermuscular connective tissue (?).
(3) The axial skeleton.
(4) The muscular and connective-tissue wall of the
alimentary tract.
(5) A great part of the peritoneal epithelium.
_General Considerations._ In the history which has just been given of the
development of the mesoblast, there are several points which appear to me
to throw light upon the primitive origin of that layer. Before entering
into these it is however necessary to institute a comparison between the
history of the mesoblast in Elasmobranchii and in other Vertebrates, in
order to distinguish as far as possible the primitive and the secondary
characters present in the various groups.
Though the Mammals are to be looked on as the most differentiated group
amongst the Vertebrates, yet in their embryonic history they retain many
very primitive features, and, as has been recently shewn by Hensen[204],
present numerous remarkable approximations to the Elasmobranchii. We find
accordingly[205] that the primitive lateral plates of mesoblast undergo
nearly the same changes in these two groups. In Mammals there is at first a
continuous cavity extending through both the parietal and vertebral
portions of each plate, and dividing the plates into a somatic and a
splanchnic layer: this cavity is the primitive body-cavity. The vertebral
portion of each plate with its contained cavity then becomes divided off
from the parietal. The later development of these parts is not accurately
known, but it seems that the outer portion of each vertebral plate,
composed of two layers (somatic and splanchnic) enclosing between them a
remnant of the primitive body-cavity, becomes separated off as a
muscle-plate. The remainder forms a vertebral rudiment, &c. Thus the
extension of the body-cavity into the vertebral portion of the mesoblast,
and the constriction of the vertebral portion of the cavity from the
remainder, are as distinctive features of Mammals as they are of the
Elasmobranchii.
Footnote 204: _Zeitschrift f. Anat. Entwicklungsgeschichte_,
Vol. 1.
Footnote 205: Hensen _loc. cit._
In Birds[206] the horizontal splitting of the mesoblast into somatic and
splanchnic layers appears, as in Mammals, to extend at first to the summit
of the protovertebræ, but these bodies become so early separated from the
parietal plates that this fact has usually been overlooked or denied; but
even on the second day of incubation the outer layer of the protovertebræ
is continuous with the somatic layer of the lateral plates, and the inner
layer and kernel of the protovertebræ with the splanchnic layer of the
lateral plates[207]. After the isolation of the protovertebræ the primitive
position of the split which separated their somatic and splanchnic layers
becomes obscured, but when on the third day the muscle-plates are formed
they are found to be _constituted of two layers, an inner and an outer,
which enclose between them a central cavity_. This remarkable fact, which
has not received much attention, though noticeable in most figures,
receives a simple explanation as a surviving rudiment on Darwinian
principles. The central cavity of the muscle-plate is, in fact, a remnant
of the vertebral extension of the body-cavity, and is the same cavity as
that found in the muscle-plates of Elasmobranchii. The two layers of the
muscle-plate also correspond with the two layers present in Elasmobranchii,
the one belonging to the somatic, the other to the splanchnic layer of
mesoblast. The remainder of the protovertebræ internal to the muscle-plates
is very large in Birds, and is the equivalent of that portion of the
protovertebræ which in Elasmobranchii is split off to form the vertebral
bodies[208] (Pl. 11, figs. 6, 7, 8, _Vr_). Thus, though the history of the
development of the mesoblast is not precisely the same for Birds as for
Elasmobranchii, yet the differences between the two groups are of such a
character as to prove in a striking manner that the Avian development is a
derivation from a more primary form, like that of the Elasmobranchii.
Footnote 206: For the history of protovertebræ and
muscle-plates in Birds, vide _Elements of Embryology_, Foster
and Balfour. The statement there made that the horizontal
splitting of the mesoblast does not extend to the summit of the
vertebral plate, must however be regarded as doubtful.
Footnote 207: Vide _Elements of Embryology_, p. 56.
Footnote 208: Dr Götte, _Entwicklungsgeschichte der Unke_, p.
534, gives a different account of the development of the
protovertebræ from that in the text. He states that the
muscle-plates do not give rise to the main dorso-lateral
muscles, but only to some superficial ventral muscles, while
the dorso-lateral muscles are according to him formed from part
of the kernel of the protovertebræ internal to the
muscle-plates. The account given in the text is the result of
my own investigations, and accords precisely with the recent
statements of Professor Kölliker, _Entwicklungsgeschichte_,
1876.
According to the statements of Bambeke and Götte, the Amphibians present
rather remarkable peculiarities in the development of their muscular
system. Each side-plate of mesoblast is divided into a somatic and a
splanchnic layer, continuous throughout the vertebral and parietal portions
of the plate. The vertebral portions (protovertebræ) of the plates soon
become separated from the parietal, and form an independent mass of cells
constituted of two layers, which were originally continuous with the
somatic and splanchnic layers of the parietal plates. The outer or somatic
layer of the vertebral plates is formed of a single row of cells, but the
inner or splanchnic layer is made up of a central kernel of cells and an
inner single layer. This central kernel is the first portion of the
vertebral body to undergo any change, and it becomes converted into the
main dorso-lateral muscles of the body, which apparently correspond with
the muscles derived from the whole muscle-plate of the Elasmobranchii. From
the inner layer of the splanchnic division there are next formed the main
internal ventral muscles, rectus abdominis, &c., as well as the chief
connective-tissue elements of the parts surrounding the spinal cord. The
outer layer of the vertebral plates forms the dermis and subcutaneous
connective tissue, as well as some of the superficial muscles of the trunk
and the muscles of the limbs.
Dr Götte appears to think that the vertebral plates in Amphibians present a
perfectly normal development very similar to that of other Vertebrates. The
divergences between Amphibians and other Vertebrates appear, however, to
myself, to be very great, and although the very careful account given by Dr
Götte is probably to be relied on, yet some further explanation than he has
offered of the development of these parts amongst the Amphibians would seem
to be required.
A primary stage in which the two layers of the vertebral plates are
continuous with the somatic and splanchnic layers of the body-wall is
equally characteristic of Amphibians, Elasmobranchii and Mammals. In the
subsequent development, however, a great difference between the types
becomes apparent, for whereas in Elasmobranchii both layers of the
vertebral plates combine to form the muscle-plates, out of which the great
dorso-lateral muscles are formed, in Amphibians what appear to be the
equivalent muscles are derived from a few of the cells (the kernel) of the
inner layer of the vertebral plates only. The cells which form the lateral
muscles in Amphibians might be thought to correspond in position with the
cells which become, in Elasmobranchii, converted into the special early
formed band of muscles (_m.p´._), rather than, as their development seems
to indicate, with the whole Elasmobranch muscle-plates[209].
Footnote 209: The type of development of the muscle-plates of
Amphibians would become identical with that of Elasmobranchii if
their first-formed mass of muscle corresponded with the
early-formed muscles of Elasmobranchii, and the remaining cells
of both layers of the protovertebræ became in the course of
development converted into muscle-cells indistinguishable from
those formed at first. Is it possible that, owing to the
distinctness of the first-formed mass of muscle, Dr Götte can
have overlooked the fact that its subsequent growth is carried
on at the expense of the adjacent cells of the somatic layer?
Osseous Fishes are stated to agree with Amphibians in the development of
their protovertebræ and muscular system[210], but further observations on
this point are required.
Footnote 210: Ehrlich, "Ueber den peripher. Theil d.
Urwirbel." _Archiv f. Mic. Anat._ Vol. XI.
