144 THE AMINO ACIDS 



sharply between a maintenance diet and one capable 

 of promoting growth. They have shown, for example, 

 that a young animal may be maintained on a certain 

 diet indefinitely without manifesting any tendency .to 

 grow. From the work of Donaldson it has been dem- 

 onstrated that the life span of the white rat is about 

 three years. Sexual maturity is reached in sixty 

 days. The first year of life for the rat corresponds 

 to the first thirty years of human life, and the curve 

 of growth for this period is reproduced below. Fig. 2. 

 As an illustration of the influence of an isolated 

 protein, casein (fed with starch, sugar, agar, lard, 

 and a salt mixture), the chart, Fig. 3, is shown. It 

 is evident that casein as the sole source of nitrogen 

 was apparently incapable of allowing normal growth 

 in a young rat during a period of forty-six days. In 

 other words, stunting occurred. In period 2, casein 

 and sugar were replaced by milk. Growth was 

 resumed. The influence of changing the salt mixture 

 content of the food intake is quite evident in periods 



3, 4, and 5. The ability of milk to furnish the necessary 

 nitrogen requirement is well shown in the chart, Fig. 



4, the curve obtained being to all intents and purposes 

 identical with the normal growth curve. 



If to the casein diet "protein-free milk" is added, 

 instead of whole milk replacing casein, normal condi- 

 tions obtain as is well illustrated in the chart, Fig. 5. 



Casein alone was found to be .unable to support 

 growth. In Fig. 6 is shown a curve in which, during 

 period 2, casein was the only source of protein and 



