516 RADIATION BIOLOGY 



arises as a position effect of structural change.) A study of over 6,000,000 

 ommatidia in eyes, the X chromosome of which had in the spermatozoon 

 stage received over 3500 r, showed no white spots attributable to gene 

 mutation. From the fact that, at the given dose, 1 spermatozoon in 

 some 6000 is caused to have a mutation to white eye, it was reckoned 

 that, if there were a gene-mutational aftereffect, it must have become 

 weakened more than a thousand times before the 15-20 nuclear divisions 

 were completed which established the ommatidial rudiments. 



Similar but less definitive evidence to the same effect is to be found in 

 experiments in which males were irradiated in different embryonic and 

 larval stages and were examined, when mature, for mutant white patches 

 m the eye (e.g., Patterson, 1929). For in such work the size of the patch 

 agrees in a general way with the stage at which the exposure occurred, 

 and there is a conspicuous absence, in the eyes of flies given the earlier 

 treatments, of singly occurring small white patches, except as presumable 

 results of the migration of nuclei in flies having large patches as well. 



A different kind of aftereffect of ionizing radiation on mutagenesis 

 might conceivably be a sensitization or desensitization of the irradiated 

 material to subsequent irradiation. It has been noted on pp. 485-486 

 that Lane has reported chromosome breakage in Tradescantia microspores 

 to be hindered by a prior irradiation, although this is disputed by Sax. 

 Gulbekian (1936a) reported the opposite effect with regard to the produc- 

 tion of lethals in Drosophila, inasmuch as males which were irradiated as 

 pupae and again as adults appeared to give a higher lethal frequency than 

 the sum of those produced by irradiating only the pupae and only the 

 adults. In this calculation he took zero as the frequency for the pupae 

 alone since he obtained no mutations at all from them, and he inferred 

 that the dose used was below the threshold for an effect, when applied to 

 this stage alone. However, Timofeeff-Ressovsky (1937a), in a retest of 

 this question, found that the individuals with the combination treatment 

 showed, well within the limits of statistical error, the value expected on 

 an additive (nonsynergistic) relation. He also found that the mutation 

 frequency obtained by irradiating the pupae alone varied linearly with 

 the dose applied to them instead of giving any evidence of a threshold. 

 Finally, he found that irradiations divided into six doses given on con- 

 secutive days, with the first dose given in the pupa stage, resulted in the 

 same lethal frequency as if the same total dose were concentrated 

 together on the last day. 



This result is in line with the other work previously cited, showing the 

 lack of effect of differences in the timing of irradiation on the production 

 of lethals in Drosophila spermatozoa. There is, however, a notable lack 

 of data on this subject for cells other than spermatozoa, and it cannot be 

 taken for granted that, in these other more typical and less dormant cells, 

 the radiosensitivity to mutagenesis might not be affected and even 



