4i6 THE PHYSIOLOGY OF REPRODUCTION 



after the disappearance of the prochorion. In the non-vascular part 

 ' it is probably transmitted through the hypoblast cells to the yolk-sac, 

 whence, in turn, it reaches the embryo either by the vitelline vessels 

 or the developing alimentary 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 the trophoblast is 

 lined by a thin layer of non-vascular somatopleur, 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 split 

 over its whole extent. Hence 

 the cojlom is gjtiall, and cor- 

 respondingly the separation - of 

 the yolk-sac and trophoblast is insignifieant (Fig. 103). The 

 allantois grows out into the coslom only to impinge on and 

 invaginate the wall of the yolk-sac. It never comes in contact with 

 the outer wall of the blastocyst. The part of the wall where the 

 mesoblast is unsplit is thrown into folds which fit into corresponding 

 furrows of the mucosa. Hence an avillous yolk-sac placenta is formed 

 (Selenka^). The nutrition in the uterus is very primitive. The-ova 

 contain a comparatively large supply of yolk granules for the initial 

 stages of development. As they travel along the oviduct and into 

 the uterus, they are invested with a thick nutritive layer, derived 



1 The name proamnion is given to that region of the amnion below the head 

 of the embryo from which the mesoderm, is absent. • In Marsupials this may 

 npTmst 



2 Selenka, Studien iiber die EntwicMungsgeschichte der Thiere, Wiesbaden. 



T?iG. 102. — 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.) 



^U, Allantois ; Apl, area placentalis ; 

 Ec, ectoderm ; ' Mes, mesoderm ; 

 Ent, extra - embryonic entoderm ; 

 Gee, ccelom ; En, entodermic cavity 

 of the embryo ; pro. A, proamnion.' 



