52 THE VITAMINS 



discussed by Mendel in the paragraph quoted above are soluble in 

 alcohol, the work of Osborne and Mendel was in accord with that 

 of Hopkins and carried the subject further in showing the existence 

 of two factors in the problem. 



A paper published by McCollum and Davis (1915) did much to 

 advance the view that "there are necessary for normal nutrition during 

 growth two classes of unknown accessory substances, one soluble in 

 fats and accompanying these in the process of isolation of fats from 

 certain foodstuffs and the other soluble in water, but apparently not 

 in fats." By means of feeding experiments with young rats, it was 

 found that polished rice could not be supplemented so as to produce 

 rations capable of inducing growth by the addition of purified protein, 

 fats which possess the growth-promoting property, and salt mixtures, 

 but that the addition of quantities of wheat embryo or of milk powder 

 as small as two per cent of a food mixture consisting of poHshed rice, 

 casein, salts and butterfat, furnished enough of an essential accessory 

 to induce growth. This substance essential for growth was shown to 

 be present in the water and alcohol extracts of wheat embryo and of 

 egg yolk and to be apparently stable to heat. That it was not identical 

 with the growth-promoting constituent of butterfat was shown by the 

 fact that the addition of 20 per cent of butterfat to the basal ration 

 did not induce growth unless the other accessory was supplied nor did 

 the addition of the water-soluble accessory suffice to support growth 

 in the absence of the fat-soluble accessory. That the amounts of the 

 water-soluble factor necessary to induce normal growth are very minute 

 was shown by the fact that amounts of an alcohol extract of wheat 

 embryo carrying as little as 0.6 gram of solids and 0.0095 gram of 

 nitrogen, equal to 0.33 per cent of the total nitrogen of the ration, 

 sufficed to induce normal growth. 



Attention was also called to the probability that lactose of the 

 ordinary purity may be contaminated with sufficient of the water- 

 soluble, growth-promoting accessory to cause pronounced increase in 

 the body weight of animals whose ration was otherwise adequate. Later 

 in the same year McCollum and Davis (1915a) reported that sup- 

 posedly pure casein as well as lactose may contain appreciable amounts 

 of the water-soluble, growth-promoting substances. Casein was there- 

 fore purified by twice repeated precipitation, followed by washing, 

 drying and grinding. With casein prepared in this way, together with 

 dextrin, butterfat and salts, they obtained no appreciable growth even 

 during the first month. Nor was growth obtained in the presence of 

 excess of the water-soluble factor with absence of the fat-soluble factor. 



