292 Cell as Basis oj Organic Activity 



Among many mammals, the allantois is important in forming the 

 fetal portion of the uterine attachment, the placenta. In man, however, 

 the allantois is rudimentary. 



The Placenta. — All higher mammals form a complex attachment 

 between the developing embryo and the maternal uterus. This struc- 

 ture, the placenta, is formed of the allantois and chorion and portions 

 of the uterine wall. In man, the allantois plays no important role, but 

 in many of the carnivores it is very large. The shape and position of 

 the placenta vary widely in different mammals. In man, however, it 

 is a disc-shaped structure. The surface of the chorion is covered with 

 numerous fingerlike villi which intermingle with the folds of the uterine 

 wall. Blood vessels from the embryo are in these villi and are in close 

 contact with the blood vessels of the uterus. By diffusion, food and 

 oxygen pass into the embryo and waste materials are removed. There 

 is no actual contact between the blood of the mother and that of the 

 embryo. As the embryo grows, it remains attached to the placenta by 

 the long umbilical cord. The umbilical cord has an outer wrapping from 

 the enveloping amnion and within it are found the allantois, the yolk 

 sac, and the umbilical blood vessels. In human beings, the umbilical 

 cord is about two feet long at birth. 



Parturition. — Parturition or birth of young marks the end of 

 interuterine development among the higher mammals. The length of 

 time intervening between fertilization and parturition varies among the 

 different mammals. This gestation period for some representative mam- 

 mals is as follows : 



At the time of parturition, the uterus undergoes a series of strong 

 contractions which bring about dilation of the cervix and rupture of 

 the amniotic sac. This latter also serves as a hydrostatic wedge in 

 aiding the dilation of the cervix. These contractions of the uterus 

 finally result in the expulsion of the fetus. Following this, the uterus 

 is greatly reduced in size, and a second series of contractions which 

 expel the embryonic membranes soon follow. These membranes, chiefly 

 consisting of the placenta, are known as the "afterbirth." 