Though the development of the general muscular system and muscle-plates
does not, according to existing statements, take place on quite the same
type throughout the Vertebrate subkingdom, yet the comparison which has
been instituted between Elasmobranchii and other Vertebrates appears to
prove that there are one or two common features in their development, which
may be regarded as primitive, and as having been inherited from the
ancestors of Vertebrates. These features are (1) The extension of the
body-cavity into the vertebral plates, and subsequent enclosure of this
cavity between the two layers of the muscle-plates; (2) The primitive
division of the vertebral plate into a somatic and a splanchnic layer, and
the formation of a large part of the voluntary muscular system out of the
splanchnic layer.
* * * * *
The ultimate derivation of the mesoblast forms one of the numerous burning
questions of modern embryology, and there are advocates to be found for
almost every one of the possible views the question admits of.
All who accept the doctrine of descent are agreed that primitively only two
embryonic layers were present--the epiblast and the hypoblast--and that the
mesoblast subsequently appeared as a distinct layer, after a certain
complexity of organization had been attained.
The general agreement stops, however, at this point, and the greatest
divergence of opinion exists with reference to all further questions which
bear on the development of the mesoblast. There appear to be four
possibilities as to the origin of this layer.
It may be derived:
(1) entirely from the epiblast,
(2) partly from the epiblast, and partly from the hypoblast,
(3) entirely from the hypoblast,
(4) or may have no fixed origin.
The fourth of these possibilities may for the present be dismissed, since
it can be only maintained should it turn out that all the other views are
erroneous. The first possibility is supported by the case of the
Coelenterata, and we might almost say by that of this group only[211].
Footnote 211: The most important other instances in addition
to that of the Coelenterata which can be adduced in favour of
the epiblastic origin of the mesoblast are the Bird and Mammal,
in which according to the recent observations of Hensen for the
Mammal, and Kölliker for the Mammal and Bird, the mesoblast is
split off from the epiblast. If the views I have elsewhere put
forward about the meaning of the primitive groove be accepted,
the derivation of the mesoblast from the epiblast in these
instances would be apparent rather than real, and have no deep
morphological significance for the present question.
Other instances may be brought forward from various groups, but
none of these are sufficiently well confirmed to be of any
value in the determination of the present question.
Amongst the Coelenterata the mesoblast, when present, is unquestionably a
derivative of the epiblast, and when, as is frequently the case, a distinct
mesoblast is not present, the muscle-cells form a specialized part of the
epidermic cells.
The condition of the mesoblast in these lowly organized animals is exactly
what might _à priori_ have been anticipated, but the absence throughout the
group of a true body-cavity, or specially developed muscular system of the
alimentary tract, prevents the possibility of generalizing for other
groups, from the condition of the mesoblast in this one.
In those animals in which a body-cavity and muscular alimentary tract are
present, it would certainly appear reasonable to expect the mesoblast to be
derived from both the primitive layers: the voluntary muscular system from
epiblast, and the splanchnic system from the hypoblast. This view has been
taken and strongly advocated by so distinguished an embryologist as
Professor Haeckel, and it must be admitted, that on _à priori_ grounds
there is much to recommend it; there are, however, so far as I am aware of,
comparatively few observed facts in its favour.
Professor Haeckel's own objective arguments in support of his view are as
follows:
(1) From the fact that some investigators derive the mesoblast with
absolute confidence from the hypoblast, while others do so with equal
confidence from the epiblast, he concludes that it is really derived from
both these layers.
(2) A second argument is founded on the supposed derivation of the
mesoblast in Amphioxus from both epiblast and hypoblast. Kowalevsky's
account (on which apparently Prof. Haeckel's[212] statements are based)
appears to me, however, too vague, and his observations too imperfect, for
much confidence to be placed in his statements on this head. It does not
indeed appear to me that the formation of the layers in Amphioxus, till
better known, can be used as an argument for any special view about this
question.
Footnote 212: Vide _Anthropogenie_, p. 197.
(3) Professor Haeckel's own observations on the development of Osseous fish
form a third argument in support of his views. These observations do not,
however, accord with those of the majority of investigators, and not having
been made by means of sections, require further confirmation before they
can be definitely accepted.
(4) A fourth argument rests on the fact that the various embryonic layers
fuse together to form the primitive streak or axis-cord in higher
vertebrates. This he thinks proves that the mesoblast is derived from both
the primitive layers. The primitive streak has, however, according to my
views, quite another significance to that attributed to it by Professor
Haeckel[213]; but in any case Professor Kölliker's researches, and on this
point my own observations accord with his, appear to me to prove that the
fusion which there takes place is only capable of being used as an argument
in favour of an epiblastic origin of the mesoblast, and not of its
derivation from both epiblast and hypoblast.
Footnote 213: Vide Self, "Development of Elasmobranch Fishes,"
_Journal of Anat. and Phys._ Vol. X. note on p. 682, and also
Review of Professor Kölliker's "Entwicklungsgeschichte des
Menschen u. d. höheren Thiere," _Journal of Anat. and Phys._
Vol. X.
The objective arguments in favour of Professor Haeckel's views are not very
conclusive, and he himself does not deny that the mesoblast as a rule
apparently arises as a single and undivided mass from one of the two
primary layers, and only subsequently becomes split into somatic and
splanchnic strata. This original fusion and subsequent splitting of the
mesoblast is explained by him as a secondary condition, a possibility which
cannot by any means be thrown on one side. It seems therefore worth while
examining how far the history of the somatic and splanchnic layers of the
mesoblast in Elasmobranchii and other Vertebrates accords with the
supposition that they were primitively split off from the epiblast and the
hypoblast respectively.
It is well to consider first of all what parts of the mesoblast of the body
might be expected to be derived from the somatic and splanchnic layers on
this view of their origin[214].
Footnote 214: Professor Haeckel speaks of the splitting of the
mesoblast in Vertebrates into a somatic and splanchnic layer as
a secondary process (_Gastrula u. Eifurchung d. Thiere_), but
does not make it clear whether he regards this secondary
splitting as taking place along the old lines. It appears to me
to be fairly certain that even if the original unsplit
condition of the mesoblast is to be regarded as a secondary
condition, yet that the splitting of this must take place along
the old lines, otherwise a change in the position of the
body-cavity in the adult would have to be supposed--an unlikely
change producing unnecessary complication. The succeeding
argument is based on the assumption that the unsplit condition
is a secondary condition, but that the split which eventually
appears in this occurs along the old lines, separating the
primitive splanchnopleure from the primitive somatopleure.
From the somatic layer of the mesoblast there would no doubt be formed the
whole of the voluntary muscular system of the body, the dermis, the
subcutaneous connective tissue, and the connective tissue between the
muscles. It is probable also, though this point is less certain, that the
skeleton would be derived from the somatic layer. From the splanchnic layer
would be formed the connective tissue and muscular layers of the alimentary
tract, and possibly also the vascular system.
Turning to the actual development of these parts, the discrepancy between
theory and fact becomes very remarkable. From the somatic layer of the
mesoblast, part of the voluntary muscular system and the dermis is no doubt
derived, but the splanchnic layer supplies the material, not only for the
muscular wall of the digestive canal and the vascular system, but also for
the whole of the axial skeleton _and a great part of the voluntary muscular
system of the body, including the first-formed muscles_. Though remarkable,
it is nevertheless not inconceivable, that the skeleton might be derived
from the splanchnic mesoblast, but it is very difficult to understand how
there could be formed from it a part of the voluntary muscular system of
the body indistinguishably fused with part of the muscular system derived
from the somatopleure. No fact in my investigations comes out more clearly
than that a great part of the voluntary muscular system is formed from the
splanchnic layer of the mesoblast, yet this fact presents a most serious
difficulty to the view that the somatic and splanchnic layers of the
mesoblast in Vertebrates are respectively derived from the epiblast and
hypoblast.
