Induction of Chromosomal Aberrations 243 



rays, and with intensities ranging from 1 to 50 r/minute, it 

 has been shown that the frequency of chromosome inter- 

 changes increases as the intensity decreases when exposures 

 were carried out in the absence of oxygen (hehum was used to 

 replace the oxygen of the cell). The reverse trend, of course, 

 holds for irradiations in air or in oxygen. The frequency of 

 aberrations obtained with an intensity of 1 r/minute in helium 

 was approximately equal to that found at an intensity of 

 25 r/minute in pure oxygen. These results are somewhat 

 unexpected, and following the line of reasoning expressed by 

 Wolff and Luippold (1955), it would appear that quite dif- 

 ferent sets of conditions prevail in the oxygen as opposed to 

 the helium series. Radiation at a comparatively high inten- 

 sity in oxygen leads to a high rate of breakage, but this is 

 offset by the fact that a large proportion of the breaks restitute 

 during the period of exposure ; high frequencies of aberrations 

 are obtained in the low intensity helium series with a consider- 

 ably lower frequency of breaks but with negligible rejoin- 

 ability during the period of radiation. It has logically been 

 assumed by Beatty, Beatty and Collins (1956), in the 1 

 r/minute helium experiments, that the circumstances of 

 anoxia are greatly exaggerated by a continued depletion of 

 residual oxygen through respiration during the 400 minutes 

 of exposure to radiation. The energy sources of the cell 

 which might otherwise be available for rejoining purposes 

 would be sharply depressed by removal of the oxygen, 

 and all breaks induced would remain open and be available 

 for rejoining when oxygen was once again added to the cell. 

 This hypothesis can be tested further. If cells are pretreated 

 for 400 minutes in an oxygen-free atmosphere, and then 

 irradiated at various intensities, they should have their 

 energy reserves at a low ebb, rejoining should not take place, 

 and no intensity effect should be observed. Or, conversely, 

 pretreatments in an oxygen-free atmosphere for varying 

 periods of time, followed by radiation at a constant intensity, 

 should yield frequencies of aberrations which increase as the 

 duration of the pretreatment increases. The latter experiment 



