FORMATION OF THE MEMBRANES 



807 



Formation of the Allantois and the Permanent Chorion. During 

 the early stages of development of the umbilical vesicle, and as it is 

 shut off from the intestine, there appears an elevation at the posterior 



Fig. 225. Human embiyo of twenty to twenty-five days (Coste) . 



Am, amnion; LL, lower limb; UA, umbilical artery; Uc, umbilical cord; UL, upper limb; Ys, 

 yolk-sac (umbilical vesicle). 



portion of the intestine, which rapidly increases in extent until it forms 

 a membrane of two layers, extending between the internal and the 

 external layers of the amnion. This membrane, the allantois, becomes 

 vascular early in its development, increases in size quite rapidly, and 

 finally it completely encloses the internal layer of the amnion and the 



