338 S. O. MAST 



MATERIAL AND METHODS 



Throughout the entire period covered by the experimental 

 work connected with this paper numerous observations were 

 made on didinia collected in various places and on mass cultures 

 kept in the laboratory under various conditions. These obser- 

 vations concerned primarily the life history and the general 

 physiology of the organisms, especially those phases of it which 

 are associated with the processes of encystment and conjuga- 

 tion and the relation between these processes and the environ- 

 ment. The main part of the experimental work consisted, 

 however, of the study of a series of groups of pure lines which 

 were started and maintained as follows. 



Four pairs of conjugating didinia were taken from a \ngorous 

 culture which had been in the laboratory for some time Each 

 of these four pairs was then put into a rectangular watch-glass 

 containing a few drops of solution and a considerable number of 

 paramecia The watch-glasses were now piled one upon the 

 other, placed into a damp chamber and later examined daily. 

 Six of the ex-con jugants died within a few days. The other two 

 divided and from each, two lines were started and labeled (conj.) 

 indicating that conjugation had occurred. Thus there were 

 four lines started with individuals which had conjugated. Simul- 

 taneously four lines were started with individuals taken from the 

 same culture, but which had not conjugated. From this group 

 of lines other groups were started from time to time as indicated 

 in table 1. 



xA.ll of the cultures were kept continuously in the same damp 

 chamber and all were treated as nearly alike as possible They 

 were fed throughout the entire experiment practically exclusively 

 on paramecia, which were obtained from four cultures kept in 

 liter jars These cultures were continuously maintained in the 

 most uniform and \dgorous condition possible During the 

 early part of the experiment one-half of the solution in each of 

 the four jars was replaced by standard timothy hay solution 

 (100 cc. water -|- 1 gram hay, boiled ten minutes) at given inter- 

 vals but that in each jar at a different time. During the latter 



