AMPHIBIA. 125 



whole edge of which the epiblast and intermediate cells are 

 continuous. 



' From the ventral lip of the blastopore the mesoblast (fig. 71, 

 ;#'), derived from the small intermediate cells, grows inwards till 

 it comes to the segmentation cavity ; the growth being not so 

 much due to an actual invagination of cells at the lip of the 

 blastopore, as to a differentiation of yolk-cells in situ. Shortly 

 after the stage represented in fig. 71 B, the plug of yolk, which 

 fills up the opening of the blastopore, disappears, and the mesen- 

 teron communicates freely with the exterior by a small circular 

 blastopore (fig. 73). The position of the blastopore is the same 

 as in other types, viz. at the hinder end of the embryo. 



By this stage the three layers of the embryo are definitely 

 established. The epiblast, consisting from the first of two strata, 

 arises from the small cells forming the roof of the segmentation- 

 cavity. It becomes continuous at the lip of the blastopore with 

 cells intermediate in size between the cells of which it is formed 

 and the yolk-cells. These latter, increasing in number by 

 additions from the yolk-cells, give rise to the mesoblast and to 

 part of the hypoblast ; while to the latter layer the yolk-cells, as 

 mentioned above, must also be considered as appertaining. 

 Their history will be dealt with in treating of the general fate of 

 the hypoblast. 



Urodela. The early stages of the development of the Newt have 

 been adequately investigated by Scott and Osborn (No. 114). The 

 segmentation and formation of the layers is in the main the same as in the 

 Frog. The ovum is without black pigment. There is a typical unsymmet- 

 rical invagination, but the dorsal lip of the blastopore is somewhat thickened. 

 The most striking feature in which the Newt differs from the Frog is the 

 fact that the epiblast is at first constituted of a single layer of cells (fig. 75, ep\ 

 The roof of the segmentation cavity is constituted, during the later stages of 

 segmentation, of several rows of cells (Bambeke, No. 95), but subsequently it 

 would appear to be formed of a single row of cells only (Scott and Osborn, 

 No. 114). 



General history of the layers. 



Epiblast : Anura. At the completion of the invagination 

 the epiblast forms a continuous layer enclosing the whole ovum, 

 and constituted throughout of two strata. The formation of the 

 medullary canal commences by the nervous layer along the 

 axial dorsal line becoming thickened, and giving rise to a some- 



