CONDITIONS FOR CONJUGATION 373 



three of these mass cultures, during approximately the same 

 period of time in each, beginning on January 30th in If^Jfb (7) 

 and lasting until February 27th. Just preceding conjugation, 

 during the tenth week, the fission-rate in the slide cultures reached 

 the highest point which it had attained since the second week 

 following the previous conjugation, as shown by the average 

 number of divisions weekly for all the lines. (This first maxi- 

 mum may possibly represent a compensatory tendency after 

 the drop immediately following conjugation, for the average rate 

 for the first two weeks is less than that for the second two weeks.) 

 New strains were again started from exconjugants of the culture 

 Jj-Jib (2) on February 2, 1920, and were designated I^J^c (1) to 

 (12). From the six which survived, mass cultures were started 

 on February 16th, and in two of these conjugation set in at the 

 end of the fourth week. The fission-rate for these two strains 

 attained its maximum during the fifth week. A similar set of 

 experiments was conducted, using forty exconjugant strains 

 derived from one of the above cultures, and designated 44^ 

 (1) to (40). Twenty mass cultures were started, and in six of 

 these conjugation occurred, setting in during the fourth week 

 in each one. The fission-rate (in four lines of each strain) 

 attained its highest point at about the third week. It is signifi- 

 cant in this connection to note that in almost every instance 

 conjugation occurred in rapidly dividing strains. Slow strains, 

 although derived originally from exconjugants of the same race, 

 do not conjugate, even after a period of three months has 

 elapsed. 



These experiments bring out another point which has already 

 been touched upon (p. 362). There appear to be racial diversi- 

 ties among the exconjugant strains, originally from the same race. 

 When a particular strain shows a tendency to undergo depression, 

 all of its four lines may be affected, and frequently all die at 

 about the same time (usually at the end of four to six weeks), 

 whereas in the four lines of some other strain no depression may 

 occur at this time. 



My experiments with Par. aurelia likewise support the view 

 set forth above: that conjugation occurs at about the time 



