AN AMICRONUCLEATE OXYTRICHA 483 



From time to time, as these pairs appeared, numbers were 

 isolated for study; in all 100 pairs have been studied. Of these, 

 3 pairs separated within a few hours after isolation, while the 

 remaining 97 pairs never separated and died in from six to forty- 

 eight hours. Of the animals which separated, one of each pair 

 was stained and the other was transferred to fresh medium on 

 a depression slide. Study of the stained specimens showed a 

 typical condition of the macronucleus in all cases and there was 

 not the slightest sign of any structure which could be interpreted 

 as a definitive micronucleus. The animals which were carried 

 on depression slides continued to divide, thus indicating that 

 whatever may have been the condition of these apparent conju- 

 gants, at any rate, it was not pathological. 



The appearance and behavior of the animals in cultures, in 

 which pairs were occurring in large numbers, was not different 

 from that of typical, healthy animals. In order to determine 

 definitely if this phenomenon was due to a depressed or patho- 

 logical state, numbers of single animals were isolated and carried 

 in lihes by daily isolations. The division rate of these subcultures 

 indicated that the organisms were in a good physiological con- 

 dition. Pairs which had been attached for a few hours were sep- 

 arated by forcibly ejecting them from a capillary pipette. Here, 

 again, the division rate did not show that the animals were in 

 a pathological state. It is therefore clear that some other expla- 

 nation must be sought, both for the onset of the tendency to pair 

 and for the high mortality in permanent pairs. 



3. Cytology of Oxytricha hymenostoma 



In Adew of the fact that the presence of the micronucleus in 

 the hypotrichous ciliates has been insisted upon so strenuously 

 by recent investigators, and since there is an ever-present possi- 

 bility of overlooking this important structure, every effort has 

 been made in this study to determine its presence. Care has 

 been taken to secure specimens in as many different stages of the 

 life-history as possible, and these have been treated with a large 

 variety of stains, as already mentioned. In addition to total. 



THE JOURXAL OP EXPERIMENTAL ZOOLOGY, VOL. 29, KO. 3 



