158 THE BIOLOGY OF THE FROG CHAP. 



If the liver of a frog be left for some hours before boiling 

 and then tested for glycogen, it will be found that the 

 amount of this substance obtained is comparatively small, 

 and if appropriate tests be applied, it may be shown that a 

 certain amount of dextrose has appeared in its stead. The 

 liver contains a ferment which has the power of converting 

 glycogen into dextrose ; as the ferment is destroyed by 

 boiling, a greater amount of glycogen can be obtained from 

 the liver if it is boiled soon after it is removed from the 

 body. 



The glycogen content of the liver not only increases in 

 the fall and decreases in the spring and summer, but it 

 undergoes changes in relation to variation in the amount of 

 food, and to changes of temperature of short duration. 

 After feeding there is a slight increase in the amount of 

 glycogen in the liver ; this slowly disappears if the frog is 

 kept several days without food. In winter, if frogs in which 

 the liver is well filled with glycogen be kept for a few days 

 in a warm room, the glycogen content of the liver rapidly 

 decreases. On the other hand, if summer frogs, which gen- 

 erally contain little glycogen, be kept at a low temperature 

 for several days, the amount of glycogen in the liver becomes 

 markedly increased. 



The glycogen stored in the liver may be given out slowly 

 into the blood in the form of dextrose, into which it is 

 changed by an enzyme in the hepatic cells. The liver acts 

 as a sort of reservoir of food, storing it up in a comparatively 

 insoluble form when it is in excess, and expending it gradu- 

 ally to tide over periods of fasting. The frog begins its long 

 period of hibernation with a large reserve supply of this 

 material, which is slowly used up during the winter and 

 more rapidly consumed in the early spring. 



While glycogen occurs in greatest abundance in the liver, 



