KEJUVENESCENCE IN UROLEPTUS MOBILIS 



131 



Incidence of conjugation and encystment 



Conjugation tests in all series have been carried out at weekly 

 intervals beginning with the sixth ten-day period, during which 

 the standardized culture medium was introduced. The records 

 of these tests give the dates on which pairing occurred. With 

 only one exception, every series derived from an ex-con jugant 

 failed to give any evidence of conjugation before the fiftieth day. 

 The one exception was the I series in which a single pair of con- 

 jugating individuals were observed in a test made during the third 

 ten-day period. In the C and D series the first pairing occurred 

 in the fifth ten-day period ; in the H and L series the first pairing 

 occurred in the sixth period, and in the J and F series the first con- 

 jugation occurred in the seventh and eighth periods. In all of these 

 cases, except the I series, conjugations were of epidemic fre- 

 quency, i.e., hundreds of pairs were present in the testing dishes. 

 It is apparent, therefore, that the first sixty days, approximately, 

 represent a period of immaturit^^so far as conjugation is concerned, 

 although, during this period, vitality as indicated by the division 

 rate is at its maximum. After conjugation begins in a series, 

 the succeeding tests are usually positive unless some adverse 

 condition renders the test inconclusive. 



During the period of immaturity the history of the conjugation 

 tests was the same for all series. Two or three thousand indi- 



