360 HOYT S. HOPKINS 



they conjugated at all. In the stock cultures the paramecia con- 

 jugated little or not at all, but they remained susceptible during 

 long periods of time, yielding conjugants readily in watch-glass 

 cultures isolated from the main cultures. 



Par. aurelia, 3a. The culture was started January 23, 1919, 

 from an individual isolated from the same abandoned hay culture 

 as lb, and so it is not known with certainty whether it represents 

 a race distinct in descent from Ih, although its characteristics are 

 sufficiently distinct to render this probable. Conjugation was 

 observed in this culture while still small on February 13th, in 

 about 20 to 30 per cent of all, and again on February 15th and 

 17th in smaller numbers (3 to 5 per cent). Conjugants were 

 again seen in this culture on March 12th, about 1 per cent, and 

 later between March 17th and 22nd, in no case in more than 

 casual numbers. Isolation experiments continued to yield 

 them, however, often as many as 5 per cent, from March 15th 

 until April 18th, when the experiments were discontinued. 



Culture 3b was derived from an exconjugant of 3a isolated on 

 February 13th. One or two pairs of conjugants were seen in 

 the culture on April 7th, after long search, while isolation experi- 

 ments yielded them in small numbers (1 per cent or less) between 

 March 19th and April 10th. Except for the casual pairs noted 

 above (April 7th), the stock culture showed none from its begin- 

 ning until the time when regular observations were discontinued. 



Cultures 3c, 3d, and 3e were started on March 20th from three 

 exconjugants of 3b, and represent thus the third 'sexual genera- 

 tion' of the race. Each was cultivated under uniform conditions 

 and observed regularly. Conjugation occurred to the extent of 

 40 per cent in a watch-glass culture derived from 3d on April 

 16th, but was not discovered in the main culture during the entire 

 period of daily observation (from March 20th to April 20th), 

 nor on subsequent examination at longer intervals. Isolation 

 experiments set before and after the 16th (on April 15th, 17th, 

 and 18th) gave no conjugants. The two related cultures, 3c 

 and 3e, failed to show any conjugation, even in the isolation 

 experiments, which were continued until April 20th. 



