352 S. O. MAST 



there no tendency to conjugate later in the cycle, especially at 

 the close when all of the individuals encysted? 



An examination of table 1 shows that in Didinium there was 

 great variation in the rate of fission in different periods. These 

 variations were largely due to changes in temperature. Wood- 

 ruff maintains that there are similar variations in the rate of 

 fission in Paramecium, but he holds that there are certain fluc- 

 tuations in this rate in the absence of any variation in tempera- 

 ture or other environmental factors. These he calls rhythms. 

 He says ('11, p. 353): "[Rhythms] are due to fundamental fac- 

 tors in cell phenomena and not to extraneous causes." For all 

 that is known to the contrary there may have occurred in 

 Didinium variations similar to those called rhythms by Wood- 

 ruff. But if there were any such fluctuations they were either 

 exceedingly small or they occurred simultaneously in all of the 

 lines running parallel, regardless as to the difference in the 

 number of generations produced by these lines since conjuga- 

 tion or encystment had occurred. Consequently, if there were 

 such fluctuations of any considerable magnitude they could not 

 have been specifically related to conjugation or encystment with 

 reference to time. 



By referring to table 1, it will also be seen that in 1912, 7/15- 

 7/20, one of the groups of lines divided into two groups having 

 different rates of fission. These two groups of lines originated 

 from a single individual during a period of abnormally high tem- 

 perature. They were carried for 315 days during which time 

 one group produced a total average of 838 generations (2f per 

 day) and the other a total average of 634 (2 per day). The 

 difference in the two groups was practically the same throughout 

 the whole period and it appears to have been permanent. The 

 origin and significance of this mutation will be more fully 

 discussed in a separate paper. 



As indicated in the table the pedigree culture continued with 

 certain intermissions from April, 1910 until May 1914. During 

 this time there were produced in one of the groups of pure lines 

 an average of 1646 generations without conjugation and in the 

 same group of lines 1035 generations without encystment. The 



