There are apparent only two possible reasons for assuming a need on the part of the body for the high exogenous katabolism of proteid so commonly observed. The one is that the carbonaceous residue left after the cleavage of nitrogen from the proteid molecule is better adapted for the needs of the body than either carbohydrate or fat. Although this does not seem very probable, it is of course a possibility and merits consideration. Feeding experiments, with a comparatively small proteid intake, continued over a sufficient length of time, would show conclusively how much weight should be attached to this hypothesis. The other possibility is that the body may derive some advantage from the presence, in the tissues and fluids, of the varied nitrogenous cleavage products split off from proteid so abundantly in exogenous katabolism. These substances are mainly amino-acids on their way to urea, and there is no apparent reason why they should be of service to the organism. Still, the processes going on in the tissues and organs of the body are intricate and not wholly understood, and we can conceive of some useful function of which as yet we have no knowledge. In the construction of tissue proteid, for example, as in a possible synthesis out of the fragments formed by hydrolytic cleavage, it is not impossible that certain corner-stones are needed, and that in order to obtain these there must be a more or less wasteful breaking down of food-proteid. However improbable this may seem, it, like the preceding hypothesis, can be tested in a way by adequate feeding experiments, which shall determine the effect on the body of a low proteid intake continued over a long period of time. On the other hand, it is equally plausible, and for some reasons more probable, to assume that this excessive exogenous katabolism may be in a measure prejudicial to the best interests of the body; that the many nitrogenous fragments formed in the efforts of the organism to prevent undue accumulation of reserve proteid may in the long run do as much harm as good.