It has been stated already that Osborne and Mendel (5) reported their failure to induce growth in young rats on a diet composed of purified casein, starch, lard and the inorganic salt mixtures employed by Rohmann and by McCollum, and it was evident from their records of food intake that sufficient food had been consumed to promote growth if the quality was satisfactory.

No explanation was apparent for their failure, for the diet of McCollum, with which successful growth had been reported, consisted of two pure proteins, several carbohydrates, including milk sugar, two fats, including butter fat, and a salt mixture (see p. 11). It had been supposed that the success observed with this food mixture was due to adequate food consumption. An effort was at once begun by McCollum to discover wherein lay the lack of harmony in these observations. As the situation stood, his work was discredited, and he felt that many would be inclined to regard his reported data as falsified. It was now imperative that he should satisfactorily explain why others had failed to confirm his results.

Returning, therefore, to this field of study, he found his results irregular and confusing. Employing a diet consisting of casein, dextrinized starch, milk sugar, and salts, he and Miss Davis tried the addition of a series of different kinds of fats, to see if any special virtue could be found in any of these, and in 1912-13, found that young rats restricted to such a diet could grow well when butter fat or egg yolk fats were incorporated in this food, but that olive oil or lard did not under these conditions induce any growth. These results were not described until June, 1913. They interpreted these results to mean that there was in certain fats a dietary essential which had not hitherto been recognized.