704 



MAMMALIA. 



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 a few only 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 associ- 

 ated with placentation. (i)At 

 a very early stage the divided 



ovum of the hedeehog con- 



FIG. 347. --Two stages in seg- . f f i, 



mented ovum of hedgehog, After S1SIS cens ' an 



Hubrecht. outer layer, epiblastic or ecto- 



Ep., Epibiast; //>., hypobiast, dermic, enclosing another 



aggregate the future inner 



layer, endoderm or hypobiast (Fig. 347, I.). (2) The 

 epiblast divides into an embryonic disc, which will form 

 the epidermis, nervous system, etc., of the embryo, and 

 an external layer, the wall of the embryonic sac or blasto- 

 cyst, with which the disc retains a slight connection until 

 the protective amnion is formed. In the outer epiblastic 

 wall lacunas develop, which are bathed by the maternal 

 blood, and the pillars of tissue between the lacunae 

 grow out into villi, which aid in this earliest connec- 

 tion between mother and offspring. Long before any 

 vascular area or foetal placenta is developed, the outer 

 epiblastic wall has the above nutritive function, and deserves 

 its name of trophoblast (Fig. 345, 7>.). (3) The hypobiast 



