624 



THE PLACENTA. 



with an abundant network of dilated capillaries, derived from the 

 bloodvessels of the original decidua. At this time, therefore, each 

 vascular loop of the foetal chorion is covered, first, with a layer 

 forming the wall of the villus. This is in contact with the lining 

 membrane of a uterine follicle, and outside of this again are the 

 capillary vessels of the uterine mucous membrane ; so that two 

 distinct membranes intervene between the walls of the fcetal capil- 

 laries on the one hand and those of the maternal capillaries on the 

 other, and all transudation must take place through the substance 

 of these two membranes. 



As the formation of the placenta goes on, the anatomical arrange- 

 ment of the fcetal vessel remains the same. They continue to 

 form vascular loops, penetrating deeply into the decidual mem- 

 brane; only they become constantly more elongated, and their 

 ramifications more abundant and tortuous. The maternal capilla- 

 ries, however, situated on the outside of the uterine follicles, become 

 considerably altered in their anatomical relations. They enlarge 

 excessively ; and, by encroaching constantly upon the little islets 

 or spaces between them, fuse successively with each other; and, 

 losing gradually in this way the characters of a capillary network, 



become dilated into wide 



Fig- 226. sinuses, which communicate 



freely with the enlarged vessels 

 in the muscular walls of the 

 uterus. As the original capil- 

 lary plexus occupied the entire 

 thickness of the hypertrophied 

 decidua, the vascular sinuses, 

 into which it is thus converted, 

 are equally extensive. They 

 commence at the inferior sur- 

 face of the placenta, where it is 

 in contact with the muscular 

 walls of the uterus, and extend 

 through its whole thickness, 

 quite up to the surface of the 

 foetal chorion. 



As the maternal sinuses grow upward, the vascular tufts of the 

 chorion grow downward, and extend also through the entire thick- 

 ness of the placenta. At this period, the development of the blood- 

 vessels, both in the fcetal and maternal portions of the placenta, is 

 so excessive that all the other tissues, which originally co-existed 



Extremity of FCETAL TUFT, from human pla- 

 centa at term, in its recent condition, a, a. Capil- 

 lary bloodvessels. Magnified 135 diameters. 



