The Works of Francis Balfour 3-11
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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.
- Cephalochorda | Urochorda | Elasmobranchii | Teleostei | Cyclostomata | Ganoidei | Amphibia | Aves | Reptilia | Mammalia | Comparison of the Formation of Germinal Layers and Early Stages in Vertebrate Development | Ancestral form of the Chordata | General Conclusions | Epidermis and Derivatives | The Nervous System | Organs of Vision | Auditory, Olfactory, and Lateral Line Sense Organs | Notochord, Vertebral Column, Ribs, and Sternum | The Skull | Pectoral and Pelvic Girdles and Limb Skeleton | Body Cavity, Vascular System and Glands | The Muscular System | Excretory Organs | Generative Organs and Genital Ducts | The Alimentary Canal and Appendages in Chordata
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
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Vol. III. A Treatise on Comparative Embryology 2 (1885)
CHAPTER XI. COMPARISON OF THE FORMATION OF THE GERMINAL LAYERS AND OF THE EARLY STAGES IN THE DEVELOPMENT OF VERTEBRATES
ALTHOUGH the preceding chapters of this volume contain a fairly detailed account of the early developmental stages of different groups of the Chordata, it will nevertheless be advantageous to give at this place a short comparative review of the whole subject.
In this review only the most important points will be dwelt upon, and the reader is referred for the details of the processes to the sections on the development of the individual groups.
The subject may conveniently be treated under three heads.
(1) The formation of the gastrula and behaviour of the blastopore : together with the origin of the hypoblast.
(2) The mesoblast and notochord.
(3) The epiblast.
At the close of the chapter is a short summary of the organs derived from the several layers, together with some remarks on the growth in length of the vertebrate embryo, and some suggestions as to the origin of the allantois and amnion.
Formation of the gastrula. Amphioxus is the type in which the developmental phenomena are least interfered with by the presence of food-yolk.
In this form the segmentation results in a uniform, or nearly uniform, blastosphere, one wall of which soon becomes thickened and invaginated, giving rise to the hypoblast ; while the larva takes the form of a gastrula, with an archenteric cavity opening by a blastopore. The blastopore rapidly narrows, while the
1 8 2
THE GASTRULA OF AMPHIOXUS.
embryo assumes an elongated cylindrical form with the blastopore at its hinder extremity (fig. 169 A). The blastopore now passes to the dorsal surface, and by the flattening of this surface a medullary plate is formed extending forwards from the blasto
FIG. 169. EMBRYOS OF AMPHIOXUS. (After Kowalevsky.) The parts in black with white lines are epiblastic; the shaded parts are hypoblastic.
A. Gastrula stage in optical section.
B. Slightly later stage after the neural plate np has become differentiated, seen as a transparent object from the dorsal side.
C. Lateral view of a slightly older larva in optical section.
D. Dorsal view of an older larva with the neural canal completely closed except for a small pore (no) in front.
E. Older larva seen as a transparent object from the side.
bl. blastopore (which becomes in D the neurenteric canal) ; ne. neurenteric canal ;
- //. neural or medullary plate; no. anterior opening of neural canal; ch. notochord;
so 1 , so", first and second mesoblastic somites.
pore (fig. 169 B). On the formation of the medullary groove and its conversion into a canal, the blastopore opens into this canal, and gives rise to a neurenteric passage, leading from the neural canal into the alimentary tract (fig. 169 C and E). At a later period this canal closes, and the neural and alimentary canals become separated.
Such is the simple history of the layers in Amphioxus. In the simplest types of Ascidians the series of phenomena is almost the same, but the blastopore assumes a more definitely dorsal position.
COMPARISON OF THE GERMINAL LAYERS.
Here also the blastopore lies at the hinder end of the medullary groove, and on the closure of the groove becomes converted into a neurenteric passage.
In the true Vertebrates the types which most approach Amphioxus are the Amphibia, Acipenser and Petromyzon. We may take the first of these as typical (though Petromyzon is perhaps still more so) and fig. 170 A B C D represents four diagrammatic longitudinal vertical sections through a form
FIG. 170. DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF BOMBINATOR AT TWO STAGES, TO SHEW THE FORMATION OF THE GERMINAL LAYERS. (Modified from Gotte.)
ep. epiblast ; m. dorsal mesoblast ; m'. ventral mesoblast ; hy. hypoblast ; yk. yolk ; x. point of junction of the epiblast and hypoblast at the dorsal side of the blastopore ; al. mesenteron ; sg. segmentation cavity.
378 THE GASTRULA OF AMPHIBIA.
belonging to this group (Bombinator). The food-yolk is here concentrated in what I shall call the lower pole of the egg, which becomes the ventral aspect of the future embryo. The part of the .egg containing the stored-up food-yolk is, as has already been explained in the chapter on segmentation (Vol. II. pp. 94 and 95), to be regarded as equivalent to part of those eggs which do not contain food-yolk ; a fact which requires to be borne in mind in any attempt to deal comparatively with the formation of the layers in the Vertebrata. It may be laid down as a general law, which holds very accurately for the Vertebrata, that in eggs in which the distribution of food-yolk is not uniform, the size of the cells resulting from segmentation is proportional to the quantity of food-material they contain. In accordance with this law the cells of the Amphibian ovum are of unequal size even at the close of segmentation. They may roughly be divided into two categories, viz. the smaller cells of the upper pole and the larger of the lower (fig. 170 A). The segmentation cavity (sg) lies between the two, but is unsymmetrically placed near the upper pole of the egg, owing to the large bulk of the ventrally placed yolk-segments. In the inequality of the cells at the close of segmentation the Amphibia stand in contrast with Amphioxus. The upper cells are mainly destined to form the epiblast, and the lower the hypoblast and mesoblast.
The next change which takes place is an invagination, the earliest traces of which are observable in fig. 170 A. The invagination is not however so simple as in Amphioxus. Owing in fact to the presence of the food-yolk it is a mixture of invagination by epibole and by embole.
At the point marked x in fig. 170 A, which corresponds with the future hind end of the embryo, and is placed on the equatorial line marking the junction of the large and small cells, there takes place a normal invagination, which gives rise solely to the hypoblast of the dorsal wall of the alimentary tract and to part of the dorsal mesoblast. The invaginated layer grows inwards from the point x along what becomes the dorsal side of the embryo ; and between it and the yolk-cells below is formed a slit-like space (fig. 170 B and C). This space is the mesenteron. It is even better shewn in fig. 171 representing the
COMPARISON OF THE GERMINAL LAYERS. 279
process of invagination in Petromyzon. The point x in fig. 170 where epiblast, mesoblast and hypoblast are continuous, is homologous with the dorsal lip of the blastopore in Amphioxus. In the course of the invagination the segmentation cavity, as in Amphioxus, becomes obliterated.
While the above invagination has been taking place, the epiblast cells have been simply growing in an epibolic fashion round the yolk; and by the stage represented in fig. 170 C and D the exposed surface of yolk has become greatly diminished ; and an obvious blastopore is thus established. Along the line of the growth a layer of mesoblast cells (iri\ continuous at the sides with the invaginated mesoblast layer, has become differentiated from the small cells (fig. 170 A) intermediate between the epiblast cells and the yolk.
Owing to the nature of the above process of invagination the mesenteron is at first only provided with an epithelial wall on its dorsal side, its ventral wall being formed of yolk-cells (fig. 170). At a later period some of the yolk-cells become transformed into the epithelial cells of the ventral wall, while the remainder become enclosed in the alimentary cavity and employed as pabulum. The whole of the yolk-cells, after the separation of the mesoblast, are however morphologically part of the hypoblast.
