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 
