The effect of altering the light regimen after copulation upon the duration of pregnancy in mice was investigated. It was found that mice bore young at a constant interval after copulation irrespective of the prevailing light conditions. No evidence was obtained that a light-dependent circadian mechanism determined the delivery time, although the possibility that such a mechanism might be established in the period immediately before mating was not ruled out by the present study. A significant negative correlation of litter size on gestation length was noted.
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