Recently metamorphosed female Xenopus laevis toads were injected with tritiated thymidine. Animals were kept at 20°C and were sacrificed 1–23 days after isotope injection. Radio-autographs of squash preparations of the ovaries were made. The progress of labeled germ cell nuclei was followed to obtain information on the time course of early meiosis and extra-chromosomal DNA synthesis. Premeiotic S was estimated to take not more than 7 days. Leptotene takes 4 days, zygotene takes 5 days, and pachytene was estimated to be completed in about 18 days. The major period of amplification of the extrachromosomal DNA occurs in pachytene and takes about 13 days. A low level of synthesis was observed before and after this period, in zygotene and late pachytene-early diplotene, extending the total time for extrachromosomal DNA synthesis during meiosis to about 18 days. These data allowed the calculation to be made that one round of replication of the amplified DNA takes between 1.2 and 3.0 days. It was also found that in both oogonial and premeiotic interphases, the nucleolus-associated DNA shows asynchronous (probably late) labeling with respect to the chromosomes.

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