Kaufmann (9) conducted the first experiment of a type which has been greatly extended by others. He fed dogs on diets in which gelatin was supplemented with tyrosin, cys-tin and tryptophan to the extent of 4.2 and 1 per cent respectively of the total nitrogen of the diet, the remainder being supplied by gelatin. It was found possible with this mixture to replace fully the protein lost through metabolism. These results he confirmed by experiments on himself. In 1905, when his investigations were made, no one suspected the necessity of including in the diet the three unidentified dietary factors fat-soluble A, water-soluble B and water-soluble C, which are concerned with the prevention of the development of a type of ophthalmia, polyneuritis and scurvy, respectively. His experiments were apparently so short that these omissions in the diet did not seriously interfere with the outcome of the study.

Abderhalden (8) sought to remove certain amino-acids from completely digested proteins. By means of feeding experiments with the protein digestion mixtures rendered incomplete, in that one or more of the complexes contained in tissue proteins were lacking, he sought to determine which, if any, of the digestion products are indispensable in the diet. The object was to determine whether, in certain cases, it is possible for the body to convert one amino-acid into another, that is, to synthesize a missing essential complex. He was able to remove practically all of both tyrosin and tryptophan from his digestion mixtures. The former of these is a cyclic compound and the latter a still more complex substance containing two types of cyclic structures in its molecule. Abderhalden demonstrated that these two amino-acids must be furnished in the diet if satisfactory nutrition is to be attained.

Henrique and Hansen (10) published similar studies which led them to believe that the three diamino-acids, arginin, his-tidin and lysin, are not necessary in the nutrition of the rat. Their experiment consisted in keeping a record of the nitrogen intake and output of a rat during a period of 23 days while the animal was restricted to a diet containing only digested protein from which these three digestion products had been removed. It is very difficult to avoid loss of a part of the nitrogen of the urine in experiments with such a small animal, and it seems probable that their work was not free from error, since their results are at variance with those of other investigators. They may possibly have failed to remove completely from their protein the amino-acids in question.