GENETICS OF INFECTIOUS DISEASES 397 



There were four X-ray exposure doses : 0, 320, 480 and 640 roentgens. The levels of 

 X-ray dosages were chosen to span the range from no effect to nearly complete 

 lethality when the mice were exposed to whole-body irradiation. 



There were eight combinations of exposures to the X-ray effects: none, head, mid, 

 rear, head-mid, head-rear, mid-rear, and whole-body exposure. The head region 

 or anterior third of the body extended into the thorax. The middle third of the body 

 (mid region) included the lower thorax and abdominal cavity containing stomach 

 and upper intestinal tract, liver, spleen, adrenals, ovaries, and kidneys. The posterior 

 third of the body (rear region) included the lower intestinal tract, bladder, and urinary 

 system and testes of the males. 



The eight groups of different regional exposures were treated with 320 r, 480 r, 

 and 640 r, making a total of 24 different X-ray treatment groups. In addition, the 

 mice of one group were put in tubes and completely lead-covered for a time comparable 

 to that of the longest dose, 640 r, but were not exposed. This group acted as a control 

 on handling as well as for the unirradiated, r group. 



There were 25 treatment groups with 5 strains and 2 sexes making up 250 cells 

 in the experiment. Each cell represented a different strain, sex, and treatment. A 

 minimum of 25 mice were treated in each cell. Some cells contained a few extra 

 animals. The completed experiment involved a total of 6,904 mice. 



The immediate effects of X ray were largely completed 12 days following irradia- 

 tion. Although there were marked differences in survival times between the different 

 strains, few deaths occurred after the twelfth day. A 1 5-day interval following irradia- 

 tion was allowed to cover the direct effects of exposure. 



The mice were then inoculated to test for their resistance to murine typhoid. 

 Different lines of the bacterial species S. typhimurium cover the full range in virulence 

 or pathogenicity due to differences in their genetic constitutions. 468 ' 1467 This factor 

 has been controlled in this work by limiting all the disease tests to one of our lines, 

 11C, of S. typhimurium. 



The data showed that the sexes were not significantly different in this experiment 

 in their reactions to this disease. The physiologic sex differences introduced by the 

 chromosomal differences played little part in the outcome of disease. Strain differences 

 were obvious. The high innate resistance of the S strain prevailed over the latent effects 

 from the X-ray exposures except in the severest treatments, those of whole-body 

 exposure to 480 r and 640 r. The S strain was more than twice as resistant to the disease 

 as were the Z and K strains. The survival curves of the S mice showed less marked 

 reductions in survival due to X-ray treatments than those of the other strains. The 

 differences in the disease-resistance levels of the Z and K strains and the level of the Q 

 strain was even greater. 



Because of our previous knowledge of the susceptibilities of the Q and Ba strains, 

 the Q strain received only one-hundredth the dose and the Ba only one-thousandth 

 the dose of S. typhimurium 1 1C which was given to the Z, K, and S mice. The extreme 

 susceptibility of the Ba mice under normal conditions allows no opportunity to 



