DEVELOPMENT OF OTHER VERTEBRATES 435 



are also present in the mammal, but they are modified as the 

 embryonic portion of the placenta. The presence of a yolk stalk 

 and a greatly reduced yolk sac, containing no yolk, completes 

 the resemblance. It therefore appears that the higher mammals 

 possess the embryonic membranes and yolk sac of birds and rep- 

 tiles, although these structures are modified to suit the peculiar 

 conditions of mammalian development. These facts, along with 

 the actual existence of mammals that lay eggs, lead zoologists to 

 believe that the ancestors of the higher mammals once laid eggs 

 like those of birds and reptiles, and that their earlier mode of 

 development has become modified in the course of their evolution. 

 If this be true it shows that the embryonic stages of animals, no 

 less than their adult structures, may undergo profound evolution- 

 ary changes. 



If we attempt, in conclusion, to picture the probable evolution 

 of the mode of development in reptiles, birds, and mammals, it 

 appears that the aquatic ancestors of these animals laid eggs, some- 

 what in the manner of the Amphibia of the present day. With the 

 specialization of the adult for terrestrial conditions, there was 

 developed an egg protected against evaporation by a shell, and 

 the embryo came to be surrounded by membranes which served 

 as adaptations to this new mode of development. In the evolu- 

 tion of higher mammals, the yolk disappeared in favor of another 

 method of nutrition, which insured a greater degree of protection, 

 since the embrj^o could develop within the body of the parent. 

 Just as the bird cares for its young after hatching, so in the Mam- 

 maUa there is a period of infancy during which the young are fed 

 and tended by the parent until better able to care for themselves. 

 The culmination of such an evolution in the mode of development 

 is seen in species like the elephant, with its long period of associa- 

 tion between parent and young, and finally in the human family. 