The final fate of the blastopore is nearly the same as in Amphioxus. It gradually narrows, and the yolk-cells which at first plug it up disappear (fig. 170 C and D). The neural groove, which becomes formed on the dorsal surface of the embryo, is continued forwards from the point x in fig. 170 C. On the conversion of this groove into a canal the canal freely opens behind into the blastopore ; and a condition is reached in which the blastopore still opens to the exterior and also into the neural canal fig. 170 D. In a later stage (fig. 172) the external opening of the blastopore becomes closed by the medullary folds meeting behind it, but the passage connecting the neural and alimentary canals is left. There is one small difference between the Frog and Amphioxus in the relation of the neural canal to the blastopore. In both types the medullary folds embrace and meet behind it, so that it comes to occupy a position at the hind extremity of the medullary groove. In Amphioxus the closure
THE GASTRULA OF AMPHIBIA.
of the medullary folds commences behind, so that the external opening of the blastopore is obliterated simultaneously with the commencing 7rl /
formation of the medullary canal ; but in the Frog the closure of the medullary folds commences anteriorly and proceeds backwards, so that the obliteration of the external opening of the blastopore is a late event in the formation of the medullary canal.
The anus is formed (vide fig. 172) some way in front of the blastopore, and a post-anal gut, continuous with the neurenteric canal, is thus established. Both the postanal gut and the neurenteric canal eventually disappear.
The two other types classed above with the Amphibia, viz. Petromyzon and Acipenser, agree sufficiently closely with them
FIG. 171. LONGITUDINAL VERTICAL SECTION THROUGH AN EMBRYO OF PETROMYZON OF 136 HOURS.
me. mesoblast ; yk. yolk-cells ; al. alimentary tract ; bl. blastopore ; s.c. segmentation cavity.
FIG. 172. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF BOMBINATOR. (After Gotte.)
- //. mouth ; an. anus ; /. liver ; ne. neurenteric canal ; me. medullary canal ;
ch. notochord ; pn. pineal gland.
to require no special mention ; but with reference to both types it may be pointed out that the ovum contains relatively more food-yolk than that of the Amphibian type just described, and
COMPARISON OF THE GERMINAL LAYERS. 28 1
that this leads amongst other things to the lower layer cells extending up the sides of the segmentation cavity, and assisting in forming its roof.
The next type to be considered is that of Elasmobranchii. The yolk in the ovum of these forms is enormously bulky, and the segmentation is in consequence a partial one. At first sight the differences between their development and that of Amphibia would appear to be very great. In order fully to bridge over the gulf which separates them I have given three diagrammatic longitudinal sections of an ideal form intermediate between Amphibia and Elasmobranchii, which differs however mainly from the latter in the smaller amount of food-yolk; and by their aid I trust it will be made clear that the differences between the Amphibia and Elasmobranchii are of an insignificant character. In fig. 174 A B C are represented three diagrammatic longitudinal sections of Elasmobranch embryos, and in fig. 173 A B C three longitudinal sections of the ideal intermediate form. The diagrams correspond with the Amphibian diagrams already described (fig. 170). In the first stage figured there is present in all of these forms a segmentation cavity (sg) situated not centrally but near the surface of the egg. The roof of the cavity is thin, being composed in the Amphibian embryo of epiblast alone, and in the Elasmobranch of epiblast and lower layer cells. The floor of the cavity is formed of so-called yolk, which forms the main mass of the embryo. In Amphibia the yolk is segmented. In Elasmobranchii there is at first a layer of primitive hypoblast cells separating the segmentation cavity from the yolk proper; this however soon disappears, and an unsegmented yolk with free nuclei fills the place of the segmented yolk of the Amphibia. The small cells at the sides of the segmentation cavity in Amphibia correspond exactly in function and position with the lower layer cells of the Elasmobranch blastoderm.
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 yolk-cells of the Amphibian embryo. The only essential difference between the two embryos arises from the roof of the segmentation cavity being formed in the Elasmobranch embryo of lower layer cells, which are absent
THE GASTRULA OF ELASMOBRANCHIL
in the Amphibian embryo. This difference no doubt depends upon the greater quantity of yolk in the Elasmobranch ovum, and a similar distribution of the lower layer cells is found in Acipenser and in Petromyzon.
In the next stage for the Elasmobranch (fig. 173 and 174 B) and for the Amphibian (fig. 170 C) or better still Petromyzon
FIG. 173. THREE DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH AN IDEAL TYPE OF VERTEBRATE EMBRYO INTERMEDIATE IN THE MODE OF FORMATION OF ITS LAYERS BETWEEN AMPHIBIA OR PETROMYZON AND ELASMO BRANCH1I.
s.if. segmentation cavity; ep. epiblast; m. mesoblast; hy. hypoblast; nc. neural canal; al. mesenteron; . nuclei of the yolk.
(fig. 171) the agreement between the three types is again very close. For a small arc (x) of the edge of the blastoderm the epiblast and hypoblast become continuous, while at all other
COMPARISON OF THE GERMINAL LAYERS. 283
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 embryo form a comparatively small mass, and are therefore rapidly enveloped ; while in the case of the Elasmobranch embryo, owing to the greater mass of the yolk, the same process occupies a long period. The portion of the blastoderm, where epiblast and hypoblast become continuous, forms the dorsal lip of an opening the blastopore which leads into the alimentary cavity. This cavity has the same relation in all the three cases. It is lined dorsally by lower layer cells, and ventrally by yolk-cells or what corresponds with yolk-cells ; a large part of the ventral epithelium of the alimentary canal being in both cases eventually derived from the yolk. In Amphibia this epithelium is formed directly from the existing cells, while in Elasmobranchii it is derived from cells formed around the nuclei of the yolk.
As in the earlier stage, so in the present one, the anatomical relations of the yolk to the blastoderm in the one case (Elasmobranchii) are nearly identical with those of the yolk-cells to the blastoderm in the other (Amphibia).
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 Amphibia 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 Elasmobranchs, owing probably to the larger bulk of the lower layer cells, the primitive hypoblast cells 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 space between the blastoderm and the yolk. The homology of this space with the primitive invagination cavity 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 : (i) The continuity of epiblast and hypoblast at the dorsal lip of the blastopore. (2) The continuous conversion of primitive
284 THE GASTRULA OF ELASMOBRANCHII.
hypoblast cells into permanent hypoblast, which gradually extends inwards towards the segmentation cavity, and exactly represents the course of the invagination whereby in Amphibia 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.
In the next stage there appear more important differences between the two types than in the preceding stages, though here again the points of resemblance predominate.
Figs. 170 D and 174 C represent longitudinal sections through embryos after the closure of the medullary canal. The neurenteric canal is established ; and in front and behind the epithelium of the ventral wall of the mesenteron has begun to be formed.
The mesoblast is represented as having grown in between the medullary canal and the superjacent epiblast.
There are at this stage two points in which the embryo Elasmobranch differs from the corresponding Amphibian embryo, (i) In the formation of the neurenteric canal, there is no free passage leading into the mesenteron from the exterior as in Amphibia (fig. 170 D). (2) The whole yolk is not enclosed by the epiblast, and therefore part of the blastopore is still open.
The difference between Amphibia and Elasmobranchii in the first of these points is due to the fact that in Elasmobranchii, as in Amphioxus, the neural canal becomes first closed behind ; and simultaneously with its closure the lateral parts of the lips of the blastopore, which are continuous with the medullary folds, meet together and shut in the hindmost part of the alimentary tract.
