588 RADIATION BIOLOGY 



were of types which occupied "strategic" positions. However, despite 

 this expectation it is very difficult to obtain empirical evidence bearing on 

 this matter, a good deal more so than in the case of administered radio- 

 isotopes. This is because many of the atoms which can be transmuted 

 by thermal neutrons are, like boron, not present in the cell in the genetic 

 material or in compounds likely (when thus altered) to affect the genetic 

 material, although at the same time the ionizing radiation generated in 

 connection with these transmutations is so considerable that it would 

 produce many gene mutations. On the other hand, those transmutable 

 elements, such as nitrogen, which probably do occupy strategic locations, 

 are also so abundantly represented in other parts of the cell that the 

 mutations produced via the ionizing radiation would in this case too be 

 likely far to outweigh in numbers any produced by the transmutation 

 itself, occurring in these relatively rare locations. Thus despite the work 

 (Conger and Giles, 1950) indicating the high mutagenicity of thermal 

 neutrons and other work indicating their peculiar damaging effects, there 

 is as yet no real evidence that thermal neutrons produce a significant 

 proportion of mutations by virtue of the chemical changes resulting from 

 the transmutations of atomic nuclei engendered by them. The ionizing 

 particles, by virtue of which they produce most of their mutations, result 

 for the most part in densely ionized tracks, which have mutagenic effi- 

 ciencies and frequency-dosage relations similar to those of other fast 

 charged particles. 



It remains to consider the possibility of producing mutations by forms 

 of radiation other than ionizing radiation and ultraviolet and by other 

 influences of a nature primarily physical. The question of the mutagenic 

 influence of visible light has already been considered (pp. 541-542). That 

 of radiation longer than visible light has been studied on various occasions. 

 Infrared, although increasing the frequency of chromosome changes pro- 

 duced by ionizing radiation, has always been found to be mutagenically 

 ineffective when given alone. Exposure of Drosophila to extremely 

 powerful electromagnetic vibrations of the order of 100-1000 meters in 

 length (radiowaves), by placing them in a very steep, rapidly alternating 

 electric gradient, failed to produce mutations in experiments carried out 

 by Horlacher (1930) or by Schmitt and OHver (1932), although the treat- 

 ment was so drastic as to kill or sterilize the majority of the flies. Nega- 

 tive results of high-frequency alternating potentials on mutation in 

 Drosophila were also obtained by Kerkis (1935b) and by Pickhan, 

 Timofeeff-Ressovsky, and Zimmer (1936). The waves used by the last 

 three investigators were only 6 meters in length. Ives and Muller in 

 1944 (unpublished work) exposed Drosophila males for over a week to very 

 intense radiation, of still shorter wave length, about 10 cm long, emitted 

 by a klystron. In these experiments also no effect on mutation rate 

 could be observed, either in mature spermatozoa or in spermatogonia. 



