NUTRITION AND GROWTH 139 



molecular groupings needed to satisfy specific needs, 

 and is thus superior to fats and carbohydrates as a 

 protein-sparer: it lacks, on the other hand, certain 

 necessary groupings, fails therefore to supply all such 

 needs, and thus cannot replace protein." 



These considerations served as the basis for the 

 experiments described by Willcock and Hopkins. Mice 

 kept under exactly similar conditions were fed with a 

 diet having zein as its source of nitrogen. In certain 

 instances small quantities of tyrosine or tryptophane 

 were added to the dietary. The results of the influ- 

 ence of such diets were measured by the "survival 

 period" that is, the period necessary to cause the 

 death of the animal. In Fig. 1 is reproduced a dia- 

 gram illustrating very clearly the influence of tryp- 

 tophane upon the survival period. With zein as the 

 only nitrogenous component of the diet young mice 

 were shown to be unable to maintain growth. Tryp- 

 tophane addition does not make zein capable of main- 

 taining growth, but does greatly prolong the survival 

 period. In Fig. 1 the survival periods of mice fed 

 upon zein alone are not given, for they were identical 

 with those obtained with mice fed zein plus tyrosine. 

 Although added tyrosine exerted no influence upon 

 the survival period, it must not be inferred that this 

 amino acid is without specific effect on metabolism: 

 it evidently played little role here because zein fed 

 supplied sufficient tyrosine, hence an excess was with- 

 out special influence. In reality tyrosine was added as 

 a control to tryptophane addition, in order to determine 



