114 EDWARD MCCRADY, JR. 



The extra-embryonic membranes. Though there will be 

 changes in the proportions of the parts in later stages, there 

 will be no changes in the constitution of the amnion, chorion, 

 yolk sac, and body stalk after the closure of the amniopore 

 in stage 29. 



In the description of stage 26 (chapter IX) the history of 

 the constitution of the blastocyst wall from the earliest stages 

 up to the beginning of amnion formation was summarized. 

 It was pointed out that the ventral two-thirds of the vesicle 

 is never invaded by mesoderm, and so represents a persistent 



Fig. 38 Photographs of external views of stage 29. A, ventral view of 16153 

 after removal of non-vascular chorion. B, lateral view of 16153 with vascular 

 chorion and head amnion also removed, but with trunk amnion intact. C, 

 dorsal view of 16154 to show closing of amniopore. 



portion of the original bilaminar vesicle yolk sac endoderm 

 covered by ectoderm. We shall call this the non-vascular yolk 

 sac chorion (fig. 40, nv.ch.). In the dorsal third of the 

 vesicle the mesoderm spreads between the ectoderm and the 

 yolk sac endoderm, and provides a vascular supply for part 

 of the yolk sac. This part is the area vasculosa or vascular 

 yolk sac chorion (v.ch.), and the embryo proper is located ap- 

 proximately in the center of it, though slightly nearer its 

 caudal edge. Finally, in a very restricted region within the 

 embryonic body and immediately around its borders, the 



