248 C. P. SwANSON AND Bengt Kihlman 



those which may persist for some time. The studies of Wolff 

 and Luippold (1955) and Beatty, Beatty and CoUins (1956) 

 with ionizing radiations and those of Kihlman (1955, and 

 unpublished) with chemical mutagens make stage IV a neces- 

 sary part of the chain of events. There is no need here to dis- 

 cuss the physical and radiochemical aspects of this system 

 since they have been adequately covered by Gray elsewhere 

 (1953; 1954a and b), but it is of interest to assess our present 

 knowledge of the relationship between the Latar jet-Gray 

 chain of events and those taking place at the chromosomal 

 level. Stages I-III are obviously related to breakage, but to 

 weigh the contribution of each stage to breakage would re- 

 quire that one determine the relative importance of direct 

 versus indirect effects of ionizing radiations. At present it 

 would appear that we can only state that the indirect effects 

 seem to outweigh the direct effects if only because we realize 

 that a small amount of radiant energy introduced into a cell 

 can lead to an inordinate amount of damage. No direct and 

 convincing evidence is yet at hand which permits us to state 

 that metabolic systems in the cell have anything to do with 

 the breakage of chromosomes by ionizing radiations. It is 

 true, of course, that the sensitivity of the chromosomes 

 changes with the stages of cell division, but this variable 

 sensitivity may well be due to the state of the chromosome 

 rather than to any metabolic system which contributes 

 directly or indirectly to breakage. However, the fact that 

 spontaneous breakage is a variable phenomenon from species 

 to species, and often within species, suggests that it may be 

 due to altered physiological conditions such as those brought 

 on by nutritional deficiencies (Steffensen, 1953). On the other 

 hand, it seems quite certain that the metabolism of the cell 

 governs the action of certain chemical mutagens. Thus, as 

 indicated earlier, MH and EOC are both dependent upon the 

 oxygen of the cell for effective action, and the effects of both 

 are suppressed by inhibitors of oxidative phosphorylation. 

 The chemical mutagens, to be sure, would not involve stages 

 I-III in acting on chromosomes, but in their place it is prob- 



