AMPHIBIA. 



103 



with the exterior by a small circular blastopore (fig. 73). The position 

 A. B. 



Ay 



Fig. 71. Diagrammatic longitodinai. sections thbough the embkyo of a Frog 



AT TWO stages, TO SHEW THE FORMATION OF THE GERMINAL LAYERS. (Modified from Gottc.) 



cp. ei^iblast; m. dorsal mesoblast; m. ventral mesoblast ; /(?/. hypoblast; yfc. yolk; 

 X. point of junction of the epiblast and hypoblast at the dorsal side of the blastopore ; 

 al. mesenteron ; s<j. segmentation cavity. 



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 esta- 

 blished. 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 inter- 

 mediate 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 treat- 

 ing of the general fate of the hypoblast. 



Urodela. The early stages of the developmeait of the Newt have been 

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

 and formation of the layers is in the mam the same as in the Frog. The 

 ovum is without black pigment There is a typical unsymmetrical in- 

 vagination, but the dorsal lip of the blastopore is somewhat thickened. 

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

 fact that the epiblast is at first constituted of a single layer of cells (tig- \ 

 75, ep). The roof of the segmentation cavity is constituted, during the 

 later stages of the 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 liistory of the layers. 



Epiblast: Auura. At the completion of the invagination the 

 epiblast forms a continuous layer enclosing the whole ovum, and con- 



