Gonadal Development and Germ Cell Kinetics in the Course of Sex Differentiation in the Salamander Onychodactylus japonicus

Gonadal development was studied histologically in Onychodactylus japonicus, and changes in the number of germ cells in the course of sex differentiation were examined. The mode of sex differentiation is of the sexually differentiated type. Gonadal sex differentiation occurred directly from the sexually indifferent primordial gonad, and this differentiation was recognized in the larvae of 19-22mm in snoutvent length (developmental stages 67-68). Slight local variations in external and histological structures were observed in the primordial gonad. No sexual difference in the number of germ cells was found when sexual differentiation was beginning. Just after sex differentiation, secondary oogonia appeared in the ovarian primordia, and proliferated temporarily. Germ cells in the ovarian primordia therefore became more numerous than those in the testicular primordia. When auxocytes appeared in the ovaries, oocytes in earlier stages decreased in number, and degenerative germ cells were seen. In the testes, a positive correlation was recognized between the number of germ cells and body length.