FURTHER STUDIES ON THE LIFE CYCLE OF 



PARAMECIUM. 



LORANDE LOSS WOODRUFF. 



I. Introduction 287 



II. Methods 288 



III. Description of Cultures 292 



IV. Discussion 295 



V. Conclusions 306 



VI. Literature 307 



I. INTRODUCTION. 



The life cycle of infusoria has been the subject of numerous 

 investigations since Ehrenberg suggested on a priori grounds that 

 the protozoa are so simply organized that they are not subject to 

 natural death, and Dujardin opposed the view and maintained 

 that the life history of infusoria comprises a cyclical change in 

 vitality which terminates in death. 



Butschli ('76), Engelmann ('76), Maupas ('88 ; '89), Joukowsky 

 ('98), Simpson ('01), Calkins ('02 ; '04), Woodruff ('05), Popoff 

 ('07) and Gregory ('09) have all advanced evidence tending to 

 show that infusoria when bred under somewhat constant culture 

 conditions pass through a more or less definite physiological 

 cycle. This cycle is characterized by an initial high potential of 

 division which gradually is expended until reproduction finally 

 ceases, and death puts an end to the cycle unless conjugation is 

 permitted or artificial stimuli are employed. Characteristic mor- 

 phological changes, both cytoplasmic and nuclear, appear in 

 many cases as "senile degeneration " increases. 



Enriques ('08) in a recent paper has again opposed the idea 

 of old age and physiological death in protozoa and has con- 

 tended that the results which support the cyclical character of 

 the infusorian life history have been obtained by faulty culture 

 methods. The conclusion of Enriques is, I believe, somewhat 

 too sweeping, and is based in part on a misunderstanding of the 

 methods by which the most extensive cultures have been con- 



287 



