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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 

 very different from a simple 

 case like that of Amphi- 

 oxus. In the latter, all the 

 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. (i) 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 



FIG. 398. Two stages in segmented 

 ovum of hedgehog. After 

 Hubrecht. 

 Ep. t Eoiblast ; Hy., hypoblast. 