In spite, therefore, of general _à priori_ considerations of a very
convincing kind which tell in favour of the double origin of the mesoblast,
this view is supported by so few objective facts, and there exists so
powerful an array of facts against it, that at present, at least, it seems
impossible to maintain it. The full strength of the facts against it will
appear more fully in a review of the present state of our knowledge as to
the development of the mesoblast in the different groups.
To this I now pass.
In a paper on the "Early stages of Development in Vertebrates[215]" a short
_resumé_ was given of the development of the mesoblast throughout the
animal kingdom, which it may be worth while repeating here with a few
additions. So far as we know at present, the mesoblast is derived from the
hypoblast in the following groups:
Echinoderms (Hensen, Agassiz, Metschnikoff, Selenka, Götte), Nematodes
(Bütschli), Sagitta (Kowalevsky, Bütschli), Lumbricus and probably other
Annelids (Kowalevsky), Brachiopoda (Kowalevsky), Crustaceans (Bobretzky),
Insects (Kowalevsky, Ulianin, Dohrn), Myriapods (Metschnikoff), Tunicates
(Kowalevsky, Kuppfer), Petromyzon (Owsjanikoff), Osseous fishes (Oellacher,
Götte, Kowalevsky), Elasmobranchii (Self), Amphibians (Remak, Stricker,
Götte).
Footnote 215: _Quart. Jl. of Micros. Science_, July, 1875.
[This Edition, No. VI.]
The list includes members from the greater number of the groups of the
animal kingdom; the most striking omissions being the Coelenterates,
Mollusks, and the Amniotic Vertebrates. The absence of the Coelenterates
has been already explained and my grounds for regarding the Amniotic
Vertebrates as apparent rather than real exceptions have also been pointed
out. The Mollusks, however, remain as a large group, in which we as yet
know very little as to the formation of the mesoblast.
Dr Rabl[216], who seems recently to have studied the development of Lymnæus
by means of sections, gives some figures shewing the origin of the
mesoblast; they are, however, too diagrammatic to be of much service in
settling the present question, and the memoirs of Professor Lankester[217]
and Dr Fol[218] are equally inconclusive for this purpose, for, though they
contain figures of elongated and branched mesoblast cells passing from the
epiblast to the hypoblast, no satisfactory representations are given of the
origin of these cells. I have myself observed in embryos of Turbo or
Trochus similar elongated cells to those figured by Lankester and Fol, but
was unable clearly to determine whence they arose. The most accurate
observations which we have on this question are those of Professor
Bobretzky[219]. In Nassa he finds that the three embryonic layers are all
established during segmentation. The outermost and smallest cells form the
epiblast, somewhat larger cells adjoining these the mesoblast, and the
large yolk-cells the hypoblast. These observations do not, however,
demonstrate from which of the primary layers the mesoblast is derived.
Footnote 216: _Jenaische Zeitschrift_, Vol. IX.
Footnote 217: _Quart. Jl. of Micros. Science_, Vol. XXV. 1874,
and _Phil. Trans._ 1875.
Footnote 218: _Archives de Zoologie_, Vol. IV.
Footnote 219: _Archiv f. Micr. Anat._ Vol. XIII.
The evidence at present existing is clearly in favour of the mesoblast
being, in almost all groups of animals, developed from the hypoblast, but
strong as this evidence is, it has not its full weight unless the actual
manner in which the mesoblast is in many groups derived from the hypoblast,
is taken into consideration. The most important of these are the
Echinoderms, Brachiopods and Sagitta.
In the Echinoderms the mesoblast is in part formed by cells budded off from
the hypoblast, _the remainder, however, arises as one or more diverticula
of the alimentary tract_. From the separate cells first budded off there
are formed the cutis, part of the connective tissue and the calcareous
skeleton[220]. The diverticula from the alimentary cavity form the
water-vascular system and the somatic and splanchnic layers of mesoblast.
_The cavity of the diverticula after the separation of the water-vascular
system, forms the body-cavity. The outer lining layer of the cavity forms
the somatic layer of mesoblast and the voluntary muscles; the inner lining
layer the splanchnic mesoblast which unites with the epithelium of the
alimentary tract._ Though this fundamental arrangement would seem to be
universal amongst Echinoderms, considerable variations of it are exhibited
in different groups.
Footnote 220: The recent researches of Selenka, _Zeitschrift
f. Wiss. Zoologie_, Vol. XXVII. 1876, demonstrate that in
Echinoderms the muscles are derived from the cells first split
off from the hypoblast, and that the diverticula only form the
water-vascular system and the epithelial lining of the
body-cavity.
There is _one_ outgrowth from the alimentary tract in Synapta; _two_ in
Echinoids, Asteroids and Ophiura; _three_ in Comatula, and four (?) in
Amphiura. The cavity of the outgrowth usually forms the body-cavity, but
sometimes in Ophiura and Amphiura (Metschnikoff) the outgrowths are from
the first or soon become solid, and only secondarily acquire a cavity,
which is however homologous with the body-cavity of the other groups.
In Sagitta[221] the formation of the mesoblast and the alimentary tract
takes place in nearly the same fashion as in the Echinoderms. The simple
invaginate alimentary cavity becomes divided into three lobes, a central
and two lateral. The two lateral lobes are gradually more and more
constricted off from the central one, and become eventually quite separated
from it; their cavities remain independent, _and form in the adult the
body-cavity_, divided by a mesentery into two distinct lateral sections.
_The inner layer of each of the two lateral lobes forms the mesoblast of
the splanchnopleure, the outer layer the mesoblast of the somatopleure._
The central division of the primitive gastræa cavity remains as the
alimentary tract of the adult.
Footnote 221: Kowalevsky, "Würmer u. Arthropoden," _Mém. Acad.
Pétersbourg_, 1871.
The remarkable observations of Kowalevsky[222] on the development of the
Brachiopoda have brought to light the unexpected fact that in two genera at
least (Argiope and Terebratula) the mesoblast and body-cavity develop as
paired constrictions from the alimentary tract in a manner almost
identically the same as in Sagitta.
Footnote 222: "Zur Entwicklungsgeschichte d. Brachiopoden",
Protokoll d. ersten Session der Versammlung Russischer
Naturforscher in Kasan, 1873. Published in _Kaiserliche
Gesellschaft Moskau_, 1874 (Russian). Abstracted in Hoffmann
and Schwalbe, _Jahresbericht f._ 1873.
It thus appears that, so far as can be determined from the facts at our
disposal, the mesoblast in almost all cases is derived from the hypoblast,
and in three widely separated groups it arises as a pair of diverticula
from the alimentary tract, each diverticulum containing a cavity which
eventually becomes the body-cavity. I have elsewhere suggested[223] that
the origin of the mesoblast from alimentary diverticula is to be regarded
as primitive for all higher animals, and that the more general cases in
which the mesoblast becomes split off, as an undivided layer, from the
hypoblast, are in reality derivates from this. The chief obstacle in the
way of this view arises from the difficulty of understanding how the whole
voluntary muscular system can have been derived at first from the
alimentary tract. That part of a voluntary system of muscles might be
derived from the contractile diverticula of the alimentary canal attached
to the body-wall is not difficult to understand, but it is not easy to
believe that the secondary system so formed could completely replace the
primitive muscular system, derived, as it must have been, from the
epiblast. In my paper above quoted will be found various speculative
suggestions for removing this difficulty, which I do not repeat here. If it
be granted, however, that in Sagitta, Brachiopods, and Echinoderms we have
genuine examples of the formation of the whole mesoblast from alimentary
diverticula, it is easy to see how the formation of the mesoblast in
Vertebrates may be a secondary derivate from an origin of this nature.
