Chapter III: Formation of the Layers (2)

Footnote 152: _Entwicklungsgeschichte der Unke._ The important
researches of Götte on the development of the ovum, though
meriting the most careful attention, do not admit of discussion
in this place.

As soon as the fact is recognised that both meroblastic and holoblastic eggs have the same fundamental constitution, the admission follows, naturally, though not necessarily, that the eggs belonging to these two classes differ solely in degree, not only as regards their constitution, but also as regards the manner in which they become respectively converted into the embryo. As might have been anticipated, this view has gained a wide acceptance.

Amongst the observations, which have given a strong objective support to this view, may be mentioned those of Professor Lankester upon the development of Cephalopoda[153], and of Dr Götte[154] upon the development of the Hen's egg. In Loligo Professor Lankester shewed that there appeared, in the part of the egg usually considered as food-yolk, a number of bodies, which eventually developed a nucleus and became cells, and that these cells entered into the blastoderm. These observations demonstrate that in the eggs of Loligo the so-called food-yolk is merely equivalent to a part of the egg which in other cases undergoes segmentation.

Footnote 153: _Annals and Magaz. of Natural History_, Vol. XI.
1873, p. 81.

Footnote 154: _Archiv f. Mikr. Anat._ Vol. X.

The observations of Dr Götte have a similar bearing. He made out that in the eggs of the Hen no sharp line is to be found separating the germinal disc from the yolk, and that, independently of the normal segmentation, a number of cells are derived from that part of the egg hitherto regarded as exclusively food-yolk. This view of the nature of the food-yolk was also advanced in my preliminary account of the development of Elasmobranchii[155], and it is now my intention to put forward the positive evidence in favour of this view, which is supplied from a knowledge of the phenomena of the development of the Elasmobranch ovum; and then to discuss how far the facts of the growth of the blastoderm in Elasmobranchii accord with the view that their large food-yolk is exactly equivalent to part of the ovum, which in Amphibians undergoes segmentation, rather than some fresh addition, which has no equivalent in the Amphibian or other holoblastic ovum.

Footnote 155: _Quart. Journ. of Micr. Science_, Oct. 1874.

Taking for granted that the ripe ovum is a single cell, the question arises whether in the case of meroblastic ova the cell is not constituted of two parts completely separated from one another.

Is the meroblastic ovum, before or after impregnation, composed of a germinal disc in which _all_ the protoplasm of the cell is aggregated, and of a food-yolk in which _no_ protoplasm is present? or is the protoplasm present _throughout_, being simply _more concentrated_ at the germinal pole than elsewhere? If the former alternative is accepted, we must suppose that the mass of food-yolk is a something added which is not present in holoblastic ova. If the latter alternative is accepted, it may then be maintained that holoblastic and meroblastic ova are constituted in the same way and differ only in the proportions of their constituents.

My own observations in conjunction with the specially interesting observations of Dr Schultz[156] justify the view which regards the protoplasm as present throughout the whole ovum, and not confined to the germinal disc. Our observations shew that a fine protoplasmic network, with ramifications extending throughout the whole yolk, is present both before and after impregnation.

Footnote 156: _Archiv f. Mikr. Anat._ Vol. XXI.

The presence of this network is, in itself, only sufficient to prove that the yolk _may_ be equivalent to part of a holoblastic ovum; to demonstrate that it is so requires something more, and this link in the chain of evidence is supplied by the nuclei of the yolk, which have been so often referred to.

These nuclei arise independently in the yolk, and become the nuclei of cells which enter the germ and the bodies of which are derived from the protoplasm of the yolk. Not only so, but the cells formed around these nuclei play the same part in the development of Elasmobranchii as do the largest so-called yolk cells in the development of Amphibians. Like the homologous cells in Amphibians, they mainly serve to form the ventral wall of the alimentary canal and the blood-corpuscles. The identity in the fate of the so-called yolk cells of Amphibians with the cells derived from the yolk in Elasmobranchii, must be considered as a proof of the homology of the yolk cells in the first case with the yolk in the second; the difference between the yolk in the two cases arising from the fact that in the Elasmobranch ovum the yolk-spherules bear a larger proportion to the protoplasm than they do in the Amphibian ovum. As I have suggested elsewhere[157], the segmentation or non-segmentation of a particular part of the ovum depends solely upon the proportion borne by the protoplasm to the yolk particles; so that, when the latter exceed the former in a certain fixed proportion, segmentation is no longer possible; and, as this limit is approached, segmentation becomes slower, and the resulting segments larger and larger.

