D. W. Cutler 431 



of organisms in a unit volume of a solution is needed. Secondly, the 

 factors governing the relation of the protozoa to the soil particles require 

 elucidation in order to explain why it is almost impossible to find the 

 organisms in any quantity by direct examination under the microscope, 

 although the same soil sample can be shown to contain tens of thousands, 

 if a dilution method is employed. 



The protozoa chosen for the experiments were obtained from Broad- 

 balk field soil, and were as follows: 



Amoebae Flagellates 



A. Imvesiana, Goodey Monas termo 



A. glebae, Dobell Bodo sp. 



A. sj). Cercomonas sp. 



Oicomonas sp. 

 No attempts were made to separate these one from another and grow 

 them in "pure" culture. Although this course presents disadvantages, 

 the treatment of the forms "en masse" more faithfully reproduces 

 field conditions, as these organisms are representative of the soil proto- 

 zoan fauna at Rothamsted. 



The average sizes of the active and cystic states are : 



Active amoebae, 12-5 /x; cystic stage 10-7 ^it. 

 Active flagellates 8-5 /x; cystic stage 4-7 /x. 



The investigation on the cihates detailed in Part II of this paper 

 was carried out upon Colpoda cucullus, which measured in the active 

 condition about 45 /i and in the cystic one from 40-45 fx. 



PART I. 

 Method for Counting Protozoa. 



KopelofE, Lint and Coleman(4) have described a direct method for 

 estimating the numbers of protozoa in a suspension which does not 

 involve plating on culture media and subsequent incubation. As this 

 seemed satisfactory it was compared with the dilution method in use 

 at Rothamsted. 



The apparatus consists of a thick glass slide in the centre of which is 

 a hollow of depth 0- 1 mm. Round this hollow is a deep groove to receive 

 any excess fluid that may be released when a cover-glass is placed upon 

 the shde. The hollow in the centre of the shde is divided into 625 squares, 

 each of which is 1/25 sq. mm. 



A volume of the fluid to be examined, and sufficient in amovmt to 

 ensure perfect contact between the cover-glass and shde, is placed in 



