250 BULLETIN 120, UNITED STATES NATIONAL MUSEUM. 



9. It is also of interest to note that there is. in the Prot oofolininae 

 at least, no rotation of the nuclei Avithin the body, the major axis of 

 each nucleus maintaining a constant relation to the major axis of 

 the body, except for a slight swaying, of not more than twenty de- 

 grees, at the time of lingitudinal fission. This constancy of orienta- 

 tion both of nuclei and of chromosomes within the nucleus opens up 

 interesting possibilities of cytological study. 



10. The Opalinidae. especially the Protoopalininae, are unique 

 among organisms in that, in the case of many species, their nuclei 

 are not customarily found in a reticulate condition, but rather come 

 to "rest" in some other phase of the mitotic cycle, the particular 

 mid-mitotic phase chosen being characteristic of the species. Species 

 whose nuclei thus regularly " rest " in some mid-mitotic condition 

 are very numerous in the genus Protoopalina, and are less numerous 

 in the genus ZeUenella. Among the multinucleate forms, the Opa- 

 Hninae, species with nuclei resting in a mid-mitotic condition are 

 fairly numerous in the genus Cepedea. but are fewer in the genus 

 Opalina. The ellipsoidal form of the nuclei in some species of 

 Opalinhme is a readily observed indication of probable mid-mitotic 

 condition. 



It is this peculiar habit, of having their nuclei come to rest before 

 having completed their mitosis, M-hich gives probablj^ the chief key 

 to the speciation in the family. Furthermore it seems to tell us 

 much about the origin of the Ciliata^ whose higher members, the 

 Euciliata, have perhaps the most peculiar nuclear conditions known 

 among organisms. The Opalinidae help us to interpret the nuclear 

 phenomena in the Euciliata. and they enable us to judge something 

 as to the ancestral conditions and the evolution of this unique group. 



In the description of the species of the genus Protoopaliva wc 

 began with Awerinzew's uninucleated P. prhnordialis and passed to 

 the eight (±)-nucleated P. axonucleata through a series of species 

 with nuclei in intermediate conditions. Let us rapidly review the 

 several steps in this ascending series, turning as we do so to the 

 figures of the several species named. 



Protoopalina prhnordialis^ nuclear condition not adequately de- 

 scribed, but said to have but one nucleus, in dumb-bell condition, 

 that is, in the midst of mitosis. 



P. diplocarya (fig. 11, p. 33), a species with two spherical nuclei, 

 still united, however, by a thread; that is. the two daughter nuclei 

 are not yet completely separated. Each nucleus is in a very early 

 metaphase, the four chromosomes being still distinct. 



P. papuensis (fig. 12, p. 35), a species with two pear-shaped nuclei 

 united by a thread ; that is, the two daughter nuclei have not com- 

 pletely separated. Each nucleus is usuallv in a skein condition. 



