254 BULLETIN 120, UNITED STATES NATIONAL MUSEUM. 



In the multinucleate forms, the Opalinhuie^ there is still further 

 inhibition of the division of the body, until, in the most extreme 

 species, we find individuals with several hundred nuclei. In Cepedea 

 segmentata (fig. 140, p. 173) we see the inhibition of the division 

 of the body carried to the greatest extreme, many fissions having 

 begim but remaining uncompleted, so that the body shows a series 

 of segments. 



But hoAvever much ths division of the body may be hindered and 

 j)ostponed, in the end each suppressed division succeeds in taking 

 place; for, as a result of the rapidly occurring divisions, preceding 

 the sexual phenomena, there are produced ultimately uninucleate 

 gametes. The body divisions are not lost, they are merely tem- 

 porarily suppressed. Even the first body division, whose suppres- 

 sion gives the Protoopalinas and the Zelleriellas their binucleated 

 character, reappears in the end, for the gametes of these genera are 

 regularly uninucleate. 



It is very interesting to find in the Euciliata indication of similar 

 suppression, but not absolute obliteration, of that division of the 

 body which, if it occurred, Avould make the Euciliates uninucleate 

 rather than binucleate forms. In the year 1912 Dr. Peebles showed 

 that when the posterior third or quarter, say, of a Paramecium is cut 

 off, the anterior moiety does not regenerate until a division occurs 

 along the line in which the next regular fission would have taken 

 place if the animal had not been mutilated.-* There is in Para- 

 mecium a potential division plane already physiologically laid down, 

 and under the stimulus of mutilation the division itself appears. 

 This seems to be the suppressed division whose omission under un- 

 disturbed conditions makes Paramecium a binucleated instead of a 

 uninucleated organism. In the Euciliates, under normal conditions, 

 there is complete suppression of one division of the body, and the 

 animals have two or more nuclei in all phases of the life historjr. 

 In the Opalinidae, on the other hand, all suppressed divisions finally 

 appear, and the organisms during the sexual phase of the life cycle 

 become uninucleate. The Opalinidae are pseudopleurinucleate forms, 

 the Euciliata, on the other hand, are permanently binucleated. 



Connected with this difference between true and false binucleation 

 in the Euciliates and Opalinids are the relations which obtain in 

 the two groups between the nuclei and the planes of division in 

 fission. In the Euciliata both macronucleus and micronucleus elon- 

 gate and become dumb-bell shaped at the time of fission, and the 

 division plane passes between and separates the two daughter nuclei 

 of each pair, large and small, so that each daughter cell receives 

 one-half of each of the two former nuclei. In the Opalinidae, on 



-* Biological Bulletin, vol. 23, 1912. 



