234 THE GASTIiULA OF ELAiiMOBRANCllIl. 



which separates them I have given three diagrammatic longitudinal 

 sections of an ideal form intermediate between Amphibia and Elasmo- 

 branehii, which diifers 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 ABC three longitudinal sections of tlie ideal inter- 

 mediate form. The diagrams correspond with the Amphibian dia- 

 grams 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 seg- 

 mentation cavity from the yolk proper; this however soon disappears, 

 and an unsegmented yolk with free nuclei fills the place of the seg- 

 mented 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 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. 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 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 



