A B ST R ACT Fruit flies live longer at the partial pressure of oxygen found in air than at either larger or smaller partial pressures. Flies exposed to 1 atm of oxygen for 8 hr every day do not recover completely in the remaining 16 hr. In general, intermittent exposures to 1 atm of oxygen are better tolerated than continuous exposure to the same average oxygen concentration per day, but exposures to higher pressures of 2-5 atm of oxygen for as little as a half hour every two days markedly shorten the life-span. Older flies consume more oxygen per minute and are more sensitive to oxygen poisoning than young flies, and the rate of dying in 6 atm of 02, or the reciprocal of the survival time, is a linear function of the age. The oxygen pressure-time curve can be well expressed by the general empirical equation (Po2) ~ X time = 120 where P is in atmosphere and survival time in hours. The progress of oxygen poisoning appears to be linear with time rather than exponential.T h e most i m p o r t a n t previous study of oxygen poisoning in adult fruit flies is by Williams and Beecher (1944). Clark et al. (1954Clark et al. ( , 1960Clark et al. ( , 1961 have studied particularly the sensitivity of insect p u p a e which, in special cases at least, are irreversibly d a m a g e d by as little as 1 rain exposure to 30 psi of oxygen. Goldsmith and S c h n e i d e r m a n (1956) have also observed the poisoning of p u p a e of Morrnoniella by 1.6-10 a t m of oxygen. In some respects our findings m e r e l y confirm those of Williams and Beecher, b u t the life-span at a wide r a n g e of different oxygen tensions a n d the effects of intermittent exposures have not been previously investigated. It does not seem to have been d e m o nstrated previously, for a n y species, t h a t the oxygen tension in air at sea level is, in fact, optimal for survival. O u r studies of the rates of recovery f r o m oxygen poisoning and the effects of high pressures of inert gases at different pressures of oxygen will be presented in later papers. M E T H O D SThe flies used in these experiments came from one culture of Drosophila melanogaster, the Swedish R wild type, obtained by Mr. R. C. Baxter of the Department of Radiation Biology, in 1956, from Cold Spring Harbor Laboratory. The flies were raised in t693

Time‐lapse photomicrography and autoradiography were used to study the initial events leading to the first cell division following transfer from red to white light. The first responses to white light, an apical swelling and a reduced rate of elongation, were often observed within the first 1–2 hr after the transfer; these two changes were not always initiated at the same time. Only the dome region of the cell participated in the elongation in red, as demonstrated by the displacement of anion exchange resin markers. The dome region as well as the region proximal to the dome was associated with the apical swelling, including regions formed in red. The average duration of the G1, S, G2, and M phases was found to be approximately 4, 3, 3.5, and 1 hr, respectively.

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