DEVELOPMENT. 



519 



somatic wall of yolk-sac cf a vine embryos, Fig. 271, Sh). Later 

 on, the mesoblast of the peripheral part of the allantois becomes 

 applied to the subzonal membrane and the two structures 



Fig. 271. Diai^ramiuatic figures illustratins; the formation of the foetal memhranea of a 

 mammal (after Kulliker). a embryo before appearance of amnion ; 6 embryo with yolk- 

 sac and developing amnion ; c embryo with amnion closing and developing allantois ; 

 d embryo with villous subzonal membrane, and with mouth and anus ; e embryo in which 

 the vascular layer of the allantois is applied to the subzonal membrane, and has "rown 

 into the villi of the latter, yolk-sac reduced, the amniotic cavity is increasing ; A embry- 

 onic thickening of the external layer ; Ah amniotic cavity ; Al allantoic stalk ; Am amnioni ; 

 Ch chorion ; Cliz chorionic villi ; D, D' zona radiata ; D(f umbilical stalk ; Dh intestinal 

 cavity ; Ds cavity of the embryonic (blastodermic) vesicle, later of the volk-sac (umbilical 

 vesicle) : E embryo ; J embryonic thickening of the inner layer, M of the middle layer • 

 SA.subzonal membrane (serous envelope) ; Sz villi of subzonal membrane. 