The second point is of some importance for understanding the relations of the formation of the layers in the amniotic and the non-amniotic Vertebrates. Owing to its large size the whole of the yolk in Elasmobranchii is not enclosed by the epiblast at the time when the neurenteric canal is established ; in other words a small posterior and dorsal portion of the blastopore is shut off in the formation of the neurenteric canal. The remaining ventral portion becomes closed at a later period. Its closure takes place in a linear fashion, commencing at the hind end of the embryo, and proceeding apparently backwards ; though, as this part eventually becomes folded in to form the ventral wall of the embryo, the closure of it really travels forwards. The
COMPARISON OF THE GERMINAL LAYERS.
process causes however the embryo to cease to lie at the edge of the blastoderm, and while situated at some distance from the edge, to be connected with it by a linear streak, representing the coalesced lips of the blastopore. The above process is diagrammatically represented in fig. 175 B; while as it actually occurs
DIAGRAMMATIC LONGITUDINAL SECTIONS OF AN ELASMOBRANCH
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 the segmentation cavity enclosed in the lower layer cells (primitive hypoblast).
B. Older blastoderm with embryo in which hypoblast and mesoblast are distinctly formed, and in which the alimentary cavity has appeared. The segmentation cavity is still represented, though by this stage it has in reality disappeared.
C. Older blastoderm with embryo in which the neural canal is formed, and is continuous posteriorly with the alimentary canal. The notochord, though shaded like mesoblast, belongs properly to the hypoblast.
it is shewn in fig. 30, p. 63. The whole closure of the blastopore in Elasmobranchii is altogether unlike what takes place in Amphibia, where the blastopore remains as a circular opening which
286 THE GASTRULA OF THE SAUROPSIDA.
gradually narrows till it becomes completely enveloped in the medullary folds (fig. 175 A).
On the formation of the neurenteric canal the body of the embryo Elasmobranch becomes gradually folded off from the yolk, which, owing to its great size, forms a large sack appended to the ventral side of the body. The part of the somatopleure, which grows round it, is to be regarded as a modified portion of the ventral wall of the body. The splanchnopleure also envelops it, so that, morphologically speaking, the yolk lies within the mesenteron.
The Teleostei, so far as the first formation of the layers is concerned, resemble in all essential features the Elasmobranchii, but the neurenteric canal is apparently not developed (?), owing to the obliteration of the neural canal ; and the roof of the segmentation cavity is formed of epiblast only.
In the preceding pages I have attempted to shew that the Amphibia, Acipenser, Petromyzon, the Elasmobranchii and the Teleostei agree very closely in the mode of formation of the gastrula. The unsymmetrical gastrula or pseudo-gastrula which is common to them all is, I believe, 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 true 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. It is this fact which causes the asymmetry of the gastrula, since it is not possible for the part of the ovum, which will become the ventral wall of the alimentary tract, and which is loaded with food-yolk, to be invaginated in the same fashion as the dorsal wall.
Sauropsida. The comparison of the different types of the Ichthyopsida is fairly simple, but the comparison of the Sauropsida with the Ichthyopsida is a far more difficult matter. In all the Sauropsida there is a large food-yolk, and the segmentation agrees closely with that in the Elasmobranchii. It might have been anticipated that the resemblance would continue in the subsequent development. This however is far from being the
COMPARISON OF THE GERMINAL LAYERS. 287
case. The medullary plate, instead of lying at the edge of the blastoderm, lies in the centre, and its formation is preceded by that of a peculiar structure, the primitive streak, which, on the
FIG. 175. DIAGRAMS ILLUSTRATING THE POSITION OF THE BLASTOPORE, AND THE RELATION OF THE EMBRYO TO THE YOLK IN VARIOUS MEROBLASTIC VERTEBRATE OVA.
A. Type of Frog. B. Elasmobranch type. C. Amniotic Vertebrate. mg. medullary plate ; ne. neurenteric canal ; bl. portion of blastopore adjoining the neurenteric canal. In B this part of the blastopore is formed by the edges of the blastoderm meeting and forming a linear streak behind the embryo ; and in C it forms the structure known as the primitive streak, yk. part of the yolk not yet enclosed by the blastoderm.
formation of the medullary plate, is found to lie at the hinder end of the latter and to connect it with the edge of the blastoderm.
The possibility of a comparison between the Sauropsida and the Elasmobranchii depends upon the explanation being possible of (i) the position of the embryo near the centre of the blastoderm, and (2) the nature of the primitive streak.
The answers to these two questions are, according to my view, intimately bound together.
288 THE GASTRULA OF THE SAUROPSIDA.
I consider that the embryos of the Sauropsida have come to occupy a central position in the blastoderm owing to the abbreviation of a process similar to that by which, in Elasmobranchii, the embryo is removed from the edge of the blastoderm ; and that the primitive streak represents the linear streak connecting the Elasmobranch embryo with the edge of the blastoderm after it has become removed from its previous peripheral position, as well as the true neurenteric part of the Elasmobranch blastopore.
This view of the nature of the primitive streak, which is diagrammatically illustrated in fig. 175, will be rendered more clear by a brief review of the early developmental processes in the Sauropsida.
After segmentation the blastoderm becomes divided, as in Elasmobranchii, into two layers. It is doubtful whether there is any true representative of the segmentation cavity. The first structure to appear in the blastoderm is a linear streak placed at the hind end of the blastoderm, known as the primitive streak (figs. 175 C, /5/and 176, pr). At the front end of the primitive streak the epiblast and hypoblast become continuous, just as they do at the dorsal lip of the blastopore in Elasmobranchii. Continued back from this point is a streak of fused mesoblast and epiblast to the under side of which a linear thin layer of hypoblast is more or less definitely attached.
A further structure, best developed in the Lacertilia, appears in the form of a circular passage perforating the blastoderm at the front end of the primitive streak (fig. 176, ne). This passage is bounded anteriorly by the layer of cells forming the continuation of the hypoblast into the epiblast.
In the next stage the medullary plate becomes formed in front of the primitive streak (fig. 175 C), and the medullary folds are continued backwards so as to enclose the upper opening of the passage through the blastoderm. On the closure of the medullary canal (fig. 177) this passage leads from the medullary canal into the alimentary tract, and is therefore the neurenteric canal ; and a post-anal gut also becomes formed. The latter part of the above description applies especially to the Lizard: but in Chelonia and most Birds distinct remnants (vide pp. 162 164) of the neurenteric canal are developed.
On the hypothesis that the Sauropsidan embryos have come
COMPARISON OF THE GERMINAL LAYERS. 289
to occupy their central position, owing to an abbreviation of a process analogous to the linear closing of the blastopore behind the embryos of Elasmobranchii, all the appearances above described receive a satisfactory explanation. The passage at the front end of the primitive streak is the dorsal part of the blastopore, which in Elasmobranchii becomes converted into the neurenteric canal. The remainder of the primitive streak represents, in a rudimentary form, the linear streak in Elasmobranchii, formed by the coalesced edges of the blastoderm, which connects the hinder end of the embryo with the still open yolk blastopore. That it is in later stages not continued to the edge of the blastoderm, as in Elasmobranchii, is due to its being a rudimentary organ. The more or less complete fusion of the layers in the primitive streak is simply to be explained by this structure representing the coalesced edges of the blastopore ; and the growth outwards from it of the mesoblast is probably a remnant of a primitive dorsal invagination of the mesoblast and hypoblast like that in the Frog.
FIG. 176. DIAGRAMMATIC LONGITUDINAL SECTION OF AN EMBRYO OF LACERTA. //. body cavity; am. amnion; ne. neurenteric canal; ch. notochord; hy. hypoblast; ep. epiblast; pr. primitive streak. In the primitive streak all the layers are partially fused.
The final enclosure of the yolk in the Sauropsida takes place at the pole of the yolk-sack opposite the embryo, so that the blastopore is formed of three parts, (i) the neurenteric canal, (2) the primitive streak behind this, (3) the blastopore at the pole of the yolk-sack opposite the embryo.
