Opalina. 267 
we may probably assume the former existence of such simple nuclei, 
even though they may not exist to-day. 
One naturally conceives a series of stages in which both the 
plastin and the chromatin constituents of the nucleus are becoming 
more highly developed. On the one hand the phylogenetic develop- 
ment of the chromatic structures probably showed at one time a 
stage with the chromatin in the form of diffuse granules not grouped 
into chromosomes, and this may have been succeeded by a stage 
such as we now seem to have in Trypanosomes, in which we have 
diffuse granules irregularly arranged during the vegetative mitosis 
(Sauvin Moore & Brernu), but true chromosomes during some of 
the divisions preceeding conjugation (ScCHAUDINN, v. PRoWAZzEK). A 
further evolution gives definite chromosomes persisting throughout 
the whole life of the cell. Ultimately the chromosomes show morpho- 
logical differences corresponding to their differences in function. On 
the other hand we conceive the plastin elements of the nucleus as 
giving rise to an intranuclear centrosome’), which soon becomes 
developed to the point of containing a centriole. The final develop- 
ment, showing a spindle and astral rays, is best seen when the 
centrosome becomes extra nuclear. The original centronucleus has 
thus evolved into an elaborate double set of structures, one con- 
sisting of the chromatin elements associated with some indifferent 
plastin, and the other being the kinetic plastin in the form of the 
centrosome, whose structure in some phases of mitosis becomes 
elaborately developed. 
The conditions in Opalina seem to throw light upon this phylo- 
geny, though its own nuclear structure seems aberrant and not to 
correspond to any stage in the phylogeny of the mitosis of higher 
forms. It seems to have substituted another and simpler mechanical 
device for the centrosome; has developed its mitosis to a Certain 
point and has stopped there, unable to go further because of the 
absence of developed centrosomes. Its method of mitosis is simple 
and is efficient to a degree, but is incapable of producing the 
remarkably perfect results reached by those cells which kept and 
further evolved their centrosomes. 
The indication that in the mitosis of Opalina the emphasis seems to 
be placed upon the chromosomes and not upon the chromioles is 
1) The karyosom of Plasmodroma seems to consist of both plastin and chro- 
matin and to be therefore more than a centrosome. It contains the centrosome. 
These relations are shown with especial clearness in an as yet unpublished paper 
by Harrmann upon Amoeba tetragena. See also Harrmann & vy. Prowazex 1907. 
