Abstract
Summary
When a large group of CF1 female mice are followed individually for their reproductive performance, it becomes evident that the first litter size is the smallest and the size gets progressively larger at least to the 4th litter. Among the 13,508 normal offspring examined, following continuous mating procedures, 52.8% were males and 47.2% were females. The number of abnormal mice found in otherwise untreated females of the CF1 strain is never high (2.29% to 3.26%), but it does increase with successive litters from the 1st to the 4th. Among the other (“not normal”) mice there were dead and/or stunted fetuses, some were eaten at birth by the mother for reasons unknown, some had persistent amnions, and others had anomalies, primarily of the central nervous system. Virgin mice becoming pregnant for the 1st time, whether young (3-5) months or older (7-9 months) do not have as many implantation sites as do the multipara females of 7-9 months. The average litter size is greatest among the multipara mice, more so than among the ex-breeders or virgins of any age. While males are always produced in greater mimbers than females, in all 4 categories, for some reason the multiparas approach closest to a 50/50 ratio. It must be pointed out, however, that these data involve only 128 litters so that the data may not be as reliable as that presented above, from a vastly larger group of mice.
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