Mammalia. The features of the development of the placental Mammalia receive their most satisfactory explanation on the hypothesis that their ancestors were provided with a large-yolked ovum like that of the Sauropsida. The food-yolk must be supposed to have ceased to be developed on the establishment of a maternal nutrition through the uterus.
On this hypothesis all the developmental phenomena subseB. in 19
quently to the formation of the blastodermic vesicle receive a satisfactory explanation.
The whole of the blastodermic vesicle, except the embryonic area, represents the yolk-sack, and the growth of the hypoblast and then of the mesoblast round its inner wall represents the
FIG. 177. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR END OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS.
ep. epiblast ; Sp.c. spinal canal ; ch. notochord ; n.e. neurenteric canal ; hy. hypoblast ; p.a.g. post-anal gut ; pr. remains of primitive streak folded in on the ventral side ; al. allantois ; me. mesoblast ; an. point where anus will be formed ; p.c. perivisceral cavity am. amnion; so. somatopleure ; sp. splanchnopleure.
corresponding growths in the Sauropsida. As in the Sauropsida it becomes constricted off from the embryo, and the splanchnopleuric stalk of the sack opens into the ileum in the usual way.
FIG. 178. OPTICAL SECTIONS OF A RABBIT'S OVUM AT TWO STAGES CLOSELY
FOLLOWING UPON THE SEGMENTATION. (After E. van Beneden.) ep. epiblast; hy. primary hypoblast; bp. Van Beneden's so-called blastopore. The shading of the epiblast and hypoblast is diagrammatic.
COMPARISON OF THE GERMINAL LAYERS.
In the formation of the embryo out of the embryonic area the phenomena which distinguish the Sauropsida from the Ichthyopsida are repeated. The embryo lies in the centre of the area ; and before it is formed there appears a primitive streak, from which there grows out the greater part of the mesoblast. At the front end of the primitive streak the hypoblast and epiblast become continuous, though a perforated neurenteric blastopore has not yet been detected.
All these Sauropsidan features are so obvious that they need not be insisted on further. The embryonic evidence of the common origin of Mammalia and Sauropsida, both as concerns the formation of the layers and of the embryonic membranes, is as clear as it can be. The only difficulty about the early development of Mammalia is presented by the epibolic gastrula and the
FIG. 179. RABBIT'S OVUM BETWEEN 70 90 HOURS AFTER IMPREGNATION.
(After E. van Beneden.)
bv. cavity of blastodermic vesicle (yolk-sack) ; ep. epiblast ; hy. primitive hypoblast ; Zp. mucous envelope.
formation of the blastodermic vesicle (figs. 178 and 179). That the segmentation is a complete one is no doubt a direct consequence of the reduction of the food-yolk, but the growth of the epiblast cells round the hypoblast and the final enclosure of the latter, which I have spoken of as giving rise to the epibolic gastrula, are not so easily explained.
292 MESOBLAST AND NOTOCHORD.
It might have been supposed that this process was equivalent to the growth of the blastoderm round the yolk in the Sauropsida, but then the blastopore ought to be situated at the pole of the egg opposite to the embryonic area, while, according to Van Beneden, the embryonic area corresponds approximately to the blastopore.
Van Beneden regards the Mammalian blastopore as equivalent to that in the Amphibia, but if the position previously adopted about the primitive streak is to be maintained, Van Beneden's view must be abandoned. No satisfactory phylogenetic explanation of the Mammalian gastrula by epibole has in my opinion as yet been offered.
The formation of the blastodermic vesicle may perhaps be explained on the view that in the Proto-mammalia the yolk-sack was large, and that its blood-vessels took the place of the placenta of higher forms. On this view a reduction in the bulk of the ovarian ovum might easily have taken place at the same time that the presence of a large yolk-sack was still necessary for the purpose of affording surface of contact with the uterus.
The formation of the Mesoblast and of the Notochord.
Amphioxus. The mcsoblast originates in Amphioxus, as in several primitive invertebrate types, from a pair of lateral
FIG. 180. SECTIONS OF AN AMPHIOXUS EMBRYO AT THREE STAGES. (After Kowalevsky.)
A. Section at gastrula stage.
B. Section of an embryo slightly younger than that represented in fig. 169 D.
C. Section through the anterior part of an embryo at the stage represented in fig. 169 !:.
/. neural plate ; nc. neural canal ; mes. archenteron in A and B, and mesenteron in C; ch. notochord ; so. mesoblastic somite.
COMPARISON OF THE GERMINAL LAYERS. 293
diverticula, constricted off from the archenteron (fig. 180). Their formation commences at the front end of the body and is thence carried backwards, and each diverticulum contains a prolongation of the cavity of the archenteron. After their separation from the archenteron the dorsal parts of these diverticula become divided by transverse septa into successive somites, the cavities of which eventually disappear ; while the walls become mainly converted into the muscle-plates, but also into the tissue around the notochord which corresponds with the vertebral tissue of the higher Chordata.
The ventral part of each diverticulum, which is prolonged so as to meet its fellow in the middle ventral line, does not become divided into somites, but contains a continuous cavity, which becomes the body cavity of the adult. The inner layer of this part forms the splanchnic mesoblast, and the outer layer the somatic mesoblast.
The notochord would almost appear to arise as a third median and dorsal diverticulum of the archenteron (fig. 1 80 ch). At any rate it arises as a central fold of the wall of this cavity, which is gradually constricted off from before backwards.
Urochorda. In simple Ascidians the above processes undergo a slight modification, which is mainly due (i) to a general simplification of the FIG igj TRANSVERSE OPTI . organization, and (2) to the non- CAL SECTION OF THE TAIL OF AN continuation of the notochord into ^SSSSSSSST'
the trunk. The section is from an embryo
The whole dorsal wall of the of the same age as fig. 8 iv. posterior part of the archenteron is * converted into the notochord (fig. bla st of tail. 181 ck), and the lateral walls into the mesoblast (me) ; so that the original lumen of the posterior part of the archenteron ceases to be bounded by hypoblast cells, and disappears as such. Part of the ventral wall remains as a solid cord of cells (al 1 ) The anterior part of the archenteron in front of the notochord passes wholly into the permanent alimentary tract.
The derivation of the mesoblast from the lateral walls of the
MESOBLAST AND NOTOCHORD.
posterior part of the archenteron is clearly comparable with the analogous process in Amphioxus.
Vertebrata. In turning from Amphioxus to the true Vertebrata we find no form in which diverticula of the primitive alimentary tract give rise to the mesoblast. There is reason to think that the type presented by the Elasmobranchii in the formation of the mesoblast is as primitive as that of any other group. In this group the mesoblast is formed, nearly coincidently with the hypoblast of the dorsal wall of the mesenteron, as two lateral sheets, one on each side of the middle line (fig. 182 m). These two sheets are at first solid masses ; and their differentiation commences in front and is continued backwards. After their formation the notochord arises from the axial portion of the hypo
FlG. 182. TWO TRANSVERSE SECTIONS OF AN EMBRYO PRISTIURUS OF THE SAME AGE AS FIG. 17.
A. Anterior section.
B. Posterior section.
mg. medullary groove ; ep. epiblast ; hy. hypoblast ; n.al cells formed round the nuclei of the yolk which have entered the hypoblast ; m. mesoblast.
