658 RADIATION BIOLOGY 



the duration of a given dose or the number of treatments into which it is 

 divided, and is not appreciably modified by temperature within the 

 narrow range that can be utilized while maintaining viability (and, in 

 some cases, fertility) of the test organisms (summary of evidence in 

 Catcheside, 1945). These experimental results add further weight to 

 the argument that chromosome breakage is not primarily attributable to 

 a long chain of chemical reactions operating over considerable distances 

 within the cell. But, as Fano, Caspari, and Demerec (1950) have 

 pointed out, experiments on time distribution could only show interaction 

 between separate events whose activity, even though not permanent, 

 lasts much longer than that of a chemical activation. Phenomena 

 exhibiting such properties are not among the usual physical effects of 

 radiation on matter and must therefore be characteristic of biological 

 materials. These authors have also pointed out that the direct propor- 

 tionality between dosage and genetic effect would hold for indirect as 

 well as direct action of radiation, provided that the efficiency of the 

 indirect mechanism was not sensitive to changes in temperature and 

 other factors. 



The more rigid concept of the target theory, that a single ionization 

 within a sensitive zone suffices to induce a break, was an outgrowth of 

 earlier studies of the conditions governing the induction of gene mutations 

 (summary of experimental data in Timofeeff-Ressovsky, 1937; and 

 critical evaluation in Muller, 1940, 1950a; Fano, 1942; Fano, Caspari, 

 and Demerec, 1950). This view has been tempered in recent years by 

 development of the interpretation that chromosome breakage is refer- 

 able to a cluster of ionizations produced by the passage of a single ion- 

 izing particle. Such clustering takes place at the tail end of the electron 

 track, or at places where the main track branches out into other short 

 tracks; it may occur along the entire length of the track of a recoil proton 

 (Lea and Catcheside, 1942; Fano, 1943a). Relative efficiencies of differ- 

 ent wave lengths and types of radiation have been explained on this 

 basis (Giles, 1940; Lea and Catcheside, 1942; Lea, 1946, 1947a; Gray, 

 1946; Catcheside, 1948). For example, soft X rays of wave length 

 4 A produce more breaks per roentgen than harder X rays, because the 

 spacing of ionizations is such that the whole track corresponds to the 

 "tail" end of tracks of hard X rays. If the spacing is too close there is a 

 loss of efficiency owing to oversaturation. Thus, wave length 8 A is less 

 effective per roentgen than other wave lengths. Additional evidence in 

 support of the concept that the large concentrations of energy resulting 

 from clusters of ionizations are required to induce chromosome breakage 

 has been obtained from comparative studies of X rays and neutrons 

 (summarized in Fano, Caspari, and Demerec, 1950). Thus the concept 

 that a single ionization serves to induce a chromosome break no longer 

 seems tenable. "When the identification of a hit as a chemical activa- 



