MANNER OF PRODUCTION OF MUTATIONS 515 



chromosome. It was found that, although many gynandromorphs were 

 produced which showed a large amount of male tissue, comprising a 

 fourth to more than half the body, and although this tissue appeared white 

 in the eyes, nevertheless there was no higher frecjuency of small white 

 patches, indicative of later loss, than in the nonirradiated material. 

 Since the cells of the optic rudiments are laid down very early and there- 

 after divide to form in each eye nearly a thousand ommatidia, all of which 

 are autonomous in regard to the red-white color difference, it can be con- 

 cluded that there is neither a short-term nor a long-term aftereffect in 

 the production of these chromosome breaks (in this connection see also 

 the discussion at the end of the second paragraph below). 



Observations of cytologists on the effects of irradiation on chromosomes 

 of kinds too varied to list have likewise provided evidence that chromo- 

 some breakage is confined to the exposed cell generation. For although 

 the breakage-fusion-bridge cycle often continues to give secondary break- 

 age, there is a conspicuous lack of good evidence that new primary 

 breakage, many breaks of which would be manifested by the apparition 

 of new acentric fragments, arises in cell generations subsequent to the 

 treated one. Corroborating this is the evidence, referred to earlier, that 

 two or more irradiations given to ordinary interphase cells at widely 

 separated intervals are unable to act synergistically in the production of 

 structural changes involving two or more breaks. If, however, breakage 

 continued to occur as an aftereffect, such synergism would result, as 

 evidenced by the fact that with short exposures these structural changes 

 increase in frequency nearly as the square of the dose. 



Except for the experiments cited on pp. 511-512, indicating the absence 

 of any aftereffect in the production of lethals that extends over one or 

 more generations of individuals, the work thus far described in this 

 section has included no reliable observations bearing on how much delay 

 the production of actual gene mutations may be subject to. For it 

 seems probable that most if not all the visibly mosaic individuals dealt 

 with in the work cited have represented position effects or manifestations 

 of aneuploidy, resulting in either case from chromosome breakage. This 

 is a matter awaiting more intensive analyses. There is, however, one 

 series of observations relevant to the question of whether gene mutations 

 arise as aftereffects of ionizing radiation. This is a study reported by 

 MuUer (1930) in which white spots were looked for in the eyes of males 

 whose X chromosomes had been derived from irradiated spermatozoa. 

 The technique was much like that in the previously cited experiment of 

 Battacharya except that an ordinary rod-shaped X chromosome was 

 employed and that male rather than female offspring were under observa- 

 tion, so that any white spots would be ascribable to gene mutation 

 (and/or ultrammute deletion) rather than to breakage followed by gross 

 chromatin loss. (In this connection it may be noted that white seldom 