Footnote 223: Comparison of Early Stages, _Quart. Jl. Micros.
Science_, July, 1875. [This Edition, No. VI.]
An attempt has been already made to shew that the mesoblast in
Elasmobranchii is formed in a very primitive fashion, and for this reason
the Elasmobranchii appear to be especially adapted for determining whether
any signs are exhibited of a derivation of the mesoblast as paired
diverticula of the alimentary tract. There are, it appears to me, several
such features. In the first place, the mesoblast is split off from the
hypoblast not as a single mass but as a pair of distinct masses, comparable
with the paired diverticula already alluded to. Secondly, the body-cavity
when it appears in the mesoblast plates, _does not arise as a single
cavity, but as a pair of cavities, one for each plate of mesoblast_, and
these cavities remain permanently distinct in some parts of the body, and
nowhere unite till a comparatively late period. Thirdly, the primitive
body-cavity of the embryo is not confined to the region in which a
body-cavity exists in the adult, _but extends to the summit of the
muscle-plates_, at first separating parts which become completely fused in
the adult to form the great lateral muscles of the body. It is difficult to
understand how the body-cavity could have such an extension as this, on the
supposition that it represents a primitive split in the mesoblast between
the wall of the gut and the body-wall; but its extension to this part is
quite intelligible, on the supposition that it represents the cavities of
two diverticula of the alimentary tract, from whose muscular walls the
voluntary muscular system has been derived. Lastly, I would point out that
the derivation of part of the muscular system from what appears as the
splanchnopleure is quite intelligible on the assumed hypothesis, but, as
far as I see, on no other.
Such are the main features presented by the mesoblast in Elasmobranchii,
which favour the view of its having originally formed the walls of the
alimentary diverticula. Against this view of its nature are the facts (1)
of the mesoblast plates being at first solid, and (2), as a consequence of
this, of the body-cavity never communicating with the alimentary canal.
These points, in view of our knowledge of embryological modifications,
cannot be regarded as great difficulties to my view. We have many examples
of organs, which, though in most cases arising as involutions, yet appear
in other cases as solid ingrowths. Such examples are afforded by the optic
vesicle, auditory vesicle, and probably also by the central nervous system,
of Osseous Fish. In most Vertebrates these organs are formed as hollow
involutions from the exterior; in Osseous Fish, however, as solid
involutions, in which a cavity secondarily appears.
The segmental duct of Elasmobranchii or the Wolffian duct (segmental duct)
of Birds are cases of a similar kind, being organs which must originally
have been formed as hollow involutions, but which now arise as solid
bodies.
Only one more instance of this kind need be cited, taken from the
Echinoderms.
The body-cavity and the mesoblast investing it arise in the case of most
Echinoderms as hollow involutions of the alimentary tract, but in some
exceptional groups, Ophiura and Amphiura, are stated to be solid at first
and only subsequently to become hollow. Should the accuracy of
Metschnikoff's account of this point be confirmed, an almost exact parallel
to what has been supposed by me to have occurred with the mesoblast in
Elasmobranchii, and other groups, will be supplied.
The tendency of our present knowledge appears to be in favour of regarding
the body-cavity in Vertebrates as having been primitively the cavity of
alimentary diverticula, and the mesoblast as having formed the walls of the
diverticula.
This view, to say the least of it, suits the facts which we know far better
than any other theory which has been proposed, and though no doubt the _à
priori_ difficulties in its way are very great, yet it appears to me to be
sufficiently strongly supported to deserve the attention of investigators.
In the meantime, however, our knowledge of invertebrate embryology is so
new and imperfect that no certainty on a question like that which has just
been discussed can be obtained; and any generalizations made at present are
not unlikely to be upset by the discovery of fresh facts.
The only other point in connection with the mesoblast which I would call
attention to is the formation of the vertebral bodies.
My observations confirm those of Remak and Gegenbaur, shewing that there is
a primary segmentation of the vertebral bodies corresponding to that of the
muscle-plates, followed by a secondary segmentation in which the central
lines of the vertebral bodies are opposite the partitions between the
muscle-plates.
The explanation of these changes is not difficult to find. The primary
segmentation of the body is that of the muscle-plates, which must have been
present at a time when the vertebral bodies had no existence. As soon
however as the notochordal sheath was required to be strong as well as
flexible, it necessarily became divided into a series of segments.
The conditions under which the lateral muscles can cause the flexure of the
vertebral column are clearly that each muscle-segment shall be capable of
acting on two vertebræ; and this condition can only be fulfilled when the
muscle-segments are opposite the intervals between the vertebræ. Owing to
this necessity, when the vertebral segments became formed, their centres
corresponded, not with the centres of the muscle-plates, but with the
inter-muscular septa.
These considerations fully explain the secondary segmentation of the
vertebræ by which they become opposite the inter-muscular septa. On the
other hand, the primary segmentation is clearly a remnant of the time when
no vertebral bodies were present, and has no greater morphological
significance than the fact that the cells to form the unsegmented
investment of the notochord were derived from the segmented muscle-plates,
and only secondarily became fused into a continuous tube.
_The Urinogenital System._
The first traces of the urinary system become visible at about the time of
the appearance of the third visceral cleft. At about this period the
somatopleure and splanchnopleure become more or less fused together at the
level of the dorsal aorta, and thus, as has been already mentioned, each of
the original plates of mesoblast becomes divided into a vertebral plate and
lateral plate (Pl. 11, fig. 6). The mass of cells resulting from this
fusion corresponds with Waldeyer's intermediate cell-mass in the Fowl.
At about the level of the fifth protovertebra the first trace of the
urinary system appears.
From the intermediate cell-mass a solid knob grows outwards towards the
epiblast (woodcut, fig. 4, _pd_). This knob consists at first of 20-30
cells, which agree in character with the neighbouring cells of the
intermediate cell-mass, and are at this period rounded. It is mainly, if
not entirely, derived from the somatic layer of the mesoblast.
From this knob there grows backwards a solid rod of cells which keeps in
very close contact with the epiblast, and rapidly diminishes in size
towards its posterior extremity. Its hindermost part consists in section of
at most one or two cells. It keeps so close to the epiblast that it might
be supposed to be derived from that layer were it not for the sections
shewing its origin from the knob above mentioned. We have in this rod the
commencement of what I have elsewhere[224] called the segmental duct.
Footnote 224: "Urinogenital Organs of Vertebrates," _Journ. of
Anat. and Phys._ Vol. X. [This Edition, No. VII.]
[Illustration: FIG. 4. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE
VISCERAL CLEFTS.
The sections are to shew the development of the segmental duct (_pd_) or
primitive duct of the kidneys. In _A_ (the anterior of the two sections)
this appears as a solid knob projecting towards the epiblast. In _B_ is
seen a section of the column which has grown backwards from the knob in
_A_.
