PETERS. — METABOLISM AND DIVISION IN PROTOZOA. 481 



later, artificial cultures that furnished material for the present woik 

 were raised in favorable salt media, as described in other sections. That 

 the earlier, natural cultures brought into the laboratory from the ponds 

 also cortaiued salts in considerable quantities is scarcely open to doubt. 

 Decaying organic matter and earth in contact with water are abundant 

 sources of salts in solution. Furthermore, evaporation of a few cubic 

 centimeters of culture medium always left a perceptible residue. Con- 

 ductivity measurements of the natural media, determined by comparison 

 with solutions of calcic chloride, usually gave a value equivalent to that of 

 a 0.00100 m. to 0.00200 m. calcic chloride solution (see p. 508). From 

 whatever culture the organisms for this experiment came, it is certain 

 that their original medium contained a greater concentration of salts than 

 the very pure distilled water to whicli they were transferred. 



The presence of salts in the medium does not necessarily imply that 

 the cell living in it contains the same salts. The discovery of the relation 

 between the salts in the medium and those contained in the free-livinw 

 cell itself, being the object of the present investigation, this relation is 

 not to be assumed. For example, in Chaetomorpha and Spirngyra, ac- 

 cording to Jansen (cited by Schafer, '98, p. 277), we have illustrations 

 of a great difference between the concentration of the salt within the cells 

 and that in the surrounding media. The two plants have about the same 

 internal osmotic pressure, although the former is a marine, the latter a 

 fresh-water form, Spirogyra thus having a greater and Chaetomorpha 

 a less pressure than their respective media. The permeability of the 

 organism, which probably governs such conditions, will receive some 

 attention for Stentor in a later section. Hei'e will be made only the 

 very probable assumption that the Stentor cell does contain a certain 

 quantity of salts. That Stentor contains physiological salts in certain 

 proportions may be inferred from the results of chemical analysis in the 

 case of all organisms obtainable in sufficient cjuantities to permit such 

 analysis, and from the results of all experiments on nutrition made with 

 reference to salts (e. g., Forster, '73, Herbst, '92-98, and others). The 

 experiments upon adjustment given in a later section of this paper also 

 lend support to the proposition that Stentor contains some of all the salts 

 found necessary for a medium in which its metabolic jDrocesses may nor- 

 mally take place. What these salts are individually in the case of Sten- 

 tor is not of importance at this stage of the discussion ; but I may point 

 out that the salts within the cell probably originate from the medium, 

 either directly, by osmotic absorption and by ingestion, or indirectly, 

 beins: contained in other organisms used as food, which orjjanisms orijxi- 



VOL. XXXIX. — 31 



