842 RADIATION BIOLOGY 



sire. This is nearly one-half the rate of induction of partial sterility at 

 the same dose level. 



The data for presterile-period matings in Table 12-8 show a higher 

 incidence of sterility in males than in females. The material is too 

 heterogeneous for an accurate test of significance, but it is noteworthy 

 that the sex difference is in the same direction in all investigations. 



The only data tabulated to show the proportion of steriles produced by 

 early and late matings within the presterile period of irradiated males are 

 those of Hertwig (1938a). Of the 10 male steriles listed in that publica- 

 tion, 8 occurred in 88 offspring of matings made from 1 to 8 days after 

 irradiation and 2 in 24 offspring of matings made from 8 to 14 days after 

 exposure. 



As shown in Table 12-8, Hertwig's results indicate that radiation- 

 induced dominant sterility does not appear in the progeny of poststerile- 

 period matings. 



Only scattered information is available on the nature of the sterility. 

 Hertwig (1935) reports that in one sterile male, spermatogonia and 

 spermatocytes were present in the testes, but all later stages of spermato- 

 genesis were lacking and no sperm could be found in the epididymis or 

 vas deferens. The histological information on the three sterile males 

 obtained by Snell (1939) in his neutron experiment is as follows: One male 

 proved to have spermatogonia and spermatocytes, but no spermatids ; one 

 had a few spermatids, some in a rather advanced stage, but no spermato- 

 zoa; and the remaining male showed a very few motile sperm in the left 

 epididymis, a few immotile sperm in the left vas, and no sperm in the 

 right reproductive tract. Of the two sterile males found by Snell and 

 Ames (1939) in the offspring of X-irradiated females, one had spermato- 

 gonia and spermatocytes, but no spermatids or spermatozoa, and the 

 other lacked testes, although epididymides and vasa deferentia were 

 present. The sterile female obtained in this experiment was small and 

 died at seven months of age. The ovaries, which were invaded by a 

 lymphoblastoma, showed primordial follicles, but no mature ones. 



The mutational changes involved in the production of dominant 

 sterility in mice are not known. Snell (1935) suggested that fragmenta- 

 tion or deletion of the Y chromosome in irradiated Y-bearing sperm could, 

 as in Drosophila, account for sterile males. This hypothesis could prob- 

 ably be tested by cytological examination. Sterility might also result 

 from damage to the X chromosome in eggs or X-bearing sperm, and this, 

 too, might be checked cytologically. Some of the animals classified as 

 sterile may have had chromosomal aberrations, such as multiple trans- 

 locations, that gave a high proportion of aneuploidy in the gametes. 

 However, the animals examined histologically were clearly sterile from 

 causes other than aneuploidy of mature gametes. More information is 

 needed on the anatomical and histological nature of the sterility in both 



