PERIODIC REORGANIZATION IN PARAMAECIUM 481 



evident not only that the reorganization process is coincident with 

 the low point between two rhythms, hut also that there is a causal 

 relation between the reorganization process and the rhythms. 



VII. THE REORGANIZATION PROCESS AND ITS RELATION TO 

 DEPRESSION PERIODS 



The results of the great majority of studies on various spe- 

 cies of Infusoria, from the pioneer experiments of BiitschH and 

 Engehiiann, have indicated that these forms will not reproduce 

 indefinitely if conjugation between two animals is prevented, 

 but that the race shows signs of lowered vitality accompanied 

 by marked morphological changes which sooner or later end in 

 death. Improved methods of conducting the cultures, however, 

 have enabled succeeding investigators to continue the races longer 

 and longer, until studies on the culture which forms the basis 

 of this paper have shown that under proper environmental con- 

 ditions Paj^amaecium aurelia can be bred indefinitely without 

 conjugation or artificial stimulation. In other words, there is 

 no evidence of a life cycle, as understood and emphasized by 

 Maupas, Calkins and other authors, which comprises a more 

 or less definite number of cell generations beginning after con- 

 jugation with the high potential of vitality of youth and maturity 

 and leading to either old age and death or conjugation and 

 rejuvenescence again. 



A critical survey of the literature, however, shows that another 

 type of variation occurs in the life history of Infusoria under 

 culture conditions which must be distinguished from the cycle. 

 In certain cases this has been recognized by the author, but as 

 a rule it has been passed over without comment owing to the 

 fact that only by the daily isolation method of culture is it 

 possible to demonstrate its reality. Apparently Hertwig ('00- 

 '04) from his study by mass culture methods of the life history 

 of Actinosphaerium eichhornii, Dileptus gigas and Paramaecium 

 caudatum was the first to contrast minor periods and deep 

 periods of physiological depression, the latter resulting in death. 

 The lesser periods were marked chiefly by a slight lowering of 



