PHYSICAL AND BIOLOGICAL FACTORS 933 



nant mouse is exposed during the postimplantation stages, the relative 

 magnitude of effects is reversed, abnormality exceeding mortality. There 

 is a striking separation in sensitivity to lethal action. Exposure to 200 r 

 on the ninth day of gestation is completely lethal while 400 r is required 

 on the tenth day of gestation (Wilson and Karr, 1950). This may be 

 related to implantation of the embryo or to beginning differentiation. 

 Malformations of the skeleton and destruction of the developing nervous 

 system are prominent sequelae of irradiation during the latter two-thirds 

 of pregnancy (Russell, 1950; Hicks, 1950). In contrast, adult bone and 

 nervous tissue are relatively radioresistant (see Chap. 13). 



There is remarkably little difference in the LD 50 for delayed deaths 

 among the chick embryo, baby chick, and adult chicken, the embryo 

 being, in fact, somewhat more resistant, perhaps because of the sterility 

 of the internal environment of the egg (Karnofsky et al., 1950; Jacquez 

 and Karnofsky, 1950). This is apparently not the case in the mammal. 

 The mammalian fetus is more susceptible than either the young or adult 

 animal, and the former is, in general, more sensitive than the latter. 

 There are some exceptions, however. Mice under 15 days of age are less 

 sensitive and survive longer than 30-day-old animals, which manifest a 

 greatly increased susceptibility (Quastler, 1945b; Abrams, 1951; Furth 

 and Furth, 1936). Sensitivity to the lethal action of X rays decreases 

 rapidly with increasing age beyond 30 days, and little difference is appar- 

 ent between two- and three-month-old animals. The acute median 

 lethal dose of y rays does not differ appreciably for mice varying in age 

 from 1.5 to 12 months (Zirkle et al., 1946). The relative resistance of the 

 newborn mouse may be related to protective influences associated with 

 suckling (Abrams, 1951). Conversely, the sensitivity of 30-day-old 

 mice may be a function of changes associated with puberty. It is of 

 interest that maximal susceptibility to radiation lymphoma also occurs 

 during the puberal period in mice (Kaplan, 1948). In contrast to these 

 results, the amount of X radiation required for minimal depression of 

 femoral growth following local exposure of the epiphyseal region is linear 

 with age (Hinkel, 1942). Minimal stunting is seen with 700 r in one- 

 month-old rats, whereas 2200 r is required at six months of age. The 

 younger rats, however, recover more quickly than older animals. 



Females may be somewhat more resistant than males, i.e., by about 

 50 r, but a sex difference is not apparent in all species or in different 

 strains of the same species (Hagen and Simmons, 1947; Hagen and 

 Sacher, 1946; Abrams, 1951; Zirkle et al, 1946). The role of body 

 weight in radiosensitivity is not well established, although it appears that 

 heavier animals tend to be less sensitive. It is not clear, however, 

 whether this is a reflection of weight or of age. The median lethal dose of 

 X rays is not very dependent upon weight in rats of approximately the 

 same age (Hagen and Simmons, 1947). However, rats of the same age ; 



