932 RADIATION BIOLOGY 



following penetrating external irradiation (Bloom, 1947; Prosser et al, 

 1947b). The wider the distribution of an internal emitter, the greater is 

 the similarity of the clinical picture to that associated with penetrating 

 radiation. 



INDIVIDUAL SENSITIVITY 



Individuals in an apparently uniform population do not respond equally 

 to radiation. Curves relating mortality to dose are of the sigmoid type 

 and are quite steep, especially for mammals (Ellinger, 1945; Boche and 

 Bishop, 1946). Variations in mortality of from to nearly 100 per cent 

 may occur in the LD 50 range (Clark and Uncapher, 1949). As might be 

 anticipated, the dose range between just lethal and completely lethal is 

 greater in hybrid than in inbred strains, and, in addition, the LD 5 o varies 

 under different laboratory conditions. Besides the variation in mortality, 

 there are impressive differences in survival time during the acute period 

 following median lethal irradiation. In the rat, deaths within 30 days 

 are most frequent between the fourth and eighth and between the tenth 

 and fifteenth postirradiation days (Hagen and Simmons, 1947). Mor- 

 tality waves have also been observed in rabbits and chickens (Karnofsky 

 et al, 1950; Jacquez and Karnofsky, 1950; Steamer, 1951; Hagen and 

 Sacher, 1946). In these animals the first peak of deaths occurs during 

 the first day after irradiation, even with an LD 50 - The existence of 

 several processes that may lead to death in the acute period is suggested 

 by these findings. Reference has already been made to the apparent 

 transition from one mechanism of death to another with increasing 

 amounts of supralethal radiation. Although there are wide variations in 

 mortality, it is rather surprising that there are few histologic differences 

 between animals dying after an LD 50 of radiation and those surviving the 

 same dose (Bloom, 1947). 



Radiosensitivity of the embryo varies with its age (Karnofsky et al, 

 1950; Wilson, 1935; Wilson and Karr, 1950). The chick embryo mani- 

 fests an increasing sensitivity to early lethality (death within 24 hours) 

 during the third to seventh day of incubation, and is most sensitive on 

 the eighth to tenth day, after which its response is stabilized at a slightly 

 more resistant level (Karnofsky et al, 1950). Depression of growth is 

 also related directly to the age of the chick embryo at the time of irradia- 

 tion. Interestingly enough, the number of dividing cells in the non- 

 irradiated embryo decreases markedly between the third and eighth days 

 of incubation (O'Connor, 1950). Oxygen consumption is unchanged, 

 but there is a threefold decrease in aerobic glycolysis during this interval. 

 Irradiation of the pregnant mouse before implantation of the embryo 

 leads to a high prenatal fetal mortality with only a negligible incidence 

 of abnormality in animals surviving to term (Russell and Russell, 1950). 

 The earliest stages after mating are the most sensitive. When the preg- 



