No. 606] MUTATION IN DIDINIUM NASUTUM 357 



close of the experiment, however, the rate of fission was 

 not as high as the temperature at this time would lead one 

 to expect. The didinia in both groups appeared to be in 

 poor condition. There were numerous very small in- 

 dividuals produced and an unusually large number of 

 monsters. Conjugation was prevalent, but it was almost 

 impossible to induce encystment. The death-rate was, 

 however, not abnormally high. Whether or not the lines 

 would have recovered from this depression if the ex- 

 periment had been continued, is a question which can not 

 be definitely answered. 



Before the experiment was closed some cysts were se- 

 cured in both groups. These were kept in a damp cham- 

 ber as usual until the following year. Then they were put 

 into culture fluids of various sorts containing paramecia ; 

 but only a few developed, all of which belonged to the 

 more rapidlj* dividing lines. From these, five new lines 

 were started and carried on for 40 days. During all this 

 time the condition of the individuals was much as it had 

 been immediately before encystment. 



Throughout the entire experiment the didinia were cul- 

 tivated in rectangular watch-glasses having a depression 

 with a curved bottom. These dishes were piled one upon 

 the other and kept in a damp chamber. All of the didinia 

 were fed with paramecia from the same cultures. At 

 each feeding an equal amount of solution was taken from 

 two of the most vigorous of four pint cultures which were 

 continuously kept in as flourishing conditions as possible 

 by adding fresh water and a little timothy hay from time 

 to time. The two equal quantities of solution were then 

 thoroughly mixed and two drops of this mixture contain- 

 ing numerous paramecia were put into each of as many 

 watch-glasses as there were didinia cultures. One drop 

 of solution containing one didinium was then taken from 

 each of the didinia cultures and added to each of the 

 watch-glasses containing the paramecia. The remaining 

 didinia, after recording the number of generations pro- 

 duced, were destroyed or used in studying conjugation 



