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 diagram- 

 matic longitudinal sections of Elasmobranch embryos, and in 

 fig. 173 A B C three longitudinal sections of the ideal inter- 

 mediate 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 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 Elasmo- 

 branch blastoderm. 



The relation of the yolk to the blastoderm in the Elasmo- 

 branch 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 



