MANNER OF PRODUCTION OF MUTATIONS 509 



0.5-A wave length the surface of the body nearer the source of radiation 

 had to be exposed to a dose 20 per cent higher than with ordinary X or 7 

 rays in order to attain the same amount of radiation within the gonads, 

 because of the greater absorption of the softer radiation by the superficial 

 layers of the body, and with the 1.25-A wave length the surface radiation 

 had to be 100 per cent greater than with ordinary X or 7 rays. The fact 

 that the mutation frequency remained unaffected by these differences in 

 superficial exposure showed (even when allowance is made for the lesser 

 exposure of the remoter side of the body when softer rays were used) that 

 the irradiation of the somatic cells did not cause a perceptible production 

 of mutations in the germ cells, despite the very small distance separating 

 them. More striking was the fact that, when still softer radiation, of 

 5-A wave length, was used, practically none of which penetrated as deep 

 as the gonads, even a superficial exposure of nearly 1,000,000 r resulted in 

 no appreciable production of mutations. 



It might be postulated that, in these experiments on the spatial Umita- 

 tion of the mutagenic influence, its diffusion to the unexposed chromo- 

 somes in nearby tissues was obstructed mainly by the presence of cell 

 boundaries. In that case, irradiation of germ cells of one sex before 

 fertilization might later result in mutations in the chromosomes supplied 

 by unexposed germ cells of the opposite sex which had engaged in fertiliza- 

 tion with the exposed germ cells, provided that this supposed indirect 

 influence was capable of persisting until after fertilization. Some data 

 bearing on this possibihty were presented in the very first experiments on 

 the production of mutations by radiation in Drosophila (Muller, 1928b). 

 These showed that the cross influence (''transverse induction"), if it 

 exists, must be a very slight one, either of the exposed female germ cells 

 on the unexposed genetic material supplied by the father or in the oppo- 

 site direction. Further tests of the matter were later carried out, in 

 which only female germ cells were irradiated since, because of their far 

 greater bulk, these cells would have a much better chance of transmit- 

 ting such an effect. It was found, in the experiments of Muller (1930), 

 of Timofeeff-Ressovsky (1931b, 1937b), of Bonnier et al. (1952), and of 

 Hollaender, Baker, and Anderson (1952), that there was no detectable 

 production of mutations in the chromosomes derived from untreated 

 spermatozoa which had entered the eggs of exposed females. 



Experiments of Timofeeff-Ressovsky (1937b) showed also that muta- 

 tions were induced by X rays in Drosophila spermatozoa with the same 

 frequency regardless of whether the eggs which they fertilized had been 

 derived from irradiated or nonirradiated females. On the other hand. 

 Bonnier et al. (1952), in another series of tests, obtained a slightly higher 

 mutation frequency in chromosomes derived from irradiated spermatozoa 

 when these had fertilized eggs from irradiated than from nonirradiated 

 females. Although this result might be interpreted as the effect of a 



