480 J. A. DAWSON 



steady downward trend until the period September 9th to Sept. 

 14th, which had an average of 1.3 divisions per day. From this 

 time on there occurred twice a general rise and fall in the division 

 rate, and at the conclusion of the second drop in the rate the four 

 lines of culture A died out." 



Although it has not been found possible to continue the race 

 by the daily isolation method, it has been kept alive to date 

 (April 12th) without conjugation and, to all appearance, in a 

 good physiological condition in the Petri-dish cultures. The 

 method of carrying these cultures was as follows. Several- 

 animals selected from stock slides were placed in a small Petri- 

 dish in about 20 cc. of the same kind of medium used for culture 

 A. This culture was examined carefully each day to preclude 

 the possibility of unobserved conjugation. When the organisms 

 had multiplied for a few days, isolation of several individuals 

 was made to a fresh culture of the same kind, and the process 

 has been repeated to date. On account of the ease with which 

 accurate observations on these cultures may be made, it is believed 

 that no ex-conjugants (assuming conjugation is possible in 

 this species) have been isolated for the continuance of the race. 



The result of this experiment on Oxytricha hymenostoma is 

 very similar to that obtained by Baitsell ('14) with Pleurotricha, 

 the life cycle of which, he found, could be prolonged indefinitely 

 b}^ use of a hay-infusion medium in test-tube cultures. It thus 



macronuclei. Fermor states that encystment took place in apparently 

 quite normal cultures and regards the process just described as a substi- 

 tute for conjugation, since the latter process did not occur in the cultures of 

 Stylonychia. Thus the possibility is indicated that in hypotrichous forms 

 during encystment a process occurs which is, in a way, analogous to endomixis 

 (Woodruff and Erdmann, '14) in Paramecium. It should be emphasized, how- 

 ever, that the process described by Fermor involves a fusion of micronuclei and 

 therefore is autogamy rather than endomixis as defined by Woodruff and Erd- 

 mann in their Paramecium work, since they found no micronuclear fusion. In 

 the organism studied in this work I have observed no encystment, and it is diffi- 

 cult to conceive how a similar nuclear reorganization process might occur in 

 Oxytricha hymenostoma, since no definitive micronucleus is present. 



2 A culture of Oxytricha sp. carried in the same medium and under abso- 

 lutely the same conditions reached 400 generations, outliving the culture (A) 

 by two and a half months, thus showing that the medium was apparently more 

 suitable for other species of hypotrichs. 



