STAGES B TO G.
The present chapter deals with the history of the development of the
Elasmobranch embryo from the period when the medullary groove first arises
till that in which it becomes completely closed, and converted into the
medullary canal. The majority of the observations recorded were made on
Pristiurus embryos, a few on embryos of Torpedo. Where nothing is said to
the contrary the statements made apply to the embryos of Pristiurus only.
The general external features for this period have already been given in
sufficient detail in the last chapter; and I proceed at once to describe
consecutively the history of the three layers.
_General Features of the Epiblast._
At the commencement of this period, during the stage intermediate between B
and C, the epiblast is composed of a single layer of cells. (Pl. 10,
fig. 1.)
These are very much elongated in the region of the embryo, but flattened in
other parts of the blastoderm. Throughout they contain numerous
yolk-spherules.
In a Torpedo embryo of this age (as determined by the condition of the
notochord) the epiblast presents a very different structure. It is composed
of small spindle-shaped cells several rows deep. The nuclei of these are
very large in proportion to the cells containing them, and the
yolk-spherules are far less numerous than in the cells of corresponding
Pristiurus embryos.
During stage C the condition of the epiblast does not undergo any important
change, with the exception of the layer becoming much thickened, and its
cells two or three deep in the anterior parts of the embryo. (Pl. 10,
fig. 2.)
In the succeeding stages that part of the epiblast, which will form the
spinal cord, gradually becomes two or three cells deep. This change is
effected by a decrease in the length of the cells as compared with the
thickness of the layer. In the earlier stages the cells are wedge-shaped
with an alternate arrangement, so that a decrement in the length of the
cells at once causes the epiblast to be composed of two rows of
interlocking cells.
The lateral parts of the epiblast which form the epidermis of the embryo
are modified in quite a different manner to the nervous parts of the layer,
becoming very much diminished in thickness and composed of a single row of
flattened cells. (Pl. 10, fig. 3.)
Till the end of stage F, the epiblast cells and indeed all the cells of the
blastoderm retain their yolk-spherules, but the epiblast begins to lose
them and consequently to become transparent in stage G.
_Medullary Groove._
During stage B the medullary groove is shallow posteriorly, deeper in the
middle part, and flattened out again at the extreme anterior end of the
embryo. (Pl. 7, fig. 10_a_, _b_, _c_.)
A similar condition obtains in the stage between B and C, but the canal has
now in part become deeper. Anteriorly no trace of it is to be seen. In
stage C it exhibits the same general features. (Pl. 10, fig. 2_a_, 2_b_,
2_c_.)
By stage D we find important modifications of the canal.
It is still shallow behind and deep in the dorsal region, Pl. 10, figs.
3_d_, 3_e_, 3_f_; but the anterior flattened area in the last stage has
grown into a round flat plate which may be called the cephalic plate,
Pl. 8, D and Pl. 10, figs. 3_a_, 3_b_, 3_c_. This plate becomes converted
into the brain. Its size and form give it a peculiar appearance, but the
most remarkable feature about it is the ventral curvature of its edges. Its
edges do not, as might be expected, bend dorsalwards towards each other,
but become sharply bent in a ventral direction. This feature is for the
first time apparent at this stage, but becomes more conspicuous during the
succeeding ones, and attains its maximum in stage F (Pl. 10, fig. 5), in
which it might almost be supposed that the edges of the cephalic plate were
about to grow downwards and meet on the ventral side of the embryo.
In the stages subsequent to D the posterior part of the canal deepens much
more rapidly than the rest (vide Pl. 10, fig. 4, taken from the posterior
end of an embryo but slightly younger than F), and the medullary folds
unite and convert the posterior end of the medullary groove into a closed
canal (Pl. 8, fig. F), while the groove is still widely open
elsewhere[180]. The medullary canal does not end blindly behind, but simply
forms a tube not closed at either extremity. The importance of this fact
will appear later.
Footnote 180: Vide Preliminary Account, etc. _Q. Jl. Micros.
Science_, Oct. 1874, Pl. 14, 8_a_. [This Edition, No. V. Pl. 3,
8_a_.] This and the other section from the same embryo (stage
F) may be referred to. I have not thought it worth while
repeating them here.
