242 THE UROGENITAL SYSTEM 



constitute the "after-birth." The placenta usually is everted so that its amniotic 

 surface is convex, its maternal surface concave. It is composed of the amnion, 

 chorion frondosum, villi with intervillous spaces incompletely divided by the 

 septa into cotyledons and includes on the maternal side the basal plate and a por- 

 tion of the spongy layer of the decidua basahs. The amnion is usually attached 

 to the chorion, but the membranes may rupture in such a way that the child is born 

 enveloped in the amnion, the part covering the head being known popularly as 

 the "caul." Near the center of the placenta is attached the umbilical cord, and 

 at its margins the placenta is continuous with the decidua vera and the remains 

 of the chorion lasve and decidua capsularis. 



The Position of the Placenta in Utero and its Variations. — The position of 

 the placenta is determined by the point at which embryo is implanted. In 

 most cases it is situated on either the dorsal or ventral wall of the uterus. Oc- 

 casionally it is lateral in position and very rarely (1 in 1600 cases) it is located 

 near the cervix and covers the internal os uteri, constituting a placenta prcevia. 

 A partially or wholly dupUcated placenta or accessory (succenturiate) placentas 

 may be formed from persistent patches of viUi on the chorion Iseve. Cases have 

 been observed in which from three to seven subdivisions of the placenta occurred. 



