232 GEORGE ALFRED BJMTSELL 



are similar to degenerate animals found by former investigators, 

 by whom they were thought to be due to an inherent condition 

 of senescence in the animals. 



7. In these experiments the results indicate that the dying out 

 of some of the cultures was due, not to a condition of inherent 

 senescence, but to the fact that the culture conditions supphed 

 were not entirely favorable to the indefinite existence of the 

 organisms under observation, since in Oxytricha fallax 'sister 

 cells' bred in a mass test tube- culture lived more than twice as 

 long as those bred in daily isolation cultures, and in Pleurotricha 

 lanceolata culture conditions have been found in which this 

 organism will apparently live indefinitely without conjugation or 

 artificial stimulation. 



