In winter the testes of the teleost, Oryzias latipes, are inactive and the primary spermatogonia Ia, Ib and secondary spermatogonia II cease to proliferate. When the males were transferred to a warm environment (25°C) from cold thermal surroundings (0 to 5°C), multiplication of the primary spermatogonia Ib occurred within a few days after the transfer. The behavior of the primary spermatogonia Ia and Ib in the testes of the fish irradiated with X rays (100 to 1000 rad) and transferred to 25°C was investigated. At 3 days after irradiation, the pyknoses were observed in the acini containing the primary spermatogonia Ib, and the surviving fraction of the cells depended on the exposure dose. A decrease in the mitotic index was also observed in the primary spermatogonia Ib. After irradiation with 100, 250, 500, and 1000 rad of X rays, the surviving fractions of the primary spermatogonia Ib were 61, 57, 48, and 40% of controls, respectively, on Day 5. In the fish irradiated with 500 and 1000 rad, thereafter, a further decrease in cell population was found, and in those with 100 and 250 rad, the number of the primary spermatogonia Ib increased to some extent. On the other hand, there was no significant reduction in the number of the primary spermatogonia Ia following administration of doses of 100 to 1000 rad of X rays.

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