Footnote 157: "Comparison," &c., _Quart. Journ. Micr.
Science_, July, 1875. [This Edition, No. VI.]

The question how far the facts in the developmental history of the various vertebrate blastoderms accord with the view of the nature of the yolk just propounded is one of considerable interest. An answer to it has already been attempted from a general point of view in my paper[158] entitled 'The Comparison of the early stages of development in Vertebrates'; but the subject may be conveniently treated here in a special manner for Elasmobranch embryos.

Footnote 158: _Loc. cit._

In the woodcut, fig. 1, _A_, _B_, _C_[159], are represented three diagrammatic longitudinal sections of an Elasmobranch embryo. _A_ nearly corresponds with the longitudinal section represented on Pl. 7, fig. 4, and _B_ with Pl. 7, fig. 7. In Pl. 7, fig. 7, the segmentation cavity has however completely disappeared, while it is still represented as present in the diagram of the same period. If these diagrams, or better still, the woodcuts fig. 2 _A_, _B_, _C_ (which only differ from those of the Elasmobranch fish in the smaller amount of food-yolk), be compared with the corresponding ones of Bombinator, fig. 3, _A_, _B_, _C_, they will be found to be in fundamental agreement with them. First let fig. 1, _A_, or fig. 2, _A_, or Pl. 7, fig. 4, be compared with fig. 3, _A_. In all there is present a segmentation cavity situated not centrally but near the surface of the egg. The roof of the cavity is thin in all, being composed in the Amphibian of epiblast alone, and in the Elasmobranch of epiblast and _lower layer cells_. The floor of the cavity is, in all, formed of so-called yolk (vide Pl. 7, fig. 4), which in all forms the main mass of the egg. In the Amphibian the yolk is segmented, and, though it is not segmented in the Elasmobranch, it contains in compensation the nuclei so often mentioned. In all, the sides of the segmentation cavity are formed by lower layer cells. In the Amphibian the sides are enclosed by smaller cells (in the diagram) which correspond exactly in function and position with the lower layer cells of the Elasmobranch blastoderm.

Footnote 159: This figure, together with figs. 2 and 3, are
reproduced from my paper upon the comparison of the early
stages of development in vertebrates.

Diagrammatic longitudinal sections of an Elasmobranch embryo.

_Epiblast_ without shading. _Mesoblast_ black with clear outlines to the cells. _Lower layer cells_ and _hypoblast_ with simple shading.

_ep._ epiblast. _m._ mesoblast. _al._ alimentary cavity. _sg._ segmentation cavity. _nc._ neural canal. _ch._ notochord. _x._ point where epiblast and hypoblast become continuous at the posterior end of the embryo. _n._ nuclei of yolk.

_A._ Section of young blastoderm, with segmentation cavity in the middle of the lower layer cells.

_B._ Older blastoderm with embryo in which hypoblast and mesoblast are distinctly formed, and in which the alimentary slit has appeared. The segmentation cavity is still represented as being present, though by this stage it has in reality disappeared.

_C._ Older blastoderm with embryo in which neural canal has become formed, and is continuous posteriorly with alimentary canal. The notochord, though shaded like mesoblast, belongs properly to the hypoblast.]

Diagrammatic longitudinal sections of embryo, which develops in the same manner as the Elasmobranch embryo, but in which the ovum contains far less food-yolk than is the case with the Elasmobranch ovum.

_Epiblast_ without shading. _Mesoblast_ black with clear outlines to the cells. _Lower layer cells_ and _hypoblast_ with simple shading.

_ep._ epiblast. _m._ mesoblast. _hy._ hypoblast. _sg._ segmentation cavity. _al._ alimentary cavity. _nc._ neural canal. _hf._ head fold. _n._ nuclei of the yolk.

The stages _A_, _B_ and _C_ are the same as in figure .][Transcriber's note: figure number is missing in the original.]

Diagrammatic longitudinal sections of Bombinator igneus. Reproduced with modifications from Götte.

_Epiblast_ without shading. _Mesoblast_ black with clear outlines to the cells. _Lower layer cells_ and _hypoblast_ with simple shading.

_ep._ epiblast. _l.l._ lower layer cells. _y._ smaller lower layer cells at the sides of the segmentation cavity. _m._ mesoblast. _hy._ hypoblast. _al._ alimentary cavity. _sg._ segmentation cavity. _nc._ neural cavity. _yk._ yolk-cells.