_spn._ rudiment of a spinal nerve; _mc._ medullary canal; _ch._ notochord;
_X._ string of cells below the notochord; _mp._ muscle-plate; _mp´._
specially developed portion of muscle-plate; _ao._ dorsal aorta; _pd._
segmental duct; _so._ somatopleura; _sp._ splanchnopleura; _pp._
pleuro-peritoneal or body-cavity; _ep._ epiblast; _al._ alimentary canal.]
My observations shew that the segmental duct is developed in the way just
described in both Pristiurus and Torpedo. Its origin in Pristiurus is shewn
in the adjoining woodcut, and in Torpedo in Pl. 11, fig. 7, _sd_.
At a stage somewhat older than I, the condition of the segmental duct has
not very materially altered. It has increased considerably in length, and
the knob at its front end is both absolutely smaller, and also consists of
fewer cells than before (Pl. 11, fig. 7, _sd_). These cells have become
more columnar, and have begun to arrange themselves radially; thus
indicating the early appearance of the lumen of the duct. The cells forming
the front part of the rod, as well as those of the knob, commence to
exhibit a columnar character, but in the hinder part of the rod the cells
are still rounded. In no part of it has a lumen appeared.
At this period also the knob, partly owing to the commencing separation of
the muscle-plate from the remainder of the mesoblast, begins to pass
inwards and approach the pleuro-peritoneal cavity.
At the same stage the first not very distinct traces of the remainder of
the urinary system become developed. These appear in the form of solid
outgrowths from the intermediate cell-mass just at the most dorsal part of
the body-cavity.
The outgrowths correspond in numbers with the vertebral segments, and are
at first quite disconnected with the segmental duct. At this stage they are
only distinctly visible in the first few segments behind the front end of
the segmental duct. A full description of them will come more conveniently
in the next stage.
By a stage somewhat earlier than K important changes have taken place in
the urinary system.
The segmental duct has acquired a lumen in its anterior portion, which
opens at its front end into the body-cavity. (Pl. 11, fig. 9, _sd._) The
lumen is formed by the columnar cells spoken of in the last stage,
acquiring a radiating arrangement round a central point, at which a small
hole appears. After the lumen has once become formed, it rapidly increases
in size.
The duct has also grown considerably in length, but its hind extremity is
still as thin, and lies as close to the epiblast, as at first. The
segmental involutions which commenced to be formed in the last stage, have
now appeared for every vertebral segment along the whole length of the
segmental duct, and even for two or three segments behind this.
They are simple independent outgrowths arising from the outer and uppermost
angle of the body-cavity, and are at first almost without a trace of a
lumen; though their cells are arranged as two layers. They grow in such a
way as to encircle the oviduct on its inner and upper side (Pl. 11, fig. 8
and Pl. 12, fig. 14_b_, _st_). When the hindermost ones are formed, a
slight trace of a lumen is perhaps visible in the front ones. At a stage
slightly subsequent to this, in Scyllium canicula, I noticed 29 of them;
the first of them arising in the segment immediately behind the front end
of the oviduct (Pl. 12, fig. 17, _st_), and two of them being formed in
segments just posterior to the hinder extremity of the oviduct.
Pl. 12, figs. 16 and 18 represent two longitudinal sections shewing the
segmental nature of the involutions and their relation to the segmental
duct.
Many of the points which have been mentioned can be seen by referring to
Pl. 11 and 12. Anteriorly the segmental duct opens into the
pleuro-peritoneal cavity. In the sections behind this there may be seen the
segmental duct with a distinct lumen, and also a pair of segmental
involutions (Pl. 12, fig. 14_a_). In the still posterior sections the
segmental duct would be quite without a lumen, and would closely adjoin the
epiblast.
It seems not out of place to point out that the modes of the development of
the segmental duct and of the segmental involutions are strikingly similar.
Both arise as solid involutions, from homologous parts of the mesoblast.
The segmental duct arises in the vertebral segment immediately in front of
that in which the first segmental involution appears; _so that the
segmental duct appears to be equivalent to a single segmental involution_.
The next stage corresponds with the first appearance of the external gills.
The segmental duct now communicates by a wide opening with the body-cavity
(Pl. 11, fig. 9, _sd_). It possesses a lumen along its whole length up to
the extreme hind end (Pl. 11, fig. 9_a_). It is, however, at this hinder
extremity that the most important change has taken place. This end has
grown downwards towards that part of the alimentary canal which still lies
behind the anus. This downgrowth is beginning to shew distinct traces of a
lumen, and will appear in the next stage as one of the horns by which the
segmental ducts communicate with the cloaca (Pl. 11, fig. 9_b_). All the
anterior segmental involutions have now acquired a lumen. But this is still
absent in the posterior ones (Pl. 11, fig. 9_a_).
Owing to the disappearance of the body-cavity in the region behind the
anus, the primitive involutions there remain as simple masses of cells
still disconnected with the segmental duct (Pl. 11, figs. 9_b_, 9_c_ and
9_d_).
_Primitive Ova._ The true generative products make their first appearance
as the _primitive ova_ between stages I and K.
In the sections of one of my embryos of this stage they are especially well
shewn, and the following description is taken from those displayed in that
embryo.
They are confined to the region which extends posteriorly nearly to the end
of the small intestine and anteriorly to the abdominal opening of the
segmental duct.
Their situation in this region is peculiar. There is no trace of a distinct
genital ridge, but the ova mainly lie in the dorsal portion of the
mesentery, and therefore in a part of the mesoblast which distinctly
belongs to the splanchnopleure (Pl. 12, fig. 14_a_). Some are situated
external to the segmental involutions; and others again, though this is not
common, in a part of the mesoblast which distinctly belongs to the
body-wall (Pl. 12, fig. 14_b_).
The portion of mesentery, in which the primitive ova are most densely
aggregated, corresponds to the future position of the genital ridge, but
the other positions occupied by ova are quite outside this. Some ova are in
fact situated on the outside of the segmental duct and segmented tubes, and
must therefore effect a considerable migration before reaching their final
positions in the genital ridge on the inner side of the segmental duct
(Pl. 12, fig. 14_b_).
The condition of the tissue in which the ova appear may at once be gathered
from an examination of the figures given. It consists of an irregular
epithelium of cells partly belonging to the somatopleure and partly to the
splanchnopleure, but passing uninterruptedly from one layer to the other.
The cells which compose it are irregular in shape, but frequently columnar
(Pl. 12, figs. 14_a_ and 14_b_).
They are formed of a nucleus which stains deeply, invested by a _very
delicate_ layer of protoplasm. At the junction of somatopleure and
splanchnopleure they are more rounded than elsewhere. Very few loose
connective-tissue cells are present. The cells just described vary from
.008 Mm. to .01 Mm. in diameter.
The primitive ova are situated amongst them and stand out with
extraordinary clearness, to which justice is hardly done in my figures.
The normal full-sized ova exhibit the following structure. They consist of
a mass of somewhat granular protoplasm of irregular, but more or less
rounded, form. Their size varies from .016 - .036 Mm. In their interior a
nucleus is present, which varies from .012 - .016 Mm., but its size as a
rule bears _no_ relation to the size of the containing cell.
This is illustrated by the subjoined list of measurements.
Size of Primitive ova in Size of nucleus of Primitive
degrees of micrometer scale ova in degrees of micrometer
with F. ocul 2. scale with F. ocul 2.
10 8
13 8
13 8
14 7
15 7
13 7-1/2
11 8
16 5-1/2
12 7
10 7
15 6
13 6
12 7
The numbers given refer to degrees on my micrometer scale.
