1. There is a distinct sexual difference in the body weight of the mouse, the males being considerably heavier than the females. As the result of gonadectomy in the male there occurs no change in the body weight, while in the female it increases rapidly after spaying.

2. A slight sexual difference is seen to exist in the body length, the females being longer than the males. Body length increases slightly after castration, while spaying produces no effect.

3. There is a distinct sexual difference in the weight of the kidney, it being strikingly heavier in the male. The kidney of the castrated male becomes greatly reduced in weight and approaches that of the female kidney; in the spayed female only a slight reduction in the weight of the kidney may be observed.

4. The thymus of the female is considerably heavier than that of the male. As in the kidney, castration is followed by a striking increase in weight which approaches that of the female thymus; spaying has but little effect.

5. There is a distinct sexual difference in the weight of the spleen, it being considerably heavier in the female. The results of castration and of spaying are the same as in the case of the thymus.

6. The results of the present study do not agree with Lipschütz's hypothesis of an "asexual" type, for as the result of gonadectomy in the male the weight of the kidney, thymus, and spleen approaches that of these organs in the female, or, in other words, gonadectomy is followed by a development of the characters typical of the opposite sex.

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