In a stage but slightly subsequent to F nearly the whole of the medullary
canal becomes formed. This occurs in the usual way by the junction and
coalescence of the medullary folds. In the course of the closing of the
medullary groove the edges of the cephalic plate lose their ventral
curvature and become bent up in the normal manner (vide Pl. 10, fig. 6, a
section taken through the posterior part of the cephalic plate), and the
enlarged plate merely serves to enclose a dilated cephalic portion of the
medullary canal. The closing of the medullary canal takes place earlier in
the head and neck than in the back. The anterior end of the canal becomes
closed and does not remain open like the posterior end.
Elasmobranch embryos resemble those of the Sturgeon (Acipenser) and the
Amphibians in the possession of a spatula-like cephalic expansion: but so
far as I am aware a ventral flexure in the medullary plates of the head has
not been observed in other groups.
The medullary canal in Elasmobranchii is formed precisely on the type so
well recognised for all groups of vertebrates with the exception of the
Osseous Fishes. The only feature in any respect peculiar to these fishes is
the closing of their medullary canal first commencing behind, and then at a
second point in the cervical region. In those vertebrates in which the
medullary folds do not unite at approximately the same time throughout
their length, they appear usually to do so first in the region of the neck.
_Mesoblast._
The separation from the hypoblast of two lateral masses of mesoblast has
already been described. Till the close of stage C the mesoblast retains its
primitive bilateral condition unaltered. Throughout the whole length of the
embryo, with the exception of the extreme front part, there are present two
plates of rounded mesoblast cells, one on each side of the medullary
groove. These plates are in very close contact with the hypoblast, and also
follow with fair accuracy the outline of the epiblast. This relation of the
mesoblast plates to the epiblast must not however be supposed to indicate
that the medullary groove is due to growth in the mesoblast: a view which
is absolutely negatived by the manner of formation of the medullary groove
in the head. Anteriorly the mesoblast plates thin out and completely
vanish.
In stage D, the plates of mesoblast in the trunk undergo important changes.
The cells composing them become arranged in two layers (Pl. 10, fig. 3), a
splanchnic layer adjoining the hypoblast (_sp_), and a somatic layer
adjoining the epiblast[181] (_so_). Although these two layers are
distinctly formed, they do not become separated at this stage in the region
of the trunk, and in the trunk no true body-cavity is formed.
Footnote 181: I underestimated the distinctness of this
formation in my earlier paper, _loc. cit._, although I
recognised the fact that the mesoblast cells became arranged in
two distinct layers.
By stage D the plates of mesoblast have ceased to be quite isolated, and
are connected with the lower layer cells of the general blastoderm.
Moreover the lower layer cells outside the embryo now exhibit distinct
traces of a separation into two layers, one continuous with the hypoblast,
the other with the mesoblast. Both layers are composed of very flattened
cells, and the mesoblast layer is often more than one cell deep, and
sometimes exhibits a mesh-like arrangement of its elements.
Coincidentally with the appearance of a differentiation into a somatic and
splanchnic layer the mesoblast plates become partially split by a series of
transverse lines of division into protovertebræ. Only the proximal regions
of the plates become split in this way, while their peripheral parts remain
quite intact. As a result of this each plate becomes divided into a
proximal portion adjoining the medullary canal, which is divided into
_protovertebræ_, and may be called the _vertebral plate_, and a peripheral
portion not so divided, which may be called the _lateral plate_. These two
parts are at this stage quite continuous with each other; and, as will be
seen in the sequel, the body-cavity originally extends uninterruptedly to
the summit of the vertebral plates.
By stage D at the least ten protovertebræ have appeared.
In Torpedo the mesoblast commences to be divided into two layers much
earlier than in Pristiurus; and even before stage C this division is more
or less clearly marked.
In the head and tail the condition of the mesoblast is by no means the same
as in the body.
In the tail the plates of mesoblast become considerably thickened and give
rise to two projections, one on each side, which have already been alluded
to as caudal or tail-swellings; vide Pl. 8, figs. D, F, and Pl. 10,
fig. 3_f_ and fig. 4, _ts_.
These masses of mesoblast are neither divided into protovertebræ, nor do
they exhibit any trace of a commencing differentiation into somatopleure
and splanchnopleure.
