Influence of photoperiod on the time of parturition in the rat. III. Comparison of different daily light lengths with changes in light timing or light pulse given during darkness

The time of birth has been determined in rats isolated and divided into diffe- rent groups according to the light regime applied throughout pregnancy (day 1 : beginning of pregnancy).The majority of deliveries in all groups occurred during two well-defined periods, the first on the afternoon of day 22 of gestation and the second on day 23 at the end of darkness or the beginning of the light phase.On day 22 of gestation, 47.1, 58.7 and 76.0 p. 100 of the births occurred in groups daily receiving 14 h (control group C), 12 h (group I) and 8 h (group S), respectively, of light ; the photoperiod finished at the same time of day in all these groups.Birth rates similar to those of group C were obser- ved on days 22 and 23 as follows : (1) in two S groups (S19 and S21) submitted to 8 h of