_A_ is the youngest stage in which the alimentary involution has not yet appeared. _x_ is the point from which the involution will start to form the dorsal wall of the alimentary tract. The line on each side of the segmentation cavity, which separates the smaller lower layer cells from the epiblast cells, is not present in Götte's original figure. The two shadings employed in the diagram render it necessary to have some line, but at this stage it is in reality not possible to assert which cells belong to the epiblast and which to the lower layer.

_B._ In this stage the alimentary cavity has become formed, but the segmentation cavity is not yet obliterated.

_x._ point where epiblast and hypoblast become continuous.

_C._ The neural canal is already formed, and communicates posteriorly with the alimentary.

_x._ point where epiblast and hypoblast become continuous.]

The relation of the yolk to the blastoderm in the Elasmobranch embryo at this stage of development very well suits the view of its homology with the large cells of the Amphibian ovum. The only essential difference between the two ova arises from the roof of the segmentation cavity being in the Elasmobranch embryo formed of lower layer cells, which are absent in the Amphibian embryo. This difference no doubt depends upon the greater quantity of yolk particles present in the Elasmobranch ovum. These increase the bulk of the lower layer cells, which are thus compelled to creep up the sides of the segmentation cavity till they close it in above.

In the next stage for the Elasmobranch, fig. 1 and 2 _B_ and Pl. 7, fig. 7, and for the Amphibian, fig. 3, _B_, the agreement between the two types is again very close. In both for a small portion (_x_) of the edge of the blastoderm the epiblast and hypoblast become continuous, while at all other parts the epiblast, accompanied by lower layer cells, grows round the yolk or round the large cells which correspond to it. The yolk cells of the Amphibian ovum form a comparatively small mass, and are therefore rapidly enveloped; while in the case of the Elasmobranch ovum, owing to the greater mass of the yolk, the same process occupies a long period. In both ova the portion of the blastoderm, where epiblast and hypoblast become continuous, forms the dorsal lip of an opening--the anus of Rusconi--which leads into the alimentary cavity. This cavity has the same relation in both ova. It is lined dorsally by lower layer cells, and ventrally by yolk or what corresponds with yolk; the ventral epithelium of the alimentary canal being in both cases eventually supplied by the yolk cells.

As in the earlier stage, so in the present one, the anatomical relations of the yolk to the blastoderm in the one case (Elasmobranch) are nearly identical with those of the yolk cells to the blastoderm in the other (Amphibian). The main features in which the two embryos differ, during the stage under consideration, arise from the same cause as the solitary point of difference during the preceding stage.

In Amphibians, the alimentary cavity is formed coincidently with a true ingrowth of cells from the point where epiblast and hypoblast become continuous, and from this ingrowth the dorsal wall of the alimentary cavity is formed. The same ingrowth causes the obliteration of the segmentation cavity.

In the Elasmobranchii, owing to the larger bulk of the lower layer cells caused by the food-yolk, these have been compelled to arrange themselves in their final position during segmentation, and no room is left for a true invagination; but instead of this there is formed a simple split between the blastoderm and the yolk. The homology of this with the primitive invagination is nevertheless proved by the survival of a number of features belonging to the ancestral condition in which a true invagination was present. Amongst the more important of these are the following:--(1) The continuity of epiblast and hypoblast at the dorsal lip of the anus of Rusconi. (2) The continuous conversion of indifferent lower layer cells into hypoblast, which gradually extends backwards towards the segmentation cavity, and exactly represents the course of the invagination whereby in Amphibians the dorsal wall of the alimentary cavity is formed. (3) The obliteration of the segmentation cavity during the period when the pseudo-invagination is occurring.

The asymmetry of the gastrula or pseudo-gastrula in Cyclostomes, Amphibians, Elasmobranchii and, I believe, Osseous Fishes, is to be explained by the form of the vertebrate body. In Amphioxus, where the small amount of food-yolk present is distributed uniformly, there is no reason why the invagination and resulting gastrula should not be symmetrical. In other vertebrates, where more food-yolk is present, the shape and structure of the body render it necessary for the food-yolk to be stored away on the ventral side of the alimentary canal. This, combined with the unsymmetrical position of the anus, which primitively corresponds in position with the blastopore or anus of Rusconi, causes the asymmetry of the gastrula invagination, since it is not possible for the part of the ovum which will become the ventral wall of the alimentary canal, and which is loaded with food-yolk, to be invaginated in the same fashion as the dorsal wall. From the asymmetry, so caused, follow a large number of features in vertebrate development, which have been worked out in some detail in my paper already quoted[160].

