It is divided into two layers—an upper one, the epiblast, formed by a single row of columnar cells; and a lower one, consisting of the remaining cells of the blastoderm.
A cavity next appears in the lower layer cells, near the non-embryonic end of the blastoderm, but the cells soon disappear from the floor of this cavity which then comes to be constituted by yolk alone.
The epiblast in the next stage is reflected for a small arc at the embryonic end of the blastoderm, and becomes continuous with the lower layer cells; at the same time some of the lower layer cells of the embryonic end of the blastoderm assume a columnar form, and constitute the commencing hypoblast. The portion of the blastoderm, where epiblast and hypoblast are continuous, forms a projecting structure which I have called the embryonic rim. This rim increases rapidly by growing inwards more and more towards the centre of the blastoderm, through the continuous conversion of lower layer cells into columnar hypoblast.
While the embryonic rim is being formed, the segmentation cavity undergoes important changes. In the first place, it receives a floor of lower layer cells, partly from an ingrowth from the two sides, and partly from the formation of cells around the nuclei of the yolk.
Shortly after the floor of cells has appeared, the whole segmentation cavity becomes obliterated.
When the embryonic rim has attained to some importance, the position of the embryo becomes marked out by the appearance of the medullary groove at its most projecting part. The embryo extends from the edge of the blastoderm inwards towards the centre.
At about the time of the formation of the medullary groove, the mesoblast becomes definitely constituted. It arises as two independent plates, one on each side of the medullary groove, and is entirely derived from lower layer cells.
The two plates of mesoblast are at first unconnected with any other cells of the blastoderm, and, on their formation, the hypoblast remains in connection with all the remaining lower layer cells. Between the embryonic rim and the yolk is a cavity,—the primitive alimentary cavity. Its roof is formed of hypoblast, and its floor of yolk. Its external opening is homologous with the anus of Rusconi, of Amphioxus and the Amphibians. The ventral wall of the alimentary cavity is eventually derived from cells formed in the yolk around the nuclei which are there present.
* * * * *
Since the important researches of Gegenbaur[149]upon the meroblastic vertebrate eggs, it has been generally admitted that the ovum of every vertebrate, however complicated may be its apparent constitution, is nevertheless to be regarded as a simple cell. This view is, indeed, opposed by His[150]and to a very modified extent by Waldeyer[151], and has recently been attacked from an entirely new standpoint by Götte[152]; but, to my mind, the objections of these authors do not upset the well founded conclusions of previous observations.
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.
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.
Taking for granted that the ripe ovum is a single cell, thequestion 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 whichallthe protoplasm of the cell is aggregated, and of a food-yolk in whichnoprotoplasm is present? or is the protoplasm presentthroughout, being simplymore concentratedat 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.
The presence of this network is, in itself, only sufficient to prove that the yolkmaybe 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 casewith 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.
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.
In the woodcut, fig. 1,A,B,C[159], are represented three diagrammatic longitudinal sections of an Elasmobranch embryo.Anearly corresponds with the longitudinal section represented onPl.7, fig. 4, andBwithPl.7, fig. 7. InPl.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. 2A,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, orPl.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 inthe Elasmobranch of epiblast andlower layer cells. The floor of the cavity is, in all, formed of so-called yolk (videPl.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.
Fig. 1.
Diagrammatic longitudinal sections of an Elasmobranch embryoDiagrammatic longitudinal sections of an Elasmobranch embryo.Epiblastwithout shading.Mesoblastblack with clear outlines to the cells.Lower layer cellsandhypoblastwith 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 an Elasmobranch embryo.
Epiblastwithout shading.Mesoblastblack with clear outlines to the cells.Lower layer cellsandhypoblastwith 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.
Fig. 2.
Diagrammatic longitudinal sections of embryoDiagrammatic 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.Epiblastwithout shading.Mesoblastblack with clear outlines to the cells.Lower layer cellsandhypoblastwith 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 stagesA,BandCare the same as in figure .[TN9]
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.
Epiblastwithout shading.Mesoblastblack with clear outlines to the cells.Lower layer cellsandhypoblastwith 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 stagesA,BandCare the same as in figure .[TN9]
Fig. 3.
