VITAMIN B {B^) 51 



as food for growing animals, even though the purified food substances 

 were themselves derived from milk. 



In 1913 they developed the idea more fully, giving experimental 

 evidence which pointed plainly to the presence in milk of a water- 

 soluble substance important for growth and different from any of the 

 known constituents of the diet. Their chief evidence on these points 

 at that time was as follows: Young rats fed solely upon milk food 

 (a paste consisting of milk powder, 60 per cent, starch, 12 per cent, 

 lard, 28 per cent) not only grew from infancy to full maturity but 

 gave birth to litters of normal young which in turn throve on a diet 

 precisely like that furnished to their parents. Therefore this food 

 was considered to contain all that is essential for both growth and 

 maintenance. 



Mixtures of starch, lard, purified protein from milk, and a salt 

 mixture made in imitation of milk ash never supported growth. But 

 on mixtures of purified protein, lard, starch, and "protein-free milk," 

 young rats grew well for 60 days to 100 days or more; when finally 

 there was a sudden decline, and death followed unless a change was 

 made in the diet. 



The observation that the nutritive decline which eventually sets 

 in on such a diet can be averted and growth renewed by the addition 

 of butter fat to the diet led to the conclusion that a substance exert- 

 ing a marked influence upon growth is present in butter and tended 

 at the time to concentrate attention upon this factor rather than to 

 emphasize the presence of another grov^rth-promoting substance in the 

 protein-free milk. In December, 1914, Mendel (1915) in a lecture on 

 Nutrition and Growth deHvered before the Harvey Society of New 

 York stated : "It is not unlikely, to speak conservatively, that there are 

 at least two 'determinants' in the nutrition of growth. One of these 

 is furnished in our 'protein-free milk' which insures proper mainte- 

 nance even in the absence of growth. When this was fed we have 

 maintained rats without growth for very long periods. Without this 

 'determinant' (as, for example, in diets of isolated food substances 

 containing artificial substitutes for 'natural' protein-free milk) the spe- 

 cial components of butter fat or cod-liver oil or egg fat induce only 

 limited gains at best. Another 'determinant' is furnished by these natural 

 fats." Meantime Hopkins (1912) had pubHshed the results of his 

 discovery dating back to 1906, of the remarkable effect of the addition of 

 small amounts of milk to a diet of purified foodstuffs and had drawn 

 the conclusion that certain natural foods contain an alcohol-soluble, 

 growth-promoting substance or substances. Since both of the substances 



