Animals were placed in a room in which the normal light-dark sequence was reversed for at least 4 days. Three or four females were placed with a male in total darkness. Copulation was determined by the presence of a vaginal plug. The middle of the “artificial night” was designated as day 0 of pregnancy. It is not considered identical with ovulation time. A late ovulation may result in considerable retardation in development. On the other hand, an early ovulation does not advance development, because the second maturation division is completed only after fertilization.