Chromosomes and Genes 103 



and Wagner et ah, 1950), indicating that the Hberation of oxygen 

 plays a major role in mutagenic action by affecting a peroxide. The 

 decisive experiment is that an irradiated medium also produces muta- 

 tion in bacteria. Whatever the role of the peroxide intermediary turns 

 out to be, it is hardly possible to reconcile the facts with the Treffer 

 theory. 



Another major group of facts has to do with agents or conditions 

 which change the radiation effect upon mutation rate. The most 

 important of these is the role of oxygen concentration in altering the 

 radiation effect (see Hollaender, Baker, and Anderson, 1951; and 

 Hollaender et al., 1951). Such a result may mean that the agent, 

 actually oxygen concentration, affects the "gene" itself so as to change 

 its X-ray sensitivity. In this case the Treffer theory would not be 

 affected. Or there are mutagenic substances produced by X-raying in 

 the chromosomal neighborhood. If so, the results would go in the 

 same direction as those of Stone, against the Treffer theory. Experi- 

 mental evidence (by Giles, Riley, and others; see Giles, 1952) indicates 

 that chromosome breaks are produced in larger quantity when ir- 

 radiation is done under oxygen tension, but not if oxygen treatment 

 precedes or follows the irradiation. This is in favor of the mutagen 

 explanation as opposed to the Treffer theory. The conclusion is greatly 

 supported by the fact that the same curve for frequency of effect after 

 irradiation with different oxygen concentrations is obtained for dif- 

 ferent effects as mutation, chromosomal breakage, and mitotic delay. 

 This would be expected only when the production of a mutagenic sub- 

 stance is decisive, since a direct change of the "gene" could hardly be 

 identical in all these circumstances (Hollaender et al., 1951). Ob- 

 viously, the mutagenic effect of the intermediary substance must occur 

 near the chromosomes, for irradiation of sperm (closely packed chro- 

 mosomes) produces the effect, while irradiation of egg cytoplasm 

 before fertilization is ineffective. Extensive work on bacterial mutation 

 leads to the same conclusions, using different media which affect the 

 sensitivity to radiation. Again there is at least a strong suspicion (see 

 Stone et al.) that a peroxide is involved which acts in statu nascendi. 

 There is much more work available in this same line of antimutagenic 

 agents and agents influencing the mutagenic actions. (See the large 

 amount of work of the Hollaender group and the Soc. Exp. Biol, sym- 

 posium on chromosome breakage, 1953.) 



However, criticism of the theory of peroxides as intermediates 

 and of the conclusions of Giles is not lacking. The details are not im- 



