108 HYPOBLAST. 



This conversion is apparently denied by Gotte, but since I find cells in all 

 stages of transition between yolk-cells and hypoblast-cells I cannot doubt 

 the fact of its occurrence. 



At first, the mesenteron. freely communicates with the exterior 

 by the opening of the blastopore. The lips of the blastopore gradu- 

 ally approximate, and form a narrow passage on the dorsal side of 

 which the neural tube opens, as has already been described (fig. 73). 

 The external opening of this passage finally becomes obliterated, 

 and the passage itself is left as a narrow diverticulum leading from 

 the hind end of the mesenteron into the neural canal (fig. 76). It 

 forms the post-anal gut, and gradually narrows and finally atrophies. 

 At its front border, on the ventral side, there may be seen a 

 slight ventrally directed 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 proctodseum, 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, 

 [t 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 um- 

 bilical 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 groivth 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 be- 

 comes more prominent (fig. 77 A), and ends behind at the blastopore 

 (bl), the lips of which are continuous 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 the unsegmented 



