96 THE STRUGGLE FOR EXISTENCE 



physicist does over his, the results tend to take on a degree of precision 

 and uniformity not so far short of that usual in the older science, as 

 we are accustomed to expect" ('28, p. 35). 



ii 



(1) Jennings pointed out the necessity of a careful control of bac- 

 teria in the cultures of Protozoa in 1908, and one of the first attempts 

 to grow Paramecia in pure cultures of bacteria was made by Hargitt 

 and Fray ('17), Oehler ('20) and Jollos ('21). Since then numerous re- 

 searches have appeared and a good review of them can be found in the 

 recently published book of Sandon ('32) The Food of Protozoa as well 

 as in Hartmann ('27) and Belar ('28). It can be noted in a quite 

 general form that in order to standardize the conditions of cultivation 



TABLE VIII 



Balanced physiological salt solution of Osterhout 



NaCl 2 .35 gr. 



MgCl 2 0. 184 gr. 



MgS0 4 0.089 gr. 



KC1 0.050 gr. 



CaCl 2 0.027 gr. 



Bidistilled water to 100 c.c. 



This solution is diluted with bidistilled water 225 times. 



of Protozoa it is necessary: (1) to standardize the quality of food — 

 to cultivate the Protozoa on bacteria of a definite species, (2) to stand- 

 ardize the quantity of food — the number of bacteria per unit of 

 volume must have a fixed value, and (3) to standardize the physico- 

 chemical conditions of the medium. These difficult and as one may 

 think hardly realizable problems have been solved very simply for 

 Oxytrichia by Johnson ('33), who has been partly preceded by Barker 

 and Taylor ('31). The method is this: a culture of a certain bac- 

 terium is made on a solid medium and then a fixed quantity of bac- 

 teria is taken off the solid medium and transferred into a balanced 

 physiological salt solution, where these bacteria do not multiply and 

 serve as food for the Protozoa. Like Johnson we used Osterhout's 

 salt solution, the composition of which is given in Table VIII. As 

 will be shown further on, in certain experiments this medium was 



