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THE statement of Ribbert(l) that the testicle undergoes hypertrophy after unilateral castration has been confirmed by several authors, lately by Sand and by ourselves. This hypertrophy is generally considered as a compensatory one. Lately Stieve(2) has argued that this is due to a greater need of a hormone secreted by the generative cells which he considers as the endocrine apparatus of the testicle. I have shown elsewhere(3) that on removal of the greater part of both testicles, the small fragments which remain, even when they are less than 1 p.c. of the testicle, do not hypertrophy, and are sufficient for the full development of those sexual characters which depend upon the internal secretion of the testicle. These results, obtained on guinea pigs and mice, fully agree with the earlier observations of Pe6zard on the cock(4). It follows that the hypertrophy which occurs after unilateral castration cannot be due to an increased demand for internal secretion. In another paper (5) I have expressed the opinion that the hypertrophy after unilateral castration is only an apparent one and that it is due to the development being quicker than normal, and that the final weight did not exceed the final weight of the normal testicle. In this paper I give experimental evidence justifying this opinion.If my view is correct, the difference in weight between the testicle of a normal and a unilateral castrated animal must be a function of the time elapsed after the operation was performed. On prepuberal unilateral castration the remaining testicle should attain quicker the maximal weight; the difference in weight of the testicle in a normal and an operated animal should be great at the time of beginning testicular maturation and should be less and less the nearer full testicular maturation is. Our assumption can be expressed in the following diagram (Fig. 1). The thick line is the real curve of growth of the testicle of normal rabbits (the body weight is taken instead of age (see below)). If our 1 A preliminary account has been published in the C. R. de la Soc. de Biol. 87, p. 60. 1922.'X

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