THE PLACENTA. 621 



CHAPTER XII. 



THE PLACENTA. 



WE have shown in the preceding chapters that the foetus, during 

 its development, depends for its supply of nutriment upon the lining 

 membrane of the maternal uterus ; and that the nutriment, so sup- 

 plied, is absorbed by the bloodvessels of the chorion, and transported 

 in this way into the circulation of the foetus. In all instances, ac- 

 cordingly, in which the development of the foetus takes place within 

 the body of the parent, it is provided for by the relation thus esta- 

 blished between two sets of membranes; namely, the maternal 

 membranes which supply nourishment, and the foetal membranes 

 which absorb it. 



In some species of animals, the connection between the maternal 

 and foetal membranes is exceedingly simple. In the pig, for ex- 

 ample, the uterine mucous membrane is everywhere uniformly 

 vascular ; its only peculiarity consisting in the presence of nume- 

 rous transverse folds, which project from its surface, analogous to 

 the valvulae conniventes of the small intestine. The external in- 

 vesting membrane of the egg, which is the allantois, is also smooth 

 and uniformly vascular like the other. No special development of 

 tissue or of vessels occurs at any part of these membranes, and 

 no direct adhesion takes place between them; but the vascular 

 allantois or chorion of the foetus is everywhere closely applied to 

 the vascular mucous membrane of the maternal uterus, each of the 

 two contiguous surfaces following the undulations presented by the 

 other. (Fig. 224.) By this arrangement, transudation and absorp- 

 tion take place from the bloodvessels of the mother to those of the 

 foetus, in sufficient quantity to provide for the nutrition of the latter. 

 When parturition takes place, accordingly, in these animals, a very 

 moderate contraction of the uterus is sufficient to expel its contents. 

 The egg, displaced from its original position, slides easily forward 

 over the surface of the uterine mucous membrane, and is at last 

 discharged without any hemorrhage or laceration of connecting 

 parts. In other instances, however, the development of the foetus 

 requires a more elaborate arrangement of the vascular membranes. 