The sections shew the origin of the mesoblast.
blast (which had no share in giving rise to the two mesoblast plates) as a solid thickening (fig. 183 //), which is separated from it as a circular rod. Its differentiation, like that of the mesoblastic plates, commences in front. The mesoblast plates subsequently become divided for their whole length into two layers, between which a cavity is developed (fig. 184). The dorsal parts of the plates become divided by transverse partitions into somites, and these somites with their contained cavities are next separated from the more ventral parts of the plates (fig. 185 mp). In the somites the cavities become eventually obliterated, and from their inner sides plates of tissue for the vertebral bodies (fig. 186 Vr) are separated ; while the outer parts, consisting of two sheets, containing the remains of the original cavity, form the muscleplates (mp).
COMPARISON OF THE GERMINAL LAYERS.
The undivided ventral portion gives rise to the general A
FIG. 183. THREE SECTIONS OF A PRISTIURUS EMBRYO SLIGHTLY OLDER THAN
FIG. 18 B.
The sections shew the development of the notochord.
Ch. notochord; CK. developing notochord; mg. medullary groove; lp. lateral plate of mesoblast ; ep. epiblast ; hy. hypoblast.
somatic and splanchnic mesoblast (fig. 185), and the cavity between its two layers constitutes the body cavity. The originally separate halves of the body cavity eventually meet and unite in the ventral median line throughout the greater part of the body, though in the tail they remain distinct and are finally obliterated. Dorsally they are separated by the mesentery. From the mesoblast at the junction of the dorsal and
FIG. 184. TRANSVERSE SECTION THROUGH THE TAIL-REGION OF A PRISTIURUS EMBRYO OF THE SAME AGE AS FIG. 28 E.
df. dorsal fin; sp.c. spinal cord; pp. body cavity; sp. splanchnic layer of mesoblast; so. somatic layer of mesoblast; mp'. commencing differentiation of muscles ; ch. notochord ; x. subnotochordal rod arising as an outgrowth of the dorsal wall of the alimentary tract ; a/, alimentary tract.
MESOBLAST AND NOTOCHORD.
ventral parts of the primitive plates is formed the urinogenital
That the above mode of origin of the mesoblast and noto chord is to be regarded as a modification of that observable in Am phioxus seems probable from the
following considerations :
In the first place, the mesoblast is
split off from the hypoblast not as a
single mass but as a pair of distinct
masses, comparable with the paired di vcrticula in Amphioxus. Secondly,
the body cavity, when it appears in
the mesoblast p\a.tes,does not arise as a
single cavity, but as a pair of cavities,
one for each plate of mesoblast ; and
these cavities remain permanentlydis tinct in some parts of the body, and
nowhere unite till a comparatively
late period. Thirdly, the primitive body cavity of the embryo is not confined to the region in which a body cavity exists in the adult, but extends to the summit of tJie muscleplates, at first separating parts which become completely fused in the adult to form the great lateral muscles of the body.
It is difficult to understand how the body cavity could thus extend into the muscle-plates on the supposition that it represents a primitive split in the mesoblast between the wall of the gut and the body-wall ; but its extension to this part is quite intelligible, on the hypothesis that it represents the cavities of two diverticula of the alimentary tract, from the muscular walls of which the voluntary muscular system has been derived ; and it may be pointed out that the derivation of part of the muscular system from what is apparently splanchnic mesoblast is easily explained on the above hypothesis, but not, so far as I see, on any other.
FIG. 185. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNG KK
THAN 28 F.
sp.c. spinal canal; W. white matter of spinal cord ; pr. posterior nerve-roots ; ch. notochord ; x. subnotochordal rod ; ao. aorta ; vip. muscle-plate ; mp'. inner layer of muscle-plate already converted into muscles ; Vr. rudiment of vertebral body ; si. segmental tube ; sd. segmental duct ; sp.v. spiral valve ; v. subintestinal vein ; p.o. primitive generative cells.
COMPARISON OF THE GERMINAL LAYERS.
Such are the main features, presented by the mesoblast in Elasmobranchii, which favour the view of its having originally formed the walls of the alimentary diverticula. Against this view of its nature are the facts (i) of the mesoblast plates being at first solid, and (2) of the body cavity as a consequence of this never communicating with the alimentary canal. These points, in view of our knowledge of embryological modifications, cannot be regarded as great difficulties in my hypothesis. We have many examples of organs, which, though in most cases arising as involutions, yet appear in other cases as solid ingrowths. Such examples are afforded by the optic vesicle, auditory vesicle, and probably also by the central nervous system of Osseous Fishes. In most Vertebrates these organs are formed as hollow involutions from the exterior ; in Osseous Fishes, however, as solid involutions, in which a cavity is secondarily established.
There are strong grounds for thinking that in all Vertebrates the mesoblast plates on each side of the notochord originate independently, much as in Elasmobranchii, and that the notochord is derived from the axial hypoblast ; but there are some difficulties in the application of this general statement to all cases. In Amphibia, Ganoids, and Petromyzon, where the dorsal hypoblast is formed by a process of invagtnation as in Amphioxus, the dorsal mesoblast also owes its origin to this invagination, in that the indifferent invaginated layer becomes divided into hypoblast and mesoblast. Amongst these forms the mesoblast sheet, when separated from the hypoblast, is certainly not continuous across the middle line in Petromyzon (Calberla) and the Newt (Scott and Osborn), and doubtfully so
FIG. 1 86. HORIZONTAL SECTION THROUGH THE TRUNK OF AN EMBRYO OF SCYLLIUM CONSIDERABLY YOUNGER THAN 28 F.
The section is taken at the level of the notochord, and shews the separation of the cells to form the vertebral bodies from the muscle-plates.
ch. notochord ; ep. epiblast ; Vr. rudiment of vertebral body ; mp. muscle-plate ; mp'. portion of muscle-plate already differentiated into longitudinal muscles.
298 MESOBLAST AND NOTOCHORD.
in the other forms. It arises, in fact, as in Elasmobranchii, as two independent plates. The fact of these plates originating from an invaginated layer can only be regarded in the light of an approximation to the primitive type found in Amphioxus.
In Petromyzon and the Newt the whole axial plate of dorsal hypoblast becomes separated off from the rest of the hypoblast as the notochord, and this mode of origin for the notochord resembles more closely that in Amphioxus than the mode of origin in Elasmobranchii.
In Teleostei, there is reason to think that the processes in the formation of the mesoblast accord closely with what has been described as typical for the Ichthyopsida, but there are still some points involved in obscurity.
Leaving the Ichthyopsida, we may pass to the consideration of the Sauropsida and Mammalia. In both of these types there is evidence to shew that a part of the mesoblast is formed in situ at the same time as the hypoblast, from the lower strata of segmentation spheres. This mesoblast is absent in the front part of the area pellucida, and on the formation of the primitive streak (blastopore), an outgrowth of mesoblast arises from it as
FIG. 187. TRANSVERSE SECTION THROUGH AN EMBRYO RABBIT OF EIGHT DAYS. ep. epiblast ; me. mesoblast ; ky. hypoblast ; mg. medullary groove.
in Amphibia, etc. From this region the mesoblast spreads as a continuous sheet to the sides and posterior part of the blastoderm. In the region of the embryo, its exact behaviour has not in some cases been quite satisfactorily made out. There are reasons for thinking that it appears as two sheets not tinited in the axial line in both Lacertilia (fig. 126) and Mammalia (fig. 187), and this to some extent holds true for Aves (vide p. 156). In Lacertilia (fig. 188) and Mammalia, the axial hypoblast becomes wholly converted into the notochord, which at the posterior end of the body is continued into the epiblast at the dorsal lip of the blastopore ; while in Birds the notochord is formed by a very similar (fig. 189 cfi) process.
COMPARISON OF THE GERMINAL LAYERS.
