X-RAYS 423 



ment ; seeds mutate less freely in dormancy than during germina- 

 tion. Organisms also vary greatly in their susceptibility, fungi 

 being affected only by very high dosages (Dickson, 1933). It 

 follows from these considerations that although the types of gene 

 mutant induced are the same as the natural ones, the mechanism 

 of change is different (c/. App. II). 



Side by side with gene changes, structural changes are induced 

 in the chromosomes by irradiation. Simple deficiencies perhaps 

 occur like gene mutations in proportion to the dose (Stadler, 1932), 

 but more complicated changes do not show these simple relation- 

 ships {cf. Ch. XII). 



In order to compare the effect of treatment on structural change 

 with that on gene-change we must remember that the chromosomes 

 may break (by definition) betw^een any two genes. Muller and his 

 collaborators (1935) have, in fact, shown in particular segments in 

 Drosophila that any pair of adjacent chromomeres distinguishable 

 in ultra-violet light may be separated by X-ray breakage. The 

 natural frequency of breakage in any given interstitial segment 

 is therefore probably lower than that of any genes whose mutation 

 rate has yet been accurately measured. The frequency of breakage 

 that may be induced is, on the other hand, perhaps as high as that 

 of any genes. An exact comparison is impossible as we shall see 

 later on account of differential mortality at different stages of 

 development in different organisms. It seems, however, that in 

 general the differences in the natural order of mutability ; mutable 

 gene — stable gene — gene thread are reduced by treatment. 



The kinds and degrees of change seen in the chromosomes after 

 treatment depend on three factors : (i) how far the general condition 

 of the cell or the organism has been modified ; (ii) the stage of the 

 nuclear cycle, whether meiosis or mitosis, that has been treated ; 

 (iii) the extent of the nuclear cycle or the life-cycle which has elapsed 

 between treatment and observation. We wiU first consider the 

 effect of this interval. 



When root-tips or spermatogonia are studied a few hours after 

 treatment we can be sure that the nuclei are passing through their 

 first mitosis. We then see that chromosomes of new shapes and 

 sizes are produced. These have been described as the result of 



