MANNER OF PRODUCTION OF MUTATIONS 581 



much greater than that which can be demonstrated to have been produced 

 in the medium by the given irradiation. Catalase appUed to either the 

 irradiated or the hydrogen peroxide-treated medium prevented the kill- 

 ing, which therefore was caused by peroxides (or substances inter- 

 changable with peroxides and in equilibrium with them) in the medium. 

 Moreover, the reducer, sodium hydrosulfite (group 2), introduced into 

 the medium, afforded protection against the killing effect. However, 

 neither irradiated nor hydrogen peroxide — containing medium gave rise 

 to mutations; these were produced only by X rays received within the 

 cell. Corresponding with this, the hydrosulfite did not protect against 

 the mutagenic action of the X rays. Although it might be (jonjectured 

 that peroxides are not mutagenic in Paramecia, it seems more likely that 

 those in the medium become reduced before reaching the genetic material, 

 their killing action in this organism being on more superficially lying sub- 

 stances, and it may well be that the hydrosulfite also fails to penetrate so 

 far unchanged. 



On the other hand, ethyl alcohol (0.7 ill), representing group 3, did 

 reduce the frequency of the mutations induced in Paramecia, but it did 

 not protect against the killing action of the X rays. This finding is 

 understandable if the killing depended on reactions occurring in super- 

 ficially lying material since the alcohol in the medium, not having been 

 subjected to the required metabolic changes, would not be expected to be 

 effective. More surprising was the lack of protective action of formate 

 (group 4), not merely against the killing effect of the X rays (a fact which 

 fitted in with the above results) but also against their mutagenic effect. 

 If the formate had really penetrated unchanged, this result would indicate 

 that group 3 substances do not act by forming group 4. At any rate, it is 

 evident that differences in metabolism and in permeability must play 

 important roles in determining the effectiveness of any given substance in 

 affording protection against mutagenesis in any given type of organism. 



18. MUTAGENESIS BY COSMIC AND OTHER NATURAL RADIATION 



The question of the extent to which the ionizing radiation present in 

 nature is the cause of spontaneous mutations was first raised by Muller 

 (1927, 1928b) and has since been brought up on various occasions (e.g., 

 Olson and Lewis, 1928; Thomas, 1936). Although there is no doubt that 

 some mutations must originate as effects of natural radiation, it was cal- 

 culated by Muller and Mott-Smith (1930) that in Drosophila considera- 

 bly less than a thousandth of the natural mutations can be of this origin. 

 In this work all natural types of radiation were taken into account and 

 given maximum values, including those'of cosmic and earth origin and 

 those derived from radioactive substances (potassium, radium, etc.) 

 within the organism. Measurements of the radioactivity of Drosophila 



