28 agents were applied to the zygotes for a 20- to 30-min time period shortly following fertilization. The zygotes were subsequently irradiated at different times in the cell cycle—in some cases during the treatment period. A comparison of the radiation sensitivity curves of treated and untreated zygotes for the agents tested is given below. Figure 23 shows the effect of treating Lyltechinus zygotes for twenty minutes with 10-4 Mribonuclease after fertilization. The solid curve depicts the re- sponse of control zygotes to irradiation with 5000 R at the times indicated on the abcissa. The dashed curve depicts the response of zygotes to the same dose of radiation after the ribonuclease treatment. The responsible for the shape of the response curve apr to be delayed principally during the time of tr« ment, but in this case the magnitude of radiat response is appreciably less during and after the tre ment. A similar result is shown in Figure 26 for tre ment with 0.1. beta mercaptoethanol. There is possibility, however, that even though in each ¢ the fertilized egg suspension was rinsed, enough che ical remained to protect some critical structures fr the action of the radiation. As shown in Figure 27, treatment with EDTA mg/ml) of Arbacia zygotes appeared to protect zygotes both before and after application. In this stance, however, the prolongation of the cell divis solid and dashed arrows represent the division times period seemed to occur in the latter part of the - shifted in time only by the approximate length of the served when colchicine was used. This is shown Figure 28 which depicts treatment of the same ; metes in morning and afternoon experiments with t different colchicine concentrations. Only the lov of the unirradiated control and treated zygotes; thus, a 20-min treatment with ribonuclease delayed the unirradiated cell division time by about 18 min. The response curve for the drug-treated zygotes resembles closely that of the untreated ones and appears to be treatment time. This would be expected if the treatment with ribonuclease delayed the cell progression process only during the time of application and did not affect the system’s subsequent response to radiation. Figure 24 shows similar results when Arbacia zygotes are temporarily immersed in 99.8% deuterated sea water. On the other hand, Figure 25 depicts the radiation response of Arbacia zygotes after a 30-min treatment with 2.5 x 1073M sodium azide. Again, the reactions cle—after the chemical was apparently removed dilution. A similar, but less dramatic, result was | concentration was protective. The super-imposition the control curves in the two experiments, howev illustrates the precision of the measurements. He ever, again the complete removal of the chemic from sensitive structures by the rinsing process v not assured or tested, and, therefore, the reduction radiation effect observed might be the result of pi tection of relevant entities by the chemical pres: during and after the treatment period. It is felt that the action of nitrogen is immedia yy) Oo Ww o + oO om oO rep) oO RADIATION -INDUCED CLEAVAGE DELAY IN MINUTES 70 o oO ° CONTROL DIVISION TIMES i + 10 20 40 30 50 60 70 80 MINUTES AFTER FERTILIZATION ZYGOTES IRRADIATED Fic. 23.—Thesensitivity to radiation-induced cleavage delay as a function of the postfertilization time Lytechinus zygotes a: irradiated. The dashed curve showsthe effect of treatment with 10-4 M ribonuclease for 20 min right after fertilization.