564 PROTOZOA 



tow-nettings from the Cherwell in river water {pR approx. 8) to 

 which wheat grains were added. The pelagic Stentor settle down, 

 secrete houses and multiply. However, after ten days or so this 

 dominant form is replaced by Actinosphsrium, which feeds on it. 

 The prevention of cycles of organisms, is, in fact, the chief difficulty 

 in obtaining permanent mass cultures of any special form. 



1110. Cultivation of Amoeba proteus. Taylor {Quart. Journ. 

 Micr. Sci., 1924, pp. 69, 119) has since 1915 investigated the 

 conditions under which this organism will complete its life history 

 in artificial culture. 



The medium used is boiled rain-water,* containing about 

 twenty-two wheat grains to the litre (the wheat is boiled to 

 prevent germination). The optimum reaction of a flourishing 

 culture containing numerous large active amoebae is about pH 6-6, 

 and sub-cultures need only be made every three months or so. 

 Into the new culture fluid should be carried over several cubic 

 centimetres of the old culture, including organisms on which the 

 amoebae are feeding, as well as alga;, to keep the water oxygenated. 



The cultures are best kept in large covered glass vessels in 

 which the medium has a depth of 2 or 3 in., though smaller 

 vessels may be used. They should be kept towards the back of 

 a room away from bright light. Under these conditions the 

 cultures can be kept with very little trouble indefinitely. 



Sometimes difficulty is experienced in starting a culture owing 

 to the presence of enemies to the amoeba, such as oligochaetes, 

 ostracods, etc., and it may be necessary to make several sub- 

 cultures to eliminate these. f If only few specimens can be found 

 with which to start a culture, it is well to use only a small amount 

 of medium and place it in a test-tube or Petri dish until the correct 

 conditions are established. 



Conditions which induce encystment of A. proteus are still 

 being investigated. I understand from Sister Monica Taylor 

 that these rest with the individual amoeba rather than with the 

 culture medium, and that the only way by which one can hope 

 to encourage encystment is to starve large specimens which 



* In some districts the tap-water is suitable, but here in Oxford 

 and many other places the water is " hard." Part of this hardness 

 (temporary hardness — so called because it can be removed by boiling) 

 is due to the presence of carbonates of the alkali and alkaline earth 

 metals, kept in solution as bicarbonates so long as there is COg in the 

 water. The pH may or may not be too high for the cultivation of any 

 special protozoa to start with (here it is generally 7-6). However, when 

 placed in shallow vessels, water containing COj will tend to lose it, and 

 consequently the water will become more alkaline (see below, § 1111, 

 under Spirostomum). 



t I understand that small rotifers, especially the red rotifer (of rain 

 barrels), have proved themselves to be useful in small numbers as 

 scavengers in amoeba cultures. 



