GENETIC EFFECTS IN MAMMALS 837 



rected figures. In 102 offspring of matings made within three weeks after 

 exposure of male rats to doses of from 100 to 1000 r, Henson (1942) was 

 able to raise only 46 per cent to weaning as against 79 per cent of 96 

 controls. An effect of radiation on postnatal viability is also indicated in 

 the few litters allowed to come to term in the work of Amoroso and 

 Parkes (1947) on irradiation of rabbit spermatozoa in vitro. Thus the 

 data so far obtained indicate an appreciable manifestation of dominant, 

 deleterious, postnatal effects in offspring of presterile-period matings of 

 irradiated males. The relative importance of chromosomal aberrations 

 and of semilethal and subvital point mutations as causes of the postnatal 

 mortality is not known. 



Offspring of Irradiated Females. Information on the radiation induc- 

 tion of dominant lethals in females does not go beyond the observation of 

 a reduction in litter size. Snell and Ames (1£39) state that, as the dose is 

 increased, litter size in the mouse falls off more rapidly for females than 

 for males, and they attribute the greater effect to a radiation damage of 

 the mother which interferes with development of the young. Another 

 factor that should be kept in mind is that, as the second maturation 

 division of oogenesis in the mouse is not completed until after fertiliza- 

 tion, the germ cells in the female are irradiated in oocyte stage. The 

 results obtained may, on this account alone, prove to be different from 

 those observed for irradiated sperm. 



Offspring of Poststerile-period Matings of Irradiated Males. In contrast 

 to the marked effect of irradiation of the male on litter size in presterile- 

 period matings, there is little reduction in litter size in matings made in 

 the post sterile period (Table 12-7). The germ cells utilized in poststerile- 

 period matings received the radiation in spermatogonial stages. There is 

 no reason to doubt that dominant lethals are induced in spermatogonia, 

 but, as suggested by Strandskov (1932), it is quite likely that many of 

 them would fail to pass through the cell divisions between spermatogonia 

 and sperm. Chromosome aberrations of the types which, when induced 

 in sperm, cause breakdown in early cleavages would seem to be especially 

 subject to elimination by germinal selection if they are induced in 

 spermatogonia. That such elimination is occurring is suggested by the 

 observed reduction in number of spermatogonia following irradiation. 

 Schinz and Slotopolski (1925), Hertwig (1938b) and Fogg and Cowing 

 (1952) have reported finding degenerating cells at short intervals after 

 irradiation. The fact that Eschenbrenner and Miller (1950) found little 

 evidence of degenerating spermatogonia at later times (one week or more) 

 after irradiation of mice with 400 r hardly justifies the authors' conclusion 

 that the effect on the spermatogonia "is not one of cell death but is one 

 of inhibition of division." There may well be some inhibition of mitosis, 

 but it is difficult to see how this could result in a reduction in number of 

 spermatogonia to 1.5 per cent of normal. The results of Eschenbrenner 



