266 M. M. Mercar 
of the neighboring chromatin masses to form the nuclear net which 
is always recognisable at all stages of mitosis, however faint the 
transverse fibrils may become. The chromatin masses of the neigh- 
boring units send out also broad plates of chromatin (Text Fig. VI, D) 
which soon completely unite them into a chromatin ribbon (Text 
Fig. VI,E). Preparatory to the next mitosis the chromatin masses 
of neighboring units again separate (Text Fig. VII, F and G) and 
each constricts into two thus producing sixteen chromatin masses, 
whose branches are all interconnected, each unit of course having 
two chromatin masses (Text Fig. VI,A). The eight units now 
draw in most of the substance of their lateral branches and reassume 
their position side by side in eight more distinct parallel lines 
stretching from pole to pole of the nucleus, each line having upon 
to two chromatin masses (Text Fig. VI, B). The cycle then repeats 
itself. 
Upon this interpretation it would be seen that the division is 
not a highly developed mitosis, but that, still, by means of the per- 
sistence of the longitudinal chromatin fibres in all stages, even in 
the resting nucleus, and through their retention of their connection 
with the two poles of the ovoid nuclear membrane, the chromatin 
masses, after they divide, are able to send one of their daughter 
masses to each pole of the nucleus, securing thus a result somewhat 
similar to that obtained in the fully developed mitoses of higher 
animals. There is no means in this division to secure the distribution 
of one half of each granule of each chromosome to the daughter 
nucleus, but each daughter nucleus does receive about half of the 
mass of each chromosome of the parent nucleus. In this case, we 
see that the emphasis is upon the chromosomes and not upon the 
chromioles. . 
The evolution of mitosis. 
Is this type of mitosis in Opalina aberrant or does it correspond 
to a stage in the phylogeny of the more highly developed mitoses 
of Metazoa? The nuclei of many, probably of all, Plasmodroma are 
centronuclei (Bovert 1900) each containing, in addition to the chro- 
matin elements and indifferent plastin, a differentiated karyosom 
which functions as a more or less perfect centrosome. Compare 
Euglena (KeutTEN 1895), Trypanosoma (Scuaupinn 1904, v. PROWAZEK 
1905, Satvin Moore & Brent 1907), Amoeba (ScHaupinn 1894, 
Hartmann & v. Prowazex 1907). It is somewhat doubtful whether 
the simplest nuclei have differentiated karyosomes, but at any rate, 
