S32 



RADIATION BIOLOGY 



He concluded: "The most plausible interpretation seems to be the 

 induction of dominant lethal mutations." Snell (1932, 1933b), working 

 on mice, also found a reduction in litter size in early matings of X-irradi- 

 ated males and no effect on motility of sperm. In the same material he 

 found that many embryos died usually at, or shortly after, implantation. 

 He also demonstrated the presence of both male and female pronuclei in 

 all of fourteen eggs fixed and sectioned 16 hours after impregnation. 

 Snell concluded that the reduction in litter size, caused by the death of 

 embryos, is most plausibly explained by the induction of chromosome 

 abnormalities. Final proof of the correctness of the interpretations of 

 Strandskov and Snell was provided by Hertwig (1935), Hertwig and 

 Brenneke (1937) and Brenneke (1937). In the first place, it was demon- 

 strated that fertilizing capacity of mouse and rat sperm is unaltered even 

 by high doses. Table 12-4 shows the results obtained for doses up to 



Table 12-4. Lack of Effect of Radiation on the Percentage of Unfertilized 

 Eggs in Presterile-period Matings of X-irradiated Male Mice and Rats 



(Data from Brenneke, 1937) 



a Percentages given in Brenneke's table have been changed to agree with the num- 

 bers given by her. 



2200 r in the mouse and 1800 r in the rat. Hertwig and Brenneke (1937) 

 state that even with 4000 r, the highest dose used, there was still no effect 

 on fertilizing ability of mouse sperm. Second, the authors showed that 

 as soon as the eggs cleave, differences between experimentals and controls 

 become apparent. Supernumerary nuclei were found sometimes in the 

 blastomeres of the two-cell stage and more often in the cells of later 

 stages. In metaphase of cleavage mitoses, portions of chromatin lying 

 away from the equatorial plate could be found, and, in anaphase, lagging 

 chromosomes were observed. Amoroso and Parkes (1947), using arti- 



