86 GROWTH 



and incorporated into their synthetic diets, and they then ob- 

 tained at least a moderate degree of success. They observed, 

 however, that in a few months young animals ceased growth and 

 declined, so evidently the problem was not yet solved. Unfortu- 

 nately, it will be necessary to pass over many important contri- 

 butions, but we will take time to mention a publication of F. G. 

 Hopkins. 24 He, too, used synthetic diets, and in some cases his 

 experimental animals did fairly well. He remarked on the fact, 

 however, that the successes were obtained when he employed 

 relatively crude materials. It seemed that the more he purified 

 the components the poorer were the results. Hopkins was con- 

 vinced that his failures were due to the absence of some essen- 

 tial organic factor, and made it clear that the materials must 

 have a high degree of potency, in very small amounts. Because 

 of the small quantity necessary it seems that Hopkins believed 

 the role of these accessory factors was that of a hormone or 

 catalyst, and he suggested that they may serve as a stimulus to 

 some of the organs of internal secretion. We may add that this 

 role is just as obscure now as when postulated by Hopkins four- 

 teen years ago. 



We now enter on the period when a temporary solution of the 

 problem was reached. In 19 13 McCollum and Davis 25 reported 

 that animals may be maintained for some time on a diet of 

 casein, lactose, lard, and salts. If milk fat or &gg fat were in- 

 corporated in the ration, however, fairly satisfactory growth 

 followed. The month following the publication of McCollum 

 and Davis' work, Osborne and Mendel 26 published a paper 

 which confirmed it, in regard to the unique role of milk fat. 

 Following these papers there was some confusion for two years 

 as several workers were unable to repeat McCollum's work. 

 However, in 19 15, McCollum and Davis 27 demonstrated that 

 two factors were concerned in the earlier failures. One is pres- 

 ent in milk fat, as they had previously stated. The other was 

 present in skim milk, and in yeast, as Funk had consistently 

 maintained. It is obvious enough now that the earlier successes 



