REPRODUCTION 209 



which they can be cultivated in the laboratory, the majorhVy of 

 Protozoa used in the study of the problem have been free-living 

 freshwater ciliates that feed on bacteria and other microorganisms. 



The very first extended study was made by Maupas (1888) who 

 isolated Stylonychia pustulata on February 27, 1886, and observed 

 316 binary fissions until July 10. During this period, there was noted 

 a gradual decrease in size and increasing abnormality in form and 

 structure, until the animals could no longer divide and died (Fig. 

 93). A large number of isolation culture experiments have since been 

 carried on numerous species of ciliates by many investigators. The 

 results obtained are not in agreement. However, the bulk of ob- 

 tained data indicates that the vitality of animals decreases with the 

 passing of generations until finally the organisms suffer inevitable 

 death, and that in the species in which conjugation or other sexual 

 reproduction occurs, the declining vitality often becomes restored. 

 Perhaps the most thorough experiment was carried on by Calkins 

 (1919, 1933) with Uroleptus mobilis. Starting with an exconjugant on 

 November 17, 1917, a series of pure-line cultures was established by 

 the daily isolation method. It was found that no series lived longer 

 than a year, but when two of the progeny of a series were allowed to 

 conjugate after the first 75 generations, the exconjugants repeated 

 the history of the parent series, and did not die when the parent 

 series died. In this way, lines of the same organism have lived for 

 more than 12 years, passing through numerous series. In a series, 

 the average division for the first 60 days was 15.4 divisions per 10 

 days, but the rate gradually declined until death. Woodruff and 

 Spencer (1924) also found the isolation cultures of Spathidium 

 spathula (fed on Colpidium colpoda) died after a gradual decline in 

 the division rate, but were inclined to think that improper environ- 

 mental conditions rather than internal factors were responsible for 

 the decline. 



On the other hand, Woodruff (1932) found that 5071 generations 

 produced by binary fission from a single individual of Paramecium 

 aurelia between May 1, 1907 and May 1, 1915, did not manifest any 

 decrease in vitality after eight years of continued asexual reproduc- 

 tion. Other examples of longevity of ciliates without conjugation 

 are: Glaucoma for 2701 generations (Enriques, 1916), Paramecium 

 caudatum for 3967 generations (Metalnikov, 1922), Spathidium spa- 

 thula for 1080 generations (Woodruff and Moore, 1924), Didinium 

 nasutum for 1384 generations (Beers, 1929), etc. With Actinophrys 

 sol, Belaf (1924) carried on isolation cultures for 1244 generations for 

 a period of 32 months and noticed no decline in the division rate. 



