834 RADIATION BIOLOGY 



ficial insemination of rabbit spermatozoa irradiated in vitro, have con- 

 firmed both the lack of effect of high doses on fertilizing capacity of sperm 

 and the occurrence of blastomeres with supernumerary nuclei. 



Turning to some of the details of the experimental findings, data for 

 effect on litter size in presterile-period matings of irradiated males are 

 given in Table 12-5. The litter sizes are based on the numbers of litters 

 actually born. This underestimates the effect of radiation, particularly 

 at higher doses, because some pregnancies, the proportion increasing with 

 dose, fail to produce any young at term. Even without this correction, 

 the data in Table 12-5 clearly show that litter size is dependent on dose. 

 An effect of dose on litter size is also indicated in the few litters allowed to 

 go to term in the work of Amoroso and Parkes (19-47) on artificial insemi- 

 nation of rabbit spermatozoa irradiated in vitro. In the data of both 

 Snell and Hertwig, the reduction in litter size is greater for the later 

 matings within the presterile period. The proportion of pregnancies that 

 fail to reach term is, according to Hertwig (1938a), also greater in the 

 later matings. Although the results of Russell et at. do not show a greater 

 drop in litter size in the later matings, it should be pointed out that they 

 were obtained from an experiment in which each male was mated to only 

 one female in each of the two weeks and, therefore, possibly expended his 

 store of sperm at a slower rate than that occurring in the experiments of 

 Snell and Hertwig. The data of Brenneke (1937) show that the propor- 

 tion of eggs having abnormal cleavages may increase both with number 

 of matings and time. These data also provide the most direct evidence 

 that the increased effect in later matings is attributable to a higher inci- 

 dence of dominant lethals. 



Information on the time of death of the embryos carrying dominant 

 lethals is given in Table 12-6. Additional information is provided by the 

 studies of Brenneke (1937) on the proportion of abnormal cleavages found 

 at various times after fertilization. Brenneke 's results for 800 r showed 

 abnormality in 23.2 per cent of 186 two-cell stages, 44.4 per cent of 90 

 three- to six-cell stages and 41.9 per cent of 105 seven- to twelve-cell 

 stages. Because of the possibility of further cleavage of some of the 

 abnormal embryos found at any one stage, the total percentage of 

 abnormality cannot be accurately estimated from these data. However, 

 it is clearly higher than the percentage of preimplantation death esti- 

 mated from the 800 r data of Snell and of Russell et at. If the difference 

 proves to be real, it would indicate that some of the embryos showing 

 abnormal cleavages actually survive to implantation. Most of the 

 dominant lethal types that survive to implantation were found by Snell 

 to die shortly thereafter. This appeared to be true in the material of 

 Russell et al, but, with the females opened at a later stage in pregnancy, 

 the time of death could not be accurately determined. Henson found 

 degeneration occurring shortly after implantation in six of eleven rat 