Diagrammatic longitudinal sections of Bombinator igneusDiagrammatic longitudinal sections of Bombinator igneus. Reproduced with modifications from Götte.Epiblastwithout shading.Mesoblastblack with clear outlines to the cells.Lower layer cellsandhypoblastwith 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.Ais the youngest stage in which the alimentary involution has not yet appeared.xis 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.
Diagrammatic longitudinal sections of Bombinator igneus. Reproduced with modifications from Götte.
Epiblastwithout shading.Mesoblastblack with clear outlines to the cells.Lower layer cellsandhypoblastwith 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.
Ais the youngest stage in which the alimentary involution has not yet appeared.xis 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 2BandPl.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].
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.
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.
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 dasProperistom, nach innen um und eine dünne Zellenschicht wächst als directe Fortsetzung desselben, wie ein immerenger werdendes Diaphragma, in die Keimhöhle hinein. Diese Zellenschicht ist das entstehende Entoderm (Fig. 64i, 74i). 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. 64w, 74w), 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 - 0.009Mm., und haben ein klares, sehr feinkörniges Protoplasma. Die Entodermzellen (Fig. 78) messen 0.02 - 0.03Mm.und ihr Protoplasma ist mehr grobkörnig und trüber. Letztere bilden auch den grössten Theil des Randwulstes, den wir nunmehr alsUrmundrandder 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.
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 theirdiscussion 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.
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].
Apart from disputed points of development, it appears to me that a comparative account of the development of the meroblasticvertebrate 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].
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].
Owing also to the respective situations of the embryo in theblastoderm, the alimentary and neural canals communicate posteriorly in Elasmobranchii and Osseous Fishes, butnotin 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].
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 (videProfessor Haeckel's paper,Taf.II.fig. 30). In these instances there is no commencement of an ingrowth in themanner supposed for meroblastic ova.
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, itremains 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].
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.
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. 2a. Portion of same blastoderm highly magnified, to shew the characters of the nuclei of the yolkn´and the nuclei in the cells of the blastoderm.
Fig. 2b. Large knobbed nucleus from the same blastoderm, very highly magnified.
Fig. 2c. 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. 5a. 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 onPl.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 onPl.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. 7a. 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. 8a, 8b, 8c. 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. 8ais the most posterior of the three sections. In it the mesoblast forms a large mass on each side, imperfectly separated from the hypoblast. In 8b, 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 8ca cap of mesoblast is present, similar to that in 8b, though much smaller. The sections of these embryos were somewhat oblique, and it has unfortunately happened that while in 8aone side is represented, in 8band 8cthe other side is figured, had it not been for this the sections 8band 8cwould have been considerably longer than 8a.
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. 10a, 10b, 10c. 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.
10a. The most posterior of the three sections. It shews the posterior flatness of the medullary groove and the two isolated vertebral plates.
10b. 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.
10cshews 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.
[116]Qy. Journal of Microsc. Science,Oct.1874. [This Edition,No.V.]
[117]Centr. f. Med. Wiss.No.38, 1875.
[118]Professor Bambeke (“Poissons Osseux,”Mém. Acad. Belgique1875) describes a cavity in the blastoderm of Leuciscus rutilus, which he regards as the true segmentation cavity, but not as identical with the segmentation cavity of Osseous Fishes, usually so called. Its relations are the same as those of my segmentation cavity at this stage. This paper came into my hands at too late a period for me to be able to do more than refer to it in this place.
[119]Loc. cit.
[120]Loc. cit.
[121]Loc. cit.
[122]Qy. Journal of Micros. Science,Oct.1874. [This Edition,No.V.]
[123]Loc. cit.Probably Dr Schultz, here as in other cases, has mistaken nuclei for cells.
[124]Loc. cit.
