728 MAMMALIA 
(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 foetal 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 hedge- 
hog, which is at once a 
simple and central type. 
Before doing so, it may 
be well to note briefly 
certain facts in regard to 
the early development of 
the egg. In  Eutheria, 
segmentation is holoblastic 
and yolk is absent, but the 
process of development is 
ee, F nteq Very different from a simple 
TiC: Gram of hedgehog. © After case like that of Amphi: 
Hubrecht. oxus. In the latter, all the 
Ep., Epiblast ; Ay., hypoblast. cells of the blastosphere 
form part of the embryo; 
in the former, only a few take a direct part in the process ; 
the remainder form the wall of the embryonic sac or 
blastocyst, from which the yolkless yolk-sac or umbilical 
vesicle is later developed. A process of folding-off of the 
embryo occurs therefore in Mammals as in Birds and 
Reptiles, the chief difference being that, roughly speaking, 
in the former the yolk-sac has a cellular wall from the first, 
in the latter the germinal layers slowly spread over the yolk 
as development proceeds. 
Bearing these facts in mind, let us then seek to define 
the embryonic and maternal structures which are associated 
with placentation. (1) At a very early stage the divided 
ovum of the hedgehog consists of a sac of cells, an outer 
layer, epiblastic or ectodermic, enclosing another aggregate 
