_).
They appear to be distinct vital parts of the nucleus, and thus
different from certain other solid bodies which are simply excreted
material, and hence lifeless. Further, we find that the reticulum within
the nucleus is made up of two very different parts. One portion is
apparently identical with the reticulum of the cell substance (Fig. 23,
_d_). This forms an extremely delicate network, whose fibres have
chemical relations similar to those of the cell substance. Indeed,
sometimes, the fibres of the nucleus may be seen to pass directly into
those of the network of the cell substance, and hence they are in all
probability identical. This material is called _linin_, by which name we
shall hereafter refer to it. There is, however, in the nucleus another
material which forms either threads, or a network, or a mass of
granules, which is very different from the linin, and has entirely
different properties. This network has the power of absorbing certain
kinds of stains very actively, and is consequently deeply stained when
treated as the microscopist commonly prepares his specimens. For this
reason it has been named _chromatin_ (Fig, 23, _e_), although in more
recent times other names have been given to it. Of all parts of the cell
this chromatin is the most remarkable. It appears in great variety in
different cells, but it always has remarkable physiological properties,
as will be noticed presently. All things considered, this chromatin is
probably the most remarkable body connected with organic life.

[Illustration: FIG. 26.--Different forms of nucleii.]

The nucleii of different animals and plants all show essentially the
characteristics just described. They all contain a liquid, a linin
network, and a chromatin thread or network, but they differ most
remarkably in details, so that the variety among the nucleii is almost
endless (Fig. 26). They differ first in their size relative to the size
of the cell; sometimes--especially in young cells--the nucleus being
very large, while in other cases the nucleus is very small and the
protoplasmic contents of the cell very large; finally, in cells which
have lost their activity the nucleus may almost or entirely disappear.
They differ, secondly, in shape. The typical form appears to be
spherical or nearly so; but from this typical form they may vary,
becoming irregular or elongated. They are sometimes drawn out into long
masses looking like a string of beads (Fig. 24), or, aga