The sensitivity to x-ray-induced mitotic delay of the sea urchin egg was measured at different times during the mitotic cycle. In the first division cycle of Arbacia punctulata the sensitivity initially increases and then decreases biphasically to "insensitivity." However, cells irradiated late in the first cycle which are not delayed at their first division are delayed in their second mitotic cycle. The rise in sensitivity with regard to the delay of the second mitosis begins during the phase of "insensitivity" to delay of the first mitosis and extends a few minutes into the second division cycle. The fact that the sensitivity increase cannot be described by either a linear or a first-order exponential equation is discussed with reference to Lea's classic recovery hypotheses. It is suggested that the partially repairable damage by ionizing radiation may affect two different events in prophase, whereas, nonrepairable uv damage can be measured only with respect to one mitotic event. A correlation exists between periods of sensitivity to the induction of mitotic delay and of multipolar mitoses, suggesting an influence of radiation on the multiplication of centrioles. Attention is directed to possible differences between the radiation-induced mitotic delay expressed during the prophase of the sea urchin eggs and delays found at other mitotic stages in cells which must grow as well as divide.

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