PETERS. — METABOLISM AND DIVISION IN PROTOZOA. 493 



constitutive proportion referred to in the preceding section. A complete 

 knowledge of an animal's relation to substances, even if only to a class, 

 for example to physiological salts, would require more experiments than 

 I have been able to make. But I have extended my studies far enough 

 to illustrate this method of attacking some of the problems of the internal 

 chemical conditions of a free-living cell. 



Polished watch-glasses, closed by piling them on top of one another, 

 were used to contain the salt media and the Stentors. It was cus- 

 tomary to make the test for each substance with fifty animals, ten each 

 in five different watch-glasses. In the following records, unless a state- 

 ment to the contrary is made, there were ten animals to a watch-glass. 

 All the Stentors for a given experiment came from the same culture and 

 they were therefore subject to like conditions. But differently numbered 

 experiments frequently included animals of different origin. Their com- 

 parability has been previously discussed. The number of Stentors sur- 

 viving after the expiration of stated intervals of time are written in their 

 respective columns. 



Expt. No. U, April 2, 1902. 



The difficulty in finding a uniformly successful method of raising 

 Stentors in mass-cultures finally led me to give special attention to the 

 salt-content of the media so used. Many such cultures were carried on 

 parallel to the following experiments and with media whose composition 

 was based upon them. From among the salt solutions in common use 

 for the water-culture of plants, I selected Pfeffer's medium (PfefFer : 00, 

 p. 240), made as follows. Of calcic nitrate 4 grm. ; of potassic nitrate, 

 magnesic sulphate, acid phosphate of potassium, each 1 grm. ; of potassic 

 chloride 0.5 grm. ; all dissolved in 7 litres of distilled water. A prelim- 

 inary trial showed that much dilution was necessary for use with Stentor. 

 The above medium was diluted to seven times its original volume. It is 

 not necessary for my purpose to determine what may be the chemical 

 condition of the above salts after their interaction in dilute solution. 

 Stentors placed in this medium gave the following results: 



One Seventh Pfeffer's Medium. 



Time hr. 15 hr. 4G hr. 10 da. 15 da. 



No. of Stentors . 50 55 96 99 76 



The medium contained no food in the ordinary sense of the term and 

 the numerical results are another illustration of the propriety, under this 



