682 COLLEGE ZOOLOGY 



This, later, becomes coated wholly or in part on its inner side 

 by somatic mesoblast, and constitutes the membrane known as 

 the subzonal membrane. . . . Later on, the mesoblast of the 

 peripheral part of the allantois becomes applied to the subzonal 

 membrane and the two structures constitute the embryonic 

 membrane called the chorion. . . . The chorion" develops vas- 

 cular villi which enter into close relation with the uterine wall. 

 In this manner there is developed a relatively large surface, 

 permeated with branches from the foetal vessels, the blood of 

 which is in intimate osmotic connection with the blood of the 

 uterine wall. This connection of the chorion of the foetus with 

 the uterine walls gives rise to the placenta, by means of which 

 the nourishment and respiration of the foetus are provided for 

 in the body of the mother. . . . The placenta presents great 

 variations, in the individual orders, in its special development 

 and in the mode of its connection with the uterine walls." 

 (Sedgwick.) 



d. Hibernation 



The problem of maintaining life during the winter is solved 

 by most birds by migrating. Mammals, on the other hand, 

 usually remain active, like the rabbit, or hibernate. During 

 hibernation the temperature of the body decreases and the ani- 

 mal falls into a profound torpor. A cold-blooded animal, like 

 the frog, can be almost entirely frozen without being injured, 

 but warm-blooded animals must protect themselves from the 

 cold; they therefore seek a sheltered spot, such as a burrow in 

 the ground, in which to spend the winter. Furthermore, at this 

 time the fur of mammals is very thick and consequently helps 

 to retain the body heat. 



The temperature of the body of hibernating animals becomes 

 considerably lower than normal ; for example, a ground squirrel 

 which hibernated in a temperature of 35.6 F. had a body 

 temperature exactly the same. (Semper.) Respiration almost 

 ceases; the heart beats very slowly; and no food is taken into 



