848 RADIATION BIOLOGY 



accumulated exposure within each group. Seven partially sterile ani- 

 mals were found in 3072 tested offspring. This rate does not differ sig- 

 nificantly from that of two in 2755 found in the controls. It is not 

 possible in Charles' data to separate accurately the dose received by 

 spermatozoal stages from that received by prespermatozoal stages, a dis- 

 tinction which, as has already been shown, is all important. It is prob- 

 able that any real excess over the controls was due solely to that portion 

 of the total irradiation which was received by the sperm but, even if this 

 is assumed, the data are not suitable, or extensive enough, for answering 

 the question as to whether or not fractionated doses to the sperm are less 

 effective than a single dose. 



It may be concluded from Charles' data and from the work of Snell, who 

 found no translocations in 196 control mice in all his experiments com- 

 bined, that the rate of occurrence of spontaneous translocations is quite 

 low in mice. Charles has not yet presented any detailed information on 

 the two spontaneous cases reported by him. The only spontaneous 

 mammalian translocation thoroughly analyzed is one in the rat investi- 

 gated by Waletzky and Owen (1942), Tyler and Chapman (1948), and 

 Bouricius (1948). The pattern of fertility reduction, abnormal embryos, 

 and cytological aberrations described falls well within the range of 

 variability found for X-ray-induced translocations in the mouse. 



Turning to a consideration of the characteristics of the translocations, 

 the degree of effect on the fertility of the animals that are heterozygous 

 for a translocation is of first importance. Hertwig (1940) found that the 

 fertility, expressed as percentage of control litter size at birth, of trans- 

 location heterozygotes in eleven of her X-ray-induced partially sterile 

 lines of mice ranged from 42 to 59 per cent. The mean of these lines was 

 45 per cent. Uterine dissections in the same eleven lines and a few others 

 showed similar reductions in fertility as measured by number of living 

 embryos. Hertwig concluded that the differences between lines could 

 be attributed to chance. The mean fertility of heterozygotes in three 

 translocation lines of mice studied by Roller (1944) was 30-39 per cent in 

 one line, 46 per cent in another, and 44-46 per cent in the third. Cyto- 

 logical analysis showed correlation between reduction in fertility and fre- 

 quency of nondisjunctional coorientation of chromosomes in the ring-of- 

 four in first meiotic division. In the six translocation stocks investigated 

 in detail by Snell (1946), the fertility of heterozygotes ranged from 38 to 

 62 per cent for females and from 41 to 69 per cent for males. Differences 

 between lines were significantly greater than could be expected by chance. 

 In five of the six lines, the percentage fertility of males was higher than 

 that of females and the difference was significant in two of these lines. 

 The assumption that the percentage fertility corresponds to the per- 

 centage of orthoploid gametes is supported by the observed proportion of 

 normal embryos in one of the lines, and is tenable as part of the explana- 



