CONDITIONS FOR CONJUGATION 345 



tozoans as regards resistance must be taken into account in 

 order to understand why one succumbs in an environment in 

 which another grows and apparently thrives. If two races 

 distinct in size, one of Par. caudatum, the other of aurelia, are 

 reared together in the same culture dish, hence in the same 

 medium, one may survive, the other die, or both may live to- 

 gether. Like differences undoubtedly exist between races of the 

 same species, for my work indicates that such races show constant 

 natural differences as regards their resistance to specific reagents 

 and their ability to grow readily in slide cultures as well as in 

 mass cultures. 



During the course of development of these cultures records 

 were taken of races under similar growth conditions, in order 

 to determine in advance which ones were subject to periodic 

 epidemics of conjugation and the nature of these rhythmic 

 changes, so as to compare their behavior under usual conditions 

 with that to be obtained later in the experiments. Certain 

 cultures were subjected to daily observation for weeks at a time 

 with this object in view, and record kept of sporadic occurrences 

 and of conjugation en masse. This revealed what races were 

 subject to frequent epidemics and in what ones conjugation was 

 rare or lacking. Most of these observations were made daily 

 under a binocular microscope of wide field of vision, upon cul- 

 tures growing in Stender dishes measuring 5 cm., 6 cm., and 10 

 cm. in diameter, so that any but rare conjugating pairs could 

 hardly be overlooked. The same method of daily observation 

 was adopted during the course of experiments with small cultures, 

 and since these cultures were comparatively free from sediment, 

 it was an easy matter to conduct thorough observations upon 

 them during the few days required for each experiment. 



Many of the experiments referred to in this paper were carried 

 out after the procedure recommended by Zweibaum (p. 341), 

 using salt solutions,- with the object of showing whether each 

 race would not yield conjugants when the conditions which he 

 found most favorable were supplied. For this purpose sub- 

 cultures were derived from the main cultures by diluting 50 to 

 100 cc. of the culture containing many paramecia with an equal 



