PLACENTA 



821 



In giving an account of the placentation of the Eutheria, 

 we shall mainly follow Hubrecht in his account of the 

 placentation of the hedgehog, 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 Amphioxus. 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 blasto- 

 cyst, from which the yolk- 

 less yolk-sac or umbilical 

 vesicle is later developed. 

 A process of folding-off of 

 the embryo occurs there- 

 fore 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, ectodermic or epiblastic, enclosing another aggregate 

 — the future inner layer, endoderm or hypoblast (Fig. 494, 

 I.). (2) The ectoderm divides into an embryonic disc, which 

 will form the epidermis, nervous system, etc., of the em- 

 bryo, and an external layer, the wall of the embryonic sac 

 or blastocyst, with which the disc retains a slight connec- 

 tion until the protective amnion is formed. In the outer 

 ectodermal wall lacuna; develop, which are bathed by the 

 maternal blood, and the pillars of tissue between the 



Fig. 494. — Two stages in segmented 

 ovum of hedgehog. — After 

 Hubrecht. 



Ep., Epiblast or ectoderm ; Hy., hypoblast 

 or endoderm. 



