AMICRONUCLEATE RACES OF INFUSORIA 331 



are consistent, since, for example, the study of two races of the 

 same species under identical conditions invariably revealed one 

 micronucleate and the other amicronucleate.'* 



OXYTRICHA FALLAX 



A specimen of Oxj^tricha fallax was isolated from a laboratory 

 aquarium on January 28, 1920, and a pedigree culture 

 started by the usual daily isolation method. Standard beef 

 extract was employed as the culture medium.^ A cytological 

 study disclosed no micronuclei, and therefore the culture was 

 not employed for the purpose intended, but carried on for a 

 thorough examination of its nuclear apparatus and its life-history. 



The graph of the rate of reproduction of the lines of this culture 

 is given in figure 1. It exhibits no unusual features, except 

 possibly greater fluctuation in division rate than one usuall}^ 

 finds in hypotrichous ciliates when bred under essentially con- 

 stant culture conditions. The great fall in the rate of division 

 during the fifth five-day period, however, is clearly related to 

 the fact that during the previous five days daily isolations were 

 not made and fresh culture medium was not supplied. The 

 cause of the sudden death of the race on May 4, 1920, at the 

 246th generation is not apparent, since the vitality as measured 

 by division rate of the previous five-day period was not low. 

 Twice during the life of the culture a few pairs of conjugants 

 were observed in the mass cultures formed of animals discarded 

 from the pedigreed lines at the daily isolations. The conjugants 

 failed to live when isolated, and when preserved revealed no 

 evidence of micronuclei. 



In a word, this race consistently failed to reveal a morpho- 

 logically differentiated micronucleus, though it possessed the 

 potentiahty to produce by division at least the number of de- 

 scendants represented by 2 to the 246th power. 



* Miss Hope Spencer, of this laboratory, has assisted me in conducting the 

 various pedigree lines and'in making permanent preparations. Professor Bait ell 

 oriented, by his plasma method (Proc. Soc. Exp. Biol, and Med., 1921, vol. IS, 

 p. 172), certain animals for sectioning. 



'^ L. L. Woodruff and G. A. Baitsell, The reproduction of Paramecium aurelia 

 in a constant culture medium of beef extract. Jour. Exp. Zool., 1911, vol. 11, 

 p. 135. 



