820 



PHYLUM CHORDATA : CLASS MAMMALIA 



ascribed to degeneration, and not to a primitive condition. 

 The presence of a yolk-sac placenta in Marsupials is not 

 in itself of great importance, for a connection between the 

 yolk-sac of the embryo and the wall of the oviduct exists 

 in two Elasmobranch 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. 



Fig. 493. — Embryo of Perameles with its foetal membranes. 



—After Hill. 



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

 of yolk-sac ; ch., cliorion or false amnion ; s.t., sinus terminalis ; 

 b.c, extra-embryonic body cavity ; v.o., vascular omphalopleura, 

 or area of non-separation between yolk-sac wall and chorion, 

 constituting the yolk-sac placenta. The endodenn is dotted 

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

 of the embryo into it. 



{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. 

 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. 



