Change in radiosensitivity of sea-urchin eggs during early cleavage stages. III. Effects of split-dose irradiation with 137Cs gamma-rays.

When sea-urchin eggs were irradiated with 137Cs gamma-rays, their radiosensitivity, expressed by the percentage which formed pluteus larvae, fluctuated during the early cleavage cycle. Split-dose irradiations were made both in the sensitive and resistant phases. For eggs in the sensitive phase, the effect of the first exposure of 500 rad was not diminished during the interval before the second exposure. Eggs irradiated in the resistant phase were only slightly damaged. Results imply that fluctuations in radiosensitivity of sea-urchin eggs are caused mainly by different degrees of non-repairable damage in each phase of cleavage rather than by different recovery abilities.