676 



RADIATION BIOLOGY 



as compared with values obtained from controls that received only the 

 X-ray treatment. 



Comparable results were obtained when near-infrared radiation was 

 used in combination with nitrogen mustard in treating spermatozoa; pre- 

 treatment effected an increase of about 50 per cent in the frequency of 

 rearrangements, but there was no significant increase when near-infrared 

 radiation was used after the spermatozoa had been exposed to the 

 chemical (Kaufmann, Gay, and Rothberg, 1949). 



Since pretreatment did not increase the frequency of single-break 

 dominant lethals, no general increase in number of all types of breaks was 

 involved in its effect. The action of near-infrared radiation in "sensitiz- 



Table 9-7. Effects of Treatment with X Rays and with X Rays plus Near- 

 infrared Rays 

 (Kaufmann, Hollaender, and Gay, 1946) 



Treatment of males 



24-72 hr of infrared + 4000 r 

 of X rays 



4000 r alone 



4000 r of X rays 4- 24-72 hr 

 of infrared 



Frequency of chromosomal breaks 



Total 

 sperms 



721 

 549 



483 



Rearrange- 

 ments 



312 

 169 



133 



Per cent 

 rearrange- 

 ments 



43.27 ± 1.8 

 30.78 ± 2.0 



27.50 ± 2.0 



No. of 

 breaks 



850 

 435 



386 



Breaks 

 per 100 

 sperms 



117.9 

 79.2 



79.9 



ing" the chromosomes therefore appears to be restricted to the control 

 of processes involved in the formation of chromosomal aberrations. In 

 other words, the subsequent behavior of some breakage ends is modified 

 by conditions created as a consequence of pretreatment, whereas that of 

 others is not. In explanation of this difference it has been suggested that 

 the single-break, dominant-lethal aberrations originate at the time of 

 irradiation of the spermatozoa, whereas the recombination of other break- 

 age ends is delayed until after fertilization. Supplementary lines of 

 evidence in support of this interpretation have been presented in detail 

 by Kaufmann and Wilson (1949). 



In the light of these observations, the breaks induced by irradiation 

 have been classified as "complete," or thoroughgoing, and "potential" 

 (the latter in conformity with the concept developed by Muller, 1940, 

 1941; Kaufmann, 1941b; Fano, 1941). Various lines of experimental 

 evidence indicate that the potentialities of individual breaks to become 

 restituted or to participate in structural rearrangement can be modified 

 by various kinds of supplementary treatment (e.g., by treatment of 

 spermatozoa with radiation of wave length 2537 A after treatment with 



