PHYSIOLOGICAL VARIATIONS IN PARAMECIUM 505 



more carefully guarded methods, the individual cells being daily 

 transferred to fresh twenty-four-hour hay infusion. Under these 

 conditions the race continued to live for twenty-three months. 

 During this period, however the entire race would have died at 

 any one of three successive periods had not precautionary meth- 

 ods been used. These periods, marked by, sluggish metabolism 

 and by a greatly diminished division rate, were termed 'depres- 

 sions' and they occurred with surprising regularity once in six 

 months. Minor fluctuations in vitality which Woodruff ('05) 

 called 'rhythms' were self -regulated by the organisms. The de- 

 pressions were overcome by the use of beef extract, or by simple 

 salts such as potassium phosphate or potassium chloride. At 

 each depression period save the first one when this condition of 

 the organisms was novel, half of the race were continued on the 

 hay diet and half were treated with beef extract and salts for a 

 short period. In all cases the first half died within three weeks, 

 the other half continued to live with renewed metabolic power 

 and with high division rates. In the final period of depression, 

 however, the use of salts or extracts was not followed by renewal 

 of vitality and the entire race died in the 742d generation. 



Further observations and experiments of similar nature con- 

 firmed the earlier conclusion of Maupas. Woodruff ('05) car- 

 ried Oxy tricha f allax through 860 generations when the race died ; 

 Pleurotricha lanceolata through 440 generations and Gastrostyla 

 through 288 generations; Gregory ('09) carried Tillina magna 

 through 548 generations when the race died; Moody ('12) carried 

 Spathidium spatula through 218 generations until it died; Cal- 

 kins (unpublished) carried Actinobolus radians through 8 months 

 of culture when the race died in the 446th generation, and also 

 carried Blepharisma undulans through eleven months of culture 

 when this race died in the 224th generation. 



Several different observers with several different infusoria have 

 thus reached the conclusion that a race of protozoa has a certain 

 potential of vitality which under the conditions of culture weakens 

 until the organisms die. 



These conclusions have been criticized by Enriques, Jennings 

 and others on the ground that the conditions under culture are 



