248 THE BIOLOGY OF STENTOR 



respiration or metabolism, which produce shedding of the mem- 

 branellar band in Stentor followed by a wave of disintegration 

 passing over the ectoplasm towards the posterior pole. I am 

 therefore inclined to agree with Weisz that the localized disintegra- 

 tive action of various solutions is a function of the special state of 

 organization of the different parts of the cell cortex. 



9. Acquired tolerance to external agents 



Pre-treatment of organisms with sub-lethal concentrations of 

 killing agents generally increases subsequent tolerance of originally 

 lethal concentrations of the same substances ! This adaptation has 

 also been demonstrated in Stentor. Davenport and Neal (1896) 

 succinctly summarized their studies on coeruleus. ''Stentors reared 

 for two days in a culture solution containing 0-00005% mercuric 

 chloride resist a killing solution of o -001% HgCl2 nearly four times 

 as long as those reared in water. Similar results were obtained by 

 use of quinine. " This was not due to the selection of resistant 

 individuals but a genuine acclimatization, because no deaths 

 occurred at the lower concentration and the same individuals were 

 carried into the higher. Nor was this a general adaptation to 

 increased osmotic pressure ; for the concentrations used were very 

 low, and NaCl solutions of the same osmotic pressure gave no 

 increased tolerance to the killing agents. The increased immunity 

 was acquired rapidly, measurable resistance developing after i or 

 2 hours exposure to the sub-lethal concentrations, gradually 

 increasing thereafter until exposure of longer than 96 hours gave 

 no further resistance. The stronger the acclimatizing solution the 

 greater the resistance developed until the strength was such that 

 the lethal effects were additive. Killing, by disintegration of the 

 cortex of the cell, occurred about three times more rapidly at 23° 

 than at 15 °C, indicating that death was caused by a chemical 

 reaction. 



Similar effects were studied in the response of coeruleus to 

 alcohols and glycerine by Daniel (1909). Animals lived well for 

 weeks in 1% ethanol, were destroyed by 6 hours in 2%, and died 

 in 2 hours at 3%. At lethal concentrations the body cilia soon 

 stopped beating but the membranelles remained active up to the 

 time of death. Two different stocks showed notable differences in 

 regard to acquired tolerance. In the first stock, 1% solutions 



