in, resembling
minute coiled worms (Fig. 21), while in still other cells they may be
branching like the twigs of a tree. The form and shape of the chromatin
thread differs widely. Sometimes this appears to be mere reticulum (Fig.
23); at others, a short thread which is somewhat twisted or coiled (Fig.
26); while in other cells the chromatin thread is an extremely long,
very much twisted convolute thread so complexly woven into a tangle as
to give the appearance of a minute network. The nucleii differ also in
the number of nucleoli they contain as well as in other less important
particulars. Fig. 26 will give a little notion of the variety to be
found among different nucleii; but although they thus do vary most
remarkably in shape in the essential parts of their structure they are
alike.

==Centrosome.==--Before noticing the activities of the nucleus it will be
necessary to mention a third part of the cell. Within the last few years
there has been found to be present in most cells an organ which has been
called the _centrosome._ This body is shown at Fig. 23, _g_. It is found
in the cell substance just outside the nucleus, and commonly appears as
an extremely minute rounded dot, so minute that no internal structure
has been discerned. It may be no larger than the minute granules or
microsomes in the cell, and until recently it entirely escaped the
notice of microscopists. It has now, however, been clearly demonstrated
as an active part of the cell and entirely distinct from the ordinary
microsomes. It stains differently, and, as we shall soon see, it
appears to be in most intimate connection with the center of cell life.
In the activities which characterize cell life this centrosome appears
to lead the way. From it radiate the forces which control cell activity,
and hence this centrosome is sometimes called the dynamic center of the
cell. This leads us to the study of cell activity, which discloses to us
some of the most extraordinary phenomena which have come to the
knowledge of science.

==Function of the Nucleus.==--To understand why it is that the nucleus has
taken such a prominent position in modern biological discussion it will
be only necessary to notice some of the activities of the cell. Of the
four fundamental vital properties of cell life the one which has been
most studied and in regard to which most is known is reproduction. This
knowledge appears chiefly under two heads, viz., _cell division_ and the
_fer