1911.] 



Knowledge of the Protozoa of the Soil. 



173 



rapidly, and for this reason it did not seem possible to cause protozoa to 

 collect at a given point by directly applying the current to the soil. 



The following method was finally arrived at : — In a large Petri dish, 

 11 cm. in diameter and 2 cm. deep, a fairly large quantity (2 — 5 grm.) of the 

 required soil is placed : 20 c.c. of sterile 1-per-cent. hay infusion is then 

 added and the soil quickly teased out as finely as possible. The culture is 

 then put into the 30° C. incubator — incubation has in all cases been carried 

 out at this temperature. After a time, the culture is carefully taken from 

 the incubator and the supernatant liquid pipetted off, without much 

 agitation, so as to avoid taking up any quantity of soil particles. 



This liquid is put into the two tubes of a small hand centrifugal machine, 

 and centrifugalised at high speed for a few minutes in order to bring down 

 any protozoa present to the bottom of the tubes. The bulk of the liquid is 

 then taken up as quickly as possible by means of a pipette, and put back 

 into the culture in the large dish. The remaining few cubic centimetres at 

 the bottom of the tubes are then agitated and taken up by a small pipette 

 and put into a shallow Petri dish 6 cm. in diameter. This is placed on the 

 stage of the microscope, and submitted to the electric current for a few 

 minutes. The current passes between the two non-polarisable electrodes 

 dipping into the liquid and connected up with the storage battery and the 

 milliammeter. An E.M.F. of 12 volts and a current of - 0002 to 

 0"00028 ampere have been used in most of the work. By focussing a low- 

 power objective (1-inch) on to the cathode or close to it, it is possible to 

 watch the ciliates come swimming into this region under the influence of the 

 electric stimulus. 



It has been possible by this means to find, in a few minutes, a few or 

 even solitary ciliates in 5 or 6 c.c. of culture liquid. If, after the passage of 

 the current for about 5 minutes, no protozoa are found in the region of the 

 cathode, the conclusion is that none are present, and the liquid is put back 

 into the main culture. 



For further observation, under high-power objectives, of the ciliates 

 caused to aggregate at the cathode, it has been necessary to extract them 

 from the culture liquid. For this purpose capillary pipettes have been used. 

 Placing the fine end of one of these under the low power, and then having 

 the required organism in focus, following it carefully by moving the dish 

 with the left hand, the pipette is brought down on to it, with the result that 

 the organism is suddenly sucked up into the tube. From this it is usually 

 transferred to a cover-slip by blowing down the broad end of the tube. Here 

 it can be treated in whatever way is desired. 



As mentioned in Part I, the protozoa in the ordinary soil culture do not 



2 



