Superfoetation has been induced experimentally in the mouse by treating females with gonadotrophins twice within a short period. Successive ovulations resulted from the treatments. After each treatment with gonadotrophins the females were paired with males, or were artificially inseminated, the spermatozoa carrying distinctive gene markers so that the paternity of offspring could be identified. If females were mated after one treatment and artificially inseminated after the other, embryos differing in age by 2½-3 days could develop from conception to birth in the same female. No examples of superfoetation were found when females mated in response to both treatments or when the difference in age of embryos from the first and second treatments was 3½-4 days; few females, however, could be induced to mate in response to each treatment.
In the experiments in which the age difference of the embryos was 2½-3 days, the second treatment with gonadotrophins delayed the transit of the first set of embryos along the uterine tubes. This often resulted in the blastocysts from the first treatment and the recently ovulated pronucleate eggs from the second treatment being found together in the tubes. Both groups of embryos apparently then moved normally along the tubes, implanted in the uterus at or about the same time, and were probably born simultaneously. If the age difference was 3½-4 days, transit of the first set of embryos was not delayed and many of these embryos were destroyed in the uterus.
The incidence of successive ovulations and the response of the female tract to repeated injections of gonadotrophins, the duration of the fertilizing capacity of spermatozoa in the female tract, aspects of sperm transport, and the possibility of increasing the incidence of mixed litters are discussed.
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