126 



A. I. RIXGEii 



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 only 

 three died before .the 

 twentieth day and all 

 the others lived from 

 twenty-four to forty-five 

 days. 



There is therefore 

 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., t h e 

 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 (1011). 

 T&ey 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 

 substances. They found 

 tht? 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- 

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



UAY5 



4 



8 13 16 30 34 38 33 36 40 44 48 



DIAGRAM I. Diagram constructed from the results 

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

 heavy lines show the survival periods (in days) of 

 twenty -one individual mice upon the zein diet with 

 tyrosiii. The light lines show the same for nineteen 

 mice upon the zein diet with tryptophane. 



