As can be seen by the titles of several of the papers cited in connection with the feeding of isolated and purified food substances, one of the earliest explanations which occurred to investigators as the possible cause for the failures was the probable need of the animal for phosphorus in certain organic combinations. Some of the experiments were interpreted as supporting the view that proteins containing phosphorus as a part of the molecule were better utilized for growth than were phosphorus-free proteins. Between 1895 and 1910 there was a widespread belief among students of this subject that the presence of organically bound phosphorus might be a factor of great importance in nutrition. It is easy to appreciate how this idea gained credence. Casein, the principal protein of milk, is a phosphorus-containing protein, and the yolk of egg is very rich in both the phosphorized protein, vitellin, and in phosphorized fats. Milk and the egg yolk are peculiarly constituted for the role they play in reproduction and the nutrition of the young during the earliest period of post-natal life when growth is most rapid. It seems significant that in these two substances organically bound phosphorus is very abundant, whereas it is abundant nowhere else among our natural foods. It has never been conclusively shown up to the present time that a protein occurs in plant tissues which contains the element phosphorus as a part of its molecule. Other organic phosphorus compounds (lecithins, nucleic acids) do exist in plant tissues but never in great abundance.

Among the experiments cited above in which animals were confined to diets of purified proteins, carbohydrates, fats and mineral salts, those of Hall, Steinitz, Leipziger, Zadik and Ehr-lich were directed towards ascertaining the importance of phosphorus in organic as contrasted with inorganic combinations. Other investigators working along the same lines were Korn-auth (16), Gottstein (17), Ehrstrom (18), Hirschler and Ter-ray (19), Gilbert and Posternak (20), Tunnicliffe (21), LeClerc and Cook (22), Koch (23), Hart, McCollum and Fuller (24), and Forbes (25). The evidence either seemed definitely to support the view that organic phosphorus compounds were essential in the diet or else it failed to establish the fact that they were dispensable.