Sex-chromatin frequency and cell number were determined for 250 rabbit embryos recovered 58 to 136 h after fertilization and consisting each of about 40 to 23,000 cells. Of the 37 embryos recovered up to 83 h after fertilization, all but one, an exceptionally large embryo, were sex chromatin negative. Three of the 99 embryos recovered between 84 and 99 h after fertilization had sex-chromatin frequencies exceeding 10 Sex-chromatin frequency increased rapidly between 100 and 120 h post-fertilization, reaching a maximum (up to 87) in some embryos at 114 to 136 h after fertilization. Nuclei containing sex chromatin were uniformly distributed throughout the embryos. The frequency of sex-chromatin bodies not located at the nuclear periphery increased more rapidly during development than did the frequency of peripheral sex chromatin. These observations agree in general with previous reports and with the expectation, based on the Lyon hypothesis, that up to a certain early stage in the development of female mammals both X chromosomes are isopycnotic and presumably genetically active.
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