REPRODUCTIVE ORGANS IN THE FEMALE. 329 



layer and an inner connective-tissue layer, the latter 

 being vascular. It is this vascular chorion that enters 

 into intimate relations with the uterine mucosa 

 to form the placenta, the former the fetal part and 

 the latter the maternal part. These parts become 

 intimately associated. 



The chorion very early produces a large number of 

 villi which invade the mucosa, where they ultimately 

 become large and much branched, like the root of a 

 tree. These are the chorionic villi and belong to 

 the fetal pla- 

 centa. Each Blood capillaries. Zellknoten. 



villus consists 

 of a connective- 

 tissue core with 

 an epithelial 

 lining. The 

 fetal blood cir- 

 culates through 

 the connective- 

 t issue core 



while the maternal blood bathes the external sur- 

 faces of the villi. The terminal ramifications of 

 each villus becomes firmly anchored in the uterine 

 mucosa, while the branched lateral twigs float freely 

 in the maternal blood spaces or intermllus sinuses. 

 These villi serve a double purpose. They attach 

 the fetal placenta firmly to the uterus, and establish 

 a close relation between fetal and maternal blood 

 whereby the embryo receives proper nourishment. 

 Upon closer examination each villus should present 

 in cross section an outer layer of simple squamous 



Epithelial cells. 



Fig. 236. Cross section of two human choriomc 

 villi at end of pregnancy. 



