MAJ^NER OF PRODUCTION OF MUTATIONS 561 



very different effective dose in the two oases. This would have influenced 

 the relative frequencies of the two types in an unknown way, especially in 

 view of the lack of knowledge of the chromosomal pattern which these 

 types actually represented. It therefore seems premature to accept these 

 results as throwing doubt on the conclusion of Riley, Giles, and Beatty, 

 which was based on the examination of structural changes of known types. 



Not all tests with oxygen and irradiation have given positive results. 

 Russell, Kile, and Russell (1951) reported that reduction of the oxygen 

 content of air to 5 per cent failed to decrease the frequency of dominant 

 lethals (presumably caused by chromosome changes) induced in sperma- 

 tozoa of mice, even though the reduction in somatic damage to the adult 

 mice was marked. On the other hand, the influence of oxygen during 

 irradiation in promoting chromosome aberrations has been extended to 

 Drosophila by Grodner and by Baker and Edington (unpublished, cited by 

 Hollaender, Baker, and Anderson, 1952). A plotting of the curve relating 

 frequency to oxygen concentration shows it to be strikingly similar to 

 that obtained for Tradescantia multihit aberrations by Giles and Beatty, 

 with its maximum almost reached at the oxygen pressure of air. Since 

 this curve, in turn, is similar to the frequency-oxygen curve obtained by 

 Gaulden and Nix (1950) for mitotic inhibition, the interpretation is 

 favored of an indirect action, similarly exerted in both cases by active 

 substances produced in the intracellular medium. It is also pointed out 

 by Hollaender, Baker, and Anderson that in this case the application of 

 the idea that the effect is on joining rather than on breakage would 

 require the assumption that the change in properties of the broken pieces 

 was retained for days, until after fertilization, since joining does not 

 occur in the spermatozoon stage. 



Another objection to applying the idea to Drosophila would seem to lie 

 in the indirect evidence obtained by Baker and Sgourakis (1949, 1950) 

 and the direct evidence by Baker and Von Halle (1952) that the presence 

 of oxygen as opposed to nitrogen during X irradiation results in the 

 production of more dominant lethals. Since these lethals are caused 

 mainly by unrestituted single breaks, this result may be interpreted as 

 meaning that oxygen acts by increasing breakage. However, certain 

 features of the data of Baker and Von Halle have caused them to question 

 this conclusion. One of the features mentioned by them, namely, the 

 smaller amount of increase of dominant lethals than of chromosome 

 aberrations by oxygen, is only to be expected on any view, because of the 

 former being mainly one-hit and the latter multihit phenomena. The 

 other feature, the lesser amount of oxygen effect on sperm used the second 

 day as compared with those used the first day after irradiation, would not 

 seem to offer greater difficulties on the view of oxygen affecting breakage 

 than on that of its affecting union. Similarly, in studies by Haas et al. 

 (1952) on the X-ray induction of translocations in Drosophila, the pro- 



