¹ From the Laboratory of General Physiology, Harvard University, Cambridge

The geotropic orientation of young rats of a closely inbred line P, separated 6 years ago from a stock which gave rise also to the closely inbred lines A and B previously studied, has been measured. The curve connecting orientation-angle with inclination of substratum is in a general way similar to those for A, B, and with them differs markedly from that for race K of totally different ancestry. The variability of the response exhibits similar affinities. Although the orientation-angles are rather close to those obtained with A, the threshold slope for P is that for B; analysis of the curve discloses how this fact is related to the character of the presumptive distributions of thresholds for the respective arrays of tension-receptor units. It is pointed out that these considerations prevent loose comparisons of "geotropic sensitivity" in different lines of rats, but make it plain why comparisons in terms of thresholds for response or of magnitude of response at the same threshold are ineffective. The estimated "number of excitation units," however, is found to correspond to the variability of orientation, and to the manner and extent whereby the dispersions of the measured orientation-angles are governed by the magnitude of the gravitational vector.

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