No. 500] LIFE CYCLE OF PARAMECIUM 



twenty- two in December, 1907, when the culture was sub- 

 jected to a particularly uniform culture medium. The 

 highest rate of division so far attained occurred at 

 period ten in July, 1907. The average rate of division 

 for the entire year is obviously considerably above one 

 division per day, the organisms being in the 465th genera- 

 tion on May 6, 1908. That this is not the maximum rate 

 of division of which the culture is capable is shown by 

 the fact that a culture isolated line by line from the one 

 under consideration and treated daily for six months with 

 alcohol is at the same date in the 505th generation. 



The major fluctuations in the division rate which have 

 occurred in the life of this culture are all "rhythms" 

 (using this term in the sense in which I employed it in 

 discussing the life history of Oxytricha) and so far no 

 "cycle" has been completed. Calkins in his earlier 

 papers on Paramecium believed the cycle to be of three 

 months' duration, agreeing in this regard with certain of 

 the earlier investigators on this form. In his last paper 

 on the subject, however, as has been noted, he interpreted 

 the tri-monthly fluctuations as simply minor changes from 

 which recovery was autonomous and regarded the cycle as 

 the larger semi-annual fluctuations, recovery from which 

 was only brought about by stimuli. I have previously in- 

 terpreted these trimonthly depressions as rhythms and 

 the results obtained from this culture would seem to show 

 that the semi-annual cycles of Calkins are also merely 

 rhythms— recovery from which was not autonomous under 

 the conditions of a constant environment. This culture 

 shows that the cycle of Paramecium under a varied 

 environment may be considerably over a year in duration, 

 since the culture at present shows no sign of waning 

 vitality. 



This suggests the much-discussed question as to whether 

 the protozoa are potentially immortal, and the role of 

 conjugation in the life history. Up to the present time 

 there has been no tendency among the individuals of this 

 culture to conjugate, although in the "stock" cultures, 