In the head, so far as I have yet been able to observe, the mesoblastic
plates do _not_ at this stage become divided into protovertebræ. The other
changes exhibited in the cephalic region are of interest, mainly from the
fact that here appears a cavity in the mesoblast directly continuous with
the body-cavity (when that cavity becomes formed), but which appears at a
very much earlier date than the body-cavity. This cavity can only be looked
on in the light of a direct continuation of the body or peritoneal cavity
into the head. Theoretical considerations with reference to it I propose
reserving till I have described the changes which it undergoes in the
subsequent periods.
Pl. 10, figs. 3_a_, 3_b_ and 3_c_ exhibit very well the condition of the
mesoblast in the head at this period. In fig. 3_c_, a section taken through
the back part of the head, the mesoblast plates have nearly the same form
as in the sections immediately behind. The ventral continuation of the
mesoblast formed by the lateral plate has, however, become much thinner,
and the dorsal or vertebral portion has acquired a more triangular form
than in the sections through the trunk (figs. 3_d_ and 3_e_).
In the section (fig. 3_b_) in front of this the ventral portion of the
plate is no longer present, and only that part exists which corresponds
with the vertebral division of the primitive plate of mesoblast.
In this a distinct cavity, forming part of the body-cavity, has appeared.
In a still anterior section (fig. 3_a_) no cavity is any longer present in
the mesoblast; whilst in sections taken from the foremost part of the head
no mesoblast is to be seen (vide Pl. 10, fig. 5, taken from the front part
of the head of the embryo represented in Pl. 8, fig. F).
A continuation of the body-cavity into the head has already been described
by Oellacher[182] for the Trout: but he believes that the cavity in this
part is solely related to the formation of the pericardial space.
Footnote 182: _Zeitschrift f. wiss. Zoologie_, 1873.
The condition of the mesoblast undergoes no important change till the end
of the period treated of in this chapter. The masses of mesoblast which
form the tail-swellings become more conspicuous (Pl. 10, fig. 4); and
indeed their convexity is so great that the space between them has the
appearance of a median groove, even after the closure of the neural canal
in the caudal region.
In embryos of stage G, which may be considered to belong to the close of
this period, eighteen protovertebræ are present both in Pristiurus and
Torpedo embryos.
_The Alimentary Canal._
The alimentary canal at the commencement of this period (stage B) forms a
space between the embryo and the yolk, ending blindly in front, but opening
posteriorly by a widish slit-like aperture, which corresponds to the anus
of Rusconi (Pl. 7, fig. 7).
The cavity anteriorly has a more or less definite form, having lateral
walls, as well as a roof and floor (Pl. 7, figs. 10_b_ and 10_c_).
Posteriorly it is not nearly so definitely enclosed (Pl. 7, fig. 10_a_).
The ventral wall of the cavity is formed by yolk. But even in stage B there
are beginnings of a cellular ventral wall derived from an ingrowth of cells
from the two sides.
By stage C considerable progress has been made in the formation of the
alimentary canal. Posteriorly it is as flattened and indefinite as during
stage B (Pl. 10, figs. 2_b_ and 2_c_). But in the anterior part of the
embryo the cavity becomes much deeper and narrower, and a floor of cells
begins to be formed for it (Pl. 10, fig. 2); and, finally, in front, it
forms a definite space completely closed in on all sides by cells (Pl. 10,
fig. 2_a_). Two distinct processes are concerned in effecting these changes
in the condition of the alimentary cavity. One of these is a process of
folding off the embryo from the blastoderm. The other is a simple growth of
cells independent of any folding. To the first of these processes the depth
and narrowness of the alimentary cavity is due; the second is concerned in
forming its ventral wall. The combination of the two processes produces the
peculiar triangular section which characterises the anterior closed end of
the alimentary cavity at this stage. The process of the folding off of the
embryo from the blastoderm resembles exactly the similar process in the
embryo bird. The fold by which the constricting off of the embryo is
effected is a perfectly continuous one, but may be conveniently spoken of
as composed of a head fold and two lateral folds.
Of far greater interest than the nature of these folds is the formation of
the ventral wall of the alimentary canal. This, as has been said, is
effected by a growth of cells from the two sides to the middle line
(Pl. 10, fig. 2). The cells for this are however not derived from
pre-existing hypoblast cells, but are formed spontaneously around nuclei of
the yolk. This fact can be determined in a large number of sections, and is
fairly well shewn in Pl. 10, fig. 2, _na_. The cells are formed in the
yolk, as has been already mentioned, by a simple aggregation of protoplasm
around pre-existing nuclei.
