656 RADIATION BIOLOGY 



40-70 per cent lower if the flies are irradiated in an atmosphere of nitrogen 

 than when they are irradiated in an atmosphere of oxygen. (Similar and 

 more extensive results have been obtained in studies of plant chromo- 

 somes; e.g., by Thoday and Read, 1947, 1949, and by Giles and Riley, 

 1949.) 



One of the criteria of the indirect effect is the protection afforded by 

 dissolved substances, other than those being tested, present in the aque- 

 ous medium. Further evidence, therefore, of the indirect nature of the 

 reaction system is afforded by the findings that various substances, such 

 as cysteine, glutathione, thiourea, ethyl alcohol, azides, and cyanides, 

 when used in proper concentrations, afford protection against the effects 

 of ionizing radiations. Pioneering work along these lines with respect to 

 biologically active materials has been done on isolated enzymes (e.g., the 

 studies of Dale, 1940, 1942; Dale, Davies, and Meredith, 1949; and 

 McDonald, reported in Kaufmann et al., 1950), and has recently been 

 extended to whole-body irradiation (reviewed in Bacq, 1951). From the 

 results obtained in irradiating entire animals, which indicated that the 

 median lethal dose is much higher when protective agents are used, it can 

 only be inferred that chromosome breaks are involved, although the 

 dosage in most of these experiments was of the order of magnitude that is 

 known to produce chromosomal rearrangements. 



Whatever the precise course of events in the production of any specific 

 break may be, it appears probable that molecules near a high concentra- 

 tion of ionizations may undergo chemical change although not themselves 

 ionized, and that a large number of chemical activations per chromosome 

 is required before any observable breaks occur (Fano, Caspari, and 

 Demerec, 1950). 



If an indirect reaction system can produce chromosome breaks, it 

 may conceivably originate in some part of the cell other than the chromo- 

 some (a remote effect). Duryee (1939, 1949) has reported that the 

 chromosomes of isolated oocyte nuclei of amphibia are much less sensitive 

 to radiation damage than chromosomes in the intact cell. Chromosomal 

 injuries are also produced by exposure of isolated nonirradiated nuclei to 

 irradiated cytoplasmic brei. Although these findings may suggest "that 

 nuclear damage results mainly from chemical change in the cytoplasm," 

 as Duryee has proposed, the intense X-ray doses used and the surgical 

 procedures employed in isolating nuclei involve a reaction system differ- 

 ent from that which leads to the formation of viable chromosomal 

 rearrangements in the intact cell. It has been shown, however, that the 

 types of viable aberrations induced by ionizing radiations can be pro- 

 duced by treating cells with chemical mutagens, which do not reach the 

 nucleus directly but diffuse into it, either in their original form or as 

 degradation products. For example, chromosome breaks have been 

 induced in Drosophila by mustard gas and nitrogen mustard (Auerbach 



