FCETAL ENVELOPES 



38s 



centa). In a few carnivores the conditions are intermediate be- 

 tween the discoidal and the zonary types, the zone being incomplete, 

 with a gap of some extent. 



It should be noted that in several forms with an allantoic placenta (horse, 

 carnivores, rodents, and insectivores) there is also present a more or less well- 

 developed vitelline placenta. In the case of the placenta vera there may be 

 merely an invasion of the connective tissue by the chorionic villi, or the walls 

 of the uterine blood-vessels may break down, so that the villi extend into the 

 blood stream of the mother. In the primates the chorion at first has villi all 

 over the surface, but later they are restricted as described above, so that there 



Fig. 406. — Schematic representation of the placenta and embryo of the rabbit, 

 after Bonnet, am, amnion; al, allantois; b, blood sinuses; c, chorion;//*, foetaljplacenta; 

 mp, maternal placenta; ms, muscular waJl of uterus; tnc, mucosa of uterus; s, sinus ter- 

 minalis; u, umbilcial cord; y, yolk sac. 



are recognized two regions, the chorion frondosum, with villi, and the chorion 

 laeve, where they are absent. 



That portion of the uterine wall which, in animals with a placenta vera, is 

 cast off at birth is called the decidua. In man and the higher apes the embryo 

 and its envelopes become enclosed (fig. 405, 0) in the uterine wall, and in this 

 case the portion which bounds the embryo toward the free surface is called the 

 decidua capsularis (or reflexa), that opposite, the decidua serotina, while the 

 rest of the mucosa is the decidua vera. In animals with a true placenta there 

 would be extensive hemorrhage at the time of parturition, were it not that the 

 contraction of the uterine muscles closed the blood-vessels. After birth, the 

 lining epithelium of the uterus is restored by outgrowths from the bottoms of 

 the uterine glands which are not cast off with the decidua. 



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