New York Zoological Society 



A VIVIPAROUS SNAKE 



In the garter snake and in some other species, the fertilized eggs remain within the 

 body of the mother until the young are developed to the adult form 



Most reptiles and some birds leave the eggs, which are hatched by the heat 

 of the sun or at ordinary temperatures. Most of the common birds, however, 

 build more or less elaborate nests and keep the eggs warm during hatching. 

 And most birds care for the fledglings, as well as the eggs. In many species the 

 young learn their "song" from the parents or other older birds. Domesticated 

 canaries are sometimes trained as songbirds, or at least protected against un- 

 desirable noises, since they are very imitative. The feeding of young birds 

 by the parents is a very interesting operation to watch, as it shows the de- 

 velopment of rather complex instincts. 



Infancy among Mammals We consider the mammals the highest class 

 of vertebrates — and indeed of all living things. In this class the dependence 

 of the young upon their parents is carried to the greatest extreme. In the 

 entire class the female develops special milk glands — hence the name, from 

 mammae, the breasts or teats. Related to the presence of the milk gland is the 

 infant's equipment of nerve-and-muscle sucking mechanism, and his complete 

 dependence upon the milk supply for his nourishment. 



In the marsupials the egg hatches within the uterus of the mother, as in 

 the "true mammals", but the fetus leaves at a rather early stage — in the 



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