Since it is the ratio alone which it is necessary to call attention to, the
numbers are not reduced to decimals of a millimeter. Each degree of my
scale is equal, however, with the object glass employed, to .002 Mm.
This series brings out the result I have just mentioned with great
clearness.
In one case we find a cell has three times the diameter of the nucleus
16 : 5-1/2; in another case 10 : 8, the nucleus has only a slightly smaller
diameter than the cell. The irrationality of the ratio is fairly shewn in
some of my figures, though none of the largest cells with very small nuclei
have been represented.
The nuclei are granular, and stain fairly well with hæmatoxylin. They
usually contain a single deeply stained nucleolus, but in many cases,
especially where large (and this independently of the size of the cell),
they contain two nucleoli (Pl. 12, figs. 14_c_ and 14_d_), and are at times
so lobed as to give an apparent indication of commencing division.
A multi-nucleolar condition of the nuclei, like that figured by Götte[225],
does not appear till near the close of embryonic life, and is then found
equally in the large ova and in those not larger than the ova which exist
at this early date.
Footnote 225: _Entwicklungsgeschichte der Unke_, Pl. 1, fig. 8.
As regards the relation of the primitive ova to each other and the
neighbouring cells, there are a few points which deserve attention. In the
first place, the ova are, as a rule, collected in masses at particular
points, and not distributed uniformly (fig. 14_a_). The masses in some
cases appear as if they had resulted from the division of one primitive
ovum, but can hardly be adduced as instances of a commencing coalescence;
since if the ova thus aggregated were to coalesce, an ovum would be
produced of a very much greater size than any which is found during the
early stages. Though at this stage no indication is present of such a
coalescence of cells to form ova as is believed to take place by Götte,
still the origin of the primitive ova is not quite clear. One would
naturally expect to find a great number of cells intermediate between
primitive ova and ordinary columnar cells. Cells which may be intermediate
are no doubt found, but not nearly so frequently as might have been
anticipated. One or two cells are shewn in Pl. 12, fig. 14_a_, _x_, which
are perhaps of an intermediate character; but in most sections it is not
possible to satisfy oneself that any such intermediate cells are present.
In one case what appeared to be an intermediate cell was measured, and
presented a diameter of .012 Mm. while its nucleus was .008 Mm. Apart from
certain features of the nucleus, which at this stage are hardly very
marked, the easiest method of distinguishing a primitive ovum from an
adjacent cell is the presence of a large quantity of protoplasm around the
nucleus. The nucleus of one of the smallest primitive ova is not larger
than the nucleus of an ordinary cell (being about .008 Mm. in both). It is
perhaps the similarity in the size of the nuclei which renders it difficult
at first to distinguish developing primitive ova from ordinary cells.
Except with the very thinnest sections a small extra quantity of protoplasm
around a nucleus might easily escape detection, and the developing cell
might only become visible when it had attained to the size of a small
typical primitive ovum.
It deserves to be noticed that the nuclei even of some of the largest
primitive ova scarcely exceed the surrounding nuclei in size. This appears
to me to be an argument of some weight in shewing that the great size of
primitive ova is not due to the fact of their having been formed by a
coalescence of different cells (in which case the nucleus would have
increased in the same proportion as the cell); but to an increase by a
normal method of growth in the protoplasm around the nucleus.
It appears to me to be a point of great importance certainly to determine
whether the primitive ova arise by a metamorphosis of adjoining cells, or
may not be introduced from elsewhere. In some of the lower animals, _e.g._
Hydrozoa, there is no question that the ova are derived from the epiblast;
we might therefore expect to find that they had the same origin in
Vertebrates. Further than this, ova are frequently capable in a young state
of executing amoeboid movements, and accordingly of migrating from one
layer to another. In the Elasmobranchii the primitive ova exhibit in a
hardened state an irregular form which might appear to indicate that they
possess a power of altering their shape, a view which is further supported
by some of them being at the present stage situated in a position very
different from that which they eventually occupy, and which they can only
reach by migration. If it could be shewn that there were no intermediate
stages between the primitive ova and the adjoining cells (their migratory
powers being admitted) a strong presumption would be offered in favour of
their having migrated from elsewhere to their present position. In view of
this possibility I have made some special investigations, which have
however led to no very satisfactory results. There are to be seen in the
stages immediately preceding the present one, numerous cells in a
corresponding position to that of the primitive ova, which might very well
be intermediate between the primitive ova and ordinary cells, but which
offer no sufficiently well marked features for a certain determination of
their true nature.
In the particular embryo whose primitive ova have been described these
bodies were more conspicuous than in the majority of cases, but at the same
time they presented no special or peculiar characters.
In a somewhat older embryo of Scyllium the cells amongst which the
primitive ova lay had become very distinctly differentiated as an
epithelium (the germinal epithelium of Waldeyer) well separated by what
might almost be called a basement membrane from the adjoining
connective-tissue cells. Hardly any indication of a germinal ridge had
appeared, but the ova were more definitely confined than in previous
embryos to the restricted area which eventually forms this. The ova on the
average were somewhat smaller than in the previous cases.
In several embryos intermediate in age between the embryo whose primitive
ova were described at the commencement of this section and the embryo last
described, the primitive ova presented some peculiarities, about the
meaning of which I am not quite clear, but which may perhaps throw some
light on the origin of these bodies.
Instead of the protoplasm around the nucleus being clear or slightly
granular, as in the cases just described, it was filled in the most typical
instances with numerous highly refracting bodies resembling yolk-spherules.
In osmic acid specimens (Pl. 12, fig. 15) these stain very darkly, and it
is then as a rule very difficult to see the nucleus; in specimens hardened
in picric acid and stained with hæmatoxylin these bodies are stained of a
deep purple colour, but the nucleus can in most cases be distinctly seen.
In addition to the instances in which the protoplasm of the ova is quite
filled with these bodies, there are others in which they only occupy a
small area adjoining the nucleus (Pl. 12, fig. 15_a_), and finally some in
which only one or two of these bodies are present. The protoplasm of the
primitive ova appears in fact to present a series of gradations between a
state in which it is completely filled with highly refracting spherules and
one in which these are completely absent.
This state of things naturally leads to the view that the primitive ova,
when they are first formed, are filled with these spherules, which are
probably yolk-spherules, but that they gradually lose them in the course of
development. Against this interpretation is the fact that the primitive ova
in the younger embryo first described are completely without these bodies;
this embryo however unquestionably presented an abnormally early
development of the ova; and I am satisfied that embryos present
considerable variations in this respect.
If the primitive ova are in reality in the first instance filled with
yolk-spherules, the question arises as to whether, considering that they
are the only mesoblast cells filled at this period with yolk-spherules, we
must not suppose that they have migrated from some peripheral part of the
blastoderm into their present position. To this question I can give no
satisfactory answer. Against a view which would regard the spherules in the
protoplasm as bodies which appear subsequently to the first formation of
the ova, is the fact that hitherto no instances in which these spherules
were present have been met with in the late stages of development; and they
seem therefore to be confined to the first stages.
_Notochord._
The changes undergone by the notochord during this period present
considerable differences according to the genus examined. One type of
development is characteristic of Scyllium and Pristiurus; a second type, of
Torpedo.
