26 JeNKINSON, Geinninal Layers of Vertebrates. 



to the animal hemisphere of the frog's &gg, the 

 yolk to the vegetative hemisphere. 



(2) Hence the edge of the blastoderm in the former 



is equivalent to the subequatorial bounding line 

 between animal and vegetative hemispheres in 

 the latter. 



(3) In both this bounding line becomes in its 



entirety the lip of the blastopore, the posterior 

 point of the edge in the Elasmobranch being the 

 dorsal lip of the frog, the anterior point of the 

 edge the ventral lip. 



(4) In both the blastopore closes bilaterally, growth 



being greatest at the dorsal lip. 



(5) In both the material for the germinal layers is 



laid down during the process ; the notochord 

 lies in front of the dorsal lip, the mesoderm at 

 the lateral lips in two sheets continuous with 

 one another behind in the ventral lip. 

 The principal points of difference are two. First, 

 in the Elasmobranchs the closure of the blastopore is 

 divided (as in Bdellostoma, and also, as we shall see in 

 a moment, in the Teleostei) into two periods ; in the 

 first, the overgrowth is almost confined to the dorsal lip 

 and produces the material for the formation of the 

 embryo ; in the second, the yolk is gradually covered by 

 an extension of the blastoderm in which the lateral and 

 anterior margins are alone concerned. Secondly, in the 

 Elasmobranch a part only of the blastoporic lip is in- 

 volved in the formation of the embryo, the lateral and 

 ventral lips remaining wholly extra-embryonic, 



TELEOSTEI. 

 The ^^^ of the Teleostei is, like that of the Elasmo- 

 branchs, megalecithal and meroblastic. Segmentation 



