Embryonic area 



Outer germ layer (Ectoderm). 



Middle germ layer (Mesoderm) 

 " Inner germ layer (Endoderm) xV 



Blastodermic vesicle 

 later becoming 

 yolk sac 



Chorion 



Ectoderm 

 Endoderm 



Embryo 



Amniotic 

 fold 



Gut of embryo 

 Allantois 



Closing amniotic folds 



Allantois joining with 

 chorionic membrane 



Extra embryonic 

 space 



Allantoic 

 stalk 



Yolk sac 



Amniotic cavity 



Allantois 

 Amniotic cavit 



orionic villi 



Head of 

 embryo 



Placenta 

 fetal 



,Body stalk, 

 vS^S^ umbilicus 



Degenerating 

 yolk sac 



Extra-embryonic space 

 Fig. 176. — Diagrammatic sections showing the development of the egg and 

 embryonic membranes of a Mammal. A, blastula showing fundament of 

 embryo at top, before the appearance of the amnion. B, embryo outlined, with 

 developing amnion and yolk sac. C, embryo with amnion further developed 

 and allantois appearing. D, embryo with amnion closing, and allantois joining 

 with outer membrane, or chorion. E, F, embryos in which the vascular layer 

 of the allantois is applied to the chorion and growing into the villi of the latter 

 to form the fetal placenta; yolk sac reduced; amniotic cavity increasing; 

 mouth and anus established. 



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