The cells being described are in most cases formed close to the
pre-existing hypoblast cells, but often require to undergo a considerable
change of position before attaining their final situation in the wall of
the alimentary canal.
I have already alluded to this feature in the formation of the ventral wall
of the alimentary cavity. Its interest, as bearing on the homology of the
yolk, is considerable, owing to the fact that the so-called yolk-cells of
Amphibians play a similar part in supplying the ventral epithelium of the
alimentary cavity, as do the cells derived from the yolk in Elasmobranchii.
The fact of this feature being common to the yolk-cells of Amphibians and
the yolk of Elasmobranchii, supplies a strong argument in favour of the
homology of the yolk-cells in the one case with the yolk in the other[183].
Footnote 183: Nearly simultaneously with Chapter III. of the
present monograph on the Development of Elasmobranchii, which
dealt in a fairly complete manner with the genesis of cells
outside the blastoderm, there appeared two important papers
dealing with the same subject for Teleostei. One of these, by
Professor Bambeke, "Embryologie des Poissons Osseux," _Mém.
Cour. Acad. Belgique_, 1875, which appeared some little time
before my paper, and a second by Dr Klein, _Quart. Jour. of
Micr. Sci._ April, 1876. In both of these papers a development
of nuclei and of cells is described as occurring outside the
blastoderm in a manner which accords fairly well with my own
observations.
The conclusions of both these investigators differ however from
my own. They regard the finely granular matter, in which the
nuclei appear, as pertaining to the blastoderm, and
morphologically quite distinct from the yolk. From their
observations we can clearly recognise that the material in
which the nuclei appear is far more sharply separated off from
the yolk in Osseous Fish than in Elasmobranchii, and this sharp
separation forms the main argument for the view of these
authors. Dr Klein admits, however, that this granular matter
(which he calls parablast) graduates into the typical
food-yolk, though he explains this by supposing that the
parablast takes up part of the yolk for the purpose of growth.
It is clear that the argument from a sharp separation of yolk
and parablast cannot have much importance, when it is admitted
(1) that in Osseous Fish there is a gradation between the two
substances, while (2) in Elasmobranchii the one merges slowly
and insensibly into the other.
The only other argument used by these authors is stated by Dr
Klein in the following way. "The fact that the parablast has,
at the outset, been forming one unit with what represents the
archiblast, and, _while increasing has spread_ i.e. _grown over
the yolk_ which underlies the segmentation-cavity, is, I think,
the most absolute proof that the yolk is as much different from
the parablast as it is from the archiblast." This argument to
me merely demonstrates that certain of the nutritive elements
of the yolk become in the course of development converted into
protoplasm, a phenomenon which must necessarily be supposed to
take place on my own as well as on Dr Klein's view of the
nature of the yolk. My own views on the subject have already
been fully stated. I regard the so-called yolk as composed of a
larger or smaller amount of food-material imbedded in
protoplasm, and the meroblastic ovum as a body constituted of
the same essential parts as a holoblastic ovum, though divided
into regions which differ in the proportion of protoplasm they
contain. I do not propose to repeat the positive arguments used
by me in favour of this view, but content myself with alluding
to the protoplasmic network found by Schultz and myself
extending through the whole yolk, and to the similar network
described by Bambeke as being present in the eggs of Osseous
Fish after deposition but before impregnation. The existence of
these networks is to me a conclusive proof of the correctness
of my views. I admit that in Teleostei the 'parablast' contains
more protoplasm than the homologous material in the
Elasmobranch ovum, while it is probable that after impregnation
the true yolk of Teleostei contains little or no protoplasm;
but these facts do not appear to me to militate against my
views.
I agree with Prof. Bambeke in regarding the cells derived from
the sub-germinal matter as homologous with the so-called
yolk-cells of the Amphibian embryo.
