Reproduction 295 



to amnion (Fig. 241), and the stalks of the yolk-sac and allantois with 

 their vitelline and allantoic blood-vessels. 



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

 young mammal is expelled, along with the amnionic fluid, by muscular 

 contraction of the uterine walls. The amnion, chorion, extra-fetal part 

 of the allantois, fetal placenta, and more or less uterine tissue in a 

 placenta of the deciduous type are discharged later as the " afterbirth." 

 The umbilical cord is severed. That portion of the allantois remaining 

 within the body undergoes degeneration except that more or less of its 

 proximal region may persist as the distal portion of the urinary bladder. 



Functions of Embryonic and Fetal Membranes. The amnion 

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

 the embryo is suspended and protected from deforming pressures. The 

 amnionic mesoderm produces nonstriated muscle-fibers which render 

 the amnion contractile. Its gentle undulations keep the fluid moving, 

 thus preventing local stagnation and possible adhesions of embryonic 

 parts. 



The chorion, in conjunction with the allantois (rarely the yolk- 

 sac) serves to produce nutritive, respiratory, and excretory membranes. 



The allantois (in conjunction with the chorion) of reptiles and 

 birds is the respiratory organ. In viviparous reptiles its circulation may 

 pick up some nutriment and oxygen from the uterine wall or fluids. 

 The allantoic cavity is a receptacle for waste from the embryonic 

 kidneys. Its proximal or coelomic portion, after hatching or birth, per- 

 sists to greater or less extent (varying in different animals) as the 

 distal portion of the urinary bladder of reptiles and mammals. Or, 

 instead of discussing the possible part played by the allantois in form- 

 ing the urinary bladder, it may come nearer the truth to regard the 

 allantois as merely an exaggerated urinary bladder, developed pre- 

 cociously, and temporarily subserving some vital needs of the embryo. 



In mammals the allantois is the essential part of the placenta. 

 The placental villi are primarily chorionic, but their vascular struc- 

 tures are derived mainly or entirely from the allantois. In the placenta 

 are concentrated all the vital functions, nutrition, respiration, and, at 

 least in part, excretion. 



The vascular splanchnopleural wall of the yolk-sac is the essential 

 nutritive membrane in reptiles, birds, and primitive mammals. In some 

 early mammals the yolk-sac may have produced a placenta. In later 

 mammals it is vestigial. 



Summary and Some Evolutionary Implications 



Cleavage is a process of mobilizing the agencies within the fertilized 

 egg, always a relatively large cell. Metabolism, necessarily involving 



