Parameciidae 123 



into infusions such as the above. Certain investigators adjust osmotic 

 pressures, pH, or other ionic concentrations by adding salts or other 

 compounds. Such techniques are not essential to the production of ex- 

 cellent mass-cultures of Paramecia, although they will enhance the uni- 

 formity of the medium. 



The present discussion is limited to cultures of P. multimicronucleatum 

 grown in infusions prepared by boiling hay or hay-flour combinations in 

 water. The following is a formula which I have employed frequently 

 and successfully: 



1 gram hay 

 0.1 grams white flour 

 700 cc. distilled water 



Stir the flour through the hay. Bring the water to a boil. Add the hay-flour 

 mixture. Boil 10 minutes. Cool. Add distilled water to replace that evaporated. 

 Seed with 200 Paramecia on the second day. 



The above formula may be considerably altered, especially in routine 

 work, without materially interfering with the production of a satisfactory 

 population. Permissible variations include the use of hay up to 4 grams 

 if flour be omitted (Jones, 1930) ; the use of tap or pond water if non- 

 toxic; and variation in the date of seeding and the number of seed Para- 

 mecia introduced. 



Additions of hay or flour in excess of the amounts stated will produce 

 a hydrogen-ion concentration which will destroy the Paramecia on the 

 fourth or fifth day (Jones, 1930) (pH 4.83 or less) . The "seeding" must, 

 under such circumstances be delayed until the pH exceeds 5.0. In the 

 presence of such excesses of food, certain other unidentified split products 

 which result may prove to be toxic, even if the pH be satisfactory. In- 

 troduction of excesses of food material must therefore be avoided, either 

 when originally preparing the infusion, or later, if feeding techniques be 

 employed. 



Cultures should not be covered if populations of maximum concentra- 

 tion are desired. Non-evaporating cultures made as described above will 

 produce populations of approximately 300 Paramecia per cc. of solution 

 as a maximum, whereas evaporating cultures may eventually yield from 

 2000 to 4000 per cc. 



MAINTAINING AND REHABILITATING MASS-CULTURES 



Mass-cultures, especially of the non-evaporating type (in which the 

 cover is placed on the jar, but not screwed down) , may be revived by feed- 

 ing when they approach the point in the cycle of culture conditions at 

 which the Paramecia would normally disappear. When a population 

 of Paramecia is present in a culture it may frequently be increased by 

 the same means. At various times I have with success employed flour, 