I have recently, in some of the later stages of development,
met with very peculiar nuclei of the yolk immediately beneath
the blastoderm at some little distance from the embryo, Pl. 10,
fig. 8. They were situated not in finely sub-germinal matter,
but amongst large yolk-spherules. They were very large, and
presented still more peculiar forms than those already
described by me, being produced into numerous long filiform
processes. The processes from the various nuclei were sometimes
united together, forming a regular network of nuclei quite
unlike anything that I have previously seen described.
The sub-germinal matter, in which the nuclei are usually
formed, becomes during the later stages of development far
richer in protoplasm than during the earlier. It continually
arises at fresh points, and often attains to considerable
dimensions, no doubt by feeding on yolk-spherules. Its
development appears to be determined by the necessities of
growth in the blastoderm or embryo.
The history of the alimentary canal during the remainder of this period may
be told briefly.
The folding off and closing of the alimentary canal in the anterior part of
the body proceeds rapidly, and by stage D not only is a considerable tract
of alimentary canal formed, but a great part of the head is completely
folded off from the yolk (Pl. 10, fig. 3_a_). By stage F a still greater
part is folded off. The posterior part of the alimentary canal retains for
a long period its primitive condition. It is not until stage F that it
begins to be folded off behind. After the folding has once commenced it
proceeds with great rapidity, and before stage G the hinder part of the
alimentary canal becomes completely closed in.
The folding in of the gut is produced by two lateral folds, and the gut is
not closed posteriorly.
It may be remembered that the neural canal also remained open behind. Thus
both the neural and alimentary canals are open behind; and, since both of
them extend to the posterior end of the body, they meet there, their walls
coalesce, and a direct communication from the neural to the alimentary
canal is instituted. The process may be described in another way by saying
that the medullary folds are continuous round the end of the tail with the
lateral walls of the alimentary canal; so that, when the medullary folds
unite to form a canal, this canal becomes continuous with the alimentary
canal, which is closed in at the same time. In whatever way this
arrangement is produced, the result of it is that it becomes possible to
pass in a continuously closed passage along the neural canal round the end
of the tail and into the alimentary canal. A longitudinal section shewing
this feature is represented on Pl. 10, fig. 7.
This communication between the neural and alimentary canals, which is
coupled, as will be seen in the sequel, with the atrophy of a posterior
segment of the alimentary canal, is a feature of great interest which ought
to throw considerable light upon the meaning of the neural canal. So far as
I know, no suggestion as to the origin of it has yet been made. It is by no
means confined to Elasmobranchii, but is present in all the vertebrates
whose embryos are situated at the centre and not at the periphery of the
blastoderm. It has been described by Goette[184] in Amphibians and by
Kowalevsky, Owsjannikow and Wagner[185] in the Sturgeon (Acipenser). The
same arrangement is also stated by Kowalevsky[186] to exist in Osseous
Fishes and Amphioxus. The same investigator has shewn that the alimentary
and neural canals communicate in larval Ascidians, and we may feel almost
sure that they do so in the Marsipobranchii.
Footnote 184: _Entwicklungsgeschichte der Unke._
Footnote 185: _Mélanges Biologiques de l'Académie
Pétersbourg_, Tome VII.
Footnote 186: _Archiv. f. mikros. Anat._ Vol. VII. p. 114. In
the passage on this point Kowalevsky states that in
Elasmobranchii the neural and alimentary canals communicate.
This I believe to be the first notice published of this
peculiar arrangement.
The Reptilia, Aves, and Mammalia have usually been distinguished from other
vertebrates by the possession of a well-developed allantois and amnion. I
think that we may further say that the lower vertebrates, Pisces and
Amphibia, are to be distinguished from the three above-mentioned groups of
higher vertebrates, by the positive embryonic character that their neural
and alimentary canals at first communicate posteriorly. The presence or
absence of this arrangement depends on the different positions of the
embryo in the blastoderm. In Reptiles, Birds and Mammals, the embryo
occupies a central position in the blastoderm, and not, as in Pisces and
Amphibia, a peripheral one at its edge. We can, in fact, only compare the
blastoderm of the Bird and the Elasmobranch, by supposing that in the
blastoderm of the Bird there has occurred an abbreviation of the processes,
by which the embryo Elasmobranch is eventually placed in the centre of the
blastoderm: as a result of this abbreviation the embryo Bird occupies _from
the first a_ central position in the blastoderm[187].
