1236 BIOLOGICAL EFFECTS OF RADIATION 



males, for the X-chromosome, only mutation can be accountable. Soma- 

 tic mutations occur and, as Patterson found, the extent of the size of 

 the mosaic patch depends on the time of raying. The earlier the treat- 

 ment, the greater the number of cell divisions subsequent to it, and the 

 larger the mosaic patch accordingly. To compute the frequency of 

 somatic mutation is a diflficult matter; it is necessary to refer the muta- 

 tion back to the total number of cells in the anlage when it occurred, 

 in order to obtain a proper value. Patterson's attempt to reach a value 

 in the percentage of the total number of adult units which have mutated 

 is difficult to accept, even as a rough approximation. Further data are 

 necessary. 



It may be remarked that the same experiments have been used to 

 demonstrate chromosome breakage in the somatic cells. The data are 

 not quite satisfactory, owing to Stern's (168) recent demonstration of 

 somatic crossing over which is probably also involved in Patterson's 

 cases of somatic segregation (132). The data interpreted as resulting 

 from chromosome breakage show suggestive similarities to those of 

 Stern, and while it is probable that there is a high frequency of chromo- 

 some breakage somatically, further data are needed to show what propor- 

 tion of the mosaics are due to somatic crossing over, and what to breakage. 

 In the one case where a distinction between the two is possible in Patter- 

 son's experiments (the homozygous eosin female) no breakage was 

 detected. Patterson's explanation on the grounds of genie balance 

 relations which would give this result can be shown to be invalid. 



On the whole, it appears that the existing data can best be understood 

 in terms of similar rates of mutation in all cells. That this is not neces- 

 sarily true in general is shown by the experiments of Demerec (18, 19, 20) 

 on frequently mutating genes in Drosophila virilis, where definite genetic 

 factors have been shown to influence the stability of the genes in different 

 tissues. 



THE MODE OF ACTION OF RADIATION 

 GENERAL CONSIDERATIONS 



The previous sections have been devoted to a description of the 

 relations of internal factors in the mutation process. This is essential 

 for the understanding of the mode of action of radiation. But the proper 

 analysis of the process from this point of view involves quantitative 

 determinations of mutation rates under varied conditions of radiation. 



It may be well first to consider what to expect in such experiments; 

 how in general does radiation produce its effects and, among the different 

 possibilities, which lead to different experimental results? The first 

 result of radiation is the ionization of matter, the release of electrons; 

 the resultant chemical reactions are to be attributed to this ionization. 

 Secondary effects are then possible by way of interaction between prod- 



