294 



Basic Structure of Vertebrates 



sizes — the cotyledonary placenta. In carnivores the placenta usually 

 takes the form of a broad band or zone encircling the chorion at a 

 position about midway between head and tail of the fetus — the zonary 

 placenta (Fig. 242). A discoidal placenta, in which villi are restricted 

 to a single relatively large area of the chorion, occurs in insectivores, 

 bats, rodents, and higher primates, including man. 



A fetal placenta whose villi do not penetrate deeply into the uterine 

 wall separates from it readily and without loss of uterine material. 

 Such a placenta, called nondeciduate, occurs in most ungulates, in 

 the whale and dugong, and in lemurs. When, however, the fetal villi 

 are deeply embedded in the uterine wall, at time of birth the involved 

 layer of the uterus is split off and discharged with the fetal placenta. 



Fig. 242. Fetus of cat, removed from uterus without 

 rupturing chorionic sac (C), showing zonary distribution 

 of placental villi. (Courtesy, Neal and Rand: "Chordate 

 Anatomy," Philadelphia, The Blakiston Company.) 



This deciduate condition occurs in carnivores, in the elephant, and 

 commonly in animals having a discoidal placenta. 



In marsupial mammals the chorio-allantoic membrane forms at most 

 (in the bandicoot, Perameles) only a very limited vascular area. Usually 

 there is merely contact of a nonvascular chorion and the uterine wall 

 without villi or other connection. It is a noteworthy fact that in the 

 "Australian cat," Dasyurus, the splanchnopleure of the yolk-sac joins 

 the chorion and produces a placenta-like vascular area in relation to 

 the uterine wall. It is possible that in early mammals both yolk-sac 

 and allantois were potentially placenta-forming. In higher primates, 

 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. 



The mammalian umbilical cord is the much-elongated and atten- 

 uated connection between the body of the fetus and the extra-fetal 

 membranes. It contains somatopleure extending from fetal body-wall 



