478 J. A. DAWSON 



cultures of Pleurotricha lanceolata for 458 generations without 

 conjugation or degeneration. Woodruff ('05), working with 

 Oxytricha, Pleurotricha, and Gastrostyla, found that the number 

 of generations in the life cycles of these species was not at all 

 constant. The culture of Oxytricha was longest lived (860 

 generations) and its life was prolonged by artificial stimulation. 

 The same author ('13), referring to his early culture of Oxytricha, 

 stated that he believed 'Hhat if an entirely suitable environment 

 had been secured this culture would have given evidence of 

 unlimited power of reproduction by division without conjugation 

 as my present P. aurelia has done.' ' Popoff ('07) kept a culture 

 of Stylonychia mytilus for three and a half months, at the end 

 of which time it died during a deep 'depression period.' He 

 concluded that the cause of depression was not accidental changes 

 in environment, but lay in the organism itself. Enriques ('03, 

 '05, etc.), working with Oxytricha and Stylonychia, concluded 

 that the cause of depression and subsequent death was prolonged 

 poisoning with bacterial poisons, and that ''agamic reproduction 

 can be continued as long as one likes if the technique is good 

 and bacteria are not too numerous. No change of food is neces- 

 sary. " Baitsell ('12) kept Stylonychia pustulata for 572 gene- 

 rations. He did not observe any "appearance of abnormal or 

 degenerating animals," but the fission rate of the cultures grad- 

 ually declined until death occurred. This he interpreted as due 

 to unsuitable cultural conditions. In later work ('14) he suc- 

 ceeded in keeping a culture of Pleurotricha in a beef-extract 

 medium for 943 generations. He also carried mass cultures of 

 the same organism in test-tubes and without conjugation for 

 twenty- two months. From this he concluded that Pleurotricha 

 would "apparently live indefinitely without conjugation or arti- 

 ficial stimulation. " 



The evidence from these investigators proves that, if a life 

 'cycle' exists in hypotrichs, it is subject to great variation, de- 

 pending largely, if not entirely, upon the environment. Arti- 

 ficial stimulation has been shown to have the effect of prolonging 

 the life 'cycle,' and the most recent work indicates that this 

 'cycle' may be prolonged indefinitely if the organism is provided 



