FACTORS INFLUENCING RATE OF METABOLISM 47 



ties, and snakes are apparently as effective for digestion 

 as those of homoiotherms and show their greatest degree 

 of activity at about the same temperature, 37 deg. C. 

 (Miiller, 1922; Kenyon, 1925). The action of such sub- 

 stances as thyroidin, epinephrin, and insulin is appar- 

 ently quite similar throughout the series of chordate 

 animals. There are here and there exceptions to these 

 general statements concerning dietary requirements, 

 enzymes, and hormones, but they do not on the whole 

 seem to point to any essential diiference between poikilo- 

 therms and homoiotherms. For example, vitamin defi- 

 ciency will not cause scurvy in rats, but pigeons and rab- 

 bits are quite subject to such disease. Magath and Mann 

 (1923) believe that the metabolism of carbohydrates is 

 perhaps different in homoiotherms and poikilotherms 

 because they found that after complete excision of the 

 liver the giving of glucose would restore a dog or a 

 goose somewhat, but had no effect on a gar, frog, or 

 turtle. In any vertebrate the mass of protoplasmic sub- 

 stance determines the height of metabolism and the rate 

 and amount of heat production. The rate of metabolism 

 in poikilotherms is in general correlated with the tem- 

 perature at which it takes place, but in homoiotherms it 

 usually takes place at a rather constant and compara- 

 tively high temperature. Enzymes and hormones may 

 modify the rate, but they are not the fundamental cause 

 of it. 



Desiccation. — ^Lack of available water may retard the 

 rate of metabolism. Arthropods and vertebrates com- 

 monly prepare for hibernation or aestivation by losing 

 water. A dry rotifer may live in a dormant state for 

 months and, when wet again, complete its life cycle of a 

 few weeks. Desiccation probably causes poor elimina- 

 tion of wastes and inadequate distribution of food and 



