CONDITIONS FOR CONJUGATION 359 



previous to the 19th, when one or two pairs were noted. It 

 .was recorded at intervals during March and April, but on each 

 occasion in very small numbers (involving usually less than 1 

 per cent). During a period of thirty-two days (March 17th 

 until April 18th) very frequent recourse was had to isolation 

 experiments, in watch-glass cultures, in order to determine 

 whether the organisms continued susceptible. Every one of 

 these experiments yielded conjugants, in numbers ranging from 

 about 5 per cent to as high as 50 or 60 per cent in some instances. 

 Throughout most of this period no conjugation was noted in the 

 stock culture, either on the day preceding each isolation experi- 

 ment or on the same day. It frequently happened that in a 

 dense culture set aside in a watch-glass conjugation set in almost 

 immediately, so that at the end of one-half hour 10 per cent of 

 the paramecia would be found in union, although no pairs had 

 been seen in the beginning. Other races responded similarly 

 in these experiments, but not so quickly nor in such large 

 numbers. 



The strain lb was subjected to experimentation on September 

 30, 1919, when renewed cultures were started in small Stender 

 dishes with material taken from the stock culture. This had 

 been allowed to stand for over three months without renewal, 

 although considerable hay was added to it previously. The 

 animals were found to be in a very healthy state, showing no 

 effects of starvation, although they were not dividing. Practi- 

 cally no conjugants had been observed in the culture since May, 

 1919, when casually examined. On the second day after 

 renewal conjugation was obtained (on October 2nd) in the culture 

 Ih-A to which no electrolytes had been added. In the other 

 cultures, containing NaNOs, pairing took place in smaller num- 

 bers (table 7). 



As was pointed out above (p. 358), the reaction of these organ- 

 isms to the presence of NaNOs in the renewed culture is in marked 

 contrast with that of the other races to be described later, for 

 which a low concentration of this salt acts as a stimulant to 

 conjugation. The animals of culture lb (and of Ic) always 

 conjugated most readily in an aqueous medium, whenever 



