Induced chromosomal aberrations in pronuclei, 2-cell stages and morulae of mice

C₃H female mice, 10–12 weeks old, were treated in the pre-ovulatory phase of oogenesis or in the first mitotic interphase after fertilization with amethopterin (M) at 200 mg/kg body weight. The females were mated with untreated males of the same age group and strain and were checked hourly for vaginal plugs. At different times after fertilization the oviducts of the females of experiment and control were flushed out and the cleavage stages obtained were examined cytologically. Chromosome analysis was performed in the pronucleus stage, in the 2-cell stage and in morulae. Separate analyses of maternal and paternal chromosomes of the pronucleus stage can show whether the spontaneous or induced aberrations are derived from oogenesis or spermatogenesis. After treatment the frequency of numerical and structural aberrations increased compared with the matched control. During the early prenatal development at least 2 steps seem to be important for the elimination of chromosomal aberrations: 1, from the unfertilized mII oocyte to the pronucleus stage; and 2, from early morulae to the registration of dominant lethal mutations.