External Factors that Influence Growth 255 



continue to supply nourishment to the young in the uterus ; and 

 while the embryos might not grow as fast as do those in a well- 

 nourished individual, they would probably grow at least as long 

 as reserve material was present in the mother, and perhaps even 

 while the formed tissues of the mother were slowly being used up. 

 It would be interesting to carry out a series of exact experiments 

 of this kind on such animals as rats, mice, or dogs especially 

 on forms that can go a long time without food. 



These conditions show plainly that growth depends on other 

 factors as well as on the amount of food that an animal obtains, 

 or has in reserve. 



Not only the amount of food but the kind of food also has 

 an important bearing on the rate of growth. Yung fed tadpoles 

 on several kinds of food, including aquatic plants, the jelly of 

 frogs' eggs, yolk of hens' eggs, white of hens' eggs, fish, and beef. 

 The results are shown in the following table : 



Fed on beef the tadpoles grew nearly three times as fast as on 

 plant food. The experiments were continued, in some cases, 

 to the time of metamorphosis. The large tadpoles transformed 

 first, but the smaller ones not until they had grown large. 

 Cuenot has also reared tadpoles under different conditions, and 

 has recorded the time of their metamorphosis. He also found 

 that the well-fed animals transformed first. Barfurth has shown, 

 on the other hand, that starvation just prior to the time of meta- 

 morphosis hastens the change. 



Experiments made by Lawes and Gilbert have shown that 

 "growth tends to increase with the quantity of nitrogeneous 

 food." A curious instance confirmatory of this conclusion is 

 given by Prosher. He compared the times required by different 

 mammals to double their birth rate with the composition of the 

 milk of the mother. His results are given in the following table. 



