230 



W. L. RUSSELL AND E. F. OAKBERG 



conflicts, and the implied complexity, evaporate when it is realized that the 

 predominant effect of radiation on female fertility is a shortening of the 

 breeding period (RiisseU et al., 1959a). There are, of course, other effects, for 

 examjjle, some reduction in litter size as a result of dominant lethal mutation 

 induction in the oocytes, but the most striking is the shortening of the 

 reproductive life-span. 



When the results of dose fractionation and of various intensities of con- 

 tinuous irradiation were investigated, the effects on the reproductive life-span 

 formed a consistent picture of lowered damage compared with that from a 

 single dose of acute irradiation (Russell et al., 1959a). After a moderate dose 

 of acute irradiation, productivity may be cut down to only one or two 

 litters; whereas, with moderate doses of chronic irradiation, there may be 

 merely a slight curtailment of the normal reproductive life. An example of 

 this effect is given in Table I which contains hitherto unpublished data from 



Table I, Number of litters produced by control and irradiated female 101 x 



C3H Fi mice-f 



Age of female 



at mating 



(days) 



No. of 

 females 



Mean No. 



of litters 



per female 



t y-Rays from ^^"Cs source. Total dose 258 r. Dose-rate 0-009 r/min. Matings were made 

 at the completion of irradiation to multiple recessive test stock males used in specific locus 

 mutation experiments. Females that died before 620 days of age are excluded. 



one of our mutation experiments. Here, the mean number of litters per 

 female is about 40% less in the irradiated group. In this experiment, as in 

 others referred to, litter size and number of females having litters, stayed 

 normal, or near normal, until the beginning of a sharp decline that ended in 

 sterility. Thus, little or no effect is observed until the onset of this decline, 

 which, as is apparent for the data in Table I, can be long delayed. 



What is the aetiology of this late effect? The known killing of oocytes by 

 acute irradiation the fact that there is no replacement of oocytes in the adult 

 mouse (Brambell and Parkes, 1927; Schugt, 1928; Murray, 1931; Oakberg, 

 1960) and rat (Ingram, 1958), and the observation that fertility is high until 



