CONNECTION BETWEEN EMBRYO AND MOTHER. 727 



(b) In the Marsupials the embryo is born prematurely 

 after a short gestation. It is very small and helpless. Till 

 recently it was believed that during its intra-uterine life it 

 was either not attached to the wall of the uterus at all, or 

 only to a slight extent by a yolk-sac placenta. It is now 

 known, however, that, in Perameles at least, there is not 

 only an efficient yolk-sac placenta, but a distinct, though 

 small, allantoic placenta. The general absence of a 



am ic 



st 



FIG. 397. Embryo of Perameles with its foetal membranes. 

 After Hill. 



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

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

 terminalis ; &.c., extra-embryonic body cavity ; v.o. y vascular 

 omphalopleura, 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. 



placenta in Marsupials, and the small size of the allantois, 

 must therefore be 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 in- 

 disputably to a common origin for the two structures. 



