REPRODUCTION 



167 



which germ and soma cells are differentiated. Since that time, the 

 problem of potential immortality of Protozoa has been a matter 

 which attracted the attention of numerous investigators. Because of 

 large dimensions, rapid growth and reproduction, and ease with 

 which they can be cultivated in the laboratory, the majority 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 Februar}^ 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 



Fig. 84. Degeneration or aging in Stylonychia pustulata. X340 (Maupas, 

 modified), a, Beginning stage with reduction in size and completely 

 atrophied micronucleus; b, c, advanced stages in which disappearance of 

 the frontal zone, reduction in size, and fragmentation of the macronucleus 

 occurred; d, final stage before disintegration. 



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

 84). 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 becomes restored. Perhaps 

 the most thorough experiment was carried on by Calkins (1919, 

 1933) with Uroleptus mohilis. Starting with an exconjugant on 



