Tritiated-thymidine autoradiography was used to follow the progress of cells through spermatogenesis in the axolotl, Ambystoma mexicanum. Results indicate that, at 18"C, the most rapidly maturing germ cells spend 5 days in leptotene, 5 days in zygotene, 22 days in pachytene, 1 day in diplotene, 6 days completing meiosis I, 10 days as secondary spermatocytes, and about 12 days in spermiogenesis. In these laboratory bred and raised urodeles, mature sperm spend an additional 60 days in the testes before the first label appears in spermatophores. The data suggest that, at least for some animals, there is minimal storage of mature sperm in the vas deferens. Cyst counts on longitudinal sections through the testes revealed large differences in the proportion of cells at different stages in animals examined during the same season. The results show that spermatogenesis passes through the testes of each animal in a wave that, however, is not coordinated with that of other animals in the colony. The overall timing of spermatogenesis is consistent with the results of mutagenesis studies with the alkylating agent, EMS. EMS reacts preferentially with late spermatids and a dominant lethal effect is seen about 60 days after the mutagen treatment, corresponding to the time interval between late spermatids and mature sperm ready to be laid in spermatophores.
'Judged primarily from plumpness of the inguinal region in females and prominence of cloaca1 glands in males.
Address correspondence t o Mary Jane Miltner.