682 RADIATION BIOLOGY 



activity of a tissue indicates that many of its cells are dividing, it seems 

 probable that cells in the course of mitosis are especially sensitive to the 

 deleterious effects of radiations. This is, in fact, clearly demonstrated by 

 experiments which show that a much larger dose of radiation is required 

 to kill a cell immediately than to cause its death at or after its next divi- 

 sion. As an example, 2500 r or more is required to kill chick tissue- 

 culture cells while they are in resting stages, but irradiation of these 

 cells with 100 r is sufficient to kill a high proportion of the cells when they 

 subsequently attempt to divide (Lasnitzki, 1943a, b). Irradiation of 

 Pandorina, a colonial member of the Volvocales, with doses of X rays 

 ranging from 3000 to 300,000 r did not kill the cells immediately, but 

 death occurred when they subsequently attempted to divide (Halber- 

 staedter and Back, 1942). In another study, hematopoietic cells of tad- 

 poles of the bullfrog exposed to 500 r of X rays showed visible damage 

 only upon entering prophase (Schjeide and Allen, 1950). Some of the 

 latent damage that causes cell destruction during division in these cases 

 is presumably due to chromosomal injury. Many of the physiological 

 disturbances effected in tissues and organisms by ionizing radiations have 

 also been referred to alterations in the structure of chromosomes. In 

 determining the basis for differences in the response of tissues and cells to 

 ionizing radiations it thus becomes essential to ascertain to what extent 

 the observations of such differences are dependent on differences in break- 

 ability of chromosomes, to what extent on opportunities for recombina- 

 tion of breakage ends, and to what extent on experimental procedures 

 that permit identification of aberrations more readily in some types of 

 cells than in others. 



4-1. RELATIVE SENSITIVITY OF DIFFERENT ORGANISMS 



A few selected illustrations will serve to indicate the considerable differ- 

 ences in frequencies of radiation-induced chromosomal aberrations in 

 different species. About 24 r of X rays will induce breaks in 1 per cent 

 of the chromosomes in the neuroblast cells of Chortophaga (Carlson, 

 1941a). About 400 r is required to produce breaks in 1 per cent of the 

 X chromosomes of the spermatozoa of Drosophila, as determined by fre- 

 quencies of dominant lethals (Fano and Demerec, 1941 ; Pontecorvo, 

 1942). At the other extreme it was reported by Cleveland and Day 

 (cited in Sax and Swanson, 1941) that doses of X rays in the range 

 between 3000 and 20,000 r did not produce any chromosome aberrations 

 in the protozoon Holomastigotoid.es. 



Blumel (1950) reported a difference in the survival of eggs of D. virilis 

 and D. melanogaster exposed to comparable doses of /3 radiation from 

 P 32 combined in H 3 P0 4 added to the culture medium. Pairs of mature 

 flies were placed in shell vials, and the cultures were subsequently 

 examined. Few flies hatched in the cultures of virilis, many in the 



