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 subzoaal membrane and the two structures 



« M ^ 



Fio. 271. Diasrammatic figures illustratins the formation of the foetal membranes of a 

 mammal (after KoUiker). o embrj-o before appearance of amnion ; 6 embryo with yolk- 

 sac and developing amnion ; c embrj'o with amnion closing and developing allantois ; 

 d embrvo with vUlous 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, volk-sac reduced, the amniotic cavity is increasing ; A embry- 

 onic thickening of the externa! laver ; JA amniotic cavity ; Al allantoic stalk ; Am amuioni; 

 Ch chorion- Ckz chorionic villi"; D, D' zona radiata ; Dq umbilical stalk; Dh intestinal 

 cavitv • Ds 'cavity of the embryonic (blastodermic) vesicle, later of the yolk-sac (umbilical 

 vefticie)' • E embrvo ; J embryonic thickening of the inner layer, M of the middle layer ; 

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



