SYNTROPHOBLAST ■ /• 



ENDODERM 



Fig. 172. Formation of the yolk sac, growth of the mesoderm, and differen- 

 tiation of the trophoblast into the syntrophoblast and the chorion. 



MUSCLE LAYER 



O O O O O o- 



BLOOD VESSELS 



CAVIT* or UTERUS 



Fig. 173. Primitive-streak stage of human embryo, together with the extra- 

 embryonic membranes and the uterine wall. 



268 



