[Sex difference in the mouse milk spots, with special reference to its development and formation (author's transl)].

As reported previously, the omental milk spots in the mouse are morphologically classified into two types; type I and II. Type II milk spots are distributed on the dorsal layer of the omentum, and they are more numerous in females than in males. The type II milk spots appear at first at 10 days of age, and then gradually increase in number equally in both sexes until 2 months. After 2 months, however, the density of the spots is significantly greater in females than in males. In male mice which have been gonadectomized at 1 month, the type II spots show remarkable increase in density, and they are as frequent as those in normal females. Administration of testosterone to gonadectomized males and normal females causes a significant decrease in density of the spots. Estriol, progesterone and gonadotropin exhibit no significant effect on the density of the spots. From the results obtained, it can be concluded that the existence of testis, namely testosterone, plays a major role in development of sex difference in type II milk spots.