700 SEXUALITY 



process of conjugation and nuclear reconstitution require one day, there 

 could have been only about a day between successive conjugations, a 

 period in which at most only three or four fissions could take place. 



In P. bursaria, Jennings (1939a, 1939b) reports a regularly occurring 

 period of immaturity. In group I it lasts for from two weeks to several 

 months; in group II all clones under investigation were still immature at 

 last reports, eight months after their origin at conjugation. Periods of 

 immaturity have also been found regularly in P. caudatum by Oilman 

 (1939) and in Euplotes by Kimball (1939c). In none of these species 

 has there as yet been any report that maturity is followed by a period of 

 senescence, with loss of ability to conjugate. Many of Jennings's clones 

 of P. bursaria have been mature for over two years, without loss of sexual 

 vigor; and in this species endomixis is so rare as scarcely to account for 

 the results. 



Thus age sometimes is and sometimes is not a factor in determining 

 conjugation; the Maupasian life cycle is not an invariable feature of 

 ciliate life. Immaturity may be absent, short, or long; maturity may be 

 coextensive with life, or it may be simply preceded by a period of im- 

 maturity; or it may be delimited on either side by periods of immaturity 

 and senescence. 



THE ROLE OF ENVIRONMENTAL CONDITIONS IN DETERMINING CON- 

 JUGATION 



Maupas (1889) recognized the importance of environmental condi- 

 tions in determining conjugation, and most subsequent workers have 

 been in more or less agreement on this point; but some have carried this 

 view to the extreme of ascribing to environmental conditions alone the 

 determination of conjugation. The preceding account has shown that 

 this cannot always be true, for hereditary and developmental internal 

 factors have been demonstrated as playing a decisive role in many of the 

 races and species. Nevertheless, environmental conditions, such as nutri- 

 tion, temperature, and light, do have marked limiting effects on the 

 occurrence of conjugation. 



In P. aurelia (Sonneborn, 1938a), the mating reaction does not take 

 place in cultures that are either overfed or completely starved. Inter- 

 mediate nutritive conditions are most favorable for its occurrence. More- 



