COMPETITION FOR COMMON FOOD IN PROTOZOA 97 



buffered and kept at a definite hydrogen ion concentration (pH). 

 Special experiments made by Johnson showed that the bacteria do 

 not multiply in this medium, and that their number scarcely changes 

 within 24 hours. 



Beginning our experiments on the growth of pure and mixed popu- 

 lations of Paramecium caudatum, Paramecium aurelia and Stylonychia 

 pustulata, we devoted a certain time to finding a culture of bacteria 

 suitable as food for all three species of Protozoa. Using the data 

 published by Philpot ('28) we chose finally the pathogenic bacterium 

 Bacillus pyocyaneus, which was cultivated in Petri dishes at 37°C. on 

 a solid medium of the following composition : peptone, 1 gr. ; glucose, 

 2 gr.; K2HPO4, 0.02 gr.; agar-agar, 2 gr., per 100 cm 3 of tap-water. 

 One standardized uniformly filled platinum loop of fresh Bacillus 

 pyocyaneus taken off the solid medium was placed in 10 cm 3 of Oster- 

 hout's salt solution. This mixture was prepared anew every day, 

 and we will speak of it as the "one-loop" medium. 



(2) Such a standard and convenient technique of cultivation en- 

 ables us to approach the experimental investigation of an important 

 problem: the course of the process of competition for a source of 

 energy kept continually at a certain level. With this object the 

 cultivation was carried on in graduate tubes for centrifugation of 

 10 c.c. capacity, which were filled with nutritive medium up to 5 

 c.c. and closed with cotton wool stoppers. Twenty individuals of the 

 corresponding species were placed in every tube, or 20 plus 20 in 

 case of a mixed culture. The medium was changed daily in the 

 following manner. The tube was placed in a centrifuge, and after 

 two minutes of centrifugation with 3500 revolutions per minute the 

 infusoria fell to the bottom, the liquid above was very gently drawn 

 off by means of a pipette with a caoutchouc ball and a freshly made 

 nutritive medium was poured in. Besides this, every other day each 

 culture was washed with the salt solution free of bacteria, in order to 

 prevent the accumulation of waste products in the few drops of liquid 

 remaining at the bottom of the tube with the Paramecia at the mo- 

 ment when the medium was changed. For this purpose after the 

 pouring off of the old medium the tubes were filled with a pure salt 

 solution, centrifuged and the liquid was drawn off a second time. 

 Every day before the medium was changed each culture was carefully 

 stirred up, 0.5 c.c. of the liquid was taken out and the number of in- 

 fusoria in it counted. After counting the sample was destroyed. All 



