4.24 



THE MAINTENANCE OF SPECIES 



brain 



^pirzccL CarcC 



digestive, 

 •trcxct/ 



the yolk sac. Gradually 

 blood vessels develop 

 in the mesenchyme of 

 the yolk sac, facilitat- 

 ing the transportation 

 of food to the develop- 

 ing embryo. 



Amnion and Chorion 



In addition to the 

 protection afforded by 

 egg membranes or shells 

 and the yolk sac, the 

 higher vertebrates, 

 namely, the reptiles, 

 birds, and mammals, 

 elaborate additional 

 embryonic membranes 

 that serve not only as 

 supplementary protec- 

 tive devices to keep the 

 embryos from mechan- 

 ical injury but also tem- 

 porarily handle the problems of respiration, excretion, and nutrition. 

 In order to understand their functions, and the fact that their 

 evolution is intimately tied up with that of the land-inhabiting 

 reptiles, birds, and mammals, one must trace their embryological 

 development. 



As long as organisms returned to the water during the breeding 

 season, as the amphibians still do, the exchange of gases and elimination 

 of wastes takes place directly, since the surrounding water not only 

 contains sufficient dissolved oxygen but also it soon dissipates waste 

 products which are passed through the egg membranes and elimi- 

 nated. With the acquisition of a land habitat, the inability to return 

 to the water to spawn presented new problems, centering about the 

 control of metabolism in the embryo. These needs were met through 

 the elaboration of a series of embryonic membranes, which were 

 apparently developed to facilitate the carrying on of normal metabolic 

 processes through a permeable egg shell. They occur in modified 

 forms in all land vertebrates. 



SomotopW 



5plar2diT7op'l<aure 



Diagram of a developing fish embryo. Note the 

 " contained " yolk sac. What is its ultimate fate ? 



