142 On the Genetics of the Ciliate Protozoa 



A. The Asexual Period. 



24. We may first consider the duration of the period of multiplica- 

 tion by fis.sion. It is important to inquire how far Maupas was justified 

 in concluding that this period is of a limited duration, and that — ■ 

 without conjugation — old age and death form the natural termination 

 of the ciliate life-cycle. 



25. The first experiments to be considered are those of Joukowsky 

 (1898), who kept four cultures — each started from an exconjugant — of 

 Pleurotricha lanceolata. One of these continued for 458 generations', 

 without conjugation or degeneration. This line was still developing 

 normally when it was abandoned. Another line persisted for 250 

 generations, a third for 442 — both finally dying. The fourth was 

 followed to the 220th generation. It developed further, but abnormally. 

 Two similar cultures of Paramecium, caudatum reached 150 and 170 

 generations, and then died. Conjugation was observed twice, and 

 degenerate forms were found at the end of the time. The first culture 

 {Pleurotricha) is interesting, on account of its long duration ; but from 

 the others no definite conclusions can be drawn. 



26. Kulagin (1899) studied Paramecium, and suggested that 

 " senescence " in old cultures is really due to the accumulation of 

 harmful substances in the bodies of the organisms or the surrounding 

 medium. Satisfactory evidence for this conclusion he does not give. 



27. Simpson (1901) found that in his cultures of Paramecium the 

 rate of division of the organisms gradually decreased, until they finally 

 died. He did not attempt to ascertain the determining factors. 



28. Calkins (1902 (with Lieb, 1902), 1902 a, 1904), also investigated 

 Paramecium, and made a similar observation. He succeeded in keeping 

 one line for 742 generations, without conjugation occurring; others for 

 shorter periods. In all cases he found that the rate of fission gradually 

 became slower, and the animals entered a '-period of depression." From 

 this they were able to recover if stimulated with various chemicals and 

 extractives. " The well-marked cycles, with periods of depression which 

 demanded stimulation of a decided character, were approximately of six 

 months duration, while intermediate cycles of le.ss importance were 

 about three months long" (Calkins, 1904). "One natural way in 



' It should be remembered that all "generations" are asexually produced among the 

 ciliates. The term does not connote any of those secondary meanings associated with 

 the conception of "generation" in sexually reproducing organisms. 



