6o4 Lorande Loss Woodruff. 



the "age" of the culture with reference to the last conjugation 

 period. The number of generations from the recovery of my 

 Oxytricha A-culture to its extinction gives the number of divisions 

 in a cycle of an artificially stimulated line, but it remains to be 

 shoMm that this is directly comparable to "rejuvenescence" by 

 conjugation. 



Joukowsky carried a series of Pleurotricha lanceolata through 

 458 generations and found no signs of degeneration, and he sug- 

 gested that degeneration depends not on the number of divisions 

 only, but on the rapidity w^ith which they succeed each other. 

 This conclusion, on a priori grounds, would seem reasonable, 

 but my cultures give no evidence to substantiate it. Calkins, on 

 the other hand, lays more emphasis on the duration in time of the 

 cycles than on the number of generations passed through, and he 

 showed ('04) that about six months is the period of the cycle in 

 Paramoecium, but it would seem that about three months was 

 the result reached by other workers on this species. Calkins 

 ('02, I, 2, 3) himself, in his earlier studies on Paramoecium, 

 believed that the cycle in this species was approximately of three 

 months' duration, as he interpreted the smaller trimonthly fluc- 

 tuations as the cycles. I am satisfied that these' periodic lesser 

 changes in vitality which are so conspicuous in his culture- 

 curves are identical with what I have termed rhythms in my 

 cultures, and in the light of his results with Paramoecium, it is 

 probable that the earlier workers on this species, Joukowsky and 

 Simpson, have been dealing with rhythms rather than cycles. 

 I believe that it is essential to recognize a sharp distinction between 

 "rhythms" and "cycles," which may be defined as follows: 



A rhythm is a rnuior periodic rise and fall of the fission-rate, due 

 to some unknown factor in c ell-met aholism, from which recovery 

 is autonomous. 



A cycle IS a periodic rise and fall of the fission-rate, extending over 

 a varying number of rhythms, and ending in the extinction of the race 

 unless it IS ^^rejuvenated" by conjugation or changed environment. 



The question of the number of generations, as well as the time 

 duration, of a life-cycle, is very uncertain and extremely difficult 

 to determine as it is probably dependent upon more than one 

 factor. My cultures lead me to believe, with Simpson, that the 

 personal equation, if I may use that term, of the individual selected 

 to start a culture has the most influence in determining the number 



