MANNER OF PRODUCTION OF MUTATIONS 489 



cent higher than that given by the two shorter wave lengths, whereas 

 8.3 A gives a far lower frequency than any of the others. Lea (1946) 

 interprets these results as meaning that about 15 ionizations are usually 

 necessary to produce a break; for he calculates that, considering the 

 thickness of a chromatid thread (reckoned to be 0.1 n), groups of ioniza- 

 tions of at least this size (15 ionizations), lying at the ends of tracks, fall 

 within a thread in correspondingly greater abundance (i.e., about 60 per 

 cent oftener) when 4.1-A than when 1.5-A radiation is used, while with 

 the 8.3-A treatment, a group of ionizations as large as this comes to lie 

 inside a 0.1-m thread far less often. Curiously enough, he finds a wave- 

 length-frequency curve of the same shape, with a peak at 4.1 A, on cal- 

 culating the probabilities, at the different wave lengths, of a large enough 

 cluster coming to lie simultaneously within each of two contiguous sister 

 chromatids, so as to give rise to isochromatid breaks. 



In considering the meaning of these data it should be borne in mind 

 that, at the stage at which the irradiations were carried out and for some 

 time thereafter, the two sister chromatids derived from each chromosome 

 were in a relatively extended condition, with their corresponding parts 

 lying in close juxtaposition with each other. Evidently they were held 

 together by surrounding material and/or by forces of attraction, very 

 likely by those exerted between homologous genes. Hence, in cases in 

 which only one chromatid of a pair of sister threads was broken, the two 

 broken ends would be more likely to meet again because of the binding 

 action exerted by the other chromatid, so as to cause restitution of the 

 broken thread before the stage at which cytological observations were 

 made, than if both chromatids had been broken at once at nearby points ; 

 if both had been broken, the splintlike influence of one chromatid, tend- 

 ing to pull the pieces of its broken sister thread back into a position for 

 restitution, would be absent or much weakened. 



Now if the action of the ionizations in causing breakage is narrowly 

 localized, as may be inferred on other grounds (see pp. 517-525) as 

 well as on those provided by this experiment, then those types of radi- 

 ation which give, in proportion to the total ionization produced by 

 them, more ion clusters of a length great enough to be within "striking" 

 range of the two sister chromatids at once, would tend to cause more 

 cases of breakage of both chromatids simultaneously, in relation to cases 

 of breakage of single chromatids. Thus the reason that radiations of 1.5 

 and 0.15 A are less effective than that of 4.1 A in causing simultaneous 

 breaks of two sister chromatids must be essentially that a larger propor- 

 tion of the ionizations produced by the 1.5- and 0.15-A radiation are 

 scattered, while that of 8.3 A is much less effective than 4.1 A because, 

 although all its ionizations occur in clusters, these are usually too short 

 (Lea, 1946) to be able to affect both sister chromatids. 



It can be shown that this effect of the wave length of the radiation 



