402 University of California PuMications in Zoology [Vol. 20 



gfreater the number of samples examined, the greater the percentage 

 of infections found (see Kofoid, 1920). In the Department of Para- 

 sitology, California State Board of Health, in Berkeley, six routine 

 examinations of human faeces without finding infection are made 

 before a case is pronounced as probably negative. Dobell (1917 and 

 1920) has also pointed out the advisability of making six routine 

 examinations before a human case is pronounced uninfected and in 

 1917 states that "the expectation for the average infected case is 

 that it vrill be found positive twice in every five examinations." 



Routine examination of the normal faeces of rats and mice has 

 shown this method to be less accurate with these animals than is 

 probably the case with man. Two reasons may be given for this fact. 

 First, the number of amoebae found in the rodents is considerably 

 less than is found in man, and, second, the active stages of the intes- 

 tinal Protozoa found normally in rats and mice inhabit the small 

 intestine and caecum in the main and are seldom found in the large 

 intestine, while in man, amoebic infections, including their cysts, are 

 common in the colon. 



Brug (1919) kept records of routine examinations for amoebae 

 in the normal faeces of six rats and concluded that this method is 

 not thoroughly reliable. His results are included in table 1 for the 

 sake of comparison with similar conclusions which were drawn early 

 in the investigation by the present writer. 



It will be seen from the accompanying table that only 16.3 per cent 

 of all the examinations made gave positive results, though all the 

 animals examined were known to be infected with amoebae. Brug 

 (1919) states that "the cysts may be found for several days in suc- 

 cession, then for several days, weeks or months in succession nothing 

 may be found." Similar results, but not for intervals so long as 

 months, were obtained in the present investigation. Further, the 

 number of cysts, when present, may vary greatly. At times only 

 one cyst may be found on the entire cover glass preparation, while 

 at other times as many as three or four cysts may be found with the 

 4 mm. objective in one field of the microscope. As a rule, cysts only 

 were found in the faeces, though on three occasions motile amoebae 

 were present. These facts point to the conclusion that there is no 

 regular periodicity in the discharge of intestinal amoebae in these 

 rodents. 



It is evident from these data that the small number of examin- 

 ations sufficing for the detection of intestinal Protozoa in man would 