Footnote 160: _Quart. Journ. of Micr. Science_, July, 1875.
[This Edition, No. VI.]

Prof. Haeckel, in a paper recently published[161], appears to imply that because I do not find absolute invagination in Elasmobranchii, I therefore look upon Elasmobranchii as militating against his Gastræa theory. I cannot help thinking that Prof. Haeckel must have somewhat misunderstood my meaning. The importance of the Gastræa theory has always appeared to me to consist not in the fact that an actual ingrowth of certain cells occurs--an ingrowth which might have many different meanings[162]--but in the fact that the types of early development of all animals can be easily derived from that of the typical gastrula. I am perfectly in accordance with Professor Haeckel in regarding the type of Elasmobranch development to be a simple derivative from that of the gastrula, although believing it to be without any true ingrowth or invagination of cells.

Footnote 161: "Die Gastrula u. Eifurchung d. Thiere,"
_Jenaische Zeitschrift_, Vol. IX.

Footnote 162: For instance, in Crustaceans it does not in some
cases appear certain whether an invagination is the typical
gastrula invagination, or only an invagination by which, at a
period subsequent to the gastrula invagination, the hind gut is
frequently formed.

Professor Haeckel[163] in the paper just referred to published his view upon the mutual relationships of the various vertebrate blastoderms. In this paper, which appeared but shortly after my own[164] on the same subject, he has put forward views which differ from mine in several important details. Some of these bear upon the nature of food-yolk; and it appears to me that Professor Haeckel's scheme of development is incompatible with the view that the food-yolk in meroblastic eggs is the homologue of part of the hypoblast of the holoblastic eggs.

Footnote 163: _Loc. cit._

Footnote 164: _Loc. cit._

The following is Professor Haeckel's own statement of the scheme or type, which he regards as characteristic of meroblastic eggs, pp. 98 and 99.

Jetzt folgt der höchst wichtige und interessante Vorgang,
den ich als Einstülpung der Blastula auffasse und der zur
Bildung der Gastrula führt (Fig. 63, 64)[165]. Es schlägt
sich nämlich der verdickte Saum der Keimscheibe, der
"Randwulst" oder das _Properistom_, nach innen um und eine
dünne Zellenschicht wächst als directe Fortsetzung
desselben, wie ein immer enger werdendes Diaphragma, in die
Keimhöhle hinein. Diese Zellenschicht ist das entstehende
Entoderm (Fig. 64 _i_, 74 _i_). Die Zellen, welche dieselbe
zusammensetzen und aus dem innern Theile des Randwulstes
hervorwachsen, sind viel grösser aber flacher als die Zellen
der Keimhöhlendecke und zeigen ein dunkleres grobkörniges
Protoplasma. Auf dem Boden der Keimhöhle, d. h. also auf der
Eiweisskugel des Nahrungsdotters, liegen sie unmittelbar auf
und rücken hier durch centripetale Wanderung gegen dessen
Mitte vor, bis sie dieselbe zuletzt erreichen und nunmehr
eine zusammenhängende einschichtige Zellenlage auf dem
ganzen Keimhöhlenboden bilden. Diese ist die erste
vollständige Anlage des Darmblatts, Entoderms oder
"Hypoblasts", und von nun an können wir, im Gegensatz dazu
den gesammten übrigen Theil des Blastoderms, nämlich die
mehrschichtige Wand der Keimhöhlendecke als Hautblatt,
Exoderm oder "Epiblast" bezeichnen. Der verdickte Randwulst
(Fig. 64 _w_, 74 _w_), in welchem beide primäre Keimblätter
in einander übergehen, besteht in seinem oberen und äusseren
Theile aus Exodermzellen, in seinem unteren und inneren
Theile aus Entodermzellen.

In diesem Stadium entspricht unser Fischkeim einer
Amphiblastula, welche mitten in der Invagination begriffen
ist, und bei welcher die entstehende Urdarmhöhle eine grosse
Dotterkugel aufgenommen hat. Die Invagination wird nunmehr
dadurch vervollständigt und die Gastrulabildung dadurch
abgeschlossen, dass die Keimhöhle verschwindet. Das
wachsende Entoderm, dem die Dotterkugel innig anhängt, wölbt
sich in die letztere hinein und nähert sich so dem Exoderm.
Die klare Flüssigkeit in der Keimhöhle wird resorbirt und
schliesslich legt sich die obere convexe Fläche des
Entoderms an die untere concave des Exoderms eng an: die
Gastrula des discoblastischen Eies oder die "Discogastrula"
ist fertig (Fig. 65, 76; Meridiandurchschnitt Fig. 66, 75).

