480 



RADIATION UIOLOGY 



The delay in the onset of cell cli\i.sion is put on a fiuantitative basis by 

 (ietermininf^ the time at which the sixlh (iJNision takes place; after this 

 (li\ision no further delay occurs. When the time of the sixth division is 

 plotted against the ultraviolet dose, for each wave length used two dilfer- 

 ent curves are obtained — one in darkness, the other after photoreacti- 

 vation (Fig. 12-9). The experimental points have high dispersion, but 

 curves drawn among the points show that the effect of photoreactivation 

 is compatible with a constant ultraviolet dose reduction. 



z 

 o 



> 



I 

 I- 



o 



a 



12 3 4 5 6 7 



DOSE, lO' ergs/mm2 



Fig. 12-9. Days to the sixth division plotted against dose of 2804 A ultraviolet. 

 Curve I, dark; curve II, light. Each point represents the arithmetical mean for 5 

 to 54 lines of descent from a single experiment. (From Kimball and Gaither, 1951.) 



Death before autogamy behaves in a similar way as the retardation of 

 cell division, and the authors consider that the two phenomena are likely 

 interdependent. 



Reduced vigor after autogamy, on the contrary, is not related to the 

 two effects already described, since it occurs in animals which had com- 

 pletely recovered from previous radiation disease. Since it occurs after 

 homozygosity has been produced, it has very likely to be attributed to 

 gene mutations; however, direct evidence for this has not been presented. 

 Reduced vigor is affected by photoreactivation in a similar way as the 

 genetic effect. 



