The early experimental work with diets consisting of relatively pure food substances had for their objective in many instances, the demonstration of the inadequacy of such "incomplete" proteins as gelatin or zein, or the relative merits of phosphorized and non-phosphorized proteins. There was a period when a series of papers were published by physiological chemists debating, on the basis of rather poorly planned experimental work and its results, the question as to whether phosphorus in the form of organic compounds such as lecithins, or phosphorized fats, the nucleic acids found in the nuclei of cells, and in phosphorized proteins such as casein of milk or vitellin of egg yolk, was superior in its nutritive value to the phosphorus of mineral salts such as calcium phosphate. The view was supported that for the construction of the highly organized structures of the body tissues such as nuclear substance, complex phosphorus-containing fats, etc., it was essential that similar highly organized phosphorus compounds should be available in the food. It was generally conceded that bone could be formed from inorganic phosphates, but most investigators doubted whether this form was as valuable as organic phosphorus in animal nutrition. This subject was for years a prominent one for discussion, although there never was a single well established fact to support the view that the highly organized forms of this element which occur in plant and animal tissues are superior in animal nutrition to the simple mineral salts of phosphorus. In 1909, as has already been described (2), Mc-Collum first published experimental work which finally disposed of this entire question, by showing that young rats could grow on diets containing phosphorus only in the form of calcium phosphate derived by grinding the naturally occurring mineral.