The above processes in the formation of the mesoblast are for the most part easily explained by a comparison with the lower types. The outgrowth of the mesoblast from the sides of the primitive streak is a rudiment of the dorsal invagination of hypoblast and mesoblast found in Amphibia ; and the apparent
FIG. 188. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH AN EMBRYO LIZARD TO SHEW THE RELATIONS OF THE NEURENTERIC CANAL (ne) AND OF
THE PRIMITIVE STREAK (pr).
am. amnion; ep. epiblast; hy. hypoblast; ch. notochord ; //. body cavity; ne. neurenteric canal ; pr. primitive streak.
outgrowth of the mesoblast from the epiblast in the primitive streak is no more to be taken as a proof of the epiblastic origin of the mesoblast, than the continuity of the epiblast with the invaginated hypoblast and mesoblast at the lips of the blastopore in the Frog of the derivation of these layers from the epiblast in this type.
The division of the mesoblast into two plates along the dorsal line of the embryo, and the formation of the notochord from the
FIG. 189. TRANSVERSE SECTION THROUGH THE EMBRYONIC REGION OF THE BLASTODERM OF A CHICK AT THE TIME OF THE FORMATION OF THE NOTOCHORD, BUT BEFORE THE APPEARANCE OF THE MEDULLARY GROOVE.
ep. epiblast; ky. hypoblast; ch. notochord; me. mesoblast; n. nuclei in the yolk of the germinal wall yk.
axial hypoblast, are intelligible without further explanation. The appearance of part of the mesoblast before the formation of the primitive streak is a process of the same nature as the
300 THE EPIBLAST.
differentiation of hypoblast and mesoblast in Elasmobranchii without an invagination.
In the Sauropsida, some of the mesoblast of the vascular area would appear to be formed in situ out of the germinal wall, by a process of cell-formation similar to that which takes place in the yolk adjoining the blastoderm in Elasmobranchii and Teleostei. The mesoblast so formed is to be compared with that which arises on the ventral side of the embryo in the Frog, by a direct differentiation of the yolk-cells.
What was stated for the Elasmobranchii with reference to the general fate of the mesoblast holds approximately for all the other forms.
The epiblast in a large number of Chordata arises as a single row of more or less columnar cells. Since the epidermis, into which it becomes converted, is formed of two more or less distinct strata in all Chordata except Amphioxus and Ascidians, the primitive row of epiblast cells, when single, necessarily becomes divided in the course of development into two layers.
In some of the Vertebrata, viz. the Anurous Amphibia, Teleostei, Acipenser, and Lepidosteus, the epiblast is from the first formed of two distinct strata. The upper of these, formed of a single row of cells, is known as the epidermic stratum, and the lower, formed of several rows, as the nervous stratum. In these cases the two original strata of the epiblast are equivalent to those which appear at a later period in the other forms. Thus Vertebrates may be divided into groups according to the primitive condition of their epiblast, viz. a larger group with but a single stratum of cells at first ; and a smaller group with two strata.
While there is no great difficulty in determining the equivalent parts of the epidermis in these two groups, it still remains an open question in which of them the epiblast retains its primitive condition.
Though it is not easy to bring conclusive proofs on the one side or the other, the balance of argument appears to me to be
COMPARISON OF THE GERMINAL LAYERS. 301
decidedly in favour of regarding the condition of the epiblast in the larger group as primitive, and its condition in the smaller group as secondary, and due to the throwing back of the differentiation of the epiblast to a very early period of development.
In favour of this view may be urged (i) the fact that the simple condition is retained in Amphioxus through life. (2) The correlation in Amphibia, and the other forms belonging to this group, between a closed auditory pit and the early division of the epiblast into two strata; there being no doubt that the auditory pit was at. first permanently open, a condition of the epiblast which necessitates its never having an external opening must clearly be secondary. (3) It appears more likely that a particular genetic feature should be thrown back in development, than that such an important feature, as a distinction between two primary layers, should be absolutely lost during an early period of development, and then re-appear in later stages.
The fact of the epiblast of the neural canal being divided, like the remainder of the layer, into nervous and epidermic parts, cannot, I think, be used as an argument in favour of the opposite view to that here maintained. It seems probable that the central canal of the nervous system arose phylogenetically as an involution from the exterior, and that the epidermis lining it is merely part of the original epidermis, which has retained its primitive structure as a simple stratum, but is naturally distinguishable from the nervous structures adjacent to it.
Where the epiblast is divided at an early period into two strata, the nervous stratum is always the active one, and takes the main share in forming all the organs derived from the layer.
Formation of the central nervous system. In all Chordata an axial strip of the dorsal epiblast, extending from the lip of the blastopore to the anterior extremity of the head, and known as the medullary plate, becomes isolated from the remainder of the layer to give rise to the central nervous axis.
According to the manner in which this takes place, three types may, however, be distinguished. In Amphioxus the axial
THE CENTRAL NERVOUS SYSTEM.
strip becomes first detached from the adjoining epiblast, which then meets and forms a continuous layer above it (fig. 190 A and B ;//). The sides of the medullary plate, which is thus shut off from the surface, bend over and meet so as to convert the
FIG. 190. SECTIONS OF AN AMPHIOXUS EMBRYO AT THREE STAGES. (After Kowalevsky. )
A. Section at gastrula stage.
B. Section of an embryo slightly younger than that represented in fig. 169 D.
C. Section through the anterior part of an embryo at the stage represented in fig. 169 E.
- //. neural plate; nc. neural canal; mes. archenteron in A and B, and mesenteron
in C ; ch. notochord ; so. mesoblastic somite.
plate into a canal (fig. 190 C nc). In the second and ordinary type the sides of the medullary plate fold over and meet so as to form a canal before the plate becomes isolated from the external epiblast.
The third type is characteristic of Lepidosteus, Teleostei, and Petromyzon. Here the axial plate becomes narrowed in such a way that it forms a solid keel-like projection towards the ventral surface (fig. 191 Me). This keel subsequently becomes separated from the remainder of the epidermis, and a central canal is afterwards developed in it. Calberla and Scott hold that the epidermic layer of the skin is involuted into this keel in Petromyzon, and Calberla maintains the same view for Teleostei (fig. 32), but further observations on this subject are required. In the Teleostei a very shallow depression along the axis of the keel is the only indication of the medullary groove of other forms.
In Amphioxus (fig. 190), the Tunicata, Petromyzon (?), Elasmobranchii (fig. 182), the Urodela and Mammalia (fig. 187), the epiblast of the medullary plate is only formed of a single row of cells at the time when the formation of the central nervous system commences; but, except in Amphioxus and the Tuni
COMPARISON OF THE GERMINAL LAYERS. 303
cata, it becomes several cells deep before the completion of the process. In other types the epiblast is several cells deep even before the differentiation of a medullary plate. In the Anura, the nervous layer of the epidermis alone is thickened in the
FIG. 191. SECTION THROUGH AN EMBRYO OF LEPIDOSTEUS ON THE FIFTH DAY
AFTER IMPREGNATION. MC. medullary cord; Ep. epiblast; Me. mesoblast ; hy. hypoblast; Ch. notochord.
formation of the central nervous system (fig. 72) ; and after the closure of the medullary canal, the epidermic layer fuses for a period with the nervous layer, though on the subsequent formation of the central epithelium of the nervous canal, there can be little doubt that it becomes again distinct.
It seems almost certain that the formation of the central nervous system from a solid keel-like thickening of the epidermis is a derived and secondary mode ; and that the folding of the medullary plate into a canal is primitive. Apart from its greater frequency the latter mode of formation of the central nervous system is shewn to be the primitive type by the fact that it offers a simple explanation of the presence of the central canal of the nervous system ; while the existence of such a canal cannot easily be explained on the assumption that the central nervous system was originally developed as a keel-like thickening of the epiblast.
