The metamorphic rate of Rana pipiens tadpoles was studied under different photoperiods, daylengths, and feeding schedules. Tail resorption and hindlimb growth and development induced by immersion in 30 pg/l thyroxine (T4) were accelerated under longer photoperiods and continuous light when 6L: 18D, 12L: 12D, 18L: 6D, and 24L regimes were compared. Constant light exposure did not produce faster development than an 18 hr photoperiod, and initially was less effective. The rate of spontaneous and T4-induced metamorphosis was greater the shorter the day on 9L: 9D, 12L: 12D, or 15L: 15D cycles, although all groups received the same overall amount of light, but in different dosages. When feeding schedule but not the LD cycle was varied, groups of tadpoles fed on 18, 24, or 30 hr regimes showed no differences in growth and development rate on 19L: 5D, and only random variations under continuous light. Differences in metamorphic rate on 18, 24, or 30 hr days are not due to the feeding schedules, but to the LD cycles. From these experiments we conclude that illumination, particularly the length and frequency of the photoperiod, affects the utilization of T4. Development rates independent of the total amount of illumination, but related to daylength and light schedule, suggest interaction of light with an endogenous timing mechanism.Light is one of the most important environmental variables that modulates growth and development in the anuran tadpole. However, a stimulatory effect of light is not universal, but apparently depends upon the species.Regimes of 24L and 14L:lOD delayed growth and development in comparison to 6L: 18D, 1L:23D, and 24D schedules in tadpoles of Discoglossus pictus (3, 9 ) . Similar inhibition of growth and development by light occurred in Xenopus Zuevis larvae. In one study, tadpoles showed the fastest rate of development on 24D, followed by 12L: 12D and 24L (4), and in another, Xenopus tadpoles developed faster on 1L:23D than on 23L: 1D (6). It has been suggested that amphibian tadpoles metamorphose when environmental conditions are no longer suitable for aquatic life (3). Perhaps in some cases, long photoperiods generate external conditions, such as warm temperature, favorable to the continued survival of the larvae.On the other hand, early work showed that continuous light promoted metamorphosis in AZytes obstetricans ( 5 ) and Rana temporaria (10). Rana ridibunda tadpoles grew faster on a 6L: 18D cycle than in constant darkness (3). A study of Ranapipiens tadpoles showed faster

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Since Rana pipiens tadpoles injected with thyroxine (T4) early in the dark develop more slowly than those injected in the light, we studied the effect of giving a light pulse of 1 hr early in the dark. Tadpoles injected under a 7.5-W red light bulb in a darkened room with 0.2 microgram T4 daily at 2200 hr went through metamorphosis faster on a 12L:3D:1L:8D cycle with a light pulse after injection than on a 12L:12D cycle without a light pulse, and even faster on a 12L:1.5D:1L:9.5D cycle with a light pulse before the injection. Thus a 1-hr light pulse counteracted the metamorphic delay resulting from administration of T4 in the dark, and set in motion the conditions that resulted in a more rapid response to an injection of T4. However, a 1-hr light pulse in the early dark had no effect on growth and development of older or younger untreated tadpoles or those constantly immersed in 30 micrograms/liter T4. Larvae on 21L:3D with T4 injection in the dark and on 12L:3D:1L:8D with T4 injection at 0700 hr just before the start of the main light phase progressed faster than 12L:3D:1L:8D with injection at 2200 hr in the dark before only a 1-hr light pulse. Thus the length of the light phase immediately after T4 injection was significant. There was no difference on 12L:12D and 12L:3D:1L:8D cycles in the effectiveness of daily injections of 10 micrograms prolactin (PRL) in the early dark at 2200 hr in promoting tail growth or antagonizing tail resorption induced by T4 immersion. Under these conditions, PRL utilization did not appear to be inhibited by the light pulse.

The influence of circadian (6L:18D, 18L:6D) and noncircadian (12L:18D, 15L:15D) light/dark (LD) schedules on the cell cycle was studied in the hindlimb epidermis of late premetamorphic tadpoles of the frog, Rana pipiens. Using tritiated thymidine autoradiography, percent labeled mitoses (PLM) curves were obtained for 3 cell cycle determinations on each LD schedule, starting at 0, 8, and 16 hr for the 24 hr, and at 0, 10, and 20 hr for the 30 hr LD regimens. From the PLM curves, the durations of the whole cycle (T), the DNA synthetic phase (S), and the pre-(G 1 +1/2M) and post-(G 2 +1/2M) synthetic gaps, which included mitotic time, were determined. The data were compared with previous work on 12L:12D. The shape of the PLM curves and the resulting phase durations were characteristic for each LD regimen. T was longer where L was less than D and shorter where L was greater than D. The relative duration of S decreased, while G 1 +1/2M increased, as L lengthened on the 24 hr regimens, whereas on the 30 hr regimens the opposite occurred. On both 24 and 30 hr schedules, as L lengthened the percentage of cell cycle time in G 2 +1/2M increased. The findings show the specific influence of a change in the LD schedule, including noncircadian regimens, on particular cell cycle phases in a vertebrate. In a separate experiment, the labeling index was studied over a 24 hr period in control tadpoles and in those treated with hydrocortisone or melatonin. Since only melatonin shifted the phase of the labeling index rhythm when injected during the light, this hormone may account for the effects of the LD schedule on the cell cycle.