PETERS. — METABOLISM AND DIVISION IN PROTOZOA. 455 



tation process is, however, indispensable, for it is the source, direct or 

 indirect, of all the nutriment of Infusoria. Success sometimes attended 

 the introduction of Stentors into a licjuid that had completed its active 

 fermentation. 



An excellent method of removing the excess of COo and noxious gases 

 from a culture, whether new or old, is to transmit a current of air 

 throuf^h it for some time. By reason of the comparatively high partial 

 pressures of the gases in contact with the air, either at the surface or 

 deeper, the whole liquid loses much of these undesirable constituents. 

 This removal of gases is probably of as much importance to the Infusoria 

 as the accompanying increase in oxygen. Unless the odor and other 

 conditions of a culture indicated the need of such aeration, it was found 

 that a o-rowing culture could be ruined by this process. In all cases an 

 estimation of acidity should be made. For some time the practice was 

 resorted to of precipitating the CO2 as calcium carbonate by the addition 

 of calcic hydroxide. Neutrality or a slight excess of calcic hydroxide 

 are not unfavorable to Stentor. The former condition cannot, of course, 

 be maintained in a culture liquid. Tliis process is inferior to aeration 

 and was abandoned in favor of the latter. 



If a culture is old and declining, it is sometimes easier to revive it than 

 to start a new one. This is probably due to the fact that the liquid of a 

 former thriving culture is a favorable salt medium for Stentor. Aeration 

 together with the addition of some brown leaves or dried reeds frequently 

 restored a culture that had run its course. If the liquid was not already 

 laden with salts, it was often found advantageous to add 100 to 200 mo- 

 lecular parts in 100,000 of the usual salts. Such addition of salts was 

 extensively practised, but to what limit it could be carried successfully 

 was never determined. By the use of any or all of the preceding methods, 

 when the conditions sugorested them, the same cultures were maintained 

 for months without emptying the contents of the jar. 



IV. Acceleration of Division. 



My earlier direct experiments upon the rate of cell-division were 

 suTcrested by the well-known examples of artificial stimulation produced 

 by the treatment of unfertilized eggs, for a short time only, with chemicals. 

 I refer to the experiments of TichomirofF('86), 0. und R. Hertwig ('87), 

 Morgan ('99),. Loeb ('99), Winkler (: 00), and others. Among the 

 substances so used occur the physiological salts, — the class in which we 

 are especially interested, — as well as acids, alkalis, extracts of physio- 



