Rabbits were mated at timed intervals after an ovulationinducing injection of chorionic gonadotrophin. Blastocysts were recovered at autopsy 6 days after mating and examined cytogenetically for chromosome abnormalities. A marked decrease in the number of blastocysts recovered occurred at delayed ovulation-mating time intervals greater than 10 hr. Thirteen chromosome aberrations arose among 135 blastocysts examined. Triploidy was the commonest anomaly and was found in six instances from rabbits mated 8 and 9 hr after injection of gonadotrophin. Fewer abnormalities occurred at the remaining delayed intervals. In addition, a pentaploid, two mixoploids and a trisomy were obtained from animals mated immediately following injection.
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