STUDIES ON GROWTH I. GROWTH FACTORS IN LIVER* By

f ~ I "MIEknown facts regarding the nutritive requirements of man and of JL the lower animals have increased remarkably within the past decade. However, this newer knowledge has not yet made possible a complete un derstanding of normal nutrition so that at the present time one is still unable to state definitely the nature of an adequate food mixture. The ex perimental data of investigations in which the white rat has been used have been made particularly confusing by the changes which have been ob served recently in the rate of growth of stock or control animals (1, 2, 3). Obviously, results become increasingly difficult to interpret as the stand ard growth curves or norms vary. An additional complicating factor in some cases has been the acceptance of a fair degree of growth of rats on an experimental diet as satisfactory even though the rate of growth was considerably below that of stock or control rats. The assumption has been made in such cases that artificial mixtures of purified food materials must of necessity be inferior to a mixture of natural foods. It appeared desirable to test the validity of this assumption and, therefore, an attempt has been made to find a satisfactory combination of supplements which would per mit optimum growth on a simple basal diet. Inasmuch as liver has been shown by Osborne and Mendel (4, 5) to exert a pronounced stimulating effect upon growth, the experiments have centered around the use of this material as a dietary supplement and an investigation is being made of its water-soluble growth factors. Twenty different combinations of supplements were tested and all but one permitted a rate of early growth as good or better than that previously recognized as normal (6). It may be stated that the capacity of a supple