Die Discogastrula unsers Knochenfisches in diesem Stadium
der vollen Ausbildung stellt nunmehr eine kreisrunde Kappe
dar, welche wie ein gefüttertes Mützchen fast die ganze
obere Hemisphäre der hyalinen Dotterkugel eng anliegend
bedeckt (Fig. 65). Der Ueberzug des Mützchens entspricht dem
Exoderm (_e_), sein Futter dem Entoderm (_i_). Ersteres
besteht aus drei Schichten von kleineren Zellen, letzteres
aus einer einzigen Schicht von grösseren Zellen. Die
Exodermzellen (Fig. 77) messen 0.006 - 0.009 Mm., und haben
ein klares, sehr feinkörniges Protoplasma. Die
Entodermzellen (Fig. 78) messen 0.02 - 0.03 Mm. und ihr
Protoplasma ist mehr grobkörnig und trüber. Letztere bilden
auch den grössten Theil des Randwulstes, den wir nunmehr als
Urmundrand der Gastrula, als "_Properistoma_" oder auch als
"RUSCONI'schen After" bezeichnen können. Der letztere
umfasst die Dotterkugel, welche die ganze Urdarmhöhle
ausfüllt und weit aus der dadurch verstopften
Urmund-Oeffnung vorragt.

Footnote 165: The references in this quotation are to the
figures in the original.

My objections to the view so lucidly explained in the passage just quoted, fall under two heads.

(1) That the facts of development of the meroblastic eggs of vertebrates, are not in accordance with the views here advanced.

(2) That even if these views be accepted as representing the actual facts of development, the explanation offered of these facts would not be satisfactory.

* * * * *

Professor Haeckel's views are absolutely incompatible with the facts of Elasmobranch development, if my investigations are correct.

The grounds of the incompatibility may be summed up under the following heads:

(1) In Elasmobranchii the hypoblast cells occupy, even before the close of segmentation, the position which, on Professor Haeckel's view, they ought only eventually to take up after being involuted from the whole periphery of the blastoderm.

(2) There is no sign at any period of an invagination of the periphery of the blastoderm, and the only structure (the embryonic rim) which could be mistaken for such an invagination is confined to a very limited arc.

(3) The growth of cells to form the floor of the segmentation cavity, which ought to be part of this general invagination from the periphery, is mainly due to a formation of cells from the yolk.

It is this ingrowth of cells for the floor of the segmentation cavity which, I am inclined to think, Professor Haeckel has mistaken for a general invagination in the Osseous Fish he has investigated.

(4) Professor Haeckel fails to give an account of the asymmetry of the blastoderm; an asymmetry which is unquestionably also present in the blastoderm of most Osseous Fishes, though not noticed by Professor Haeckel in the investigations recorded in his paper.

The facts of development of Osseous Fishes, upon which Professor Haeckel rests his views, are too much disputed, for their discussion in this place to be profitable[166]. The eggs of Osseous Fishes appear to me unsatisfactory objects for the study of this question, partly on account of all the cells of the blastoderm being so much alike, that it is a very difficult matter to distinguish between the various layers, and, partly, because there can be little question that the eggs of existing Osseous Fishes are very much modified, through having lost a great part of the food-yolk possessed by the eggs of their ancestors[167]. This disappearance of the food-yolk must, without doubt, have produced important changes in development, which would be especially marked in a pelagic egg, like that investigated by Professor Haeckel.

Footnote 166: A short statement by Kowalevsky on this subject
in a note to his account of the development of Ascidians, would
seem to indicate that the type of development of Osseous Fishes
is precisely the same as that of Elasmobranchii. Kowalevsky
says, _Arch. f. Mikr. Anat._ Vol. VII. p. 114, note 5,
"According to my observations on Osseous Fishes the germinal
wall consists of two layers, an upper and lower, which are
continuous with one another at the border. From the upper one
develops skin and nervous system, from the lower hypoblast and
mesoblast." This statement, which leaves unanswered a number of
important questions, is too short to serve as a basis for
supporting my views, but so far as it goes its agreement with
the facts of Elasmobranch development is undoubtedly striking.

