114 THE VITAMINES 



be of the greatest significance, particularly in those experiments 

 yielding negative results. It does not suffice merely to feed the 

 animal; one must also be certain that it eats enough. If no food 

 control is provided for, it is easy to make false conclusions; it would 

 be possible to designate a certain diet as insufficient, whereas the 

 true reason for the misleading experiments lies in the unsuitability 

 of the animal. This applies in general to rats that are bought; it 

 may be worth while mentioning that we have had particularly good 

 results with black and white rats, raised in the laboratory; this strain 

 seems to be unusually resistant. 



It must be emphasized once more that not all problems of nutrition 

 can be solved by the use of rats as experimental animals. To illus- 

 trate, Osborne and Mendel (316) have shown that when the growth 

 of rats is prevented by a specially chosen diet, growth may be resumed 

 after they have reached two-thirds of their size, upon the addition 

 of the missing factor. While Jackson and Stewart (317), in con- 

 firmation of these findings, concluded that this ability to resume 

 growth, after a period of repression, depends upon the age of the 

 animals and the duration of the growth inhibition (in agreement with 

 the work of Briining (318) and Aron (319) ), and under certain cir- 

 cumstances may be appreciably affected, we must nevertheless admit 

 that the rat does possess this ability for the most part. Now, it 

 would be futile to try to achieve the same results in man or other 

 mammals of a corresponding age. As a matter of fact, various kinds 

 of animals behave quite differently. In the same way, it is evidently 

 impossible to study the nature of scurvy and pellagra in these 

 animals as some investigators have done. The individual animals do 

 not compare in regard to nutrition and nutritional diseases. 



An interesting question is involved in experiments with inadequate 

 diets to show how far rats have the power to choose out of two diets, 

 the one that is adequate. This was investigated by Slonacker (320) 

 with a primitive method, and repeated later by Osborne and Mendel 

 (321). In the latter experiments, it could be shown that the rats 

 chose correctly almost invariably. 



The vitamine requirements of rats. Although the individual phases 

 of this question have been briefly considered in the historical part and 

 other sections of the book, it appears necessary to treat systematically 

 this matter which has assumed such practical as well as theoretical 

 importance. We have already said (even though the question should 



