130 



HYPOBLAST. 



of all in the middle (fig. 76). In front this process takes place 

 with the greatest rapidity. The cells of the yolk-floor become 

 continuously differentiated into hypoblast-cells, and very soon 

 the whole of the front end becomes completely lined by true 

 hypoblastic cells, while the yolk-cells become confined to the 

 floor of the middle part. 



The front portion of the mesenteron gives rise to the oeso- 

 phagus, stomach and duodenum. Close to its hinder boundary 

 there appears a ventral outgrowth, which is the commencement 

 of the hepatic di- 

 verticulum (fig. 76, 

 /). The yolk is thus 

 post-hepatic, as in 

 Vertebrates gene- 

 rally. 



The stomodae- 

 um is formed com- 

 paratively late by 

 an epiblastic inva- 

 gination (fig. 76, m). 



It should be noticed 

 that the conversion of 

 the yolk-cells into hypo- 

 blast-cells to form the 

 ventral wall of the anterior region of the alimentary tract is a closely 

 similar occurrence to the formation of cells in the yolk-floor of the 

 anterior part of the alimentary tract in Elasmobranchii. 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 

 gradually 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 



FIG. 76. LONGITUDINAL SECTION THROUGH AN 



ADVANCED EMBRYO OF BOMBINATOR. (After Gotte.) 



m. mouth ; an. anus ; /. liver ; ne. neurenteric 

 canal ; me. medullary canal ; ch. notochord ; pn. pineal 

 gland. 



