262 PROTOZOA 



to the use of different methods of culture by different investiga- 

 tors or by the same investigator at different times. Sometimes 

 cultures are transferred (fed) once a month, sometimes once a 

 day. This leads to great differences in the observed immature 

 period. With rapid growth, the immature period is very brief, 

 if it exists at all, in varieties 2, 4, and 6 (Ray, Elliott, and Clark, 

 1955); exconjugant clones of these varieties could mate again 

 within 3 to 5/2 days. In contrast, Gruchy ( 1955 ) stated that a 

 few exconjugants of variety 6 were not mature after four weeks 

 of growth, although most matured within this period. Variety 1 

 has been most fully studied in this respect. Elliott and Hayes 

 ( 1953) give two to four weeks as the immature period; but Nan- 

 ney and Caughey ( 1955 ) , growing the animals at a rate of about 

 10 fissions per day, claimed that 16% of the exconjugants mature 

 after 50 fissions, 43% after 60 fissions, 83% after 70 fissions, and 

 100% after 80 fissions. If the animals were starved before each 

 transfer (every 12-13 fissions), 100% were mature by 60 fissions. 

 Because of the time involved in starvation (transfers every third 

 day), the time till maturity was prolonged although the number 

 of fissions was reduced. Variety 8, according to Gruchy (1955), 

 matures in nine days of rapid growth, which is about the same 

 as variety 1; but with monthly transfers, they were still imma- 

 ture at six months. Gruchy further reports that exconjugants of 

 varieties 3 and 7 were still immature after eighteen and nine 

 months, respectively. These varieties seem to be difficult to cul- 

 tivate and grow slowly or erratically; this may be involved in 

 the seemingly very long period of immaturity. 



Although there is no question about the existence of an imma- 

 ture period or about its being followed by a period of maturity, 

 the possibility that maturity is followed by a period of senility is 

 just beginning to engage attention. Before inquiring into the 

 existence of varietal differences in the period of maturity, the 

 question of whether maturity ever comes to an end must be dealt 

 with. This is by no means an uncalled-for question. Cultures of 

 T. pyriformis have been maintained in axenic medium for 

 decades, and this has created widespread conviction that this 

 species is potentially immortal and hence lacking a life cycle. 



