Reproduction and Life-Cycles 95 



tain cases, small organisms (probably parasites) within the conjugants 

 were interpreted as "embryos" developing within viviparous parents. 

 These interpretations were overthrown by Biitschlii (26, 27) and Engel- 

 mann (80), who showed that the supposed ovary and testis are nuclei 

 and suggested that products of the micronuclei might be exchanged in 

 conjugation. The fusion of pronuclei in conjugation was reported a few 

 years later (133). 



Once conjugation was found to involve nuclear reorganization, and 

 occasionally the reorganization of locomotor structures, the process was 

 interpreted as a sort of rejuvenation. Engelmann (80) suggested that it 

 was unnecessary to suspect any other effect. Biitschli (27) supported a 

 physiological interpretation — ciliates become senescent during continued 

 fission and as a result reproduce less and less frequently until conjuga- 

 tion rejuvenates them and restores the normal reproductive rate. 



This question was first considered experimentally by Maupas, whose 

 isolation-culture technique (175, 176) involved tracing single ciliates from 

 one generation to the next in order to detect possible senescence. Since 

 all his strains died eventually, Maupas suggested that ciliates, like higher 

 animals, pass through a cycle of youth, maturity, and old age, ending in 

 death. The characteristic feature of maturity was assumed to be an ability 

 to conjugate normally. Conjugation was believed to rejuvenate ciliates 

 only during the phase of maturity, and therefore was a prophylactic 

 rather than a therapeutic measure. 



Biitschli (28) maintained that conjugation increased fission-rate after 

 a gradual decline. Hertwig's (113) observations on split-pairs — conjugants 

 separated at the beginning of conjugation and used for starting parallel 

 clones — indicated that fission-rates were usually higher in non-conjugant 

 than in exconjugant lines. As a result, he concluded that conjugation 

 merely regulates metabolism so as to prevent physiological exhaustion. 

 Later investigations were designed to test the theories of Biitschli, Hert- 

 wig, and Maupas. 



Joukowsky (137), after studying exconjugant and non-conjugant lines 

 of Paramecium caudatum and Pleurotricha lanceolata, concluded that the 

 degenerative changes described by Maupas were the result of unsatis- 

 factory conditions in cultures. There were no characteristic differences 

 between exconjugant and non-conjugant lines, neither type showed a 

 decreasing fission-rate, and there appeared to be no physiological cycle. 



The next important papers were those of Calkins (30, 31, 38) who 

 started isolation-cultures of Paramecium, caudatum on February 1, 1901, 

 Four lines were started from each of two ciliates and transfers were made 

 daily or every other day. After a time, recurrent "depressions* developed. 

 The early depressions, believed to represent the senescence reported by 

 Maupas, were cured by measures other than conjugation. The depression 

 of May, 1901, apparently was cured by jolting during a train ride to 



