688 RADIATION BIOLOGY 



and eggs depend on the proportion of the primary breaks that are incapa- 

 ble of restitution or participation in exchange and accordingly eventuate 

 as dominant lethals. If this proportion is high, the treated cells will 

 readily be inactivated by radiations and will yield few structural 

 rearrangements (as happens when mature eggs are irradiated). If the 

 proportion is low, the cells will be less sensitive to the dominant-lethal 

 effects of radiation and will yield more structural rearrangements (as 

 happens when mature spermatozoa are irradiated). 



4-4. CHANGES IN SENSITIVITY OF CHROMOSOMES 

 IN CELLS OF THE SAME TYPE 



4-4a. Effect of Aging. The effect of age of mature spermatozoa on the 

 frequency of induction of dominant lethals has been studied in D. melano- 

 gaster. Mature spermatozoa present in the testes at the time of emer- 

 gence will be retained if the flies are withheld from copulation. By storing 

 males, and controlling the time of irradiation, it was found that in early 

 developing males the sensitivity of the chromosomes to irradiation, as 

 determined by dominant-lethal counts of eggs, increased with increase in 

 age (Str0mnaes, 1949; cf. Dempster, 1941b). It has been suggested that 

 this dependence on age of sperm and rate of development of the males 

 may explain the discrepancies between the dose-frequency relations for 

 dominant lethals reported by different workers (Str0mnaes, 1949). 



4-4b. Effect of Stage of Mitosis. In 1906 Krause and Ziegler found evi- 

 dence that cells were most sensitive to irradiation at the time of organiza- 

 tion of the equatorial plate. During the second quarter of the century 

 extensive data assembled by a number of investigators suggested that 

 chromosomes were most sensitive to breakage during interphase or early 

 prophase stages. In some cases, studies of the same type of cell in the 

 same species led to different conclusions about relative sensitivities at 

 different mitotic phases. Obviously, the experimental methods were not 

 uniform, and no standard method had been developed for assessing the 

 full extent of radiation damage to the chromosomes. In more recent 

 years, critical studies of induced chromosomal aberrations have led to 

 conclusions agreeing with those of Krause and Ziegler. The extensive 

 literature bearing on the question has recently been surveyed by Whiting 

 (1945a), Bozeman and Metz (1949), and Sparrow (1951). Our attention 

 here will be restricted to a few illustrations of differences in sensitivity at 

 different phases of the mitotic cycle, as indicated by the production of 

 chromosomal aberrations. 



Information concerning relative sensitivities of chromosomes at differ- 

 ent stages of gametogenesis has been obtained in studies of Habrobracon. 

 In this wasp, cells in various stages of oogenesis are arranged seriatim 

 along the length of each of the four ovarioles. Criteria have been devel- 

 oped for determining whether the chromosomes of eggs at the time of 



