AN AMICRONUCLEATE OXYTRICHA 479 



with suitable environmental conditions, i.e., the 'cycle' is merely 

 an artifact resulting from unfavorable conditions. 



1. History of culture A 



This culture was begun on July 11, 1917, from the four de- 

 scendants of the Oxytricha isolated on the previous day. The 

 culture was carried by daily isolations until November 17, 1917, 

 when division stopped in all the four lines and the animals, after 

 living one to two weeks without fission, finally died. An attempt 

 was made to continue the race by isolating individuals from the 

 stock cultures of this date, but this proved unsuccessful. How- 

 ever, animals taken from stock cultures on November 2nd have 

 been kept living in small mass cultures (without conjugation) 

 and are still reproducing (April 12, 1918). This fact, reviewed 

 in the light of the work of Baitsell ('14) on Pleurotricha, seems 

 to indicate that if suitable cultural conditions are given, there is 

 a strong possibility of unlimited asexual reproduction. 



The curve (fig. 1) showing the average division rate of culture 

 A and hence the general physiological condition of the animals 

 during the life of the culture has been plotted for five-day periods. 

 This was done, according to the usual method, by averaging the 

 divisions of the four lines together and then averaging the result 

 for five-day periods. A study of this curve shows that the 

 division rate during the first month was much higher than at 

 any other time during the course of the experiments. The 

 highest point was reached between July 31st and August 5th, 

 with an average rate of 4.65 divisions per day. From this time 

 on there were fluctuations^ in the division rate which showed a 



1 A 'rhythm' was originally defined by Woodruff ('05) as "a minor periodic 

 rise and fall of the fission rate, from which recovery is autonomous." More 

 recently Woodruff and Erdmann ('14) have shown that there is a causal relation 

 between endomixis and rhythms in Paramecium. Since endomixis has not been 

 demonstrated in hypotrichs and, further, since it apparently does not occur in 

 this form, it has been deemed inadvisable to apply the term 'rhythms' to the 

 fluctuations in the division rate of Oxytricha hymenostoma. It should be 

 noted that Fermor ('13) described an internal nuclear reorganization process 

 in Stylonychia during encystment. Here the macronuclei degenerated; the 

 micronuclei fused and later by division produced new micronuclei and 