[125]Prof. Haeckel (“Die Gastrula u. die Eifurchung d. Thiere,”Jenaische Zeitschrift, Vol.IX.) has unfortunately copied a figure from my preliminary paper (loc. cit.) (repeated now), which I had carefully avoided using for the purpose of describing the formation of the layers on account of the epiblast cells in the original having been much altered by the chromic acid, as a result of which the whole section gives a somewhat erroneous impression of the condition of the blastoderm at this stage. I take this opportunity of pointing out that the colouration employed by Professor Haeckel to distinguish the layers in this section is not founded on my statements, but is, on the contrary, in entire opposition to them. From the section as represented by Professor Haeckel it might be gathered that I considered the lower layer cells to be divided into two parts, one derived from the epiblast, while the other constituted the hypoblast. Not only is no such division present at this period, but no part of the lower layer cells, or the mesoblast cells into which they become converted, can in any sense whatever be said to be derived from the epiblast.
[126]“Entwicklungsgeschichte der Najaden,”Sitz. d. k. Akad. Wien, 1875.
[127]Morphologische Jahrbuch,Vol.1.Heft3.
[128]“Développement des Mammifères,”Bul. de l'Acad. de Belgique,XL.No.12, 1875.
[129]Qy. Journal Microsc. Science,Oct.1874. [This Edition,No.V.]
[130]Götte,“Der Keim d. Forelleneies,”Arch. f. Mikr. Anat.Vol.IX.; Haeckel,“Die Gastrula u. die Eifurchung d. Thiere,”Jenaische Zeitschrift, Bd.IX.
[131]When writing my earlier paper I did not feel so confident about the mode of formation of the hypoblast as I now do, and even doubted the possibility of determining it from sections. The facts now brought forward are I hope sufficient to remove all scepticism on this point.
[132]Owing to the small size of the plates this section has been drawn on a considerably smaller scale than that represented in fig. 5.
[133]Quart. Journ. of Microscop. Science, July, 1875. [This Edition,No.VI.]
[134]Oellacher,Zeit. f. Wiss. Zoologie, Bd.XXIII.Götte,Archiv f. Mikr. Anat.Vol.IX. Haeckel,loc. cit.
[135]This floor appears in most Osseous Fish to be only partially formed.VideGötte,loc. cit.
[136]Loc. cit.
[137]Loc. cit.
[138]Loc. cit.
[139]Loc. cit.
[140]Loc. cit.
[141]ProfessorLieberkühn (Gesellschaft zu Marburg,Jan.1876) finds in Mammalia a bilateral arrangement of the mesoblast, which he compares with that described by me in Elasmobranchii. In Mammalia, however, he finds the two masses of mesoblast connected by a very thin layer of cells, and is apparently of opinion that a similar thin layer exists in Elasmobranchii though overlooked by me. I can definitely state that, whatever may be the condition of the mesoblast in Mammalia, in Elasmobranchii at any rate no such layer exists.
[142]Loc. cit.
[143]Quart. Journ. of Microsc. Science,Oct., 1874. [This Edition,No.V.]
[144]“Embryologische Studien an Würmern u. Arthropoden.”Mémoires de l'Acad. S. Pétersbourg.Vol.XIV.1873.
[145]Archiv für Mikr. Anat.Vol.VII.
[146]Jenaische Zeitschrift,Vol.IX.1875. A bilateral development of mesoblast, according to Professor Haeckel (loc. cit.), occurs in some Osseous Fish. Hensen,Zeit. für Anat. u. Entw.Vol.1., has recently described the mesoblast in Mammalia as consisting of independent lateral masses.
[147]Archiv für Mikr. Anat.Vol.XI.
[148]A protoplasmic network resembling in its essential features the one just described has been noticed by many observers in other ova. Fol has figured and described a network or sponge-like arrangement of the protoplasm in the eggs of Geryonia. (Jenaische Zeitschrift,Vol.VII.) Metschnikoff (Zeitschrift f. Wiss. Zoologie, 1874) has demonstrated its presence in the ova of many Siphonophoriæ and Medusæ. Flemming (“Entwicklungsgeschichte der Najaden,”Sitz. der k. Akad. Wien, 1875) has found it in the ovarian ova of fresh-water mussels (Anodonta and Unio), but regards it as due to the action of reagents, since he fails to find it in the fresh condition. Amongst vertebrates it has been carefully described by Eimer (Archiv für Mikr. Anat.,Vol.VIII.) in the ovarian ova of Reptiles. Eimer moreover finds that it is continuous with prolongations from cells of the epithelium of the follicle in which the ovum is contained. According to him remnants of this network are to be met with in the ripe ovum, but are no longer present in the ovum when taken from the oviduct.
