684 



RADIATION BIOLOGY 



ential sensitivity in these cases was attributed to differences in chromo- 

 some development, in speed of nuclear changes, and especially in degree of 

 freedom and capacity for chromosome movement. 



Marshak and Bradley (1945) found that the chromosomes of the 

 Walker 256 rat carcinoma were more sensitive to X rays and neutrons 

 than those of a rat lymphosarcoma throughout the greater portion of the 

 resting stage. They concluded that there are physiological differences in 



Table 9-12. Frequency of Translocations among Spermatozoa Utilized in 

 Insemination at Different Periods of Time after Irradiation 



(Catsch and Radu, 1943) 



Table 9-13. Frequency of Dominant Lethals among Spermatozoa Utilized in 



Insemination at Different Periods of Time after Irradiation 



(Timofeeff-Ressovksy, 1931) 



the chromosomes of two different tissues (lymphoid and epithelial) of the 

 same species. 



In such experiments the aberrations were scored during a later stage of 

 the same mitotic cycle in which the cells were exposed to the ionizing 

 radiations. In other cases, where the method of assay determined the 

 frequency of aberrations surviving a series of cell generations, as in 

 Drosophila studies, the possibility must be considered that the survival 

 value may differ in different types of cells. When males of Drosophila 

 are irradiated, the frequency of induced rearrangements is fairly uniform 

 among spermatozoa transferred in the earlier copulations, but signifi- 

 cantly lower among those transferred in later matings. Typical data for 

 chromosomal rearrangements are shown in Tables 9-12 and 9-13; similar 

 data for sex-linked lethals have been summarized and discussed by 

 Schultz (1936). 



