GENETICS OF INFECTIOUS DISEASES 395 



Fig. 53. Relationship between route of entry and survival of mice inoculated with 



0.002 MG. OF RICIN. 



60r 



9§ 40 



CO 



20 



LU 

 Q_ 







vm\ 



Subcutaneous Intraperitoneal Intravenous 



Direct comparisons of the products of the tests furnishes a better comparison. 

 Data on this point are not entirely satisfactory since they were collected by different 

 observers working with different populations of albino mice. Schott 1167 and later 

 Hetzer 573 selected for natural resistance to S. typhimurium by selecting for resistance 

 on the basis of survival to test dosages of 50,000 organisms introduced into the peritoneal 

 cavity. The foundation population was rather susceptible since only 17.7 per cent 

 survived the test. The first- and second-generation tests showed the families quite 

 heterogeneous for resistance. Selection for resistance was continued for six successive 

 generations when resistance of these animals gave them 75.3 per cent survival. Hetzer 

 continued the selection for two more generations with the same test dose. The sur- 

 vivors of that dose were 84 per cent. The data show that selection for resistance is an 

 efficient method for establishing natural resistance to S. typhimurium when the disease is 

 induced by intraperitoneal injection of the organisms. 



Webster 1362 selected for a susceptible and a resistant strain of mice by testing 

 previous siblings by intrastomachal inoculation of 5,000,000 organisms and then later 

 unexposed siblings for breeding on the basis of these susceptibility tests. By this test, 

 the initial population of mice was not as susceptible as that of Schott's as judged by their 

 respective survivals — 62.6 per cent on Webster's test versus 17.7 per cent on Schott's. 

 In four generations of selection Webster's selected, susceptible mice were susceptible 

 to the point where only 15 per cent survived. The selection dose was then reduced to 

 100,000 organisms. On this lower dose the survival percentage became 17.6 per cent. 

 Selection toward resistance for two generations gave a population of mice which, when 

 tested with 5,000,000 organisms, survived 88.7 per cent. The test dose was then 

 increased to 50,000,000 organisms. The survival of the fourth-generation mice treated 

 with this dose was 82.5 per cent, but the degree of resistance had reached at least a 

 temporary plateau for this method. In conclusion it may be said that the intra- 

 stomachal route proves to be a successful but slower route for selection of mice for 

 resistance and susceptibility. 



