A radioautographic study of the time course of male meiosis in the newt Triturus vulgaris.

In male Triturus vtdgaris at 16 °C the pre-meiotic synthesis of DNA (5-phase) takes 0-10 days. The 5-phase starts 1-2 days after completion of the ultimate spermatogonial mitosis, and extends into leptotene. Heterochromatic regions, most of which lie close to the centromeres, start and finish replicating about 1 day later than other parts of the chromosomes. Synapsis starts 6 days after the end of pre-meiotic S and is completed 8 days later. Pachytene lasts for 4 or s days, diplotene for 1 or 2 days, and first metaphase occurs 20 or 21 days after the end of pre-meiotic 5 . Label distribution at first metaphase in spermatocytes stemming from spermatogonia which incorporated [H]thymidine during the penultimate intermitotic 5 shows that random segregation and assortment of labelled and unlabelled chromatids took place at the ultimate spermatogonial mitosis.