PETERS. — METABOLISM AND DIVISION IN PROTOZOA. 449 



temperature standard. Perhaps distilled water was the reagent demand- 

 ing most attention. In testing waters of all sorts, as well as culture 

 liquids and reagents in special cases, the method of electrical conductivity 

 was applied. For this purpose the apparatus devised by Nernst ('94) 

 and modified by Maltby ^95) for measuring conductivity was found most 

 useful. For its construction and use the original descriptions must be 

 consulted. 



For biological purposes a great number of test-cells of various con- 

 structions was found necessary. Variation in the size and distance 

 apart of the electrodes was indispensable in some critical experiments 

 where the use of platinized electrodes was not allowable. On this 

 account and owing to the good qualities of the apparatus itself for the 

 avoidance of polarization, it was never found necessary to use platinized 

 electrodes in the test-cells. For convenience in the subsequent descrip- 

 tion of experiments I shall adopt the same notation for the measuring 

 tubes of the instruments as are given to them by Maltby ('94, Fig. 4, 

 p. 142). The tubes most referred to there are on the right-hand side, 

 the larger one being marked W" and the smaller W. 



III. Conditions of Growth in Stentor, and the Management 



OF Cultures. 



For my study it was soon found desirable to maintain a continuous 

 supply of vigorous material. Later it became evident, that it was still 

 more necessary to know the origin of the Stentors used. These require- 

 ments led to the artificial culture of Infusoria in the laboratory, instead 

 of the continual acquisition of new material from ponds in the vicinity. 

 Artificial cultures incidentally exhibit some otherwise unnoticed aspects 

 of the metabolism of Stentor, and are a valuable means of confirming by 

 mass-culture the results of special experiments. Such cultures furnish 

 to observation, and especially to chemical tests, much indication of the 

 continuous change in constitution taking place in a medium and of the 

 phases of these changes to which the different kinds of Infusoria are 

 respectively adjusted. But the practice of making artificial mass-cultures 

 had the disadvantage of consuming much time. Numerous empirical 

 trials had to be made before sufficient insight was obtained into the con- 

 ditions to which Stentors adjust themselves. This animal was observed 

 to differ from some other Infusoria (e.g., Paramaecium) in having a more 

 circumscribed ranfje of fiivorable conditions. The salt media devised as 

 the result of successive experiments, and described in the sections treat- 

 voL. XXXIX. — 29 