My observations being far more complete for Scyllium and Pristiurus than
for Torpedo, it is to the two former genera only that the following account
applies, unless the contrary is expressly stated. Only the development of
the parts of the notochord in the trunk are here dealt with; the cephalic
section of the notochord is treated of in a subsequent section.
During stage G the notochord is composed of flattened cells arranged
vertically, rendering the histological characters of the notochord
difficult to determine in transverse sections. In longitudinal sections,
however, the form and arrangement of the cells can be recognised with great
ease. At the beginning of stage G each cell is composed of a nucleus
invested by granular protoplasm frequently vacuolated and containing in
suspension numerous yolk-spherules. It is difficult to determine whether
there is only one vacuole for each cell, or whether in some cases there may
not be more than one.
Round the exterior of the notochord there is present a distinct though
delicate cuticular sheath.
The vacuoles are at first small, but during stage G rapidly increase in
size, while at the same time the yolk-spherules completely vanish from the
notochord.
As a result of the rapid growth of the vacuoles, the nuclei, surrounded in
each case by a small amount of protoplasm, become pushed to the centre of
the notochord, the remainder of the protoplasm being carried to the edge.
The notochord thus becomes composed during stages H and I (Pl. 11,
fig. 4-6) of a central area mainly formed of nuclei with a small quantity
of protoplasm around them, and of a thin peripheral layer of protoplasm
without nuclei, the widish space between the two being filled with clear
fluid. The exterior of the cells is indurated, so that they may be said to
be invested by a membrane[226]; the cells themselves have a flattened form,
and each extends from the edge to the centre of the notochord, the long
axis of each being rather greater than half the diameter of the cord.
Footnote 226: This membrane is better looked upon, as is done
by Gegenbaur and Götte, as intercellular matter.
The nuclei of the notochord are elliptical vesicles, consisting of a
membrane filled with granular contents, amongst which is situated a
distinct nucleolus. They stain deeply with hæmatoxylin. Their long diameter
in Scyllium is about 0.02 Mm.
The diameter of the whole notochord in Pristiurus during stage I is about
0.1 Mm. in the region of the back, and about 0.08 Mm. near the posterior
end of the body.
Owing to the form of its constituent cells, the notochord presents in
transverse sections a dark central area surrounded by a lighter peripheral
one, but its true structure cannot be unravelled without the assistance of
longitudinal sections. In these (Pl. 12, fig. 10) the nuclei form an
irregular double row in the centre of the cord. Their outlines are very
clear, but those of the individual cells cannot for certain be made out. It
is, however, easy to see that the cells have a flattened and wedge-shaped
form, with the narrow ends overlapping and interlocking at the centre of
the notochord.
By the close of stage I the cuticular sheath of the notochord has greatly
increased in thickness.
During the period intermediate between stages I and K the notochord
undergoes considerable transformations. Its cells cease to be flattened,
and become irregularly polygonal, and appear but slightly more compressed
in longitudinal sections than in transverse ones. The vacuolation of the
cells proceeds rapidly, and there is left in each cell only a very thin
layer of protoplasm around the nucleus. Each cell, as in the earlier
stages, is bounded by a membrane-like wall.
Accompanying these general changes special alterations take place in the
distribution of the nuclei and the protoplasm. The nuclei, accompanied by
protoplasm, gradually leave the centre and migrate towards the periphery of
the notochord. At the same time the protoplasm of the cells forms a special
layer in contact with the investing sheath.
The changes by which this takes place can easily be followed in
longitudinal sections. In Pl. 12, fig. 11 the migration of the nuclei has
commenced. They are still, however, more or less aggregated at the centre,
and very little protoplasm is present at the edges of the notochord. The
cells, though more or less irregularly polygonal, are still somewhat
flattened. In Pl. 12, fig. 12 the notochord has made a further progress.
The nuclei now mainly lie at the side of the notochord, where they exist in
a somewhat shrivelled state, though still invested by a layer of
protoplasm.
A large portion of the protoplasm of the cord forms an almost continuous
layer in close contact with the sheath, which is more distinctly visible in
some cases than in others.
While the changes above described are taking place the notochord increases
in size. At the age of fig. 11 it is in the anterior part of the body of
Pristiurus about 0.11 Mm. At the age of fig. 12 it is in the same species
0.12 Mm., while in Scyllium stellare it reaches about 0.17 Mm.
During stage K (Pl. 11, fig. 8) the vacuolation of the cells of the
notochord becomes even more complete than during the earlier stages, and in
the central cells hardly any protoplasm is present, though a starved
nucleus surrounded by a little protoplasm may be found in an occasional
corner.
The whole notochord becomes very delicate, and can with great difficulty be
conserved whole in transverse sections.
The layer of protoplasm which appeared during the last stage on the inner
side of the cuticular membrane of the notochord becomes during the present
stage a far thicker and more definite structure. It forms a continuous
layer with irregular prominences on its inner surface; and contains
numerous nuclei. The layer sometimes presents in transverse sections hardly
any indication of a division into a number of separate cells, but in
longitudinal sections this is generally very obvious. The cells are
directed very obliquely forwards, and consist of an oblong nucleus invested
by protoplasm. The layer formed by them is very delicate and very easily
destroyed. In one example its thickness varied from .004 to .006 Mm., in
another it reached .012 Mm. The thickness of the cuticular membrane is
about .002 Mm. or rather less.
The diameter of a notochord in the anterior part of the body of a
Pristiurus embryo of this stage is about 0.21 Mm. Round the exterior of the
notochord the mesoblast cells are commencing to arrange themselves as a
special sheath.
In Torpedo the notochord at first presents the same structure as in
Pristiurus, _i.e._ it forms a cylindrical rod of flattened cells.
The vacuolation of these cells does not however commence till a relatively
very much later period than in Pristiurus, and also presents a very
different character (Pl. 11, fig. 7).
The vacuoles are smaller, more numerous, and more rounded than in the other
genera, and there can be no question that in many cases there is more than
one vacuole in a cell. The most striking point in which the notochord of
Torpedo differs from that of Pristiurus consists in the fact that in
Torpedo there is never any aggregation of the nuclei at the centre of the
cord, but the nuclei are always distributed uniformly through it. As the
vacuolation proceeds the differences between Torpedo and the other genera
become less and less marked. The vacuoles become angular in form, and the
cells of the cord cease to be flattened, and become polygonal.
At my final stage for Torpedo (slightly younger than K) the only important
feature distinguishing the notochord from that of Pristiurus, is the
absence of any signs of nuclei or protoplasm passing to the periphery.
Around the exterior of the cord there is early found in Torpedo a special
investment of mesoblastic cells.
EXPLANATION OF PLATES 11 AND 12.
COMPLETE LIST OF REFERENCE LETTERS.
_al._ Alimentary tract. _an._ Point where anus will be formed. _ao._ Dorsal
aorta. _ar._ Rudiment of anterior root of spinal nerve. _b._ Anterior fin.
_c._ Connective-tissue cells. _cav._ Cardinal vein. _ch._ Notochord. _df._
Dorsal fin. _ep._ Epiblast. _ge._ Germinal epithelium. _ht._ Heart. _l._
Liver. _mp._ Muscle-plate. _mp´._ Early formed band of muscles from the
splanchnic layer of the muscle-plates. _nc._ Neural canal. _p._ Protoplasm
from yolk in the alimentary tract. _pc._ Pericardial cavity. _po._
Primitive ovum. _pp._ body-cavity. _pr._ Rudiment of posterior root of
spinal nerve. _sd._ Segmental duct. _sh._ Cuticular sheath of notochord.
