GENERAL ZOOLOGY 



Fig. 5.26. Human fetus in uterus 

 with chorion and amnion pulled 

 away; the umbilical cord attaches 

 fetus to the placenta above. The 

 crown-rump length of the fetus is 

 61 mm., and its age is 10 to 11 

 weeks. (Photograph courtesy Car- 

 negie Institution of Washington, 

 Department of Embryology.) 



The problem of meeting the metabolic requirement.s is much more com- 

 plicated for the embryo of an egg-laying terrestrial vertebrate. Certain 

 embryonic membranes always develop in such forms. They persist only dur- 

 ing development and serve to prevent drying, furnish food and oxygen, and 

 eliminate waste products (Fig. 5.24). A yolk sac is formed in reptiles and 

 birds, as in fishes, and also appears as a vestigial structure in the development 

 of mammals. In addition, the amnion with its enclosed amniotic fluid keeps 

 the embryo moist and provides a protective cushion, and the chorion forms 

 a protective membrane ne.xt to the shell of the egg of the reptile or bird. 

 A fourth membrane, the allantois, is richly supplied with blood and lies 

 next to the chorion. The allantois is a membrane which functions in gas ex- 

 change and excretion; it absorbs oxvgen and eliminates carbon dioxide, both 

 of which pass through the porous shells of reptiles' and birds' eggs. 



Among the mammals the problem is again somewhat different. Some 

 mammals lay yolky eggs as do the reptiles and birds; others store a certain 

 amount of yolk in their eggs. The opossum, for instance, gives birth to 

 very immature young which spend a considerable period in an abdominal 

 brood pouch, nourished by milk from the mammary glands (see Fig. 18.31C, 

 p. 583). In the majority of mammals, however, practically no food is stored 

 in the egg, and development is completed within the uterus. This is made 

 possible by the embryonic membranes, which are somewhat altered in func- 

 tion. During the early part of the period of cell localization the mammalian 

 embryo becomes closely associated with the lining of the uterus. The amnion 



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