> treatr treat- 29 i.v., It enters and leaves the cell in strict conformity to its concentration in the medium at any time. Therefore, the effect of nitrogen on the response curve was tested. The results are shown in Figure 29. The reduction in radiation response after the bubbling of is the nitrogen through the cell suspension has ceased and h case air has been readmitted can only mean then that the chems from system has been modified in its response to radiation. It can be envisioned that some reactions, sensitive to radiation, have proceeded and perhaps have been completed during the anaerobic period. Therefore, "A (10 ct the bthey may no longer be as vulnerable to exposure to his in- radiation. These reactions, however, do not seem to ivision be those that determine the shape of the response as obwh in le gath two lower ion of WeVEFr, Hownicals Ss was lion in f proresent diate, S are curve. Only the magnitude, therefore, and not the shape of the radiation response curve is altered by the absence of oxygen. CONTROL — DIVISION TIMES j——25'—_| + Figures 30 and 31 show the results of similar ex- periments involving prolongations of the cell cycle I time by temporary reductions of temperature. A lowering of the normal (~22° C) development tempera- KA ‘ture, even by less than 10° C, postponed the processes | determining the characteristic shape of the radiation response curve but did not modify the magnitude of the response. Since the difference in control egg divi| sion times was appreciably less than the treatment time, however, these reactions appear to have procecded more rapidly than normal once the system was ‘returned to control temperatures. Again, therefore, certain activity must have been possible during the ; period of reduced temperature which permitted the 0 0 | | i 20 30 40 30 60 §=6—70 MINUTES AFTER FERTILIZATION 10 80 4 30 Fig. 24—The sensitivity to radiation-induced cleavage delay as a function of the postfertilization time Arbacia zygotes are irradiated. The dashed curve shows the effect of immersion in 99+% deuterated sea water for 20 min right after fertilization. more rapid rate of completion of the cell cycle once RADIATION-INDUCED CLEAVAGE DELAY IN MINUTES he cyred by RADIATION-INDUCED CLEAVAGE DELAY IN MINUTES , appear ' treatdiation pie ke et Be ee RR Rede edb 50 ] | CONTROL DIVISION TIMES 1 31 ——— | = Y 0 10 20 30 40 50 60 70 | 80 90 100 MINUTES AFTER FERTILIZATION Mig, 25.—Thesensitivity to radiation-induced cleavage delay as a function of the postfertilization time Arbacia zygotes are irradinted. The dashed curve shows the effeet of treatment with 2.5 * 107? M sodium azide for 30 min right after fertilization.