500 



G. N. CALKINS AND L. H. GREGORY 



Diagram 7 



turity in a rich culture is reached after from five to seven days. 

 After this period, the conjugation power decreases. Thus eleven 

 observations on the seventh day gave 5002 pairs, whereas twenty 

 observations on the eighth day gave only 2407 pairs. There is 

 evidently a gradual lessening of the power to conjugate after 

 the optimum on the seventh day until it is practically gone by 

 the fourteenth day and entirely gone in three weeks, provided 

 the rich cultures are not re-filled. This does not mean that more 

 ex-con jugants are taken from the rich cultures at the later per- 

 iods for conjugations do not appear in the rich cultures until 

 after the seven day period in these particular tests. During the 

 later periods a few pairs may appear in the rich culture jars, 

 but they are not abundant. This is probably due t6 the fact that 

 the change in medium is so gradual in the rich cultures that con- 

 jugations are not brought about, whereas transference to a Syr- 

 acuse dish involves a relatively rapid change in medium and 

 conjugation follows, provided, of course, that it is a fertile race. 

 In the later periods of a rich culture a physiological balance is 

 established and the Paramecia then li\'e normally for months and 

 without conjugating. 



