ABSTRACT
Mitotic and meiotic activities of germ cells during early development in the medaka, Oryzias latipes, are dealt with in this report.
Primordial germ cells were obviously distinguishable from somatic cells 3 days after fertilization and began to proliferate about 7 days after fertilization. The mean number of primordial germ cells increased during a period of 7–10 days after fertilization, reaching about 90 immediately before hatching.
Newly hatched fry could be classified into two types according to the number and the nucleic activity of germ cells in the gonadal rudiment. One type consisted of fry containing about 100 germ cells and no cells in the meiotic prophase. In the other type of fry the number of germ cells increased by mitotic divisions and some of the cells began to enter into the meiotic prophase. During the course of further development the fry of the former type differen tiated into males and the latter into females.
Therefore it can be concluded that the morphological sex differentiation of germ cells occurs at the time of hatching. However, no sexual differences in the histological structure of somatic elements in the gonad are observable at that time.
