STUDIES ON THE LIFE CYCLE OF PARAMECIUM. 295 



ured by the fission rate, immediately fell below that of Culture I 

 (varied environment), and that a consistent decrease in division 

 rate was maintained until Culture I s died out on June 6, 1909, 

 at the i, 1 59th generation, after having been one hundred and 

 seven days, or a little more than three months, on the constant 

 medium ; whereas the protoplasm of Culture I maintained about 

 the same general average vitality throughout the period and had 

 attained the i,2OOth generation, a gain of forty-one generations 

 in 107 days over the I s culture. That the death of the I s culture 

 was not due to some sudden and accidental inimical change in 

 the medium is proved by the fact that the same culture medium 

 when used temporarily for the other cultures produced no dele- 

 terious effect, and also by the character of the curve of the fission 

 rate of the I s culture which has a consistent general downward 

 trend except as it is affected by the rhythms. A comparison of 

 the I s culture curve and the curves of Calkins' Paramecium 

 cultures shows a striking similarity in character. The cycles in 

 Calkins' A culture were of six months duration and varied between 

 1 26 and 200 generations in length. My I s culture passed through 

 only 138 generations, but as it actually represents only the down- 

 ward slope, or second half, of a cycle of Calkins' culture, my I s 

 cycle is really somewhat longer than those of Calkins. This 

 point is only of interest in that it indicates in a general way the 

 comparative similarity of the reactions of the protoplasm of para- 

 mecia from widely different sources to the same general condi- 

 tions ; and because it removes the possible objection that the I s 

 culture died out because it had been acclimated to the varied en- 

 vironment, and consequently it could not withstand the change 

 to a constant medium. Of course, this is only a formal objection 

 at best as there is every reason for supposing that the wild para- 

 mecia with which all cultures are started have been subjected 

 for countless generations to considerably greater variations in 

 their environment than it is possible to supply artifically. 



IV. DISCUSSION. 



Up to the present time Culture I has not completed a " cycle " 

 and all the fluctuations in vitality, as indicated by the division rate, 

 fall under the head of " rhythms," as previously defined by me, 



1 Woodruff ('05). 



