FIVE FIGURES
It has been shown (Snell, '33, a and b) that when male mice are x-rayed with a dose of about 600 Roentgen units and mated before the onset of x-ray sterility to untreated females, approximately one-fourth of their offspring show a type of induced variation which coiisists of a tendency to prodnce small litters numbering 1 to 5 o r 6 young instead of the usual 7 to 10. Such individuals have been termed semi-sterile. They produce small litters consistently, not oiily when mated inter se, but also when outcrossed to untreated stocks. Of 110 progeny of x-rayed fathers (dose from 400 to 1200 r-units) 29 were probably semi-sterile. All of 98 progeny of untreated fathers were normal. Of the 29 mice with x-ray induced semisterility, 10 were selected for an intensive study of the manner of inheritance of the trait and of its embryological significance. The following report is based on the data obtained from one of these semi-sterile individuals, 8Fl146, and his descendants.
This male was mated to a number of females from an nntreated inbred stock. By these females he had 26 young in 7 litters-an average of 3.7 young per litter. The iiormal litter size for the stock is approximately 8 young per litter.
An embryological study based on additional matings between 8 F,146 and normal females has shown that such matings result in the formation of the usual number of zygotes, National Research Council Fellow in the Biological Sciences.