AMPHIBIA. 131 



diverticulum of the alimentary tract, which first becomes visible 

 at a somewhat earlier stage (fig. 73). This diverticulum becomes 

 longer and meets an invagination of the skin (fig. 76, an), which 

 arises in Rana temporaria at a somewhat earlier period than 

 represented by Gotte in Bombinator. This epiblastic invagination 

 is the proctodaeum, and an anal perforation eventually appears 

 at its upper extremity. 



The differentiation of the hinder end of the prseanal gut 

 proceeds in the same fashion as that of the front end, though 

 somewhat later. It gives rise to the cloacal and intestinal part 

 of the alimentary tract. From the ventral wall of the cloacal 

 section, there grows out the bifid allantoic bladder, which is 

 probably homologous with the allantois of the higher Vertebrata. 

 After the differentiation of the ventral wall of the fore and hind 

 ends of the alimentary tract has proceeded for a certain distance, 

 the yolk only forms a floor for a restricted median region of the 

 alimentary cavity, which corresponds to the umbilical canal of 

 the Amniota. The true hypoblastic epithelium then grows over 

 the outer side of the yolk, which thus constitutes a true, though 

 small, and internal yolk-sack. The yolk-cells enclosed in this 

 sack become gradually absorbed, and the walls of the sack form 

 part of the intestine. 



General growth of the Embryo. 



Anura. The pyriform medullary plate, already described, 

 is the first external indication of the embryo. This plate 

 appears about the stage represented in longitudinal section in 

 fig. 71 B. The feature most conspicuous in it at first is the 

 axial groove. It soon becomes more prominent (fig. 77 A), and 

 ends behind at the blastopore (bl\ the lips of which are con- 

 tinuous with the two medullary folds. As the sides of this plate 

 bend upwards to form the closed medullary canal, the embryo 

 elongates itself and assumes a somewhat oval form. At the 

 same time the cranial flexure becomes apparent (fig. 73), and 

 the blastopore shortly afterwards becomes shut off from the 

 exterior. The embryo now continues to grow in length (fig. 

 77 B), and the mesoblast becomes segmented. The somites are 

 first formed in the neck, and are added successively behind in 



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