72 



PLACENTA 



ttie mesoderm is seen sending out buds which indent the trophoblast. By the 

 outward growth of these the epithelial strands acquire a mesodermic core, as 

 will be afterwards more fully explained ; vessels develop in the mesoderm, and the 

 result is that the whole surface of the chorion becomes occupied by vascular pro- 

 jections or villi, enclosing foetal vessels and bathed by the maternal blood. At 

 first equally distributed, the villi in the region of the decidua basalis become larger, 

 longer, and more branched, while those related to the decidua capsularis remain 

 relatively smaller and ultimately disappear by atrophy. The part of the chorion 

 in which the villi persist is known as the chorion frondosum. The remainder is 

 termed the chorion Iceve (fig. 97). In the third month the chorion frondosum 

 forms the foetal part of the definitive placenta, while the chorion Iseve, after the 

 disappearance of the decidua capsularis, comes in contact with the decidua vera, 



B.L. 



FIG. 99. PORTION OF THE TROPHOBLAST OF PETERS' OVUM. (After Peters.) 



ect, ect, chorionic ectoderm ; mes, mesoderm budding out to form, a villus core ; B.L., B.L., blood- 

 lacunae lined with a thin layer of syncytium ; tr, tr, cellular layer of trophoblast ; sij, sy, syncytium ; 

 s?y 2 , s?/ 2 , mass of syncytium invading the lumen of a maternal blood-capillary, ca, ca ; bz, boundary-zone 

 between trophoblast and decidua ; c, trophoblast-cell showing alteration of nucleus. 



and along with the amnion, as already stated, forms the double membrane lining 

 the uterine cavity. 



Having now sketched the history of the decidua and of the chorion, we are 

 in a position to study in greater detail the changes which lead to the formation of 

 the placenta. 



Placenta. We have to distinguish two phases of placentation in the human 

 subject, a primary and a secondary. In the first phase, the whole chorion is 

 covered by vascular villi, similarly related to the decidua and the maternal blood, 

 and the placenta is therefore said to be diffuse. In the second stage, after the 

 atrophy of the villi of the chorion laeve, the placenta is a discoidal plate formed 

 from one part only of the chorion (i.e. chorion frondosum), intimately united with 

 the decidua basalis. 



