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As a preliminary experiment to establish the process on the sexing of mouse embryos by chromosomal analysis, present studies were carried out with inbred (ICR, C57BL) and F1 hybrid [(ICR{times}C57BL) = F1 {times} ICR] mice to investigate the blastomere numbers and mitotic indices (M.I.) to the developmental stage of embryos recovered, the optimum periods of anti-mitotic agent administration, the successful rates of sexing and sex-ratio. The results obtained were summarized as follows: 1. The blastomere numbers (mean{pm}S.E.) of the morula and blastocyst were 18{pm}0.4 and 54{pm}0.7, respectively. 2. Whereas the M.I. of F1 hybrid (16{pm}0.2%) was higher than that fo inbred ICR (15{pm}0.1%) and C57BL (12{pm}0.6%) in the different strains, the morula (7{pm}0.6%) was higher than that of blastocyst (6{pm}0.4%) in the case of embryo stages. 3. Following to anti-mitotic agents treated, the M.I. of embryos cultured with Colcemid (17{pm}1.1%) was superior to that fo embryos cultured with Velban (12{pm}0.9%) and the Colcemid injection (7{pm}0.4%). 4. The successful rate of sexing in the blastocyst (38.7%; 124/320) was superior to the morula (35.9%; 52/145), and the F1 hybrid (48.1%) was higher than that of inbred ICR (42.4%) and C57 BL (28.2%). 5. In the successful rate of sexing to the methods of administration, the embryos cultured with Colcemid (46.0%) was superior to that of embryos cultured with Velban (39.0%) and the Colcemid injection (38.8%). 6. Of 98 embryos sexed after culture with Colcemid, 89(90.8%) were observed between 2 and 4 hrs. In the case of Velban treatment, 83.1% (74/89) was observed between 2frac{1}{2} and 4frac{1}{2} hrs. 7. Out of 761 prepared embryos it was possible to sex 311; 157 were male and 154 were female, i.e.a sex-ratio of 50% a, pp.oximately.
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