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MOST INVESTIGATORS agree that dividing cells are more sensitive to X-rays than those which are in the resting stage, but there is little agreement as to which stage in the division cycle is most sensitive (Packard 1931, Goodspeed andUber, 1939). This problem is complicated by the fact that X-rays produce two general types of response, both of which have been used as ameasure of sensitivity. The "primary" effect is produced by general physiological changes in the cell which result in a temporary cessation of mitosis and clumping and stickiness of metaphase and anaphase chromosomes. The "secondary" effect results from direct "hits" on the chromosome which may produce chromosome breaks, deletions, and gross chromosome alterations, as well as gene mutations. The primary effect is temporary following light X-ray doses, but heavy dosage may prevent recovery and have a lethal effect. The secondary effect is permanent due to chromosome alterations, and is cell lethal if essential chromosome segments are lost.Although the entire cell is affected by X-rays, the nucleus seems to be particularly sensitive. Radiation sufficient to inactivate completely the nucleus of the egg causes no permanent injury to the cytoplasm (Packard, 1931). The relative effect on nucleus and cytoplasm has been effectively demonstrated in raying sperm and eggs of Arbacia. Irradiation of either eggs or sperm causes equal delay in the time of cleavage of the fertilized egg (Henshaw and Francis, 1936). When nucleated and enucleated egg fragments are rayed and fertilized with normal sperm, delayed and abnormal development is found only in nucleated egg fragments (Henshaw, 1938). These and other experiments indicate .that a study of nuclear alterations is of primary importance in an analysis of differential sensitivity of cells to X-rays. The results obtained from a study of the effects of X-rays on individual cells can explain onlv in part the problems of radiation therapy since the effect on tissues is much more dependent upon physiological and other indirect factors.MATERIALS AND METHODs.-The differential sensitivity during the nuclear cycle was determined from an analysis of the chromosomes in a clonal line of a diploid Tradescantia species. During the summer months the microspore nucleus is in the resting stage for four or five days after microspore formation. At about thirty hours before metaphase, prophase begins as indicated by the effective splitting of the chromosomes into sister chromatids. During the winter months the nuclear cycle requires about twice this time. Nuclei rayed in the resting stage produce chromosome aberrations, most of which are ] Received for publication August 21, 1940. This work was supported, in part, by a grant from the Milton Fund of Harvard University. Miss Margery Poole has made most of the preparations, and Dr. E. V. Enzmann did the raying. 52 dependent upon two independent hits. Radiation of prophase stages results in chromatid aberrations which may be produced by single hits or by two independent hits. The aberr...

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