Sixteen pregnant female mice were operated on and H3-thymidine was injected into the amniotic cavities of the uterus. The injection was given between the 6th and 14th days of fetal life. Eighty-eight fetuses received thymidine in this way. Another series of 16 pregnant females was injected intraperitoneally with H3-thymidine between the 5th and 14th days of pregnancy. Two of these females 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 late 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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