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Sixteen pregnant fcmalc mice were opcrated on and Ha-thymidine was injected into the amniotic cavitics of the uterus. The injection was given between the 6th and 14th days of fetal life. Eighty-eight fetuses received thymidine in this way. Anothcr scries of 16 pregnant females was injected intrapcritoneally with H3-thymidine between the 5th and 14th days of pregnancy. Two of these femalcs were killed 16 days after the observation of the vaginal plug. The remaining 30 females were allowed to give birth to their progeny. The progeny was killed at birth and the ovaries of the newborn females fixed at once. Labeled oocytes at latc pachytene and early diplotene were clearly seen in individuals that received the isotope between the 10th and 12th to 13th days of fetal life, but the period of DNA synthesis preceding meiosis is at the 12th to 13th days of fetal life. Since meiosis is recognized by the 14th day, only the oocyte labeling originating from mothers injected at the 12th and 13th days may be considered as representing the DNA synthesis of the premeiotic replication.

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No abstract

Following an injection of 3H‐thymidine to mice there is no initial incorporation in small thymocytes, only in larger ones. In the course of time small thymocytes aquire the label. Whether the delayed uptake in small thymocytes is due to a direct cell to cell transfer of labelled nuclear material from inititally labelled larger cells to small thymocytes, or whether it is due to small thymocytes being formed from larger cells by mitotic division was investigated by the administration of Colcemid® immediately after one injection of 3H‐thymidine. In the absence of cell division no labelled small thymocytes appeared with time. This finding does not support the idea of a cell to cell transfer of DNA; it rather lends support to the view that small thymocytes arise by mitotic division of larger cells in the thymus. During the treatment with Colcemid® the migration of cells took place from peripheral to central cortex just like under normal conditions.