THE REPRODUCTIVE SYSTEM 377 



The decidua serotina undergoes changes connected with the 

 development of the placenta. 



The Placenta^ 



The placenta consists of two parts, one of which is of maternal 

 origin — placenta uterina- — the other of foetal origin — the placenta 

 Jcetalis. As it is in the placenta that the interchange takes place 

 between the maternal and the foetal blood, the relations between the 

 maternal and foetal parts of the placenta are extremely intimate. 

 This relation consists essentially in the growing out from the foetal 

 placenta of hnger-like projections — villi — which penetrate the mater- 

 nal placenta, the latter being especially modified for their reception. 



The Placenta Fcetalis. — This is a differentiated portion of the 

 chorion. On the surface directed toward the fcetus the chorion after 

 the third month is covered by a delicate foetal membrane, the placen- 

 tal portion of the amnion. This consists of a surface epithelium, 

 resting upon a layer of embryonal connective tissue which attaches it 

 to the chorion. The chorion consists of {a) a compact layer — the 

 membrana chorii — composed at first of embryonal, later of fibrous, 

 connective tissue, and containing the main branches of the umbilical 

 vessels and (&) an inner villous layer, which gives rise to finger-like 

 projections which extend down from the foetal into the maternal 

 placenta and serve to connect the two. 



The chorionic villi first appear as short projections composed 

 entirely of epithelium. Each of these primary villi branches dichoto- 

 mously, giving rise to a number of secondary villi. As they develop, 

 the central portion of the original solid epithelial structure is replaced 

 by connective tissue. Septa of connective tissue from the maternal 

 placenta pass down among the villi and separate them into groups or 

 cotyledons. The main or primary villi run a quite straight course from 

 the chorion into the maternal placental tissue, apparently serving to 

 secure firm union between the two. They are thus known as roots of 

 attachment or fastening villi (Fig. 268) . The secondary villi are given 

 off laterally from the primary villi, end freely in the spaces between 

 the latter — intervillous spaces (Fig. 268) — and are known as free, 

 terminal or floating villi (Fig. 268). 



The chorionic villus thus consists of a central core of connective 



^ For many facts as to the structure of the placenta, the writer is indebted to the 

 excellent chapter on the subject added by Prof. Alfred Schaper to the fifth edition of 

 Stohr's "Textbook of Histolosv." 



