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AUTHORS' ABSTRACTA comparative study of the embryology of the rabbit in races of large and of small adult size shows a consistently more rapid rate of cell multiplication and of increase in mass in largerace embryos tha.n in those of small race.This more rapid rate of growth is transmitted by and influenced equally hy sperm and egg cell, as is shown by the results of reciprocal crosses.Rate of differentiation is independent of rate of growth and unaffected by it. Consequently, embryos of the large race have attained greater size than those of the small race at corresponding stages of differentiation.The fundamental difference in rate of growth is already in evidence in forty-eight-hour embryos and becomes increasingly clear at later stages. Embryos produced by the large race have undergone about one more cell division at forty-eight hours after mating and so are atmroximatels in the thirty-two-cell stace when embryos of the small race are in the sixteen%elI stage.-For several years we have been looking for the thing named in the title of this paper and think that at last we have found it. It consists in a differential rate of cell multiplication in large-race as compared with small-race rabbits, the rate being more rapid in the former. At the same time, differentiation is no more rapid in one race than in the other. Consequently, when the same stage of development has been reached in embryos of both races, the large-race embryo contains more cells (and so larger parts) than a small-race embryo, and so produces a larger individual.This conclusion was reached by Painter ( '28) from a comparative study of twelve-day embryos of the same race which we have employed in this study. It was our purpose to extend to earlier stages the comparative study made by Painter and to learn whether his tentative conclusions held for these earlier stages of development. I n particular we were interested to find out whether an endocrine mechanism was involved in producing the differences already observable according to Painter in twelre-day embryos, or whether such 81