CONNECTION BETWEEN EMBRYO AND MOTHER. 727 
(2) 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 
an be 
Fic. 397.—Embryo of 2erameles with its foetal membranes. 
—After Hill. 
am., True amnion; ad, allantois; @/.s., allantoic stalk; y.c., 
cavity of yolk-sac ; chy chorion or false amnion; s.¢., sinus 
See 6.c., extra-embryonic body cavity; v.0., 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 toa 
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. 
