PATHOGENICITY FOR EXPERIMENTAL ANIMALS 189 



subcutaneous, intraperitoneal, or intravenous route or by all three 

 routes simultaneously, the organisms of one type would gain the 

 ascendancy and suppress the pneumococci of the other type even 

 at the site of inoculation. 



Webster and Clow 1494 found individual differences in mice to in- 

 tranasal infection with Pneumococcus. Some mice were completely 

 refractory, some became carriers, and others developed various 

 forms of infection ranging from lobar pneumonia to septicemia. 

 Animals which showed high resistance to a strain introduced 

 through the nose might exhibit moderate or high susceptibility to 

 the same strain injected intraperitoneally. Whether the grades of 

 infectivity were due to differences in the virulence of the organisms 

 or to degrees of susceptibility of the host was not ascertained. 



Distinct differences in the reactivity of mice of diverse races to 

 pneumococcal infection has been demonstrated by Rake. 1117 " 8 In 

 general, reactivity was found to be influenced by the type of or- 

 ganism used for inoculation as well as by innate characters pe- 

 culiar to the breed of the animal. For example, a single type of 

 Pneumococcus produced in mice of the same breed lesions which 

 were similar and predictable. Lesions differing quantitatively could 

 be produced in various breeds of mice by inoculation of the same 

 type of organism, but inoculation of cultures of the various types 

 into mice of a single race produced lesions differing in quality. Ex- 

 periments in which mice were infected intranasally and intrave- 

 nously revealed that lesions in the lung and other organs varied 

 with both the type of culture and the strain of mice used. 



The constancy of the results obtained in infection experiments 

 with this animal species is further conditioned by the age of the 

 individuals employed. Moreover, there is greater regularity in the 

 behavior of mice of approximately sixteen to twenty-one grams in 

 weight to artificial infection, and this fact is especially significant 

 when mice are employed for testing the potency of antipneumococ- 

 cic serum. More recently, Goodner and Miller (1935), 540 extending 

 Goodner's work on the rabbit, investigated the physiological vari- 



