CLASS MAMMALIA 445 



in which they live, and lizards and snakes hide in crevices in rock ledges 

 or crawl into holes in the ground. No birds hibernate. While many 

 mammals remain active in the winter, protected by their heat-conserving 

 covering of fur and subcutaneous fat and find a sufficient supply of food to 

 meet their needs, others hibernate during all or a part of that season. 



In preparation for hibernation a mammal becomes very fat, storing 

 up a supply of heat-producing food. During true hibernation the 

 temperature of the animal falls, frequently to within a few degrees of 

 freezing; respiration becomes very slow and shallow; the heart beats 

 slowly, and the circulation is sluggish; in fact all metabolism is carried 

 on at a very slow rate, and the temperature-regulating mechanism is 

 temporarily suspended. In other words the organism becomes for 

 the time cold-blooded. The muscles are rigid and the animal is uncon- 

 scious. The mammals of this country that may undergo true hibernation 

 include jumping mice and pocket mice, woodchucks, ground squirrels 

 (Fig. 342), chipmunks, and bats. 



Some mammals, such as the skunks, badgers, and raccoons, do not 

 undergo a tme hibernation but only spend the time in their winter 

 quarters in prolonged and profound sleep. Still others carry on various 

 activities during that time. It is while in their winter dens that female 

 bears bring forth their very small and partially developed young. 



449. Reproduction. — In all mammals fertilization is internal, the 

 sperm cells being introduced into the oviduct by copulation. The 

 egg cell of the mammal is small and possesses only a limited amount 

 of yolk. The development of the monotremes is essentially hke that 

 of reptiles and birds. The mammals of the next group, the marsupials, 

 retain the embryo for a certain period of time within the uterus, though 

 it does not become attached by a placenta such as is found in all remaining 

 mammals. 



Cleavage is apparently total and approximately equal. The egg cell 

 thus appears to be holoblastic although there are details in the develop- 

 ment which seem to indicate that it has been modified from, a meroblastic 

 type. The egg cell divides first into two and then into four cells, equal 

 in size and normally arranged (Fig. 343). As cleavage continues, 

 however, the cells shift about and finally a structure is formed which 

 consists of an outer layer of cells, called the trophoderm, and an in7ier 

 cell mass. Gradually the trophoderm and the inner cell mass become 

 separated by a cavity filled with fluid and corresponding to a blastula 

 cavity, but the two remain in contact at one pole of the vesicle. The 

 trophoderm becomes attached to the wall of the uterine cavity and from 

 it finger-like projections or papillae grow into the uterine mucosa; these 

 serve to anchor it firmly to the wall of the uterus. 



From the inner cell mass is developed not only the entire embryo 

 but also the amnion, chorion (in part), allantois, and yolk sac. In what 



