NATURE OF THE GENETIC EFFECTS 385 



broken off (leading to restitution) and, conversely, more likely to come 

 into contact with ends derived from other breaks than if they had been 

 broken during interphase, after their positions had become largely 

 stabilized. Thus diverse structural changes finally result from irradia- 

 tion during mitosis. 



The above analysis was, in fact, first arrived at through studies (Muller, 

 1939b, c, d, 1940a) of the effects of irradiation apphed to the chromosomes 

 in mature spermatozoa rather than in mitotic stages. In mature 

 spermatozoa as in mitotic stages the chromosomes are in a highly spiral- 

 ized, condensed condition. It was possible to prove, by noting the 

 effect of variations in the timing and the dosage of the radiation on the 

 frequency of structural change, that the breaks arising in these sperma- 

 tozoan chromosomes are all retained as such, without restitution, until 

 after fertilization, when the pieces become able to enter into diverse forms 

 of structural rearrangement. Hence irradiation of spermatozoa, as well 

 as that of cells in and near mitosis, is much more likely to lead to struc- 

 tural changes than is irradiation of ordinary interphase nuclei. Some 

 results have recently been obtained by Ray-Chaudhuri and Sarkar (1952) 

 which they interpret as indicating a similar delay in fusion of broken ends 

 in locust (Gesonia) spermatocytes. However, in cytes, unUke sperma- 

 tozoa, the high degree of separation of the chromosomes allows much less 

 opportunity for contact between different chromosomes or chromosome 

 regions before union occurs. In conformity with this situation it has 

 been found that most of the structural changes induced in Drosophila 

 oocytes are intrachromosomal, and mainly in the class of minute changes 

 (Glass, 1940; Muller, R. M. Valencia, and J. I. Valencia, 1950). 



Although there must be some movements of broken ends to enable ends 

 derived from different breaks to meet and result in structural changes, 

 nevertheless these movements are rather restricted in range, even in the 

 production of translocations in chromosomes derived from irradiated 

 spermatozoa. The evidence from neutron irradiation (cited in Chap. 8) 

 shows that two breaks which were produced in close proximity in the 

 spermatozoon give a much better chance of leading to a structural change 

 than do two breaks that were farther apart at the time they were pro- 

 duced. This type of spatial restriction must be far more marked when 

 the irradiation is applied to interphase nuclei, since in these the chromo- 

 somes remain, relative to their lengths, much more fixed in position 

 between the times of breakage and union. 



The influence of movement in promoting structural change is further 

 shown in Sax's (1942) treatments of Tradescantia microspores with two 

 successive irradiations. Chromosomes broken by the earlier irradiation 

 were found to have more breaks that failed to restitute produced by the 

 later irradiation in their centric fragment, which is of course more subject 

 to movement, than in their acentric fragment. Moreover, Sax (1943) 



