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EVELYN L. SHAVER

In a previous study, the effect of delayed fertilization of ova on the chromosome complement of 6-day blastocysts was investigated (Shaver & Carr, 1969). Human chorionic gonadotrophin (hcg) causes ovulation to occur in rabbits approximately 10 hr after injection. When mating was delayed at least 6 hr, 13% of the blastocysts were found to be triploid. Triploidy was not found in the control series, although blastocysts from animals mated immediately after hcg injection had a 7% incidence of other types of chromosomal anomalies. Rabbits receiving no exogenous gonadotrophin gave rise to only 2% of blastocysts with abnormal chromosomes. This difference led to an experiment in which various doses of hcg were injected before mating in order to determine whether a relationship existed between the dose level and the chromosomal abnormalities found in 6-day blastocysts.

Adult rabbits of mixed breeds purchased from a local supplier were used. Eleven rabbits were

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EVELYN L. SHAVER and D. H. CARR

Summary.

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.