FEEDING EXPERIMENTS WITH MICE 219 



Failure to grow may have a variety of causes which have been 

 discussed by Osborne and Mendel.^"* The inorganic constituents 

 may be at fault. With the Rohmann salt mixture used in their 

 earlier work with rats, Osborne and Mendel secured successful 

 maintenance;^^ but no substantial growth took place until this 

 salt mixture was replaced by their 'protein-free milk;' whereupon 

 the rate of gain was greater than that of the controls on mixed 

 food. Whether the remarkable superiority of the 'protein-free 

 milk' is due to the presence of traces of some essential substance 

 or to greater availability of the combinations in which the ions 

 are held is not yet certain. ^^ The mice did not even live on food 

 containing either the Rohmann salt mixture or one artificially 

 made up to resemble as closely as possible the 'protein-free milk,' 



Stunting may also be due to inadequacy of the proteins of 

 the food, as was the case, for example, with gliadin. Both rats^^ 

 and mice show remarkable constancy of weight when gliadin is 

 the only protein fed, the body weight often exhibiting scarcely 

 more variation than would be accounted for by the daily food 

 intake. 



In the present experiments, the stunting of the mice on the 

 casein diet already described would seem to be due to neither of 

 the above named conditions, but perhaps to an unsuitable pro- 

 portion of the ingredients of the ration for the species in question. 

 Recovery from the decline caused by feeding with gelatin, zein, 

 or salt-free foods was rapid and complete on the casein food ; but 

 the gain always stopped when the original weight was reached 

 or soon thereafter. As has been said, young mice made only a 

 very slow gain on the casein diet; but there was so little differ- 

 ence when lactalbumin, glutenin, or edestin was substituted that 

 it was evident that the trouble was not with the protein. A 

 change in the percentage of protein present made surprisingly 

 little difference ; the content of protein alone was not the difficulty. 



" Zeitschr. f. physiol. Chem., Bd. 80, p. 307, 1912. 



" See Charts xlvi and xlvii on p. 103 of Publication 156, Part II, Carnegie 

 Institution of Washington, 1911. 



i« Cf. Hopkins: Journ. Physiol., vol. 44, p. 425, 1912; Hopkins and Neville: 

 Biochem. Journ., vol. 7, p. 97, 1913. 



1^ See charts on p. 128, Osborne and Mendel, loc. cit. 