Footnote 187: Vide Note on p. 281, also p. 295, and Pl. 9,
figs. 1 and 2, and Comparison, &c., _Qy. Jl. of Micros. Sci._
July, 1875, p. 219. [This Edition, No. VI. p. 125.] These
passages give an account of the change of position of the
Elasmobranch embryo, and the Note on p. 281 contains a
speculation about the nature of the primitive streak with its
contained primitive groove. I have suggested that the primitive
streak is probably to be regarded as a rudiment at the position
where the edges of the blastoderm coalesced to give to the
embryos of Birds and Mammals the central position which they
occupy.
If my hypothesis should turn out to be correct, various, now
unintelligible, features about the primitive streak would be
explained: such as its position behind the embryo, the fusion
of the epiblast and mesoblast in it, the groove it contains,
&c.
The possibility of the primitive streak representing the
blastopore, as it in fact does according to my hypothesis,
ought also to throw light on E. Van Beneden's recent researches
on the development of the Mammalian ovum.
In order clearly to understand the view here expressed, the
reader ought to refer to the passages above quoted.
The peculiar relations of the blastoderm and embryo, and the resulting
relations of the neural and alimentary canal, appear to me to be features
of quite as great an importance for classification as the presence or
absence of an amnion and allantois.
_General Features of the Hypoblast._
There are but few points to be noticed with reference to the histology of
the hypoblast cells. The cells of the dorsal wall of the alimentary cavity
are columnar and form a single row. Those derived from the yolk to form the
ventral wall are at first roundish, but subsequently assume a more columnar
form.
_The Notochord._
One of the most interesting features in the Elasmobranch development is the
formation of the notochord from the hypoblast. All the steps in the process
by which this takes place can be followed with great ease and certainty.
Up to stage B the hypoblast is in contact with the epiblast immediately
below the medullary groove, but exhibits no trace of a thickening or any
other formation at that point.
Between stage B and C the notochord first arises.
In the hindermost sections of this stage the hypoblast retains a perfectly
normal structure and uniform thickness throughout. In the next few sections
(Pl. 10, fig. 1_c_, _ch´_) a slight thickening is to be observed in the
hypoblast, immediately below the medullary canal. The layer, which
elsewhere is composed of a single row of cells, here becomes two cells
deep, but no sign of a division into two layers exhibited.
In the next few sections the thickening of the hypoblast becomes much more
pronounced; we have, in fact, a ridge projecting from the hypoblast towards
the epiblast (Pl. 10, fig. 1_b_, _ch´_).
This ridge is pressed firmly against the epiblast, and causes in it a
slight indentation. The hypoblast in the region of the ridge is formed of
two layers of cells, the ridge being entirely due to the uppermost of the
two.
In sections in front of this a cylindrical rod, which can at once be
recognised as the notochord and is continuous with the ridge just
described, begins to be split off from the hypoblast. It is difficult to
say at what point the separation of this rod from the hypoblast is
completed, since all intermediate gradations between complete separation
and complete attachment are to be seen.
Where the separation first appears, a fairly thick bridge of hypoblast is
left connecting the two lateral halves of the layer, but anteriorly this
bridge becomes excessively delicate and thin (Pl. 10, fig. 1_a_), and in
some cases is barely visible except with high powers.
From the series of sections represented, it is clear that the notochord
commences to be separated from the hypoblast anteriorly, and that the
separation gradually extends backwards.
The posterior extremity of the notochord remains for a long time attached
to the hypoblast; and it is not till the end of the period treated of in
this chapter that it becomes completely free.
A sheath is formed around the notochord, very soon after its formation, at
a stage intermediate between stages C and D. This sheath is very delicate,
though it stains with both osmic acid and hæmatoxylin. I conclude from its
subsequent history, that it is to be regarded as a product of the cells of
the notochord, but at the same time it should be stated that it precisely
resembles membrane-like structures, which I have already described as being
probably artificial.
Towards the end of this period the cells of the notochord become very much
flattened vertically, and cause the well-known stratified appearance which
characterises the notochord in longitudinal sections. In transverse
sections the outlines of the cells of the notochord appear rounded.
Throughout this period the notochord cells are filled with yolk-spherules,
and near its close small vacuoles make their appearance in them.
An account of the development of the notochord, substantially similar to
that I have just given, appeared in my preliminary paper[188] on the
development of the Elasmobranch fishes.