It is remarkable that the primitive medullary plate rarely exhibits any indication of being formed of two symmetrical halves. Such indications are, however, found in the Amphibia (fig. 192 and fig. 72) ; and, since in the adult state the nervous cord exhibits nearly as distinct traces of being formed of two united strands as does the ventral nerve-cord of many Chaetopods, it is
304 ORGANS DERIVED FROM THE GERMINAL LAYERS.
quite possible that the structure of the medullary plate in Amphibia may be more primitive than that in other types 1 .
Formation of the organs of special sense. The more important parts of the organs of smell, sight, and hearing are derived from the epiblast ; and it has been asserted that the olfactory pit, optic vesicles and auditory pit take their origin from a special sense plate, continuous at first with this medullary plate. In my opinion this view cannot be maintained.
In the case of the group of forms in which the epiblast is early divi
FIG. 192. TRANSVERSE SECTION THROUGH THE CEPHALIC REGION OF A YOUNG NEWT EMBRYO. (After Scott and Osborn.)
In.hy. invaginated hypoblast, the dorsal part of which will form the notochord ; ep. epiblast of neural plate ; sp. splanchnopleure ; al. alimentary tract ; yk. and Y. hy. yolk-cells.
ded into nervous and epidermic layers, the former layer alone becomes involuted in the formation of the auditory pit and the lens, the external openings of which are never developed, while it is also mainly concerned in the formation of the olfactory pit.
Summary of the more important Organs derived from the three germinal layers.
The epiblast primarily gives origin to two very important parts of the body, viz. the central nervous system and the epidermis.
It is from the involuted epiblast of the neural tube that the whole of the grey and white matter of the brain and spinal cord appears to be developed, the simple columnar cells of the epiblast being directly transformed into the characteristic multipolar nerve cells. The whole of the sympathetic nervous system
1 A parallel to the unpaired medullary plate of most Chordata is supplied by the embryologically unpaired ventral cord of most Gephyrea and some Crustacea. In these forms there can be little doubt that the ventral cord has arisen from the fusion of two originally independent strands, so that it is not an extremely improbable hypothesis to suppose that the same may have been the case in the Chordata.
COMPARISON OF THE GERMINAL LAYERS. 305
and the peripheral nervous elements of the body, including both the spinal and the cranial nerves and ganglia, are epiblastic in origin.
The epithelium (ciliated in the young animal) lining the canalis centralis of the spinal cord, together with that lining the ventricles of the brain, is the undifferentiated remnant of the primitive epiblast.
The epiblast also forms the epidermis ; not however the dermis, which is of mesoblastic origin. The line of junction between the epiblast and the mesoblast coincides with that between the epidermis and the dermis. From the epiblast are formed all such tegumentary organs or parts of organs as are epidermic in nature.
In addition to the above, the epiblast plays an important part in the formation of the organs of special sense.
According to their mode of formation, these organs may be arranged into two divisions. In the first come the organs where the sensory expansion is derived from the involuted epiblast of the medullary canal. To this class belongs the retina, including the pigment epithelium of the choroid, which is formed from the original optic vesicle budded out from the fore-brain.
To the second class belong the epithelial expansions of the membranous labyrinth of the ear, and the cavity of the nose, which are formed by an involution of the epiblast covering the external surface of the embryo. These accordingly have no primary connection with the brain. ' Taste bulbs ' and other terminal nervous organs, such as those of the lateral line in fishes, are also structures formed from the external epiblast.
In addition to these we have the crystalline lens formed of involuted epiblast as well as the cavity of the mouth and anus, and the glands derived from them. The pituitary body is also epiblastic in origin.
From the hypoblast are derived the epithelium of the digestive canal, the epithelium of the trachea, bronchial tubes and air cells, the cylindrical epithelium of the ducts of the liver, pancreas, thyroid body, and other glands of the alimentary canal, as well as the hepatic cells constituting the parenchyma of the liver, developed from the hypoblast cylinders given off around the primary hepatic diverticula.
B. III. 20
306 GROWTH IN LENGTH OF THE EMBRYO.
Homologous probably with the hepatic cells, and equally of hypoblastic origin, are the spheroidal 'secreting cells' of the pancreas and other glands. The epithelium of the salivary glands, though these so closely resemble the pancreas, is probably of epiblastic origin, inasmuch as the cavity of the mouth is entirely lined by epiblast.
The hypoblast also lines the allantois. To these parts must be added the notochord and subnotochordal rod. From the mesoblast are formed all the remaining parts of the body. The muscles, the bones, the connective tissue and the vessels, both arteries, veins, capillaries and lymphatics with their appropriate epithelium, are entirely formed from the mesoblast.
The generative and urinary organs are entirely derived from the mesoblast. It is worthy of notice that the epithelium of the urinary glands, though resembling the hypoblastic epithelium of the alimentary canal, is undoubtedly mesoblastic.
From the mesoblast are lastly derived all the muscular, connective tissue, and vascular elements, as well of the alimentary canal and its appendages as of the skin and the tegumentary organs. Just as it is only the epidermic moiety of the latter which is derived from the epiblast, so it is only the epithelium of the former which comes from the hypoblast.
Growth in length of the Vertebrate Embryo.
With reference to the formation and growth in length of the body of the Vertebrate embryo two different views have been put forward, which can be best explained by taking the Elasmobranch embryo as our type. One of these views, generally held by embryologists and adopted in the previous pages, is that the Elasmobranch embryo arises from a differentiation of the edge of the blastoderm ; which extends inwards from the edge for some little distance. This differentiation is supposed to contain within itself the rudiments of the whole of the embryo with the exception of the yolk-sack ; and the hinder extremity of it, at the edge of the blastoderm, is regarded as corresponding with the hind end of the body of the adult. The growth in length takes place by a process of intussusception, and, till there are formed the full number of mesoblastic somites, it is effected, as in Chastopods, by the continual addition of fresh somites between the last-formed somite and the hind end of the body.
A second and somewhat paradoxical view has been recently brought into prominence by His and Rauber. This view has moreover since been taken up by many embryologists, and has led to strange comparisons between the
COMPARISON OF THE GERMINAL LAYERS. 307
formation of the mesoblastic plates of the Chastopods and the medullary folds of Vertebrata. According to this view the embryo grows in length by the coalescence of the two halves of the thickened edges of the blastoderm in the dorsal median line. The groove between the coalescing edges is the medullary groove, which increases in length by the continued coalescence of fresh portions of the edge of the blastoderm.
The following is His' own statement of his view: "I have shewn that the embryo of Osseous Fishes grows together in length from two symmetricallyplaced structures in the thickened edge of the blastoderm. Only the foremost end of the head and the hindermost end of the tail undergo no concrescence, since they are formed out of that part of the edge of the blastoderm which, together with the two lateral halves, completes the ring. The whole edge of the blastoderm is used in the formation of the embryo."
The edges of the blastoderm which meet to form the body of the embryo are regarded as the blastopore, so that, on this view, the blastopore primitively extends for the whole length of the dorsal side of the embryo, and the groove between the coalesced lips becomes the medullary groove.
It is not possible for me to enter at any great length into the arguments used to support this position.
They may be summarised as (i) The general appearance ; i.e. that the thickened edge of the blastoderm is continuous with the medullary fold.
(2) Certain measurements (His) which mainly appear to me to prove that the growth takes place by the addition of fresh somites between that last formed and the end of the body.
(3) Some of the phenomena of double monsters (Rauber).
None of these arguments appear to be very forcible, but as the view of His and Rauber, if true, would certainly be important, I shall attempt shortly to state the arguments against it, employing as my type the Elasmobranchii, by the development of which, according to His, the view which he adopts is more conclusively proved than by that of any other group.
