CONTROL OF CULTURES 471 



but sometimes necessary. A wild culture is selected in which the Protozoa 

 under investigation are multiplying rapidly. From the fluid of this cul- 

 ture, surface-streak plates are prepared and pure cultures of all the 

 different types of bacteria are obtained. From these pure cultures, sus- 

 pensions are systematically prepared and inoculated with the sterile 

 Protozoa. This was the method used by Johnson (1933) in his work on 

 Oxytrichd. Johnson states that his selection was made on the basis of 

 prevalence, in a thriving wild culture. In other words, the type of bac- 

 teria found in the greatest abundance he supposed to be the type upon 

 which the Protozoa were most likely to be feeding. We have found that 

 this is not always the case. In our work on the ciliate Tillina {T. canali- 

 jera, obtained from Dr. J. P. Turner and described by him in 1937) 

 we were able to obtain growth on one species only, out of twenty-six 

 types isolated from a thriving culture. This one species (a Zopfius) was 

 the least prevalent of all on our plates. The reason for this appears to be 

 that Tillina being so very selective, the Zopfius were eaten out of the 

 culture by the time we took our samples, while the other twenty-five 

 species were left to multiply. We have found this situation to hold in a 

 number of cases, so that we are of the opinion that the results obtained 

 by Johnson were due to the fact that he was dealing with a ciliate which 

 was not rigidly selective. 



The work tending to show that supplementary factors (viz., thiamin 

 and the like) are necessary for the growth of several Protozoa in pure 

 culture has been reviewed in a subsequent chapter, but several observa- 

 tions regarding the same theme may be given here, as they apply to 

 "Zwiegliedrige Kulture." These supplements may make the difference 

 between success and failure to establish a protozoan in bacteria-free cul- 

 ture. Investigations are now going on in this laboratory on the supple- 

 ment question, but they are as yet far from complete. Therefore little 

 can be said as to the exact nature of the substances or factors to be de- 

 scribed. In order to present this problem clearly to the reader, a descrip- 

 tion of a typical example will be given. 



D. M. Lilly, working in this laboratory, has studied the nutritional 

 requirements of two hypotrichous ciliates, Stylonychia pustulata and 

 Pleurotricha lanceolata. Both of these forms are bacteria-feeders in na- 

 ture, but will also become carnivorous in the presence of other small 

 ciliates. Sterilization was carried out, with the use of the dilution method, 