Footnote 167: The eggs of the Osseous Fishes have, I believe,
undergone changes of the same character, but not to the same
extent, as those of Mammalia, which, according to the views
expressed both by Professor Haeckel and myself, are degenerated
from an ovum with a large food-yolk. The grounds on which I
regard the eggs of Osseous Fishes as having undergone an
analogous change, are too foreign to the subject to be stated
here.

The Avian egg has been a still more disputed object than even the egg of the Osseous Fishes. The results of my own investigations on this subject do not accord with those of Dr Götte, or the views of Professor Haeckel[168].

Footnote 168: I find myself unable without figures to
understand Dr Rauber's (_Centralblatt für Med. Wiss._ 1874, No.
50; 1875, Nos. 4 and 17) views with sufficient precision to
accord to them either my assent or dissent. It is quite in
accordance with the view propounded in my paper (_loc. cit._)
to regard, with Dr Rauber and Professor Haeckel, the thickened
edge of the blastoderm as the homologue of the lip of the
blastopore in Amphioxus; though an invagination, in the manner
imagined by Professor Haeckel, is no necessary consequence of
this view. If Dr Rauber regards the _whole_ egg of the bird as
the homologue of that of Amphioxus, and the inclosure of the
yolk by the blastoderm as the equivalent to the process of
invagination in Amphioxus, then his views are practically in
accordance with my own.

Apart from disputed points of development, it appears to me that a comparative account of the development of the meroblastic vertebrate ova ought to take into consideration the essential differences which exist between the Avian and Piscian blastoderms, in that the embryo is situated in the centre of the blastoderm in the first case and at the edge in the second[169].

Footnote 169: I have suggested in a previous paper
("Comparison," &c., _Quart. Journal of Micr. Science_, July,
1875) that the position occupied by the embryo of Birds at the
centre, and not at the periphery, of the blastoderm may be due
to an abbreviation of the process by which the Elasmobranch
embryos cease to be situated at the edge of the blastoderm
(vide p. 296 and Pl. 9, fig. 1, 2). Assuming this to be the
real explanation of the position of the embryo in Birds, I feel
inclined to repeat a speculation which I made some time ago
with reference to the primitive streak in Birds (_Quart. Journ.
of Micr. Science_, 1873, p. 280). In Birds there is, as is well
known, a structure called the primitive streak, which has been
shewn by the observations of Dursy, corroborated by my
observations (_loc. cit._), to be situated behind the medullary
groove, and to take no part in the formation of the embryo. I
further shewed that the peculiar fusion of epiblast and
mesoblast, called by His the axis cord, was confined to this
structure and did not occur in other parts of the blastoderm.
Nearly similar results have been recently arrived at by Hensen
with reference to the primitive streak in Mammals. The position
of the primitive streak immediately behind the embryo suggests
the speculation that it may represent the line along which the
edges of the blastoderm coalesced, so as to give to the embryo
the central position which it has in the blastoderms of Birds
and Mammals, and that the peculiar fusion of epiblast and
mesoblast at this point may represent the primitive continuity
of epiblast and lower layer cells at the dorsal lip of the anus
of Rusconi in Elasmobranchii. I put this speculation forward as
a mere suggestion, in the hope of elucidating the peculiar
structure of the primitive streak, which not improbably may be
found to be the keystone to the nature of the blastoderm of the
higher vertebrates.

This difference entails important modifications in development, and must necessarily affect the particular points under discussion. As a result of the different positions of the embryo in the two cases, there is present in Elasmobranchii and Osseous Fishes a true anus of Rusconi, or primitive opening into the alimentary canal, which is absent in Birds. Yet in neither Elasmobranchii[170] nor Osseous Fishes does the anus of Rusconi correspond in position with the point where the final closing in of the yolk takes place, but in them this point corresponds rather with the blastopore of Birds[171].

Footnote 170: Vide p. 296 and Plate 9, fig. 1 and 2, and Self,
"Comparison," &c., _loc. cit._

Footnote 171: The relation of the anus of Rusconi and
blastopore in Elasmobranchii was fully explained in the paper
above quoted. It was there clearly shewn that neither the one
nor the other exactly corresponds with the blastopore of
Amphioxus, but that the two together do so. Professor Haeckel
states that in the Osseous Fish investigated by him the anus of
Rusconi and the blastopore coincide. This is not the case in
the Salmon.