(1) The general appearance of the thickened edge of the blastoderm becoming continuous with the medullary folds has been used as an argument for the medullary folds being merely the coalesced thickened edges of the blastoderm. Since, however, the medullary folds are merely parts of the medullary plate, and since the medullary plate is continuous with the adjoining epiblast of the embryonic rim, the latter structure must be continuous with the medullary folds however they are formed, and the mere fact of their being so continuous cannot be used as an argument either way. Moreover, were the concrescence theory true, the coalescing edges of the blastoderm might be expected to form an acute angle with each other, which they are far from doing.
(2) The medullary groove becomes closed behind earlier than in front, and the closure commences while the embryo is still quite short, and before the hind end has begun to project over the yolk. After the medullary canal becomes closed, and is continued behind into the alimentary canal by the neurenteric passage, it is clearly impossible for any further increase in length
308 GROWTH IN LENGTH OF THE EMBRYO.
to take place by concrescence. If therefore His' and Rauber's view is accepted, it will have to be maintained that only a small part of the body is formed by concrescence, while the larger posterior part grows by intussusception. The difficulty involved in this supposition is much increased by the fact that long after the growth by concrescence must have ceased the yolk blastopore still remains open, and the embryo is still attached to the edge of the blastoderm ; so that it cannot be maintained that the growth by concrescence has come to an end because the thickened edges of the blastoderm have completely coalesced.
The above are arguments derived simply from a consideration of the growth of the embryo ; and they prove (i) that the points adduced by His and Rauber are not at all conclusive ; (2) that the growth in length of the greater part of the body takes place by the addition of fresh somites behind, as in Chaetopods, and it would therefore be extremely surprising that a small middle part of the body should grow in quite a different way.
Many minor arguments used by His might be replied to, but it is hardly necessary to do so, and some of them depend upon erroneous views as to the course of development, such as an argument about the notochord, which depends for its validity upon the assumption that the notochord ridge appears at the same time as the medullary plate, while, as a matter of fact, the ridge does not appear till considerably later. In addition to the arguments of the class hitherto used, there may be brought against the His-Rauber view a series of arguments from comparative embryology.
(1) Were the vertebrate blastopore to be co- extensive with the dorsal surface, as His and Rauber maintain, clear evidence of this ought to be apparent in Amphioxus. In Amphioxus, however, the blastopore is at first placed exactly at the hind end of the body, though later it passes up just on to the dorsal side (vide p. 4). It nearly closes before the appearance of the medullary groove or mesoblastic somites ; and the medullary folds have nothing to do with its lips, except in so far as they are continuous with them behind, just as in Elasmobranchii.
(2) The food-yolk in the Vertebrata is placed on the ventral side of the body, and becomes enveloped by the blastoderm ; so that in all large-yolked Vertebrates the ventral walls of the body are obviously completed by the closure of the lips of the blastopore, on the ventral side.
If His and Rauber are right the dorsal walls are also completed by the closure of the blastopore, so that the whole of the dorsal, as well as of the ventral wall of the embryo, must be formed by the concrescence of the lips of the blastopore ; which is clearly a reductio adabsurdum of the whole theory. To my own arguments on the subject I may add those of Kupffer, who has very justly criticised His' statements, and has shewn that growth of the blastoderm in Clupea and Gasterosteus is absolutely inconsistent with the concrescence theory.
The more the theory of His and Rauber is examined by the light of comparative embryology, the more does it appear quite untenable ; and it may be laid down as a safe conclusion from a comparative study of vertebrate
COMPARISON OF THE GERMINAL LAYERS. 309
embryology that the blastopore of Vertebrates is primitively situated at the hind end of the body, but that, owing to the development of a large food-yolk, it also extends, in most cases, over a larger or smaller part of the ventral side.
The origin of the Allantois and Amnion.
The development and structure of the allantois and amnion have already been dealt with at sufficient length in the chapters on Aves and Mammalia ; but a few words as to the origin of these parts will not be out of place here.
The Allantois. The relations of the allantois to the adjoining organs, and the conversion of its stalk into the bladder, afford ample evidence that it has taken its origin from a urinary bladder such as is found in Amphibia. We have in tracing the origin of the allantois to deal with a case of what Dohrn would call ' change of function.' The allantois is in fact a urinary bladder which, precociously developed and enormously extended in the embryo, has acquired respiratory (Sauropsida) and nutritive (Mammalia) functions. No form is known to have been preserved with the allantois in a transitional state between an ordinary bladder and a large vascular sack.
The advantage of secondary respiratory organs during fcetal life, in addition to the yolk-sack, is evinced by the fact that such organs are very widely developed in the Ichthyopsida. Thus in Elasmobranchii we have the external gills (cf. p. 62). Amongst Amphibia we have the tail modified to be a respiratory organ in Pipa Americana ; and in Notodelphis, Alytes and Cascilia compressicanda the external gills are modified and enlarged for respiratory purposes within the egg (cf. pp. 140 and 143).
The Amnion. The origin of the amnion is more difficult to explain than that of the allantois ; and it does not seem possible to derive it from any pre-existing organ.
It appears to me, however, very probable that it was evolved part flassu with the allantois, as a simple fold of the somatopleure round the embryo, into which the allantois extended itself as it increased in size and became a respiratory organ. It would be obviously advantageous for such a fold, having once started, to become larger and larger in order to give more and more room for the allantois to spread into.
The continued increase of this fold would lead to its edges meeting on the dorsal side of the embryo, and it is easy to conceive that they might then coalesce.
To afford room for the allantois close to the surface of the egg, where respiration could most advantageously be carried on, it would be convenient that the two laminae of the amnion the true and false amnion should then separate and leave a free space above the embryo, and thus it may have come about that a separation finally takes place between the true and false amnion.
This explanation of the origin of the amnion, though of course hypothetical, has the advantage of suiting itself in most points to the actual ontogeny
310 ORIGIN OF ALLANTOIS AND AMNION.
of the organ. The main difficulty is the early development of the head-fold of the amnion, since, from the position of the allantois, it might have been anticipated that the tail-fold would be the first formed and most important fold of the amnion.
(239) F. M. Balfour. " A comparison of the early stages in the development of Vertebrates." Q:tarf. J. of Micr. Science, Vol. xv. 1875.
(240) F. M. Balfour. "A monograph on the development of Elasmobranch Fishes." London, 1878.
(241) F. M. Balfour. " On the early development of the Lacertilia together with some observations, etc." Quart, y. of Micr. Science, Vol. xix. 1879.
(242) A. Gotte. Die Entwicklungsgeschichte d. Unke. Leipzig, 1875.
(243) W. His. "Ueb. d. Bildung d. Haifischembryonen." Zeit. f. Anat, u. Entwick., Vol. u. 1877. Cf. also His' papers on Teleostei, Nos. 65 and 66.
(244) A. Kowalevsky. " Entwick. d. Amphioxus lanceolatus." Mem. Acad. des Sciences St Petersbourg, Ser. vn. Tom. xi. 1867.
(245) A. Kowalevsky. " Weitere Studien lib. d. Entwick. d. Amphioxus lanceolatus." Archivf. mikr. Anat., Vol. xiil. 1877.
(246) C. Kupffer. "Die Entstehung d. Allantois u. d. Gastrula d. Wirbelthiere." Zool. Anzeiger, Vol. II. 1879, PP- 5 2 ' 593> 612.
(247) R. Remak. Untersuchimgen iib. d. Entwicklung d. Wirbelthiere, 1850 1858.
(248) A. Rauber. Primitimtreifen u. Neurula d. Wirbelthiere. Leipzig, 1877.