LIFE HISTORY OF TWO RARE CILIATES 377 



III. GENERAL CONSIDERATIONS 

 .4. The life-cycle, senescence and rejuvenescence 



In 1838 Ehrenberg as a result of his studies of the infusoria 

 conckided that because of their simple structure and method 

 of reproduction, natural death was impossible. This view was 

 opposed by Dujardin who maintained that the life-history shows 

 definite cycles, indicative of successive periods of protoplasmic 

 vitality which eventually end in death. 



If the succession of cells formed between two consecutive 

 periods of conjugation is comparable to a metazoan, the study 

 of the cell aggregate, represented in the life-cycle is essential 

 to complete knowledge of the morphological conditions of the 

 species. It often happens in the course of the life-cycle that 

 morphological variations occur, the indications of marked differ- 

 ences in protoplasmic vitality, which, unless the Kfe history has 

 been carefully followed, may lead to confusion in regard to the 

 identification of the forms under observation. For this reason 

 many observers within the last half century have made the 

 life-history of various infusoria the subject of careful study, 

 thereby adding much to our knowledge of these forms. 



Among the earlier workers were Butschli (76) and Englemann 

 (78) who were follwed by Maupas ('88) the first to make an 

 exhaustive study of the life-cycle. Schaudinn, by his valuable 

 work on Coccidium schubergi, emphasized the importance of 

 an intimate knowledge of the life-cycle of every species. 



Interesting observations have been made since 1900 on Para- 

 moecium caudatum by Calkins ('02 and '04); on Oxj^tricha 

 fallax by Woodruff ('05); on Gastrostyla mytilus by Popoff 

 ('07); on TiUina magna by Gregory COS), and on Paramoecium 

 aureha by Woodruff ('09 and '11). With the exception of 

 Woodruff's last work, these infusoria were bred in a more or 

 less constant food medium. As a result of this method of treat- 

 ment the life-history showed a division into cycles of varying 

 vitality, measured in terms of the division rate. Enriques in 

 his paper, of 1908 criticized the results of these experiments and 

 the conclusions based upon them. He denied the existence 



