Although much information is available concerning the effects of radiations on cells, few studies have been published in which the exact stage of a mitotically active cell at the time of treatment is known. In many studies, the stage treated has been inferred on the assumption that (1) the cell sequence is not altered by irradiation; i.e., cells treated in late prophase will reach metaphase before those irradiated in middle prophase, and those irradiated in middle prophase will precede those treated in earlier stages; (2) mitotic rate is not altered by irradiation; and (3) the mitotic stage at treatment can be determined by the type of aberration produced, since chromatid and isochromatid aberrations are induced by treatment during prophase, and chromosome aberrations are induced by treatment during interphase. The experiments to be described were performed with living, unstained neuroblasts of the grasshopper Chortophaga viridifasciata. The stage treated is known from direct observation. Mitotic delay was measured in terms of the time required by cells to progress from the stage at treatment to anaphase as compared with the expected time for untreated cells. For each of the mitotic stages, chromosome breakage frequencies were determined from the acentric fragments present at the first and second anaphases after treatment.
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W. St. Amand (1956) studied this question.