_so._ Somatic layer of mesoblast. _sp._ Splanchnic layer of mesoblast.
_spc._ Spinal cord. _sp.v._ Spiral valve. _sr._ Interrenal body. _st._
Segmental tube. _sv._ Sinus venosus. _ua._ Umbilical artery. _um._
Umbilical cord. _uv._ Umbilical vein. _V._ Splanchnic vein. _v._
Blood-vessel. _vc._ Visceral cleft. _Vr._ Vertebral rudiment. _W._ White
matter of spinal cord. _x._ Subnotochordal rod (except in fig. 14_a_). _y._
Passage connecting the neural and alimentary canals.
PLATE 11.
Fig. 1. Section from the caudal region of a Pristiurus embryo belonging to
stage H. Zeiss C, ocul. 1. Osmic acid specimen.
It shews (1) the constriction of the subnotochordal rod (_x_) from the
summit of the alimentary canal. (2) The formation of the body-cavity in the
muscle-plate and the ventral thickening of the parietal plate.
Fig. 1_a_. Portion of alimentary wall of the same embryo, shewing the
formation of the subnotochord rod (_x_).
Fig. 2. Section through the caudal vesicle of a Pristiurus embryo belonging
to stage H. Zeiss C, ocul. 1.
It shews the bilobed condition of the alimentary vesicle and the fusion of
the mesoblast and hypoblast at the caudal vesicle.
Fig. 3_a_. Sections from the caudal region of a Pristiurus embryo belonging
to stage H. Zeiss C, ocul. 1. Picric acid specimen.
It shews the communication which exists posteriorly between the neural and
alimentary canals, and also by comparison with 3_b_ it exhibits the
dilatation undergone by the alimentary canal in the caudal vesicle.
Fig. 3_b_. Section from the caudal region of an embryo slightly younger
than 3_a_. Zeiss C, ocul. 1. Osmic acid specimen.
Fig. 4. Section from the cardiac region of a Pristiurus embryo belonging to
stage H. Zeiss C, ocul. 1. Osmic acid specimen.
It shews the formation of the heart (_ht_) as a cavity between the
splanchnopleure and the wall of the throat.
Fig. 5. Section from the posterior dorsal region of a Scyllium embryo,
belonging to stage H. Zeiss C, ocul. 1. Osmic acid specimen.
It shews the general features of an embryo of stage H, more especially the
relations of the body-cavity in the parietal and vertebral portions of the
lateral plate, and the early-formed band of muscle (_mp´_) in the
splanchnic layer of the vertebral plate.
Fig. 6. Section from the oesophageal region of Scyllium embryo belonging to
stage I. Zeiss C, ocul. 1. Chromic acid specimen.
It shews the formation of the rudiments of the posterior nerve-roots (_pr_)
and of the vertebral rudiments (_Vr_).
Fig. 7. Section of a Torpedo embryo belonging to stage slightly later than
I. Zeiss C, ocul. 1, reduced 1/3. Osmic acid specimen.
It shews (1) the formation of the anterior and posterior nerve-roots. (2)
The solid knob from which the segmental duct (_sd_) originates.
Fig. 8. Section from the dorsal region of a Scyllium embryo belonging to a
stage intermediate between I and K. Zeiss C, ocul. 1. Chromic acid
specimen.
It illustrates the structure of the primitive ova, segmental tubes,
notochord, etc.
Fig. 8_a_. Section from the caudal region of an embryo of the same age as
8. Zeiss A, ocul. 1.
It shews (1) the solid oesophagus. (2) The narrow passage connecting the
pericardial (_pc_) and body cavities (_pp_).
Fig. 9. Section of a Pristiurus embryo belonging to stage K. Zeiss A, ocul.
1. Osmic acid specimen.
It shews the formation of the liver (_l_), the structure of the anterior
fins (_b_), and the anterior opening of the segmental duct into the
body-cavity (_sd_).
Figs. 9_a_, 9_b_, 9_c_, 9_d_. Four sections through the anterior region of
the same embryo as 9. Osmic acid specimens.
The sections shew (1) the atrophy of the post-anal section of the
alimentary tract (9_b_, 9_c_, 9_d_). (2) The existence of the segmental
tubes behind the anus (9_b_, 9_c_, 9_d_). With reference to these it
deserves to be noted that the segmental tubes behind the anus are quite
disconnected, as is proved by the fact that a tube is absent on one side in
9_c_ but reappears in 9_d_. (3) The downward prolongation of the segmental
duct to join the posterior or cloacal extremity of the alimentary tract
(9_b_).
PLATE 12.
Fig. 10. Longitudinal and horizontal section of a Scyllium embryo of stage
H. Zeiss C, ocul. 1. Reduced by 1/3. Picric acid specimen.
It shews (1) the structure of the notochord; (2) the appearance of the
early formed band of muscles (_mp´_) in the splanchnic layer of the
protovertebra.
Fig. 11. Longitudinal and horizontal sections of an embryo belonging to
stage I. Zeiss C, ocul. 1. Chromic acid specimen. It illustrates the same
points as the previous section, but in addition shews the formation of the
rudiments of the vertebral bodies (_Vr_) which are seen to have the same
segmentation as the muscle-plates.
Fig. 12.[227] Longitudinal and horizontal section of an embryo belonging to
the stage intermediate between I and K. Zeiss C, ocul. 1. Osmic acid
specimen illustrating the same points as the previous section.
Footnote 227: The apparent structure in the sheath of the
notochord in this and the succeeding figure is merely the
result of an attempt on the part of the engraver to represent
the dark colour of the sheath in the original figure.
Fig. 13. Longitudinal and horizontal section of an embryo belonging to
stage K. Zeiss C, ocul. 1, and illustrating same points as previous
section.
Figs. 14_a_, 14_b_, 14_c_, 14_d_. Figures taken from preparations of an
embryo of an age intermediate between I and K, and illustrating the
structure of the primitive ova. Figs. 14_a_ and 14_b_ are portions of
transverse sections. Zeiss C, ocul. 3 reduced 1/3. Figs. 14_c_ and 14_d_
are individual ova, shewing the lobate form of nucleus. Zeiss F, ocul. 2.
Fig. 15. Osmic acid preparation of primitive ova belonging to stage K.
Zeiss immersion No. 2, ocul. 1. The protoplasm of the ova is seen to be
nearly filled with bodies resembling yolk-spherules: and one ovum is
apparently undergoing division.
Fig. 15_a_. Picric acid preparation shewing a primitive ovum partially
filled with bodies resembling yolk-spherules.
Fig. 16. Horizontal and longitudinal section of Scyllium embryo belonging
to stage K. Zeiss A, ocul. 1. Picric acid preparation. The
connective-tissue cells are omitted.
The section shews that there is one segmental tube to each vertebral
segment.
Fig. 17. Portion of a Scyllium embryo belonging to stage K, viewed as a
transparent object.
It shews the segmental duct and the segmental involutions--two of which are
seen to belong to segments behind the end of the alimentary tract.
Fig. 18. Vertical longitudinal section of a Scyllium embryo belonging to
stage K. Zeiss A, ocul. 1. Hardened in a mixture of osmic and chromic acid.
It shews
(1) the commissures connecting together the posterior roots of the spinal
nerves;
(2) the junction of the anterior and posterior roots;
(3) the relations of the segmental ducts to the segmental involutions and
the alternation of calibre in the segmental tube;
(4) the germinal epithelium lining the body-cavity.