Footnote 188: _Loc. cit._
To the remarks which were there made, I have little to add. There are two
possible views, which can be held with reference to the development of the
notochord from the hypoblast.
We may suppose that this is the primitive mode of development of the
notochord, or we may suppose that the separation of the notochord from the
hypoblast is due to a secondary process.
If the latter view is accepted, it will be necessary to maintain that the
mesoblast becomes separated from the hypoblast as three separate masses,
two lateral, and one median, and that the latter becomes separated much
later than the two former.
We have, I think, no right to assume the truth of this view without further
proof. The general admission of assumptions of this kind is apt to lead to
an injurious form of speculation, in which every fact presenting a
difficulty in the way of some general theory is explained away by an
arbitrary assumption, while all the facts in favour of it are taken for
granted. It is however clear that no theory can ever be fairly tested so
long as logic of this kind is permitted. If, in the present instance, the
view is adopted that the notochord has in reality a mesoblastic origin, it
will be possible to apply the same view to every other organ derived from
the hypoblast, and to say that it is really mesoblastic, but has become
separated at rather a late period from the hypoblast.
If, however, we provisionally reject this explanation, and accept the other
alternative, that the notochord is derived from the hypoblast, we must be
prepared to adopt one of two views with reference to the development of the
notochord in other vertebrates. We must either suppose that the current
statements as to the development of the notochord in other vertebrates are
inaccurate, or that the notochord has only become secondarily mesoblastic.
The second of these alternatives is open to the same objections as the view
that the notochord has only apparently a hypoblastic source in
Elasmobranchii, and, provisionally at least, the first of them ought to be
accepted. The reasons for accepting this alternative fall under two heads.
In the first place, the existing accounts and figures of the development of
the notochord exhibit in almost all cases a deficiency of clearness and
precision. The exact stage necessary to complete the series never appears.
It cannot, therefore, at present be said that the existing observations on
the development of the notochord afford a strong presumption against its
hypoblastic origin.
In the second place, the remarkable investigations of Hensen[189], on the
development of the notochord in Mammalia, render it very probable that, in
this group, the notochord is developed from the hypoblast.
Footnote 189: _Zeitschrift f. Anat. u. Entwicklungsgeschichte_,
Vol. I. p. 366.
Hensen finds that in Mammalia, as in Elasmobranchii, the mesoblast forms
two independent lateral masses, one on each side of the medullary canal.
After the commencing formation of the protovertebræ the hypoblast becomes
considerably thickened beneath the medullary groove; and, though he has not
followed out all the steps of the process by which this thickening is
converted into the notochord, yet his observations go very far towards
proving that it does become the notochord.
Against the observations of Hensen, there ought, however, to be mentioned
those of Lieberkühn[190]. He believes that the two lateral masses of
mesoblast, described by Hensen (in an earlier paper than the one quoted),
are in reality united by a delicate layer of cells, and that the notochord
is formed from a thickening of these.
Footnote 190: _Sitz. der Gesell. zu Marburg_, Jan. 1876.
Lieberkühn gives no further statements or figures, and it is clear that,
even if there is present the delicate layer of mesoblast, which he fancies
he has detected, yet this cannot in any way invalidate such a section as
that represented on Pl. X. fig. 40, of Hensen's paper.
In this figure of Hensen's, the hypoblast cells become distinctly more
columnar, and the whole layer much thicker immediately below the medullary
canal than elsewhere, and this independently of any possible layer of
mesoblast.
It appears to me reasonable to conclude that Lieberkühn's statements do not
seriously weaken the certainty of Hensen's results.
In addition to the observations of Hensen's on Mammalia, those of
Kowalevsky and Kuppfer on Ascidians may fairly be pointed to as favouring
the hypoblastic origin of the notochord.
It is not too much to say that at the present moment the balance of
evidence is in favour of regarding the notochord as a hypoblastic organ.
This conclusion is, no doubt, rather startling, and difficult to
understand. The only feature of the notochord in its favour is the fact of
its being unsegmented[191].
Footnote 191: In my earlier paper I suggested that the
endostyle of Ascidians afforded an instance of a supporting
organ being derived from the hypoblast. This parallel does not
hold since the endostyle has been shewn to possess a secretory
function. I never intended (as has been imagined by Professor
Todaro) to regard the endostyle as the homologue of the
notochord.
