872 RADIATION BIOLOGY 



tion of 419 litters (2667 young). (It must be borne in mind that the 

 table includes several genetic strains — distributed unevenly among groups 

 — with probable differences in radiation response; see a future section. 

 It is, therefore, suitable only for showing broad trends.) For irradiation 

 of postimplantation stages, doses of 100 r and below have apparently no 

 effect on prenatal viability. A dose of 200 r, which, it may be remem- 

 bered, caused 80 per cent prenatal death when applied days x /i, 1}4,, or 2^ 

 after fertilization, permits an over-all average litter size of 5.94 ± 0.27 

 when administered between days 5^ and 12^ inclusive, i.e., causes a 

 decrease of only 11 per cent from the control mean of 6.69 ± 0.18 

 (t = 2.43). This over-all slight depression is due primarily to the sig- 

 nificantly reduced litter sizes of groups irradiated near the beginning of 

 the period, namely, on days 63^, 7^2> and 8^2 (reduction for these three is 

 27 per cent; t = 4.46). The 300-r data substantiate the 200-r series in 

 again showing sensitivity to prenatal killing of individuals only in the 

 early part of the period of organogenesis. 



The findings of other investigators who have worked on the mouse are 

 in general agreement with the above results. Kaven (1938b) obtained 

 litter sizes of 4.3 and 4.7 following irradiation with 178 r to the uterus on 

 days 7 and 8, respectively. The control litter size was 6.8 (see Table 

 13-6). Litter sizes for irradiation of later stages cannot be accurately 

 calculated from his data (1938a), but where estimates are possible these 

 are usually considerably higher than the results from the 7- and 8-day 

 stages. Kosaka (1927) found that doses greater than j^ SED would give 

 100 per cent prenatal death if given between the fourth and tenth days 

 while more than 1 SED was required to accomplish the same result when 

 administered after the tenth day. 



Wilson and Karr (1951) concentrated their study on irradiation at day 

 10 after fertilization in the rat and examined for dead embryos 1-5 days 

 following irradiation. Their results for the last observation day are 

 shown in Table 13-5. These figures and the summation for all observa- 

 tion days indicate that while 50 r does not increase the percentage of 

 prenatal death at all and 100 r not significantly so (t = 1.4 for the sum of 

 all observation days), the significant (t = 5.3) killing action of 200 r 

 should result in at least 88 per cent reduction in average litter size at 

 birth (more if, as is likely, there is further death between day 15 and 

 term). For approximately comparable stages in the mouse (days 7^ and 

 83^) and the same dose, the reduction in average litter size at birth is 

 only 29 per cent. It is of interest in Wilson's data that while mortality 

 due to 200 r is spread over a 4-day interval beginning 2 days after irradia- 

 tion, all death with 400 r occurs within 24 hours of exposure. In con- 

 trast, Kosaka (1928b), in his work on rats, found that even doses which 

 kill 100 per cent of the embryos— i.e., % SED and above on any of days 

 5-10 — do not cause death until 48 to 240 hours after irradiation. In an 



