256 M. M. Mercaur 
metazoan mitoses in which one distinguishes a central and an outer 
portion in the achromatic spindle. It seems somewhat doubtful if 
the mitotic spindle in Opalina is really comparable at all to the 
mitotic spindle of a metazoan cell, though the presence of perfect 
spindles of nearly, if not exactly, the metazoan type in Sporozoa, 
and the occurrence in other Protozoa of spindles of intermediate 
character, seem to justify our regarding the structure in Opalina as 
a true, through very lowly developed, spindle. Its achromatic portion 
is evidently more nearly related to the inner than to the outer 
spindle of Metazoa, being as Boveri (1900) has shown, a “netrum”: 
The mechanism of mitosis. 
The machanism of mitosis in Opalina seems as difficult, in some 
regards, to understand as it is in other forms. One sees nothing 
in the cytoplasm which seems to be acting upon the nucleus. So 
far as one can judge, the nucleus is automatic in its movements, for 
even the separation of the daughter nuclei cannot be due to the 
pull of the cytoplasm as the body elongates, since the thread 
connecting the daughter nuclei is often long and coiled, indicating 
that it has itself been in rapid growth. The whole nucleus wanders 
forward in that daughter cell which receives the posterior nucleus 
of the parent, and some sort of contraction in the cytoplasm seems 
necessary to explain this migration, but the changes of form in the 
nucleus itself seem due to its own activity. 
In the changes of shape and in the movements accompanying 
mitosis are certain portions of the nucleus active and others passive ? 
Does the nuclear membrane elongate and become spindle-shaped 
because it is pushed upon by the fibres of the spindle forming 
within, or is the nuclear membrane the active agent, itself elongating 
by growth at the same time that it serves to supply points of 
resistence to the pull of the spindle fibers? Do the chromosomes 
migrate of their own accord along the chromatin fibres of the 
spindle; or do the latter contract and pull the chromosomes towards 
the poles of the nucleus; or do those portions of the spindle-fibres 
between the chromosomes elongate and push the chromosomes apart? 
Are both the chromatic fibres and achromatic films in the spindle 
active, or is one set of structures active and the other passive? 
A little evidence upon some of these points can be found. 
The change in torm of the nucleus from oval to elongated 
eliptical or spindle-shaped seems to be due to growth of the nuclear 
