CHROMOSOME RUPTURE 203 



ways as well (133). As already mentioned, H2O2 and possibly other 

 inactivating peroxides are not formed under the action of x-rays in the 

 absence of oxygen. 



Not less spectacular is the effect of low temperature on radiosensi- 

 tivity. When eggs of Drosophila are exposed to x-rays at 13°, 23°, and 

 28° C and incubated at room temperature, greatest injury (failure of 

 hatching) is found in eggs exposed to radiation at higher temperatures. 

 When the eggs are irradiated at room temperature and then incubated 

 at 18° and 28°, greater injury is again observed at the higher tempera- 

 ture (58). 



At low temperature or in the absence of oxygen, cell division is stopped 

 and the formation of additional cellular components is not required; 

 enzyme inactivation remains thus without far-reaching consequences. 

 If, however, the supply of pertinent components is suppressed in the 

 course of an intense synthetic process, such a suppression may lead to 

 a lethal or other far-reaching effect. 



Tissue injury of the skin of 1-day-old rats irradiated at 0-5° C with 

 dosages ranging from 300 to 3000 r is comparable to that of the rat 

 treated with only 300-600 r at 30° (59). When newly born mice are 

 kept for 10 min at 0° and irradiated for 1 min with 1500 r, they develop 

 normally, in contrast to mice irradiated at room temperature. Anes- 

 thesia with nitrogen or carbon dioxide was found by Lacassagne to have 

 the same effect as has cooling mice (60). These and numerous similar 

 results clearly bring out some connection between the intensity of nu- 

 clear synthesis and radiosensitivity. In frogs, however, toxicity was 

 found by Patt (83) not to be influenced by altering their body tempera- 

 ture during the first 24 hr after total-body irradiation with 1000 or 

 3000 r. Survival is greatly enhanced as long as the animals are kept in 

 the cold (5-6° C) continuously after the exposure. This altered sensi- 

 tivity is interpreted to be due to a prolongation of the latent period 

 (cf. also 108). A still greater difference with regard to the effect of 

 temperature is shown by plants. In Vicia faba Mottram found many 

 years ago that roots irradiated when at 0° C are markedly more sensi- 

 tive than those irradiated at 24 °C (141). 



Chromosome Rupture 



In the above consideration an attempt was made to explain the effects 

 of ionizing radiation on cell division as an interference with synthetic 

 processes taking place in the tissue cells, which ultimately induce the 

 cells to divide. Another type of interference is, however, to be con- 

 sidered also. If under the action of ionizing particles chromosome 



