XIV. X RAYS AND X IRRADIATION 481 



able changes. Two factors are important for success in the study of 

 this question — the irradiation of fairly large numbers of individuals 

 and the methods of selecting from a very large population of normal 

 individuals those few that have the genetic effects sought. There 

 are various Avays of accumulating mutations in a species. In Dro- 

 sophila a balanced lethal technique w ith inversions to prevent crossing 

 over allows a chromosome to accumulate and hold large numbers of 

 mutant genes. After the accumulation process has been carried on 

 for some time, it is possible to separate the genes by outcrossing the 

 chromosomes to normal stock and analyzing them for the gene changes 

 that have taken place. 



Another method of considerable importance is that of isolating 

 desired bacterial mutations produced through radiation effects (36). 

 On culture media bacteria of a given species tend to be selected for 

 those mutants that are favorable to this saproph3^tic life. If the 

 organism studied is a disease-producing type, this means that the 

 introduction of these nonpathogenic variants will lower the average 

 pathogenicit}' of the bacterial strains. On the other hand, when the 

 organism is in its normal host only those bacteria that have virulence 

 will survive (54), and l)e selected to continue the strain. These selec- 

 tion effects can be used in picking out the particularly desirable 

 types of organisms that may result from irradiation changes in the 

 genes. 



The significance of l)acterial mutation to the effect oi various 

 drugs in controlling pathogenicity has been studied in this manner 

 (4^). The bacterial strains are made uniform for a given type by 

 micropipet techniques in which one organism is picked out, placed on 

 suitable culture media, and allowed to grow into a colony fioni which 

 the strain is started. The progeny of this single bacterhmi are 

 considered genetically alike. The bacteria may then be treated 

 with the drug, i.e., any sulfa drug, and sho\\ii to be all susceptible. 

 The object of study may be to establish bacterial strains resistant to 

 these drugs through mutations produced in them by X rays. The 

 original bacterial culture is subdivided into many lines by selecting 

 individual organisms through micropipet methods or ])y plating and 

 isolating single colonies if the species under investigation normally 

 has only single cells. A subsample from eacli of these strains is 

 plated and examined for mutations (51). If the treatment is adap- 

 tive, the different strains should show constant, similar variations. 

 If, however, segregation of the original bacteria has taken place or 



