432 



MAMMALIA. 



known as the decidua serotina (6), and that part of the uterine 

 wall not related directly to the vesicle, the decidua vera (c). 



The chorion is directly attached to the decidua reflexa and 

 serotina. The vascular connection between the reflexa and the 

 chorion atrophies, whilst that between the serotina and the 



uv- 



FlG. 205. — DiAGBAMMATIC SectIOIT OF PREGNANT HUMAN UTERUS WITH 

 CONTAINED FCETUS. 



(Z, Allantoic stalk ; uv, umbilical vesicle ; am, aiunion ; eft, chorion ; b, decidua sero- 

 tina ; c, decidua vera ; /, decidua reflexa ; Fl, Fallopian tube ; g, cervix uteri ; a, fcetal 

 villi of placenta ; e, embryo. (From Huxley, after JJonget.) 



chorion increases, the villi becoming much branched and form- 

 ing the chorion frondosum. This latter structure and the decidua 

 serotina form the true placenta. This placenta is discoidal, but 

 it is ventral, whereas the placenta of the rabbit is dorsal. At 

 birth the placenta and the fused decidua vera and reflexa are 

 all shed, and the ruptured blood-vessels of the uterine wall 

 closed by its rapid contraction. 



3. The zonary placenta. — Here the placenta is in the form of 

 a broad band. The yolk-sac never fuses with the chorion. At 

 first the allantois fuses with the sub-zonal membrane, at one 



