Chemical studies which contribute to our knowledge of the nature of the nitrogenous constituents of "protein-free milk" have been made by Osborne and Wakeman (44), Denis and Minot (45) and Kennedy (46).

It should not be inferred from the above critical review of the role played by "protein-free milk" in their experiments, that the numerous studies conducted by Osborne and Mendel, in which this product was used, are without significance. Quite the reverse is true. They serve admirably to emphasize the variations which exist in the nutritive values of proteins from different sources. They leave no room for doubt that in a number of proteins from vegetable sources lysin is the limiting factor which determines their value for growth. They demonstrate that the sulphur-containing amino-acid, cystin, is the limiting factor in casein, and that the supplements which are essential in order to render zein nutritively complete are lysin and tryptophane. Even when these are added, however, and the protein is made adequate for growth, it is still further improved by the addition of the diamino-acid, arginin. They established the principle that the nutritive value of a protein depends upon the quality and quantity of the amino-acids which it yields. Their studies failed, because of the use of the protein-free milk, to yield any quantitative comparisons of the biological values of different proteins. But extensive as were the studies discussed in this chapter, they form but an incident in the scientific work of these distinguished investigators.

♦ Parenthetical expression the author's.

The effects of "protein-free milk" in influencing the outcome of numerous experiments has been dealt with somewhat at length because, although Osborne and Mendel have themselves furnished data which clearly indicate the part which this substance played in their work, the observations were never definitely correlated with their earlier work, so as to make clear their changed views. For the constantly increasing number of students in this field it seemed desirable to offer this digest of one phase of the literature in order to show the true status of our knowledge in this field.