192 THE REPRODUCTIVE SYSTEM. 



The chorion, the middle constituent of the foetal membrane, exhibits 

 two layers, an inner fibrous and an outer cellular layer. The fibrous 

 layer is next to the amnion, with which it is loosely united ; it is of 

 mesoblastic origin and composed of fibrous tissue. The outer or " cel- 

 lular layer " consists of several layers of large spheroidal prominently 

 nucleated cells, apparently of epiblastic origin and epithelial nature. 



Externally the chorion is closely united to the decidua vera (the 

 decidua reflexa degenerating and disappearing by about the seventh 

 month) ; the decidua vera, the mucosa of the pregnant uterus, at labor 

 is cast off with the foetal membrane as its outermost lamina. 



The placenta is formed by the union of the chorion frondosum and 

 xiecidua serotina, and hence consists partly of foetal and partly of 

 maternal tissues. It is essentially an intimate interlacement of foetal 

 and maternal blood-channels, serving alimentary, respiratory, and 

 excretory purposes for the foetus. The placenta consists of four layers, 

 which are, from the foetal surface outward, the amnion, the chorion, a 

 broad layer of chorial villi separated by maternal blood-sinuses, and 

 the decidua serotina. 



The portion of the amnion lining the foetal surface of the placenta is 

 continuous with that lining the foetal membranes, and is similar to the 

 latter in its structure, having a superficial epithelial and deeper con- 

 nective-tissue layer, and being connected with the chorion by delicate 

 muco-fibrous tissue. 



The placental portion of the chorion (chorion frondosum) presents 

 toward the amnion a fibrous layer, and beneath this the epithelioid 

 "cellular layer," as in the chorion Iseve, with which it is continuous; 

 beneath the cellular layer is a coagulum-like stratum containing pas- 

 sages and spaces, called the " canalized fibrin." 



The substance of the placenta is composed of a mass of "chorial 

 villi," with intervening maternal blood-spaces, interposed between the 

 chorion and decidua serotina. On following the foetal membranes into 

 the margin of the placenta, the chorion and decidua separate, the 

 interval between them being filled with the villi and sinuses. The 

 umbilical vessels on reaching the placenta divide and subdivide and 

 spread over its surface, running in the fibrous layer of the chorion. 

 The branches of these vessels pass outward from the chorial mem- 

 brane, toward the uterine wall, forming branching vascular tufts, the 

 chorial villi, which are covered with an epithelioid investment from 

 the outer epiblastic layer of the chorion. 



The villi begin as large coarse trunks passing from the chorion 

 toward the decidua; these divide into a great profusion of small 

 cylindrical branches and twigs. The ends of some of the villi become 

 attached to the decidua serotina, but most of the branches end in free 

 or floating blunt, round extremities. The villi are composed of arterial 

 and venous channels and capillary loops, imbedded in a loose connec- 

 tive tissue, with their surface covered with tissue of epithelial origin. 

 The foetal vessels are closed, and nowhere open into the maternal 

 blood-channels. 



The epithelioid tissue lining the surface of the villi consists variously 

 of distinct cells, a protoplasmic lamina containing numerous nuclei, 



