CHAPTER XII. 



THE PLACENTA. 



IN all instances in which, as in man and the mammalians, the em- 

 bryo is dependent, for the materials of its growth, upon nutritious fluids 

 supplied by the uterus, the communication between them is established 

 by means of two vascular membranes. One of these membranes, the 

 chorion or the allantois, is a part of the embryo; the other is the mucous 

 membrane of the uterus. By their more or less intimate juxtaposition, 

 the fluids transuded from the bloodvessels of the mother are absorbed 

 by those of the embryo, and thus a transfer of nutriment takes place 

 from the maternal to the foetal organism. 



In some species of animals, the connection between the maternal and 

 foetal membranes is a simple one. In the pig, for example, the uterine 

 mucous membrane is everywhere uniformly vascular; its only pecu- 

 liarity consisting in the presence of transverse folds, which project 

 inward from its surface, like the valvulse conniventes of the small in- 

 testine. The external investing membrane of the egg, or the allantois, 

 is also smooth and uniformly vascular. No special development of 

 tissue or of vessels occurs at any part of these membranes, and no adhe- 

 sion takes place between them. The vascular allantois of the foetus is 

 simply in close apposition with the vascular mucous membrane of the 

 uterus; each of the two contiguous surfaces following the undulations 



Fig. 270. 



Diagram oftheFoiTAL PIG, with its membranes, in, the uterus; showing the relation of 

 the allantoic and uterine surfaces. a, a, 6, 6. Walls of the uterus, c, c. Cavity of the uterus. 

 d. Amnion. e, e, Allantois. 



presented by the other. (Fig. 2TO.) By this arrangement, transudation 

 and absorption 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, a moderate contraction of the 

 uterus is sufficient to expel its contents. The egg, displaced from its 

 original position, slides forward over the surface, of the uterine mucous 



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