Dextrin . . . . . . . . 69.3 52.8 Salt mixture . . . . . . 3.7 2.6 Butter fat . . . . . . . 5.0 0.0 Agar-agar . . . . . . . 2.0 2.0 Casein . . . . . . . . . 18.0 12.6 Wheat embryo . . . . . . 2.0 30.0] These results linked up with those of Stepp and Mendel and showed that butter fat and egg yolk fat contained a growth factor which was missing in other fats. McCollum named this the "unidentified dietary factor fat- soluble A." In the same year F. G. Hopkins in England announced that the addition of 4 per cent of milk to diets consisting of purified nutrients would convert them into growth producers. This was too small an amount to admit of attributing the cause to milk proteins, fats, carbohydrates, or salts. Hopkins therefore suggested the existence of unknown factors in milk of the type to which he had earlier given the name "accessory factors." This work has recently been repeated by Osborne and Mendel who fail to find the high potency in milk ascribed to it by Hopkins but the latter's work, at that time, was accepted without question and became the impetus to important discoveries. Mendel and Osborne had meanwhile investigated more in detail their milk fractions. They obtained results that confirmed McCollum's findings for butter fat but in addition they showed that by removing all the fat and protein from milk they obtained a residue which played an important part in growth stimulation and that this factor was different from the salts present in the mixture. This specially prepared milk residue they called protein-free milk. The next few years are a melting pot of investigations. They included some sharp controversies over nomenclature and many apparently contradictory conclusions based on what we now know to be insufficient data. The principal outcome was the identification of the yeast and rice polishing substance with the factor carried by protein-free milk. On the basis of these results Funk put forward the idea that McCollum's butter-fat and egg-yolk factor was merely vitamine which clung to the fats as an adulterant. It was soon shown, however, that butter fat could be obtained that was absolutely free of nitrogen and still be stimulatory to growth. It was therefore clear that whatever the factor present it could not be the Funk vitamine. From out of the smoke of this controversy came an ultimate explanation that was very simple. There were two factors instead of one. McCollum did not discover the presence of the Funk vitamine in his mixtures at first because it was carried by the lactose and he did not know it. Finally, to cut a long story very short, these two factors or vitamines were both found to be essential to growth and in the feeding mixtures that had been used were distributed as follows _Vitamine A_ Fat-soluble Non-antineuritic Present in butter fat and egg-yolk fat