THE MAMMALS 



427 



inhale air. Mammals also use chest expansion for respiration, as is 

 done in the reptiles and birds. 



Warmblooded. Like the birds, the mammals maintain a constant 

 high body temperature. The rate of metabolism is rather high, and 

 this generates the necessary heat which holds the body temperature at 

 an even, high level when outside temperature fluctuates. 



Complex Teeth. The teeth in those lower vertebrates possessing 

 them are usually just a row of cone-like projections, with a few excep- 

 tions, such as the fangs of poisonous snakes. In mammals, the teeth are 



allantois 



Umbilical 

 cord 



Placenta 

 Amnion 



Amniotic 

 fluid 



Uterus 



Fig. 28.1. Developing embryo of a mammal. Note that the yolk sac is much smaller 

 than in a reptile or bird and the allantois is vestigial and forms a part of the umbilical 



cord. 



specialized to accomplish specific functions. There are incisors for cut- 

 ting; canine, for tearing; pre-molars and molars for grinding. These 

 are developed in varying degrees in different mammals according to dif- 

 ferent feeding habits. 



Placenta. All but the two most primitive orders of mammals have a 

 placenta, which is an organ that grows out from a developing embryo 

 and becomes firmly attached to the uterus of its mother and absorbs food 

 and oxygen from the blood of the mother. When the mother nourishes 

 the embryo in this way it is unnecessary to have a large amount of yolk 



