268 M. M. Mercarr 
not without significance. May it be that the longitudinal splitting 
of the chromosomes has been overemphasized, and is not so funda- 
mental as is often thought, having been evolved from a less definite 
simpler method of division in which the chromosomes but not their 
granules divide into equal halves? Probably the vegetative divisions 
of the micronuclei of Paramaecium are of this type. Has this simpler 
type of division itself been evolved from lowly mitoses, like those 
in many Plasmodroma, in which there is no method of securing so 
equal a distribution of the mass of each chromosome to the daughter 
nuclei, no distinct and constant chromosomes, indeed, appearing to 
be present? In such divisions as these in the Plasmodroma the 
masses of chromatin in the two daughter nuclei may be about equal. 
Possibly amitotic division stands as still more primitive. It is 
difficult to distinguish the two in some cases. Has the individuality 
of the chromosomes and perhaps of the chromioles been developed 
part passu with the elaboration of the process of division? Are the 
conditions in Opalina intermediate between ordinary amitotic division 
and highly developed mitoses, Opalina having a method of preserving 
the distinctness of the several chromosomes generation after genera- 
tion, but not having a perfect method for securing such an exact 
equality of the daughter chromosomes as results from the longitudinal 
splitting in highly developed mitoses, in which one half of each 
chromiole goes to each daughter nucleus? The latter is secured 
only in mitoses in which the chromosomes split longitudinally and this, 
it seems, is not the case in Opalina, in which the persistent attach- 
ment of the two ends of each chromosome (chromatin unit) to the 
two poles of the nucleus is the means of securing the separation 
of the daughter chromosomes and the independence of the several 
sister chromosomes. The conditions in the binucleated Opalimae seem 
to favor such a general interpretation as that here developed, but 
mitosis among the Protozoa must be better understood before one 
can accept as sufficient the evidence in favor of such a phylogeny 
of mitosis. 
Opalina seems to have the chromosomes not only distinct but 
somewhat different from one another, as is indicated by differences 
in size and form. Of course differences between the chromosomes, 
once established, could readily persist if the schema of division 
suggested for Opalina is correct, and it seems to be so, provided 
one assumption is correct, namely, the assumption that when the 
chromatin ribbon constricts to form the chromosomes for the new 
mitosis, constrictions occur at points where the eight chromosomes 
