[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).  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