348 S. O. MAST 



lines which had passed through 1559 generations and those which had 

 passed through only 850. Various means were employed in attempt- 

 ing to induce encystment, but except in the wild lines in which it oc- 

 curred freely, only a few were secured, none at all in the 850 generation 

 group. 



All of the cysts produced in the old lines were carefully pre- 

 served. The following year vigorous cultures of paramecia 

 were as usual added to all of the dishes. Active didinia ap- 

 peared in only one of twelve dishes containing cysts of the old 

 culture, while in those containing the cysts of the wild culture, 

 treated precisely the same, they appeared in practically all. From 

 the didinia thus obtained two new groups of lines were started 

 as indicated in the table. In both groups the rate of fission 

 and the condition of the didinia was practically as it had been 

 before encystment. There was no appreciable improvement in 

 the old culture. The death-rate was not very much higher in 

 this than it was in the culture from the wild race but the fission 

 rate was much lower and there was no noticeable change in the 

 condition of the specimens produced. These lines were carried 

 along for forty days and then abandoned. They did not die 

 out, but it is, of course, impossible to say whether or not they 

 would have recuperated, if the experiment had not been closed. 



As stated above, individuals recently isolated from a \vild 

 culture were much more vigorous than those in the lines long 

 continued without conjugation, but the individuals in lines 

 started from another wild culture were even less \dgorous. 

 These lines continued from 1/17 to 5/29, 1913. By referring 

 to table 1 it will be seen that in them the fission-rate was con- 

 siderably lower and the death-rate was much higher than it 

 was in the lines long continued without conjugation, and that 

 they died out at the close of the period, owing to the fact that 

 it was impossible to induce them to encyst. 



Thus we see that in these experiments there is no support for 

 Calkins' contention concerning the effect of conjugation and en- 

 cystment on the rate of fission. Nor is there any support for the 

 contention of Maupas and others that protozoa must conjugate 

 from. time to time in order to continue the race, that is, that 

 there are cycles consisting of vegetative and sexual reproduction 



