CELL DIVISION, MORPHOLOGY, VIABILITY 773 



that prophase is the mitotic stage most prolonged when the cell is irradi- 

 ated at an earlier stage. Exact comparisons, however, are not possible 

 at present, for Henshaw has not tested the immediate effect of irradiation 

 on the duration of the stage treated, and we as yet have little data on the 

 times required for neuroblasts X-rayed in different stages of mitosis to 

 complete mitosis. 



In dealing with a tissue in which the cells are in different mitotic stages 

 and in which the stage of any given cell is not known at the time of irradia- 

 tion, the experimental procedures are quite different and the interpreta- 

 tion of the results considerably more involved than when the individual 

 cells or groups of cells are in known stages during irradiation. A brief 

 survey of the postirradiation changes in the proportion of cells in different 

 mitotic stages will be followed by a comparison of the results and conclu- 

 sions of different investigators who have studied different kinds of tissues. 

 Exposure of a tissue to ionizing radiations is followed by a decrease in the 

 number of prometaphases, metaphases, anaphases, and telophases pres- 

 ent, the amount of the decrease being positively correlated with the dose 

 of radiation to which the material is exposed. If the dose is sufficient to 

 reduce the numbers of cells in these stages to zero, the length of time 

 during which they will remain at zero is directly related to the dose. In 

 such experiments the order of disappearance of the stages is prometa- 

 phases first, then metaphases, then anaphases, and finally telophases 

 (Carlson, 1942). During recovery from the irradiation effect these stages 

 reappear in the same order. As the number of cells in these stages 

 increases, it will temporarily exceed the normal if the dose is so small as to 

 produce mitotic delay only during part of the mitotic cycle. If the dose 

 is large enough to affect cells at all periods of the mitotic cycle, however, 

 the number of cells in these stages as a rule never exceeds the original. 

 With this general picture in mind of the events that follow irradiation of a 

 mitotically active tissue, we can undertake a more detailed analysis of 

 the radiation-induced changes. 



While there is general agreement that cells in prometaphase, meta- 

 phase, anaphase, and telophase at the time of irradiation subsequently 

 complete mitosis with little or no delay and that cells in interphase at 

 treatment may be prevented from entering mitosis by sufficiently large 

 doses of radiation, difference of opinion exists about the immediate reac- 

 tion of prophase cells to irradiation. Because the number of cells in 

 prophase is reduced as a result of irradiation, it has generally been 

 assumed that cells in this stage, like those in prometaphase-through- 

 telophase, are not very radiosensitive and so complete mitosis after 

 irradiation with little or no delay. This conclusion has been reached 

 from studies of mammalian tumors (Mottram, Scott, and Russ, 1926; 

 Warren, 1937), chick fibroblasts in vitro (Strangeways and Hopwood, 

 1926; Canti and Spear, 1929; Spear, 1931, 1932; Love, 1931; Lasnitski, 