Owing also to the respective situations of the embryo in the blastoderm, the alimentary and neural canals communicate posteriorly in Elasmobranchii and Osseous Fishes, but _not_ in Birds. Of all these points Professor Haeckel makes no mention.

The support of his views which Prof. Haeckel attempts to gain from Götte's researches in Mammalia is completely cut away by the recent discoveries of Van Beneden[172] and Hensen[173].

Footnote 172: "Développement Embryonnaire des Mammifères,"
_Bulletin de l'Acad. r. d. Belgique_, 1875.

Footnote 173: _Loc. cit._

It thus appears that Professor Haeckel's views but ill accord with the facts of vertebrate development; but even if they were to do so completely it would not in my opinion be easy to give a rational explanation of them.

Professor Haeckel states that no sharp and fast line can be drawn between the types of 'unequal' and 'discoidal' segmentation[174]. In the cases of unequal segmentation he admits, as is certainly the case, that the larger yolk cells (hypoblast) are simply enclosed by a growth of the epiblast around them; which is to be looked on as a modification of the typical gastrula invagination, necessitated by the large size of the yolk cells (vide Professor Haeckel's paper, Taf. II. fig. 30). In these instances there is no commencement of an ingrowth in the _manner supposed for meroblastic ova_.

Footnote 174: For an explanation of these terms, vide Prof.
Haeckel's original paper or the abstract in _Quart. Journ. of
Micr. Science_ for January, 1876.

When the food-yolk becomes more bulky, and the hypoblast does not completely segment, it is not easy to understand why an ingrowth, which had no existence in the former case, should occur; nor where it is to come from. Such an ingrowth as is supposed to exist by Professor Haeckel would, in fact, break the continuity of development between meroblastic and holoblastic ova, and thus destroy one of the most important results of the Gastræa theory.

It is quite easy to suppose, as I have done, that in the cases of discoidal segmentation, the hypoblast (including the yolk) becomes enclosed by the epiblast in precisely the same manner as in the cases of unequal segmentation.

But even if Professor Haeckel supposes that in the unsegmented food-yolk a fresh element is added to the ovum, it remains quite unintelligible to me how an ingrowth of cells from a circumferential line, to form a layer which had no previous existence, can be equivalent to, or derived from, the invagination of a layer, which exists before the process of invagination begins, and which remains continuous throughout it.

If Professor Haeckel's views should eventually turn out to be in accordance with the facts of vertebrate development, it will, in my opinion, be very difficult to reduce them into conformity with the Gastræa theory.

Although some space has been devoted to an attempt to refute the views of Professor Haeckel on this question, I wish it to be clearly understood that my disagreement from his opinions concerns matters of detail only, and that I quite accept the Gastræa theory in its general bearings.

* * * * *

Observations upon the formation of the layers in Elasmobranchii have hitherto been very few in number. Those published in my preliminary account of these fishes are, I believe, the earliest[175].

Footnote 175: I omit all reference to a paper published in
Russian by Prof. Kowalevsky. Being unable to translate it, and
the illustrations being too meagre to be in themselves of much
assistance, it has not been possible for me to make any use of
it.

Since then there has been published a short notice on the subject by Dr Alex. Schultz[176]. His observations in the main accord with my own. He apparently speaks of the nuclei of the yolk as cells, and also of the epiblast being more than one cell deep. In Torpedo alone, amongst the genera investigated by me, is the layer of epiblast, at about the age of the last described embryo, composed of more than a single row of cells.

Footnote 176: _Centralblatt f. Med. Wiss._ No. 33, 1875.

EXPLANATION OF PLATE 7.

COMPLETE LIST OF REFERENCE LETTERS.

_c._ Cells formed in the yolk around the nuclei of the yolk. _ep._ Epiblast. _er._ Embryonic ring. _es._ Embryo swelling. _hy._ Hypoblast. _ll._ Lower layer cells. _ly._ Line separating the yolk from the blastoderm. _m._ Mesoblast. _mg._ Medullary groove. _n´._ Nuclei of yolk. _na._ Cells to form ventral wall of alimentary canal which have been derived from the yolk. _nal._ Cells formed around the nuclei of the yolk which have entered the hypoblast. _sc._ Segmentation cavity. _vp._ Combined lateral and vertebral plate of mesoblast.

Fig. 1. Longitudinal section of a blastoderm at the first appearance of the segmentation cavity.