[149]“Wirbelthiereier mit partieller Dottertheilung.”Müller'sArch.1861.
[150]Erste Anlage des Wirbelthierleibes.
[151]Eierstock u. Ei.
[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.
[153]Annals andMagaz.of Natural History,Vol.XI.1873, p. 81.
[154]Archiv f. Mikr. Anat.Vol.X.
[155]Quart. Journ. of Micr. Science,Oct.1874.
[156]Archiv f. Mikr. Anat.Vol.XXI.
[157]“Comparison,”&c.,Quart. Journ. Micr. Science, July, 1875. [This Edition,No.VI.]
[158]Loc. cit.
[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.
[160]Quart. Journ. of Micr. Science, July, 1875. [This Edition,No.VI.]
[161]“Die Gastrula u. Eifurchung d. Thiere,”Jenaische Zeitschrift,Vol.IX.
[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.
[163]Loc. cit.
[164]Loc. cit.
[165]The references in this quotation are to the figures in the original.
[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.
[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.
[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 thewholeegg 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.
[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 (videp.296andPl.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.
[170]Videp.296and Plate 9, fig. 1 and 2, and Self,“Comparison,”&c.,loc. cit.
[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.
[172]“Développement Embryonnaire des Mammifères,”Bulletin de l'Acad. r. d. Belgique, 1875.
[173]Loc. cit.
[174]For an explanation of these terms,videProf. Haeckel's original paper or the abstract inQuart. Journ. of Micr. Sciencefor January, 1876.
[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.
[176]Centralblatt f. Med. Wiss.No.33, 1875.
No complete series of figures, representing the various stages in development of an Elasmobranch Embryo, has hitherto been published. With the view of supplying this deficiency Plate 8 has been inserted. The embryos represented in this Plate form a fairly complete series, but do not all belong to a single species. Figs. A, B, C, D, E, F, H, I represent embryos of Pristiurus; G being an embryo of Torpedo. The remaining figures, excepting K, which is a Pristiurus embryo, are embryos of Scyllium canicula. The embryos A-I were very accurately drawn from nature by my sister, Miss A. B. Balfour. Unfortunately the exceptional beauty and clearness of the originals is all but lost in the lithographs. To facilitate future description, letters will be employed in the remainder of these pages to signify that an embryo being described is of the same age as the embryo on this Plate to which the letter used refers. Thus an embryo of the same age as L will be spoken of hereafter as belonging to stage L.
A.
This figure represents a hardened blastoderm at a stage when the embryo-swelling (e.s.) has become obvious, but before the appearance of the medullary groove. The position of the segmentation cavity is indicated by a slight swelling of the blastoderm (s.c). The shape of the blastoderm, in hardened specimens, is not to be relied upon, owing to the traction which the blastoderm undergoes during the process of removing the yolk from the egg-shell.
B.
B is the view of a fresh blastoderm. The projecting part of this, already mentioned as the 'embryonic rim', is indicatedby the shading. At the middle of the embryonic rim is to be seen the rudiment of the embryo (m.g.). It consists of an area of the blastoderm, circumscribed on its two sides and at one end, by a slight fold, and whose other end forms part of the edge of the blastoderm. The end of the embryo which points towards thecentreof the blastoderm is the head end, and that which forms part of theedgeof the blastoderm is the tail end. To retain the nomenclature usually adopted in treating of the development of the Bird, the fold at the anterior end of the embryo may be calledthe head fold, and those at the sides theside folds. There is in Elasmobranchii no tail fold, owing to the position of the embryo at the periphery of the blastoderm, and it is by the meeting of the three above-mentioned folds only, that the embryo becomes pinched off from the remainder of the blastoderm. Along the median line of the embryo is a shallow groove (m.g.), the well-known medullary groove of vertebrate embryology. It flattens out both anteriorly and posteriorly, and is deepest in the middle part of its course.
C.