IN the study of relative growth of organs and tissues, there are certain advantages in referring the relative weights to age, throughout the life-cycle. When results are expressed according to the allometric formula1–3, y = bx α (x = weight of animal, y = weight of organ, b and α=constants), time as an independent variable tends to be eliminated. When, however, the relative weights are expressed in relation to age or its logarithm, they are found to show maximal values at definite periods of the life-cycle, specific for and characteristic of each organ4. The curves of the relative weights, as functions of the logarithm of age, tend to follow one of the types in Fig. 1. Thus, in the rabbit, that of the brain follows, from the nineteenth day of embryonic life until adult age, curve 1 of Fig. 1, that of the liver and of the lungs, curve 2, with maximal values respectively at the twenty-fourth and at the twenty-seventh day of embryonic life; that of the kidneys, curve 3, with a maximum between the second and fourth weeks after birth; that of the stomach, curve 4, with a maximum between the fifth and tenth weeks; that of the muscles, curve 5, with a maximum after puberty; and that of the perirenal adipose tissue seems to follow curve 6 with maximal values only late in the adult age. The same sequence of organs, ranged according to periods of maximal relative weights, is likely to be found in other mammals, as is seen from observations of C. M. Jackson5 on human brain, liver, lungs and kidneys the relative weights of which are maximal in the second, at the end of the third, in the fourth and between the seventh and eighth months respectively.