PETERS. — METABOLISM AND DIVISION IN PROTOZOA. 465 



preliminary preparation, much of which is described in the section on 

 General Methods and Technique (p. 444), frequently consumed the 

 major part of the time and labor expended. Secondly, the problems 

 suggested by the results with these substances could not be solved by 

 multiplication of similar experiments with other substances. A new line 

 of experimentation was required, and one suggested itself. 



Mass-cultures were carried on simultaneously with the special experi- 

 ments here described, and in most cases furnished the animals for the 

 latter. From these cultures it became more and more evident that 

 different species of Protozoa present peculiar and characteristic adjust- 

 ments. The observations that gradually impressed this fact upon me are 

 described, but not in their full detail, in the part devoted to the Condi- 

 tions of Growth in Stentor and the Management of Cultures. This 

 important fact of specific adjustment suggests that cumulative experience 

 with a single organism is the necessary preliminary basis for any reliable 

 interpretation of comparative experiments with differently adjusted Pro- 

 tozoa. Accordingly I confined my subsequent experiments almost solely 

 to Stentor coeruleus. I regret that limited time and more pressing ques- 

 tions prevented the extension of these experiments to other Protozoa. 

 However, in compensation for this deficiency I have obtained greater 

 certainty for the conclusions drawn, even upon problems of a general 

 nature, by confining my attention to Stentor. 



I selected potassic chloride, sodic chloride, calcic chloride, and magnesic 

 chloride as being four fairly representative physiological salts for animal 

 cells in general, as is shown both by ash-content and by feeding experi- 

 ments (Forster, '73, and others). Since they represent important chem- 

 ical classes of substances, it was thought that they might facilitate a 

 possible chemical interpretation of physiological action. Furthermore, 

 chlorides alone were selected in order to reduce the number of factors 

 involved and to provide a common factor that would make comparison 

 more practicable. The selection of the chloride, rather than some other 

 radical, though a matter of judgment, was to a certain extent a matter 

 of hazard. It proved to be a fortunate venture. Tests made at various 

 times between the different experiments hereafter described convinced 

 me that the chloride is better adapted to the end in view than any other 

 inorganic radical of the bases used. The chlorine ion is, upon the whole, 

 less injurious and exhibits fewer abrupt gradations in physiological 

 action from salt to salt among the salts here used. This point is always 

 capable of a satisfactory test by selecting different bases with the same 

 acid radical, and vice versa. Another organism thau Stentor might of 

 vol. xxxix. — 30 



