D. W. Cutler 



433 



2. The total number of protozoa in 500 squares is counted. This 

 represents an area of 500 x 0-04, that is, 20 sq. mm., or 2 cu. mm. The 

 factor, therefore, for estimating the number per c.c. of the suspension 

 is 2500. The two methods give concordant results: the first should be 

 used for suspensions containing over 100,000 per c.c. ; the second when 

 fewer are present. Two typical counts are shown in Tables I and II. 



The accuracy of the results was shown by checking them by a dilu- 

 tion method. If these two very difierent methods of estimation gave 

 comparable results it seemed justifiable to assume that they were fairly 

 accurate. 



Table III. 



Shoiving the resulls obtained by countimj Protozoa in a suspension by the 



direct atul imlirect method. 



10 c.c. of a I/lOO dilution was made and further diluted to the neces- 

 sary degrees. 1 c.c. of each dilution under investigation was then inoc- 

 ulated on to each of three nutrient agar plates, which were then incubated 

 at 20° C. for 21 days, and examined at intervals. If growth of protozoa 

 occurred on a 1/10,000 dilution plate, there must have been at least one 

 organism to cause this growth, and hence it was assumed that there 

 were at least 10,000 protozoa per cubic centimetre of the suspension. 

 This method clearly gives only a minimum value, but if a series of dilu- 



