PETERS. — METABOLISM AND DIVISION IN PROTOZOA. 451 



moveuieuts were thus stopped and they were pressed fiat. Finally the 

 contents of the cell escaped. Uuideutified unicellular vegetable matter 

 of low organization, Euglenae, and Arcellae were the most frequent 

 kinds of food present. These organisms were abundantly consumed. 

 Occasionally a Stentor was observed in the act of ingesting a Para- 

 maeciura. Although Stentor is omniverous, this fact was of not much 

 avail in raising a culture. Very often it was observed that an aborting 

 culture with enough Stentors still present to make a successful start con- 

 tained an abundance of the direct foods upon which the animals could 

 easily subsist. This observation was repeated so often that it was quite 

 evident that some other important condition than a supply of food had 

 not been fulfilled. 



Gradually it became clear that no other single condition is of such de- 

 termining influence for free-living cells as the salt content of their liquid 

 medium. The special experiments, subsequently described, made during 

 the period of these culture experiments, furnished direct evidence for the 

 above proposition. Hence attention was turned to the determination of 

 a favorable salt medium for Stentor. The failures and the partial suc- 

 cesses that attended this effort will not all be recorded here. They were 

 instructive in showing that the determination of an animal's adjustments 

 to substances brings to light a metabolic status not so well exhibited by 

 any other method. 



The most successful salt medium for raising cultures was obtained by 

 the experiments described in the section on Single and Combined Salts 

 (p. 497). Special tests upon the most successful medium are there 

 described. It consisted of the following combination of salts dissolved 



in w^ater : 



CaCla 00055 m. 



NaNOg 00015 m. 



MgSO^ 00015 m. 



K2HPO4 00015 m. 



The sum total of its equivalent concentration is 100 molecular parts in 

 100,000. The most important constituents are CaCl2 and K.2HPO4, 

 and the proportions, especially high for CaCls, are important. As sub- 

 sequent experiments show, Stentor can live a long time in a favorable 

 salt solution, but multiplication soon ceases. To maintain this, a food 

 supply must be added to the salt solution, and this requirement has 

 proved to be a difficulty. For the addition of any food that has been 

 found available utterly changes the salt content both qualitatively and 



