REPRODUCTION 1 21 



The terms deciduate and non-deciduate, as applied to the placenta, 

 refer to conditions which obtain at time of birth. A fetal placenta whose 

 villi do not penetrate deeply into the uterine wall separates from it readily 

 and without loss or destruction of uterine material. Such a placenta, 

 called non-deciduate, occurs in most ungulates, in the whale and dugong, 

 and in lemurs. When, however, the fetal villi are deeply imbedded in the 

 uterine wall, at time of birth the involved layer of the uterus is split off 

 and discharged with the fetal placenta. This deciduate condition of the 

 placenta occurs in the carnivores, in the elephant, and commonly in 

 animals having a placenta of the discoidal form. 



In marsupial mammals few or no villi are produced on the chorion 

 and there is merely contact of chorion and uterine wall without actual 

 connection. Usually, too, the chorion does not become vascular. It is a 

 noteworthy fact, however, that in some marsupials (Perameles) the 

 allantois, in conjunction with the chorion, forms a limited vascular area 

 which becomes closely related to the uterine wall, while in others (Dasy- 

 urus) 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. These facts suggest that in early mammals both the yolk-sac 

 and the allantois were potentially placenta-forming. In higher Primates, 

 however, the allantoic sac is rudimentary and the fetal portion of the 

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

 to bring its blood vessels into connection with the chorionic vessels of 

 the placenta. 



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 type are 

 discharged later as the "after-birth." The umbilical cord is the much 

 elongated and attenuated connexion between the body of the fetus and 

 the extra-fetal membranes. It includes somatopleure, extending from 

 the fetal body wall to the amnion, and the stalks of the yolk-sac and 

 allantois with their vitelline and allantoic blood vessels. During later 

 fetal life the yolk-sac degenerates. Its cavity is obliterated, its blood 

 vessels close and it becomes reduced to a slender solid cord contained 

 within the umbilical cord. After birth the umbilical cord is, in one way 

 or another, severed and such remnant of it as remains attached to the 

 body shrivels up and is sloughed off at the navel. That portion of the 

 allantois remaining within the body becomes the urinary bladder. 



Functions of the Embryonic and Fetal Membranes. The amnion 

 is protective in mechanical ways. It provides a fluid medium in which 

 the embryo is suspended and so protected from deforming pressures and 

 impacts. The amnionic mesoderm produces unstriated muscle fibers 



