600 RADIATION BIOLOGY 



at least) it is usually very limited in regard to the distance traversed by it. 

 Similarly, great differences may conceivably occur in the length of time 

 between its initiation and completion, as well as in the number and 

 nature of the intermediate steps. 



These steps have already been found to depend, in part, on the nature 

 of the mutagenic agent used. Thus, Auerbach's observation of the long 

 delayed mutagenesis in Drosophila which often follows treatment with 

 mustard seems to support the conclusion that some of this delay is caused 

 by the genetic material itself being thrown into a relatively unstable 

 state, inasmuch as the instability appears to become reproduced with 

 the multiplication of the cells and, in given instances, to be concentrated 

 at a particular genetic locus. On the other hand, it will be recalled that, 

 after X irradiation of Drosophila, such instability does not occur to an 

 appreciable extent. In fact, the obtaining of evidence on this point 

 constituted one of the two main objectives of Muller's first work on X-ray 

 mutagenesis in Drosophila (MuUer, 1927, 1928b, c) since at that time the 

 question of whether or not the gene was compounded of several identi- 

 cal parts or strands was an acute one, and it had been shown in 1926 

 (Muller, 1929a) that, if this were true, the appearance of a temporary 

 instability of mutants, following their origination, would probably be 

 produced. In so far as the preliminary evidence allows an opinion, genes 

 treated with ultraviolet appear to resemble those treated with X rays in 

 their stability. This would argue for the mustard instability being due, 

 not to the gene being compounded of a number of equivalent elements 

 which may become segregated at mitosis, but to an actually self-repro- 

 ducing instability of composition being induced in the gene by the 

 mustard. 



Despite the general similarity of the results of mustard mutagenesis to 

 those obtained by radiation, in most respects there are important differ- 

 ences besides the one just mentioned, which show the existence of essen- 

 tial dissimilarities in the mutagenic pathway. The study of these and 

 similar studies with other chemical mutagens promise to throw a reflected 

 light on radiation mutagenesis itself. One of these differences lies in the 

 highly synergistic mutagenic action obtaining between ultraviolet and 

 mustard, as shown when they are applied in combination (Swanson and 

 Goodgal, 1947), as compared with the very slight synergism, where it 

 occurs at all, between ultraviolet and X-ray mutagenesis. Again, 

 mustard gives no signs of causing hydrogen peroxide production and 

 therefore probably fails to activate water. In correspondence with this, 

 Auerbach and Moser (1951) have found oxygen to have no effect on 

 mutagenesis by mustard in Drosophila. Yet it has been calculated 

 (Boyland, 1952) that, for a number of different biological effects, the 

 molar concentration of mustard required for a given cjuantitative result 

 is of the same order of magnitude as the frequency of activated water 



