384 DONNELL BROOKS YOUNG 



viduals. His published evidence, however, seems somewhat 

 inconclusive. Calkins ('11) found that only a small percentage 

 of Paramecia would regenerate regardless of the position of the 

 plane of cutting. However, both nuclear elements were always 

 present in all those which did regenerate. Evidently there is 

 great variation in different species of Protozoa in regard to the 

 ability to regenerate, and it is possible that the micronucleus 

 functions differently. 



The micronucleus has long been recognized as a diagnostic 

 characteristic of the group of ciliates. In some instances the 

 micronuclei cannot be found during the vegetative stages and 

 becomes separated from the macronucleus only during conjuga- 

 tion, as Calkins ('12) demonstrated for Blepharisma undulans. 

 In Opalina, Metcalf ('09) showed that when syngamy takes place 

 the nuclei show two types of chromatin comparable to macro- 

 and micronuclei. Dawson ('20) has described an Oxytricha with 

 no micronucleus, and he has followed the life-cycle sufficiently 

 to show that the sexual phases are abortive. Here evidently 

 is a ciliate without one of the most important organelles. This 

 Oxytricha is able to live and divide without a micronucleus, but 

 such a case certainly is the rare exception. 



In Uronychia it is clear that regeneration can and does take 

 place under certain conditions without the presence of any micro- 

 nucleus. Stained preparations fail to show that micronuclei 

 have formed from the macronucleus, as Lewin ('11) suggested 

 might be the case. Usually the amicronucleate pieces became 

 abnormal if they lived for more than three or four days, so it 

 might be said that for perfect regeneration the micronucleus is 

 essential. These amicronucleate pieces apparently starved to 

 death for, as far as could be discovered, no food was taken in or 

 assimilated. In many cases no evidence was seen to indicate 

 that a mouth was formed. Stained preparations do not show 

 one, but as the mouth is not always demonstrable even in normal 

 individuals, this is not conclusive. 



The micronucleus is necessary for normal growth and division. 

 In no case in U. setigera with its one micronucleus did both pieces 

 divide, even though they did regenerate, while in binucleata, cut 



