CONJUGATION AND ENCYSTMENT 339 



part the hay and water were added directly at various intervals 

 depending upon the rate of development. This method was 

 found to maintain the cultures more nearly in a uniform con- 

 dition than the one first used. Moreover, in feeding the didinia, 

 an equal amount of solution was always taken Irom two or 

 more of the paramecia cultures and mixed in a separate dish, 

 thus increasing the uniformity of the culture medium. Two 

 drops, sometimes three, of this mixture which contained numerous 

 paramecia were put into each of the required number of clean 

 watch-glasses.. A drop of solution containing one didinium 

 was then taken from each of the pedigree cultures and put into 

 each of the watch-glasses supplied with fresh food. After re- 

 cording the number of fissions the remaining didinia were either 

 destroyed or used in making further studies of conjugation and 

 encystment. Nothing was sterilized in these experiments but 

 the same pipet was used in transferring all of the didinia. More- 

 over, from time to time a drop of solution was taken from each 

 of the pedigree cultures mixed and then a drop of the mixture 

 added to each of the new preparations. Thus all were inocu- 

 lated with the same bacteria and all were the same in other 

 respects. 



Throughout all the experiments the cultures were transferred 

 and the number of generations recorded daily, twice a day, or 

 every other day. During a greater part of the summer, when it 

 was relatively hot, the fission-rate was so high that it was found 

 advantageous to transfer twice a day, for eight or even nine gen- 

 erations, over 1000 individuals, were sometimes produced in 

 twenty-four hours. During the rest of the year they were usually 

 transferred daily. They were transferred every other day only 

 during a few short periods, when the temperature was excep- 

 tionally low. 



We shall present the results obtained in the experimental 

 observations under several headings, as follows. 



