72 



GROWTH 



The most recent study of this nature is one reported by Rose 

 and Cox. 2 They used rats as experimental animals, and supplied 

 them with more or less conventional "synthetic" diets. The ni- 

 trogenous material was supplied in three forms: first, pure 

 casein j second, casein that had been completely hydrolyzed; 

 third, hydrolyzed casein from which the amino acids, arginine 

 and histidine, had been removed. We will only say in regard to 

 this report that the animals receiving casein grew rapidly, and 



Figure 46. Zein lacks two essential amino acids, tryptophane 

 and lysine. After Osborne and Mendel. By permission of the 

 Journal of Biological Chemistry. 



those on hydrolyzed casein from which arginine and histidine 

 had been removed did not grow at all. The addition of arginine 

 did not improve the diet, but the addition of histidine enabled 

 the amino acid mixture to support growth. We may then pro- 

 visionally place histidine in our list of essential amino acids. 



This problem has been attacked in a slightly different man- 

 ner by the use of proteins known to be deficient, and this method 

 has probably been somewhat more fruitful up to date. 



Striking evidence as to the indispensability of tryptophane 

 has been published by Osborne and Mendel. 3 Rations containing 



