DEVELOPMENT. 



519 



somatic wall of yolk-sac of avine 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. Diagrammatic figures illustrating the formation of the foetal membranes of a 

 mammal (after Kolliker). a embryo before appearance of amnion ; b 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 grown 

 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 ; Chz chorionic villi ; D, D' zona radiata ;" Dq umbilical stalk ; Dh intestinal 

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

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

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



