256 



Experimental Zoology 



Man is taken as the standard, and the time required by the other 

 forms to double their birth rate is given in fractions of that of 



man. 



It will be seen that the rate of growth is proportional to the 

 amount of albumen in the milk. Thus the kitten doubles its 

 weight in 1/33 of the time required by the human infant, and the 

 milk contains five times as much albumen. It is improbable 

 that the results are due solely to this factor, but that an internal 

 factor largely regulates the growth of the young. Were this not 

 the case, we should find that a baby fed on sheep's milk would 

 doubt its weight eighteen times as fast as a baby fed on human 

 milk. 



Stimulants ajfecting Growth 



I have already stated that the rate of growth is not simply a 

 question of the amount or of the kind of food. In fact, growth 

 depends in some cases on a response to a stimulus^ and the amount 

 of food stuffs converted into tissues sometimes depends on the 

 presence of a stimulus. For example, the embryo in the uterus 

 of the mammal appears to supply such a stimulus. Many 

 chemical substances " that are not themselves food may stimulate 

 the growth processes." Just as certain poisons accelerate the 

 movements of animals, so may they also accelerate metabolic 

 processes and lead to increased growth. "Schultz found that 

 various poisons, such as corrosive sublimate, iodine, bromine, 

 arsenious acid, increase the activities of yeast in fermentation. " ^. 



^ Davenport; "Experimental Morphology." 



