252 BULLETIX 120, UNITKD STATICS NATIONAL MUSEUM. 



Oi){i]ini(l stock. This is in kooping with tlie conclusion, reached in 

 our discussion of the data from geographical distribution in section 

 7. that ZelU^riella is a comparatively modern ji^enus. None of the 

 Zellerielhis known are multinucU'ate, as are Protoopalina quadri- 

 iiucleata and P. a.ronucleata. 



Amonfj the OpaVinhde. both the Cepedeas and the Opalinas, we 

 see species with spherical. restin<^ nuclei in the j^ranular reticulate 

 condition, and other species with ellipsoidal nuclei, but we find no 

 such series of characteristic midmitotic restin^i^ conditions in the 

 several species as the ProtoopaJhuime present. The OpaJinhufe, as 

 a whole, beinir muItinuQleated, may be taken as showing: the last 

 major term in the series of nuclear conditions in tlie family. 



Tliere is evidenced in the family as a whole, when its species are 

 thus comparatively studied, a stranjjfe tendency hindering the prompt 

 completion of nuclear division, and at the same time delaying divi- 

 sion of the body. This inhibiting tendency produces results in some 

 respects unique among orgacisms. In no other group of animals 

 or plants are there known nuclei which, having once entered upon 

 mitosis, regularly come to rest without promptly completing the 

 mitotic process. It is remarkable, in the light of phenomena in other 

 groups, that there should be any such prolonged inhibition of the 

 completi(m of mitosis. It is still more remarkable to find, as we do, 

 especially among the Protoopalinas, a series of species wdiose dela^^ed 

 mitoses are interrupted at different, specifically characteristic phases 

 of the mitotic cycle, so that in some species the resting nuclei are in 

 an early prophase, in others in a late prophase, in others in about the 

 equatorial plate phase, in others in an early anaphase, in others in a 

 late anaphase, in others in an early metaphase, in others in a late 

 meta phase, Avhile still others have their nuclei resting in a reticulate 

 condition similar to the usual resting condition of the nuclei in other 

 organisms. 



Of course, in any species of Piofoopallna, or of the other genera 

 of Opalinidae, we find all stages of mitosis, but in most species there 

 is a resting condition, mid-mitotic or otherwise, as definite and 

 evident as the resting condition of the nuclei in other organisms. In 

 a few species, as for instance in Zellericlla paludicolae , either there 

 is greater latitude in the condition in which the nuclei come to rest, 

 or some of my material has been killed at a time when division was 

 very active, so that nuclei in all stages of mitosis are found in 

 abundance. The latter seems the w'holly probable explanation, for 

 not all my infections of Z. pahidieolae show this exterme diversity 

 in mitotic condition. 



The partial inhibition of nuclear division in the Opalinids is 

 paralleled l)y a partial inhibition of the division of the body. In 

 the binucleated species, and still nioiv in the multinucleated species. 



