Sex Differentiation of the Gonad in the Bullfrog, Rana catesbeiana

The bullfrogs inhabiting Lake Sagata, Niigata City, breed in June. The larvae grow quickly during the summer months. After October, the growth is gradually retarded, and stops during the winter season. The larval growth begins again in April, and the development of the hindlimb is particularly noticeable in the second-year larvae. These larvae metamorphose in June. The gonadal development notably parallels the body growth mentioned above. In the first-year larvae, irrespective of genetic sex, all the gonadal anlages develop into an ovarian structure. Almost no developmental change is observed in the winter. The gonadal development resumes in April, and the sexual difference becomes gradually clear, i. e. the growth of the oocytes is conspicuous in genetic females, whereas in genetic males, the change of the ovary into the testis takes place, and a testicular structure is observed in most specimens in June. The mode of sex differentiation in amphibians has been classified into 3 types in relation to the time of metamorphosis (Witschi, 1929), and according to this classification, the mode in the bullfrogs inhabiting Lake Sagata belongs to the semidifferentiated type. It is conceivable, however, that the conception introduced by Witschi has no significant meaning embryologically.