836 RADIATION BIOLOGY 



mammals than in Drosophila would be expected from the larger number 

 of chromosomes. Lea (1947) has speculated on the frequency of chromo- 

 some breakage that would account for the observed results. The higher 

 frequency of dominant lethals in offspring of later matings within the 

 presterile period was attributed by Hertwig (1938a) to fertilizations by 

 germ cells that had been irradiated in prespermatozoal stages which 

 were assumed to be more sensitive. Although the observed frequency of 

 major chromosomal disturbances is adequate to account for most of the 

 radiation-induced reduction in litter size, it is, however, possible that 

 dominant lethal, or subvital, point mutations also occur and contribute a 

 small portion of the cause of death of embryos. 



The percentage of stillborn offspring from presterile-period matings of 

 irradiated males is higher than in the controls in the data of both Snell 

 (1933b) and Hertwig (1938a). However, both authors believe that much 

 of this effect is not due to genetic damage in the stillborns, but to diffi- 

 culties of parturition when litter size is small and the young consequently 

 bigger at birth. Snell provides evidence for this by showing that the per- 

 centage of stillbirths is higher for the smaller litter sizes within one dose 

 group. In guinea pigs, Strandskov (1932) found no significant difference 

 between presterile-period matings and controls in percentage of still- 

 births. In the guinea pig, the normal litter size is much lower (2.79 in 

 Strandskov's material) than in the mouse, so there is presumably less 

 likelihood of parturition difficulties resulting from a reduction in litter 

 size. Among the stillborns found by Hertwig and Snell there were, how- 

 ever, a few pathological cases that could hardly be accounted for by 

 difficulties in parturition. These indicate that, as would be expected, at 

 least a portion of the stillbirths are the result of chromosomal aberrations 

 or dominant point mutations in the affected individuals. It is clear, how- 

 ever, that, because of the difficulty of distinguishing between internal and 

 external causes of death, stillbirths are particularly unsuitable material 

 for the estimation of mutation rates. 



Hertwig (1938a) found a significantly higher percentage of death 

 between birth and 75 days of age in the offspring of presterile-period 

 matings than in the controls. The percentage of death was greater for 

 later than for earlier matings within the presterile period. Autopsies did 

 not reveal the causes of death, but retarded growth was a frequent 

 characteristic of the animals that died. Strandskov (1932) found a 

 slight, but insignificant, increase in death in the 0- to 30-day age interval 

 in progeny of presterile-period matings of irradiated male guinea pigs. 

 The mean 30-day weight of the survivors was lower than that of the 

 controls, especially after correction for litter size. This is all the more 

 striking in view of the fact that the mean 0-day weight of these same 

 animals, i.e., of those raised to 30 days, was actually, though not sig- 

 nificantly, higher than the controls in both the uncorrected and cor- 



