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 fertilization of the egg.  Every animal and plant begins its life as a simple cell, and the growth of the cell into the adult is simply the division of the original cell into parts accompanied by a differentiation of the parts.  The fundamental phenomena of growth and reproduction is thus cell division, and if we can comprehend this process in these simple cells we shall certainly have taken a great step toward the explanation of the mechanics of life.  During the last ten years this cell division has been most thoroughly studied, and we have a pretty good knowledge of it so far as its microscopical features are concerned.  The following description will outline the general facts of such cell division, and will apply with considerable accuracy to all cases of cell division, although the details may differ not a little.

[Illustration:  FIG. 27.—­This and the following figures show stages in cell division.  Fig. 27 shows the resting stage with the chromatin, cr, in the form of a network within the nuclear membrane and the centrosome, ce, already divided into two.]

[Illustration:  FIG. 28.—­The chromatin is broken into threads or chromosomes, cr. The centrosomes show radiating fibres.]

==Cell Division or Karyokinesis.==—­We will begin with a cell in what is called the resting stage, shown at Fig. 23.  Such a cell has a nucleus, with its chromatin, its membrane, and linin, as already described.  Outside the nucleus is the centrosome, or, more commonly, two of them lying close together.  If there is only one it soon divides into two, and if it has already two, this is because a single centrosome which the cell originally possessed has already divided into two, as we shall presently see.  This cell, in short, is precisely like the typical cell which we have described, except in the possession of two centrosomes.  The first indication of the cell