NUTRITION AND GROWTH 615 



In attacking the problem from this viewpoint, Hopkins and Willcock 10 

 made observations on the survival period of young mice; -that is, the 

 period during which the animals survived when fed on a basal diet 

 mixed either with zein alone or with zein plus small quantities of tryp- 

 tophane. It was found that, with zein alone, the mice were unable to 

 maintain growth; they lost in weight and died in from about a week to 

 about a month. Other mice fed on the same amount of basal diet and 

 zein, but to which was also added some tryptophane, although they did 

 not grow, were capable of maintaining their body weight and lived in 

 some instances for nearly a month and a half. There were other indica- 

 tions of the difference in the efficiency of the two diets. The mice fed 

 on the zein alone were very inactive, and remained for a considerable 

 period of the time in a condition of torpor. The hair was ruffled, the 

 eyes were half closed, and the ears, feet and tail were cold. The ani- 

 mals, however, gave evidence of having a good appetite. On the other 

 hand, the mice to which tryptophane was also given manifested a strik- 

 ingly different behavior, being active and more or less normal until 

 just before death. That both groups of animals failed to live more than 

 forty-four or forty-eight days is probably to be accounted for by the 

 absence in the zein of the other unit, lysine. Had this been added along 

 with the tryptophane it is probable, in the light of Mendel and Osborne's 

 observations, that the animals would have survived much longer. 



To supply the missing unit, besides using the pure amino acid, we 

 may employ other proteins which contain the required amino acid (Curve 

 III of Fig. 184) . That mixtures of protein foodstuffs are desirable has long 

 been apparent to those who have studied practical dietetics. We must com- 

 bine the unsuitable protein with others which, although in themselves 

 perhaps also unsuitable, yet furnish us with a mixture which contains all 

 the essential units both for maintenance and growth. As Mendel points 

 out, these considerations suggest that we may be able to utilize certain 

 of the low priced protein by-products of the cereal, meat and milk in- 

 dustries. The test of the adequacy of the corrected diet must, however, 

 be determined by experiments of the type which we have just described. 

 It is probably in stock-raising rather than in connection with human nu- 

 trition that these facts will prove of practical value ; for, not only is the diet 

 of man more varied, but it contains animal proteins in which the deficien- 

 cies are not so common. 



Most important work of this character is being conducted by McCol- 

 lum and his collaborators. 12 It would take us beyond the confines of 

 this book to discuss the results in detail, but it may be mentioned that 

 they have shown that, since the adequacy of the diet depends on a 

 multiplicity of factors besides the ammo-acid make-up of proteins, 

 some of which we shall discuss immediately, very extensive observa- 



