246 C. P. SwANSON AND Bengt Kihlman 



produces a heavy concentration of aberrations in hetero- 

 chromatic segments in the middle of the long arm of S- 

 chromosomes (Loveless and Revell, 1949). MH-induced 

 breaks are also localized in heterochromatin, but in this 

 instance, the heterochromatin lying close to the centromere 

 in the nucleolar arm of the L-chromosomes is most frequently 

 involved (McLeish, 1953). EOC confines its major effect to the 

 same arm, but the aberrations involve principally the nucle- 

 olar constriction (Kihlman and Levan, 1951). The distri- 

 bution of breaks is more strikingly non-random the lower the 

 concentration of the mutagen, indicating that there are 

 preferential sites of action which may become obscured, how- 

 ever, if the concentration of mutagen is increased. 



Two other observations are of interest in this respect. The 

 localization of breaks differs also with time after treatment as 

 well as with concentration of mutagen. DEPE seems to 

 induce a lowered percentage of localized breaks at 48 hours 

 after treatment than it does at 24 hours, while EOC-induced 

 breaks are more sharply localized at 22 hours than they are 

 either earher (6 hours) or later (48 hours). What these data 

 mean in terms of a mechanism of action is not entirely clear. 

 The suggestion has often been made that chemicals and ioniz- 

 ing radiations induce breaks in interphase chromosomes 

 because of their interference with DNA synthesis. No critical 

 evidence bears on this point, but the differential times and 

 sites of action, as well as the differential effectiveness of 

 mutagens and radiations under conditions of anoxia and in 

 the presence of inhibitors of oxidative metabolism, would 

 indicate that an induced disturbance of DNA synthesis cannot 

 provide a complete and satisfactory answer as to their mode 

 of action. The seeming correspondence of many of these 

 results with those obtained by Wolff on the rejoining process 

 is striking, and suggests a common metabolic pathway some- 

 where in the chain of events leading to the observable aber- 

 rations induced by ionizing radiations and chemical mutagens, 

 but it remains to be determined whether these common path- 

 ways are at comparable stages in the chain of events. 



