The manner in which the problem was finally cleared up as to the number of unidentified factors the diet must contain in addition to the long recognized fundamental nutrients, protein, carbohydrate, fat and mineral salts, is of historical interest. During the years between 1913 and 1918 Osborne and Mendel, and McCollum and his co-workers confined their researches to different fields. The former two, believing that their diet containing a purified protein and other isolated food-stuffs, supplemented with 28 per cent of "protein-free milk," was satisfactory for the study of the comparative values of the proteins from various sources, extended their investigations in the direction of determining the biological value of each of the more important proteins isolated from natural foods. McCollum and his co-workers turned their attention to the determination of the nature from the dietary standpoint, of the deficiencies of our natural foods. It was through these studies that the next advance was gained in our knowledge of what constitutes a satisfactory diet for the rat.