PETERS. — METABOLISM AND DIVISION IN PROTOZOA. 499 



This four-salt medium represents at least one set of combinations to 

 which Steutor is well adjusted, but it may not be the only useful com- 

 bination of this description. However, numerous trials in making de- 

 partures from the qualitative or quantitative constitution of this medium 

 indicated, by their frequent failure, that both the qualitative and quanti- 

 tative range of substances to which Stentor is adjusted, is comparatively 

 limited. It would be of interest to have corresponding media well ad- 

 justed to other Protozoa tried with Stentor for the purpose of comparing 

 the results. The empirical determination of a successful combination for 

 Stentor we may now dismiss, and proceed to the consideration of some 

 general aspects of the employment of single and of combined salts. 



Vlir. General Discussion of Single and Combined Salts. 



The preceding experiments taken as a whole show that a medium 

 consisting of several salts is more favorable to the physiological processes 

 of the animal than one containing only a single salt. The physico- 

 chemical phenomena underlying this fact seem to me to be the following. 

 The experiments being made at hypisotouic concentrations, there were 

 lower partial pressures outside the cell for all its contained salts except, 

 probably, the one present in the medium. For this one salt there was 

 a lower partial pressure within the cell as soon as the concentration ap- 

 plied outside exceeded the normally low concentration within. The low 

 ash-content thus far obtained in all analyses of protoplasm shows that the 

 normal concentration of individual salts within the cell is low. It is 

 probably so low that most of the hypisotonic concentrations used in the 

 preceding experiments exceeded the physiological limit for a single salt. 

 When the contrary condition obtains, we approach, more and more as 

 the concentration lowers, the conditions shown by cells in distilled water. 

 In distilled water, as previously described, loss of salts brought about by 

 constitutional alteration of the cell-wall explains the result. But under 

 the conditions more probably existing in the above experiments we 

 should have, in consequence of differences in partial pressures, two oppo- 

 site movements of salts. Assuming permeability, the one salt applied 

 would penetrate the cell and the other physiological salts would pass into 

 the external medium. We are unable to determine how much of the 

 effect to assign to each of the two processes, but they both contribute to 

 the disturbance of the. normal proportions of physiological salts. TJiey 

 inhibit the division-reaction in Stentors by altering the constitutive propor- 

 tion of its salts. When the number of salts applied externally increases, 

 the tendency to disproportion diminishes in consequence of fewer unbal- 



