494 University of California Puhlications in Zoology [^^o^. 20 



The following conclusions may be drawn from a study of the data 

 in the accompanying tables. 



1. The incidence of infection of amoebae among culture mice in 

 this laboratory is 37.5 per cent while among rats it is 34 per cent (see 

 table 1). 



2. The incidence of infection is much lower in very old and in very 

 young mice and rats than it is in middle-aged animals (table 2). 



TABLE 2 

 Mice 



3. The incidence of infection is lower in wild mice and rats than 

 in culture mice and rats (see table 3). It is assumed that the mice 

 used by Wenyon were culture mice, and while he gives no figures of 

 the number he examined he states that he found amoebae in about half 

 of the ca.ses. The culture mice and rats examined in this laboratory 

 show an average infection of 35.7 per cent, while all the wild rats and 

 mice recorded in the literature and the 20 wild mice examined in this 

 laboratory show an average infection of 7 per cent. 



Balfour (1922) made his examinations from autopsied rats and 

 it is assumed that Rudovsky worked with dead animals. The examina- 

 tions of the wild mice in this laboratory were made after death. 

 Examination after autopsy should be just as certain a method of 

 determining infection as the epsom salt method. Brug does not state 

 how his 50 rats were examined, but, as his rate of infection is higher 

 than Rudovsky 's and only slightly lower than the lower percentage 

 recorded bj' Balfour, it would seem fair to include his results in the 

 table. These results indicate that animals living in the crowded con- 

 ditions of a laboratory colony are more liable to acquire amoebic 

 infection than those living in the open. 



