Induction of Chromosomal Aberrations 241 



sense " when viewed from the point of view of radiochemistry, 

 certain pieces of data indicate that the radiochemical events 

 in the cell can be magnified without appreciably altering the 

 final frequency of aberrations. Gray, on several occasions 

 (1953, 1954a and b), has pointed out that there exists a close 

 parallel between the number of ion pairs per unit of path 

 length and HgOg production on the one hand and the frequency 

 of aberrations on the other. It is a well known fact, however, 

 that the frequency of aberrations is proportional to the oxygen 

 concentration only at low levels; when the concentration of 

 oxygen rises above 20 per cent little increase in aberration 

 frequency is found even though HgOg production continues to 

 increase. Allen (1954) and Ebert (1955) have also shown that 

 the addition of small amounts of hydrogen to oxygenated 

 water leads to a striking increase in HgOg production, pre- 

 sumably by promoting the reaction: 2 HOg + Hg -^ 2 H2O2. 

 If it is presumed that the cell behaves as an aqueous system, 

 then increases in aberration frequency are to be expected 

 when the cells are exposed to oxygen-hydrogen mixtures. 

 Mr. T. Merz has carried out these experiments in our labora- 

 tory, using a variety of hydrogen-oxygen mixtures, and finds 

 that while in Tradescantia microspore chromosomes chromatid 

 deletions are somewhat increased whenever hydrogen is 

 added to oxygen, exchanges and isochromatid deletions 

 remain relatively unaffected. Dominant lethals in Drosophila 

 are also greater in a mixture of 20 per cent Og : 80 per cent Hg 

 than they are in air, but there is no obvious relationship 

 which suggests that the amount of radiation damage is 

 proportional to H2O2 production. These results are also 

 supported by the data of Kimball (1^55) which indicate that 

 H2O2 is not involved in the induction of genetic damage in 

 Paramecium. There is danger, of course, in forcing too close a 

 comparison between what is known to occur in oxygenated 

 water and what is expected to occur in a cell which would be 

 buff'ered against environmental change. Several interpre- 

 tations are possible here, but more important is the fact that 

 the data force us to look beyond the radiochemical events for 



