yan, A. M., and Rokitskaya, M. S., Cbmptes rendus de l'acad~mie des sciences de l' U. R. S. S., 3I: I23, I94I). It has the advantages of being large enough for observation of changes in size, shape, contractile vacuole activity, and of being easy to cultivate in large numbers.
Since propionic acid in minute traces (Miller, F. W., Jr., Agricultural News Letter, Public Relations Department, E. I. du Pont de Nemours and Co., lO: 62, 1942) is capable of inhibiting growth of mold on bread, it was thought that it might prove to be of some wdue as a therapeutic agent. Therefore it was tested in different concentrations on Paramecium caudatum. Small watch glasses were used for the experiments. These measure about20 ram. in diameter and can conveniently hold I cc. or a little more of fluid. The bottom of each glass is grooved and the upper rims are bevelled and ground so that they can be stacked one upon another, thus avoiding any great amount of evaporation.
Paramecia were cultured in small glass jars. The pH was kept at about 7.0 by addition of NaOH or NaHCO3. An original inoculation of Serratia marcescens was made to the paramecia in aquarium water, and the organisms were fed on a few flakes of dried rolled oats per jar twice a week. Subcultures were made about once a month by putting several cc. from old cultures into clean jars and adding distilled water, a few cc. per day until the jars were filled.
For testing purposes paramecia were pipetted into the watch glasses and the desired amount of propionic acid added, proportions being reached by counting the number of drops of culture and of chemical, the same pipette being used for