Fig. 2. Longitudinal section through a blastoderm after the layer of cells has disappeared from the floor of the segmentation cavity. _bd._ Large cell resting on the yolk, probably remaining over from the later periods of segmentation. Magnified 60 diameters. (Hardened in chromic acid.)

The section is intended to illustrate the fact that the nuclei form a layer in the yolk under the floor of the segmentation cavity. The roof of the segmentation cavity is broken.

Fig. 2_a_. Portion of same blastoderm highly magnified, to shew the characters of the nuclei of the yolk _n´_ and the nuclei in the cells of the blastoderm.

Fig. 2_b_. Large knobbed nucleus from the same blastoderm, very highly magnified.

Fig. 2_c_. Nucleus of yolk from the same blastoderm.

Fig. 3. Longitudinal section of blastoderm of same stage as fig. 2. (Hardened in chromic acid.)

Fig. 4. Longitudinal section of blastoderm slightly older than fig. 2. Magnified 45 diameters. (Hardened in osmic acid.)

It illustrates (1) the characters of the epiblast; (2) the embryonic swelling; (3) the segmentation cavity.

Fig. 5. Longitudinal section through a blastoderm at the time of the first appearance of the embryonic rim, and before the formation of the medullary groove. Magnified 45 diameters.

Fig. 5_a_. Section through the periphery of the embryonic rim of the blastoderm of which fig. 5 represents a section.

Fig. 6. Section through the embryonic rim of a blastoderm somewhat younger than that represented on Pl. 8, fig. B.

Fig. 7. Section through the most projecting portion of the embryonic rim of a blastoderm of the same age as that represented on Pl. 8, fig. B. The section is drawn on a very considerably smaller scale than that on fig. 5. It is intended to illustrate the growth of the embryonic rim and the disappearance of the segmentation cavity.

Fig. 7_a_. Section through peripheral portion of the embryonic rim of the same blastoderm, highly magnified. It specially illustrates the formation of a cell (_c_) around a nucleus in the yolk. The nuclei of the blastoderm have been inaccurately rendered by the artist.

Figs. 8_a_, 8_b_, 8_c_. Three sections of the same embryo. Inserted mainly to illustrate the formation of the mesoblast as two independent lateral masses of cells; only half of each section is represented. 8_a_ is the most posterior of the three sections. In it the mesoblast forms a large mass on each side, imperfectly separated from the hypoblast. In 8_b_, from the anterior part of the embryo, the main mass of mesoblast is far smaller, and only forms a cap to the hypoblast at the highest point of the medullary fold. In 8_c_ a cap of mesoblast is present, similar to that in 8_b_, though much smaller. The sections of these embryos were somewhat oblique, and it has unfortunately happened that while in 8_a_ one side is represented, in 8_b_ and 8_c_ the other side is figured, had it not been for this the sections 8_b_ and 8_c_ would have been considerably longer than 8_a_.

Fig. 9. Longitudinal section of an embryo belonging to a slightly later stage than B.

This section passes through one of the medullary folds. It illustrates the continuity of the hypoblast with the remaining lower layer cells of the blastoderm.

Figs. 10_a_, 10_b_, 10_c_. Three sections of the same embryo belonging to a stage slightly later than B, Pl. 8. The space between the mesoblast and the hypoblast has been made considerably too great in the figures of the three sections.

10_a_. The most posterior of the three sections. It shews the posterior flatness of the medullary groove and the two isolated vertebral plates.

10_b_. This section is taken from the anterior part of the same embryo and shews the deep medullary groove and the commencing formation of the ventral wall of the alimentary canal from the nuclei of the yolk.

10_c_ shews the disappearance of the medullary groove and the thinning out of the mesoblast plates in the region of the head.

Fig. 11. Small portion of the blastoderm and the subjacent yolk of an embryo at the time of the first appearance of the medullary groove × 300. It shews two large nuclei of the yolk (_n_) and the protoplasmic network in the yolk between them; the network is seen to be closer round the nuclei than in the intervening space. There are no areas representing cells around the nuclei.

Fig. 12. Nucleus of the yolk in connection with the protoplasmic network hardened in osmic acid.

Fig. 13. Portion of posterior end of a blastoderm of stage B, shewing the formation of cells around the nuclei of the yolk.

Fig. 14. Section through part of a young Scyllium egg, about 1/15th of an inch in diameter.

_nl._ Protoplasmic network in yolk. _zp._ Zona pellucida. _ch._ Structureless chorion. _fep._ Follicular epithelium. _x._ Structureless membrane external to this.