CONJUGATION AND ENCYSTMENT 347 



tion causes an increase in the rate of fission in accord with 

 Calkins' contention and the same may be said with reference to 

 encystment. 



Calkins maintains that in Didinium the rate of fission is rela- 

 tively high immediately after encystment; that after continuing 

 for about 100 generations it becomes lower and continues to 

 decrease for 30 to 50 generations when encystment again occurs, 

 during which there is a nuclear reorganization resulting in re- 

 juvenescence and an increase in the rate of fission. There is 

 not the slightest e\'idence indicating that anything of this sort 

 occurred in our experiment. If encystment affects the rate of 

 fission there ought to be some evidence of it in the periods fol- 

 lo^\ing (9/6, 1911) but there is none, as an examination of the 

 results presented in the table for these periods clearly shows. 

 Moreover, at the close of the last part of the experiment, three 

 groups of lines had passed through 1035, 831 and 850 genera- 

 tions respectively, without encystment or conjugation, proving 

 conclusively that if there are cycles with reference to encyst- 

 ment in accord with Calkins' contention, they are very different 

 from those he describes. 



It is true that toward the close of the experiment, after a long 

 period without conjugation or encystment, the didinia seemed 

 to lose their accustomed vigor, but neither conjugation nor 

 encystment served to remedy matters as a comparison with 

 the rate of fission in the wild groups during these periods shows. 

 In reference to this the following extract taken from my notes 

 is illuminating. It refers to the last periods of 1913, and gives 

 a good idea of the condition of the didinia toward the close of 

 the experiment. 



All individuals not used to continue the pedigree lines were re- 

 tained in their respective dishes. These were all examined from day 

 to day. Conjugation occurred very freely in all but the wild lines, 

 encystment very rarely. The didinia in all of these cultures eventually 

 died out except a few cysts. Conjugation did not save them. Para- 

 mecia from five different cultures were tried as food also some paramccia 

 fed on malted milk but the didinia still died. The individuals in the 

 old hnes were much smaller than those in the wild lines, the cysts were 

 also relatively small and there was a strong tendency to produce mon- 

 sters. There was no observable difference regarding this between the 



