REPRODUCTION 83 



blood-vessels of the embryo. This allantoic circulation in a reptile or 

 bird provides for respiration (see pages 33, 34). 



Before the time of hatching the shrinking yolk-sac is drawn up into 

 the growing body. The umbilical stalk — that is, the whole complex of 

 connexions between the definitive body of the embryo and the extra- 

 embryonic membranes — becomes narrowly constricted. At time of 

 hatching the amnion and the slender neck of the allantois are ruptured 

 at the umbilicus. As the young animal emerges, the amnion and chorion 

 and the extra-embryonic part of the allantois are abandoned. The 

 proximal portion of the allantois, remaining within the body, becomes 

 enlarged and serves as the urinary bladder of such adult reptiles as 

 possesses that organ. In birds, the adult having no urinary bladder, the 

 proximal remnant of the allantois degenerates. 



Among mammals there is some diversity as to the manner of origin 

 of the amnion and chorion. Once established, however, these membranes 

 possess the same relations to the germ layers and to the definitive body 

 of the embryo as in reptiles. 



The main facts concerning the development of a placenta, by the 

 chorio-allantoic membrane have already been stated (see page 35). The 

 highly vascular vilU produced by the chorio-allantoic membrane (Fig. 75) 

 may be merely lodged in depressions in the uterine wall or they may 

 pierce more or less deeply into its tissues. In extreme cases (e.g., in man) 

 there is destruction of walls of uterine blood-vessels and the extravasated 

 blood fills large sinuses in the uterine wall. The villi project into these 

 sinuses so that the villous surfaces are directly bathed by maternal blood, 

 an arrangement providing maximum efficiency in the exchange of materials 

 between fetal and maternal blood. 



Mammals exhibit various types of placenta, depending on the dis- 

 tribution of villi in the chorionic surface. When the villi are uniformly 

 distributed over the chorion, as in the horse, pig and other ungulates, 

 the placenta is called diffuse. In most ruminant ungulates, such as 

 cattle, the villi are localized in numerous patches or clusters of varying 

 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 fetus — the zonary placenta 

 (Fig. 76). 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 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 



