702 



THE FCETAL MEMBRANES. 



material of the yolk is probably absorbed by these vessels and conveyed 

 by them as nourishment into the system of the embryo. In many of 

 these animals however a quantity of the yolk substance also remains at 

 the end of incubation, and by actual transference into the intestine of 

 the embryo serves for a time as its digestible food. 



In mammals the yolk-sac grows for a time with the embryo and other 

 parts of the developing ovum, and the yolk substance within it must 

 undergo a corresponding increase. There are however great cliflFerences 

 among the difierent tribes of mammals in the extent of the development 

 of the yolk-sac during uterogestation. In some it remains large and 

 vascular, while in others it becomes atrophied and its vessels are 

 obliterated at a comparatively early period. In rodentia it attains its 



Fisr. 513. 



Fig. 513.— Magnified View op the Human Embryo of four "sveeks with the 

 Membranes opened (fi-om Leishman after Coste). 



-^iK'ni^^ ™bilical_ vesicle with the omphalo-mesenteric vessels, v, and its long tubular 

 fie ..P^rl n ^ f testiue ; c, the villi of the chorion ; m, the amnion opened ; u, cul 

 oL?! • '"""f °*°i^' ''^'"1 on each side of this the umbilical vessels passing out to the 



™ « l\ Ti T^?'°' *^^ 'y^ ' '' ^^^ ^'''- vesicle ; h, the heart ; I, tlie itver ; o, the 

 yiMestine rw'^S' ^''' W^l^^" ^^dy, in front of which the mesentery arid fold 

 ot intestine. The Wolffian duct and tubes are not represented. 



