==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.]