668 



MAMMALIA, 



branch fishes and in two lizards, but the similarity between 

 the allantoic placenta of Perameles and that of the Eutheria 

 seems to point indisputably to a common origin for the two 

 structures. 



(c) In the Eutherian Mammals, although a temporary 

 yolk-sac placenta may occur, there is always a well- 

 developed and exceedingly important allantoic placenta, 

 which is the main organ for the nutrition of the embryo. 



a/s 



Fig. 292. — Embryo of Perameles with its fcetal membranes. 

 —After Hill. 



am. t True amnion; al. t allantois ; al.s., allantoic stalk; y.c, 

 cavity of yolk-sac ; ch. t chorion or false amnion ; s.t., sinus 

 terminalis ; b.c, extra-embryonic body cavity; vo. t vascular 

 omphalopleure, or area of non-separation between yolk-sac wall 

 and chorion, constituting the yolk-sac placenta. The endoderm 

 is dotted throughout. Note the large size of the yolk-sac, and 

 the sinking of the embryo into it. 



The placenta, in rough physiological language, is a double 

 vascular sponge, partly embryonic, partly maternal, by 

 means of which the blood of the mother nourishes and 

 purifies that of the embryo. It is formed by the inter- 

 locking of fcetal and maternal tissue. 



In giving an account of the placentation of the Eutheria, 

 we shall mainly follow Hubrecht in his account of the 

 placentation of the hedgehog, which is at once a simple 

 and central type. 



