126 



A. I. RINGER 



In another series of experiments, also on young mice, they studied the 

 length of time the animals were able to survive the zein diet, and com- 

 pared it with the controls that received two per cent of tryptophan in ad- 

 dition to zein. They found that of fifteen mice kept on the zein diet all 



died between the 

 twelfth and twenty-sec- 

 ond day, whereas of the 

 fifteen on the zein plus 

 tryptophan diet onl] 

 three died before the 

 twentieth day and 

 the others lived froi 

 twenty-four to forty-five 

 days. 



There is therefoi 

 no question whatsoever 

 but that the addition of 

 tryptophan prolonged 

 the time that the ani- 

 mals could live on zein. 

 In studying the weights 

 of the animals, however, 

 they could not find any 

 differences, i.e., the 

 animals lost about as 

 much in weight with 

 the tryptophan as with- 

 out it. 



Osborne and Men- 

 del took up the study of 

 this subject on a very 

 large scale (1911). 

 They kept thousands 

 of rats for periods of 

 years, under absolutely 

 controllable conditions 

 of diet. They were thus 

 able to study the influ- 

 ence of isolated food 



8 12 16 20 24 28 32 36 40 44 48 



DIAGRAM I. Diagram constructed from the results 

 of Hopkins' and Willcock's experiments 5, 6, 7. The 

 heavy lines show the survival periods (in days) of 

 twenty -one individual mice upon the zein diet with 

 tyrosiu. The light lines show the same for nineteen 

 mice upon the zein diet with tryptophane. 



substances. They found 



the study of the changes in the body weight of the rat a most satisfactory 

 index of the rate of growth. They selected the white rat because it is 

 easily reared and cared for and because its food requirements are com- 

 paratively small. It also offers advantages because of the fact that it 



