CELL DIVISION, MORPHOLOGY, VIABILITY 



781 



The answer arrived at is 836 min, or about 14 hr for the duration of the 

 intermitotic period; yet we know from direct observations that under 

 good conditions most neuroblasts in hanging-drop preparations will go 

 through a complete mitotic cycle in 5-6 hr. Where is the error in this 

 calculation? In the first place, we know from direct timing experiments 

 that the average neuroblast passes through prometaphase, metaphase, 

 and anaphase, in 22 min while the 44 used above is approximately the 

 time taken by the slowest cells. If we use 22 instead of 44 in the above 

 equation, we arrive at an intermitotic time of about 7 hr, which is still 

 considerably higher than we observe. Probably this is not far from a 

 correct average figure when we take into account the fact that occasional 

 neuroblasts spend an excessively long time in interphase or early pro- 



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DOSE, r 



Fig. 11-6. Relation of dose of radiation to maximum depression of mitotic activity. 



Solid circle, chick fibroblast (Spear and Grimmet, 1933) ; triangle, cell of developing rat 



retina (Ta7isley, Spear, and Gliicksmann, 1937); open circle, Chortophaga neuroblast. 



phase, and so are often disregarded in the calculations as "abnormal" 

 cells. 



The efficiencies of different doses of radiation in decreasing mitotic 

 activity — as measured at the time after treatment when the effect is 

 greatest — are quite different for different kinds of cells. The mitotic 

 activity of Chortophaga neuroblasts is reduced to a much greater degree 

 than that of either rat retinal cells or chick fibroblasts by a given dose of 

 radiation (Fig. 11-6). A comparison of these efficiencies with those 

 obtained when testing the capacity of radiation to retard the mitotic 

 progress of the same cells treated in mid-mitosis points to the fallacy of 

 concluding that the relative radiosensitivities of different kinds of cells as 

 measured by one effect are necessarily correlated with their radiosensitiv- 

 ities as determined by another effect. A dose of 88 r, for example, which 

 reduces the mid-mitotic count of the neuroblast to zero and that of the 

 fibroblast to 20 per cent of normal, has no detectable effect on the mitotic 



