ssimilating food and incapable of
reproduction, and hence their life cannot continue very long. Facts like
these demonstrate conclusively the vital importance of the nucleus in
cell activity, and show us that the cell, with its power of continued
life, must be regarded as a combination of protoplasm with its nucleus,
and cannot exist without it. It is not protoplasm, but cell substance,
plus cell nucleus, which forms the simplest basis of life.

[Illustration: FIG. 25.--A cell cut into three pieces, only one of
which, No. 2, contains any nucleus. This fragment soon acquires the
original form and continues its life indefinitely, as shown at _B_. The
other two pieces though living for a time, die without reproducing.]

As more careful study of protoplasm was made it soon became evident that
there is a very decided difference between the nucleus and the
protoplasm. The old statement that the nucleus is simply a bit of dense
protoplasm is not true. In its chemical and physical composition as
well as in its activities the nucleus shows itself to be entirely
different from the protoplasm. It contains certain definite bodies not
found in the cell substance, and it goes through a series of activities
which are entirely unrepresented in the surrounding protoplasm. It is
something entirely distinct, and its relations to the life of the cell
are unique and marvelous. These various facts led to a period in the
discussion of biological topics which may not inappropriately be called
the Reign of the Nucleus. Let us, therefore, see what this structure is
which has demanded so much attention in the last twenty years.

(b) _Structure of the Nucleus_.--At first the nucleus appears to be very
much like the cell substance. Like the latter, it is made of fibres,
which form a reticulum (Fig. 23), and these fibres, like those of
protoplasm, have microsomes in intimate relation with them and hold a
clear liquid in their meshes. The meshes of the network are usually
rather closer than in the outer cell substance, but their general
character appears to be the same. But a more close study of the nucleus
discloses vast differences. In the first place, the nucleus is usually
separated from the cell substance by a membrane (Fig. 23, _c_). This
membrane is almost always present, but it may disappear, and usually
does disappear, when the nucleus begins to divide. Within the nucleus we
find commonly one or two smaller bodies, the nucleoli (Fig. 23, _f