THE DEVELOPMENT OF MAMMALS 



639 



mals. The parts of the chorion and uterine lining that are intimately 

 associated constitute the placenta. 



In vertebrates such as the frog (Fig. 6.9), gastrulation involves an 

 inpushing of certain cells of the vegetal hemisphere (invagination), a 

 growth of cells from the animal hemisphere over the vegetal cells (epi- 

 boly) and an inturning of certain of these cells (involution). These 

 complex processes are largely by-passed in mammalian development and 

 gastrulation is greatly abbreviated. In primates, cells on the lower part 

 of the inner cell mass simply differentiate as endoderm, and a small 

 space, the yolk sac, appears in their midst (Fig. 31.2). The yolk sac is 

 an embryonic vestige and is devoid of yolk. Its presence is a hold-over 



Inner 



C(zll m.ass~n 



Trophobla.st 



r~ Amniotic 

 ca.vii;y 



Aranion 

 Embryo 



i^ YolK 



r-Body 



Mesodermal ce-lls 



Chorionic 

 villu-S' 



Figure 31.2. A series of diagrams to illustrate the differentiation of the inner 

 mass into the yolk sac, amnion and embryonic disc, and to show the migration of 

 mesoderm. These changes occur in a human embryo during the second week. (Modi 

 after Patten.) 



cell 

 the 

 fied 



