96 Reproduction and Life-Cycles 



Woods Hole; that of August, 1901, by extract of raw beef; that of De- 

 cember, 1901, by beef extract; that of March, 1902, by a slight rise in 

 temperature; that of June, 1902, by brain extract. Since the lines were re- 

 juvenated by artificial means, the results were considered analogous to 

 artificial parthenogenesis (31). 



In these early papers. Calkins suggested that ciliates have the "poten- 

 tial of endless existence" without conjugation. Later on, however, the 

 depressions became more severe. The "B" lines became extinct after 16 

 months, the "A" lines in December, 1902. Attempts to rejuvenate the 

 ciliates — treatments with beef extract, pancreas, brain, mutton broth, 

 lecithin, pineapple extract, apple juice, several acids and salts, dried 

 Paramecium, the electric current and nitroglycerin — were all unsuccessful. 

 As a result, Calkins (33) was convinced that the final depressions arose 

 from "germinal exhaustion" which could not be prevented by external 

 stimulation. Therefore, strains of P. caudatum must pass through a cycle 

 of youth, maturity, and old age unless vitality is renewed by conjugation. 

 A gradual decrease in fission-rate accompanied senescence and the re- 

 juvenation by conjugation was believed to include an increase in fission- 

 rate. 



The observations of Enriques soon questioned the inevitability of 

 senescence. The first important demonstration (81) was that excessive 

 bacterial growth may lead to effects simulating senescence. Later results 

 (82, 83) included the maintenance of Glaucoma scintiUans without con- 

 jugation for almost 700 generations. The ciliates remained healthy so 

 long as fresh medium was supplied; the use of old medium induced 

 depressions. At this point, Enriques suggested that exhaustion of the in- 

 vestigators patience is a more important factor than senescence of the 

 ciliates in such investigations. 



On May 1, 1907, Woodruff started the line of Paratnecium aurelia 

 which was to deal a more serious blow to the physiological life-cycle. In 

 May, 1908, the strain had passed 490 generations (246), and at the end 

 of four years (247), had survived for 2,121 generations without conjuga- 

 tion. By this time, the evidence indicated that P. aurelia might reproduce 

 indefinitely without conjugation, or else that the "cycle" must be longer 

 than that of any ciliate investigated previously. 



The conclusion suggested by Woodruff's strain of P. aurelia did not 

 remain unchallenged. Calkins and Gregory (37) maintained that some 

 strains of Paramecium are conjugating races while others are non-con- 

 jugating, and it was argued that Woodruff's strain was a non-conjugating 

 race which should not be compared with the conjugating strain of 

 Calkins. Woodruff (248) met this objection by reporting conjugation in 

 mass-cultures started from his strain at the end of 4,102 generations. The 

 completion of 25 years without conjugation was reported in 1932 (249). 



Evidence against the physiological cycle gradually accumulated from 



