The effects of 250 r of x-rays in producing temporary cessation of mitosis in neuroblasts of the grasshopper, Chortophaga viridifasciata (De Geer), have been analyzed recently in terms of the stages of mitosis involved (Carlson, '40). It was found that a certain proportion of the cells in middle prophase at the time of treatment, instead of progressing to late prophase, reverted to earlier stages or at least underwent chromatin alterations that simulated reversion. Neuroblasts x-rayed in interphase and early prophase stages were prevented temporarily from entering middle prophase. These effects led ultimately to total temporary depletion of middle prophases and the consequent disappearance of the subsequent late prophase through early telophase stages. It was concluded from th% that middle prophase is the stage most sensitive to the effect of x-rays that causes temporary cessation of mitosis.
At the time those experiments were in progress it seemed evident that a smaller x-ray dose might serve better to emphasize the differential effects of such treatment on the mitotic 'This study was begun with the aid of a grant and the loan of a dosimeter from the Committee of Radiation of the National Research Council and completed a t the Department of Genetics of the Carnegie Institution of Washington as Rockefeller Fellow i n the Natural Sciences. 1 am especially indebted to Dr. Benjamin A. Wooten and Dr. Eric Rodgers of the Department of Physics of the University of Alabama for the use of x-ray equipment. My thanks are due Mr. Joseph Glaister f o r his aid in preparing much of the material for study. 'These stages are described in the 1940 paper. That and the present study were based on fixed and stained material, while the study of lieuroblasts in arti6cial culture medium (Carlson, '41) was based on observations of living, unstained cells; therefore different criteria had to be used to distinguish certain stages and the terminology applied to cortain of the mitotic stages differs somewhat. Late prophase in the present and the 1940 paper includes both late and very late prophase stages as reported in the 1941 paper. Early telophase in the former papers includes both early and middle telophases in the latter.