From what has been said it will be appreciated that a diet may be remarkably simple, viz.: consist of a single purified protein, a source of the sugar glucose, nine mineral elements and two un-characterized dietary factors, and be entirely adequate for the support of growth and prolonged well-being in the rat. From relatively simple substances the animal tissues are capable of synthesizing many compounds of extraordinary complexity. The limitations of the tissues are, however, sharply defined, and if the circulating fluids which constitute the environment of the living cells, deviates appreciably from very definite composition, the efficiency of their functioning is lowered, and their vitality affected. The degree to which they are affected can serve as a measure of the extent of the departure of the diet from the optimum, provided the right kind of observations are made to bring this to light.