FOETAL NUTRITION: THE PLACENTA 381 



bathed by the uterine secretion after the disappearance of the 

 prochorion. In the non-vascular part it is probably trans- 

 mitted through the hypoblast cells to the yolk-sac, whence, in 

 turn, it reaches the embryo either by the viteUine vessels or the 

 developing ahmentary canal. In the vascular part the same 

 may occur, or the nutriment may be conveyed to the embryo 

 directly by the vessels of the area vasculosa. It is in this region 

 that the foetal circulation is brought close to the maternal, and 

 gaseous exchanges may be effected. Opposite the coelom 



,-Mes. 



Fia. 77. — Diagram to illustrate the three parts of the wall of the yolk-sac 



in the rabbit. (From Minot's Human Embryology, by permission of 



William Wood* Co.) 

 Ak, allantois ; Apl., area placentalis ; Ec, ectoderm ; Mes., mesoderm ; 



Ent., extra-embryonic entoderm ; Cos., coelom ; En., entodermic cavity 



of the embryo ; Pro, A, proamnion. 



the trophoblast is lined by a thin layer of non-vascular somato- 

 pleur, through which transference of material to the coelomic 

 cavity is possible. This part is subsequently connected with 

 the embryo by the allantoic vessels. When the yolk-sac is 

 entirely separated from the outer wall, nutritive substances 

 may also be transmitted to the coelomic cavity and then to the 

 embryo or yolk-sac. 



The nutritive importance of the yolk-sac may now be con- 

 sidered in greater detail in several orders of Mammals. 



Marsupials. — In the opossum the mesoblast spreads about 

 half-way round the wall of the blastocyst, but it does not spUt 