Should it eventually turn out that the notochord is developed in most
vertebrates from the mesoblast, and only exceptionally from the hypoblast,
the further question will have to be settled as to whether it is
primitively a hypoblastic or a mesoblastic organ; but, from whatever layer
it has its source, an excellent example will be afforded of an organ
changing from the layer in which it was originally developed into another
distinct layer.
EXPLANATION OF PLATE 10.
COMPLETE LIST OF REFERENCE LETTERS.
_al._ Alimentary canal. _ch._ Chorda dorsalis or notochord. _ch´._ Ridge of
hypoblast, which will become separated off as the notochord. _ep._
Epiblast. _hy._ Hypoblast. _lp._ Coalesced lateral and vertebral plate of
mesoblast. _mg._ Medullary groove. _n._ Nucleus of yolk. _na._ Cells formed
around the nuclei of the yolk to enter into the ventral wall of the
alimentary canal. _nc._ Neural or medullary canal. _pv._ Protovertebra.
_so._ Somatopleure. _sp._ Splanchnopleure. _ts._ Mesoblast of
tail-swelling. _yk._ Yolk-spherules.
Figs. 1_a_, 1_b_, 1_c_. Three sections from the same embryo belonging to a
stage intermediate between B and C, of which fig. 1_a_ is the most
anterior. × 96 diameters.
The sections illustrate (1) The different characters of the medullary
groove in the different regions of the embryo. (2) The structure of the
coalesced lateral and vertebral plates. (3) The mode of formation of the
notochord as a thickening of the hypoblast (_ch´_), which eventually
becomes separated from the hypoblast as an elliptical rod (1_a_, _ch_).
Fig. 2. Section through the anterior part of an embryo belonging to stage
C. The section is mainly intended to illustrate the formation of the
ventral wall of the alimentary canal from cells formed around the nuclei of
the yolk. It also shews the shallowness of the medullary groove in the
anterior part of the body.
Figs. 2_a_, 2_b_, 2_c_. Three sections from the same embryo as fig. 2.
Fig. 2_a_ is the most anterior of the three sections and is taken through a
point shortly in front of fig. 2. The figures illustrate the general
features of an embryo of stage C, more especially the complete closing of
the alimentary canal in front and the triangular section which it there
presents.
Fig. 3. Section through the posterior part of an embryo belonging to stage
D. × 86 diameters.
It shews the general features of the layers during the stage, more
especially the differentiation of somatic and splanchnic layers of the
mesoblast.
Figs. 3_a_, 3_b_, 3_c_, 3_d_, 3_e_, 3_f_. Sections of the same embryo as
fig. 3 (× 60 diameters). Fig. 3 belongs to part of the embryo intermediate
between figs. 3_e_ and 3_f_.
The sections shew the features of various parts of the embryo. Figs. 3_a_,
3_b_ and 3_c_ belong to the head, and special attention should be paid to
the presence of a cavity in the mesoblast in 3_b_ and to the ventral
curvature of the medullary folds.
Fig. 3_d_ belongs to the neck, fig. 3_e_ to the back, and fig. 3_f_ to the
tail.
Fig. 4. Section through the region of the tail at the commencement of stage
F. × 60 diameters.
The section shews the character of the tail-swellings and the commencing
closure of the medullary groove.
Fig. 5. Transverse section through the anterior part of the head of an
embryo belonging to stage F (× 60 diameters). It shews (1) the ventral
curvature of the medullary folds next the head. (2) The absence of
mesoblast in the anterior part of the head. _hy_ points to the extreme
front end of the alimentary canal.
Fig. 6. Section through the head of an embryo at a stage intermediate
between F and G. × 86 diameters.
It shews the manner in which the medullary folds of the head unite to form
the medullary canal.
Fig. 7. Longitudinal and vertical section through the tail of an embryo
belonging to stage G.
It shews the direct communication which exists between the neural and
alimentary canals.
The section is not quite parallel to the long axis of the embryo, so that
the protovertebræ are cut through in its anterior part, and the neural
canal passes out of the section anteriorly.
Fig. 8. Network of nuclei from the yolk of an embryo belonging to stage H.