Ovulation and post-ovulational losses in strains of mice selected from large and small litters

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Abstract
This paper describes the aspects of fertility that had been affected by selection on litter size. For twelve generations previously the mice used as parents were chosen because they had been born in large or small litters. At the end of this time, litters in the fertile strain averaged 11·1 young born alive, while the less fertile strain averaged 5·5.It was found that male fertility and inherent viability of the young had nothing to do with the response although neither was excluded by the method of selection. Several contributions, however, were made by the females, who were affected not only in ovulation rate, but also in their control of pre-implantational losses, foetal mortality and mortality of the newly born.Females from the less fertile strain were particularly prone to pre-implantational loss of eggs. It remains to be shown whether these were due to fertilizational or implantational failure.The incidence of earlier and later embroyonic losses in females of the same strain were uncorrelated—Utters that were depleted early were neither more nor less inclined to be depleted later.