84 CHORDATE ANATOMY 



Uterus is split ofif and discharged with the fetal placenta. This deciduate 

 condition occurs in carnivores, in the elephant, and commonly in animals 

 having a discoidal placenta. 



In certain marsupials (Dasyurus) it is the splanchnopleure of the 

 yolk-sac which joins the chorion and forms a placenta-like vascular area 

 which is apposed against the uterine wall. Possibly in early mammals 

 both the yolk-sac and the allantois were potentially placenta-forming. 

 In higher primates, the allantoic sac is rudimentary and the fetal portion 



Fig. 76. — Fetus of cat, removed from uterus without rupturing chorionic sac (C), 

 showing zonary distribution of placental villi. 



of the placenta is of chorionic origin only; yet the allantois develops far 

 enough to bring its blood-vessels into connexion with the chorionic 

 vessels of the placenta. 



The umbilical cord is the much elongated and attenuated connexion 

 between the body of the fetus and the extra-fetal membranes. 



At time of birth the amnion and chorion are ruptured and the young 

 mammal is expelled, along with the amnionic fluid, by muscular contrac- 

 tion of the uterine walls. The amnion, chorion, allantois, fetal placenta, 

 and more or less uterine tissue in a placenta of the deciduous t>^e are dis- 

 charged later as the "after-birth." The umbilical cord is severed. 

 That portion of the allantois remaining within the body becomes the 

 urinary bladder. 



