THE FCETAL MEMBRANES OP MAN. 



249 



part, which has become overgrown by the decidua reflexa (fig. 143). 

 While on the latter the villi (z') cease to grow, on the former they 

 increase enormously in size and take the form of long, and at the 

 base thick, tree-like, branching structures (z), which, united into 

 tufts, project 

 far beyond the 

 surface of the 

 membrane that 

 bears them, 

 and grow into 

 pits of the ma- 

 ternal mucous 

 membrane (ds). 

 This part, to 

 which we shall 

 give more par- 

 ticular atten- 

 tion at the 

 time of inves- 

 tigating the 

 mature pla- 

 centa, is there- 

 fore distin- 

 guished as 

 chorion fron- 



dosum from the remaining larger part, the chorion Iceve 

 smooth chorion. 



The expression " smooth chorion " is, strictly speaking, not quite 

 applicable. Of the villi which are at first everywhere developed, 

 some afterwards remain preserved on the chorion laeve, especially in 

 the vicinity of the placenta. They grow into the decidua reflexa, 

 effecting a firm union with it (fig. 143 z'). 



At the same time a second distinction between chorion frondosum 

 and chorion laeve is developing. In the territory of the latter the 

 blood-vessels arising from the umbilical arteries begin to dwindle, 

 whereas the former becomes more and more abundantly supplied 

 with blood-vessels, and finally alone receives the terminal distribution 

 of the umbilical arteries. Thus the one region becomes destitute of 

 vessels, while the other becomes extraordinarily vascular, and the 

 nutritive organ for the embryo. 



Histologically the chorion laeve, which upon examination from the 



Fig. 143. Diagrammatic section through the gravid human uterus with 



contained embryo, after LONGET, from BALFOUR. 

 al, Stalk of the allantois ; rib, umbilical vesicle ; am, amnion ; h 



chorion ; ds, decidua serotina ; du, decidua vera ; dr, decidua 



reflexa ; I, Fallopian tube ; c, cervix uteri ; u, uterus ; z, villi of 



the foetal placenta ; sf, villi of the chorion laeve. 



the 



