TECHNIQUES 343 



set Up in order that the stentors may themselves possibly regulate 

 the medium to their Hking; and very little nutrient is at first added, 

 in proportion to the few stentors present. 



Progress of the starting culture can then easily be followed by 

 placing the jar briefly in front of a bright spotlight and examining 

 with a magnifying glass. At the end of a week, if the stentors are 

 multiplying, more nutrient is added, at first only a drop or two of 

 skimmed milk, but only if the water has become clear. If turbid 

 with uneaten flagellates and bacteria, the jar is let stand another 

 week before nutrifying. Since milk is a complex mixture forming 

 a nearly perfect food, it serves as a good basic nutrient and ionic 

 medium for stentors and a variety of other protozoa, including of 

 course the food organisms (Tartar, 1950). 



As the stentors increase in number, more lake or other natural 

 water which has been passed through a Millipore filter to remove 

 all protozoa and their cysts is introduced from a stock jar, with a 

 little more cotton. Eventually the culture jar will be filled to the 

 top and can be nutrified once a week with 5 or 6 drops of skimmed 

 milk. (Cream content would form a film on top and exclude the 

 air.) From the beginning the jar is covered with its original cap, in 

 the center of which is punched one hole with a large nail or ice 

 pick, the cap preventing contamination and evaporation and the 

 hole allowing gaseous exchange. 



Such cultures will remain in thriving condition for many months. 

 If removal of detrimental cohabitants was unsatisfactory, or if 

 hypotrichs, nematodes, etc., should later infest the culture, one 

 has to begin again, treating the culture as if it were a pond sample 

 and isolating stentors as before. A cardinal precaution is never to 

 over-nutrify the culture so that a distinctly putrid condition arises. 

 In the course of months the stentors may diminish in abundance 

 in spite of the regular additions of milk. When this occurs it is 

 assumed that the water should be changed. Since the stentors are 

 mostly attached to the sides and the cotton fibers, the whole jar 

 can be gently emptied, or the cotton can be retained, and then 

 immediately filled with filtered water .In the meantime the stentors 

 have remained attached to the sides and are protected by a fluid 

 film. In spite of some loss there will probably still be enough 

 animals to handle the large amount of new water. One may want 

 to add less milk now until the animals become plentiful. A con- 



