RADIATION IN PRENATAL DEVELOPMENT 877 



growth between irradiation and death. Of the animals which had 

 received 100 r, seven were raised and it was found that they made up 

 their initial weight deficiency (an average of 15 per cent at term) by the 

 age of 70 days. 



Russell (1950) presents mean birth weights for groups of animals 

 irradiated in stages differing by 24-hour intervals and ranging from day 

 3^ to day 13^2 after fertilization (see Fig. 13-3). Several variables which 

 are known to affect birth weight are controlled: (1) newborns are genet- 

 ically uniform (Fi hybrids between two inbred strains); (2) the maternal 

 environment is genetically uniform; (3) parity is controlled, all animals 

 coming from second litters. Other variables, especially litter size, could 

 not be eliminated. A secondary control line was drawn in Fig. 13-3 to 

 provide a liberal allowance for the amount of milk sucked by the time of 

 weighing. It was found that mean birth weights for the 83^- through 

 13^2-day stages of treatment fell considerably below that control line. 

 The 200-r and 300-r curves were, in general, parallel with a mean differ- 

 ence of 0.2 g. Minima for both curves lay between the 10^2 - and 11^2- 

 day stages. The short portion of the 400-r curve available paralleled the 

 300-r curve in the rising portion between days 12^ and 133^. 



Holding the time of irradiation constant, Russell, Russell, and Major 

 (1951) obtained a more complete dose-weight series for day 11^2, which 

 is close to the stage of maximum susceptibility to growth retardation. 

 Points for different doses and for the control fall on an approximately 

 straight line, weight reduction per 100 r averaging 0.22 g over the three 

 available intervals (Fig. 13-9a). 



The experiments of Raynaud and Frilley fall into a special category 

 since they involve very local irradiation with a narrow beam (0.5-3 mm 

 diameter) of very high dosage (100,000 r per exposure) in an attempt at 

 selective destruction of the pituitary. Five male mouse fetuses at term 

 which had been irradiated twice, at 12 days 6 hours and 13 days 6 hours 

 respectively, weighed, on the average, only about 60 per cent as much as 

 their control litter mates (1949a). The loss of weight due to direct 

 destruction of radiation-traversed head tissues does not account for the 

 total weight loss, which is considerable. This is also evident from the 

 fact that the irradiated animals showed a 10 per cent reduction in 

 xiphoid-anal length as well as the expected reduction in crown-anal 

 length. The results may be explained by secondary effects of pituitary 

 destruction, by such uncontrolled factors as scattered radiation to other 

 parts of the body, or, as the authors suggest, by indirect action of radia- 

 tion-produced toxins. 



3. SEX RATIO 



The only claim in the literature of an upset sex ratio following prenatal 

 irradiation is by Job et al. (1935), who state that treatment between the 



