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MAMMALIA. 



of a special adaptation the placenta. This, 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. 



As many of the most fundamental structural and func- 

 tional problems in connection with placentation are still 

 being investigated, it is impossible to discuss even the lead- 

 ing questions with defmiteness and certainty. The authority 

 here followed is Hubrecht, in his study of the placentation 

 of the hedgehog, which is at once a simple and a central 

 type. 



First, then, let us 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 the 

 future inner layer, endo- 

 derm or hypoblast (Fig. 

 226, I.). (2) The epiblast 

 divides into an embryonic 

 disc which will form the 

 epidermis, nervous system, 

 &c., of the embryo, and 

 an external layer, the wall 



of the embryonic sac or FlG> 22(5 ._ Two stages in seg . 

 blastocyst, with which the men ted ovum of Hedgehog. (After 

 disc retains a slight con- HUBRECHT.) 

 nection until the protec- E ^ Ep ibiast; //,., Hypoblast. 



tive amnion is formed. 



In the outer epiblastic wall lacunae 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 connection between mother and offspring. 

 Long before any. vascular area or foetal placenta is devel- 

 oped, the outer epiblastic wall has the above nutritive func- 

 tion, and deserves its name of trophoblast (Fig. 225, Tr.). 

 (3) The hypoblast or inner mass, which is at first a solid 

 aggregate of cells (Fig. 224, i.c.\ becomes a sac, as a morula 

 may become a blastosphere. The upper part of this sac 



