Diagram showing representative stages in mitotic or indirect cell-division: _a_, resting cell with reticular nucleus and single centrosome; _b_, the two new centrosomes formed by division of the old one are separating and the nucleus is in the spireme stage; _c_, the nuclear wall has disappeared, the spireme has broken up into six separate chromosomes, and the spindle is forming between the two centrosomes; _d_, equatorial plate stage in which the chromosomes occupy the equator of the spindle; _e_, _f_, each chromosome splits lengthwise and the daughter chromosomes thus formed approach their respective poles; _g_, reconstruction of the new nuclei and division of the cell body; _h_, cell-division completed.] The chromosomes of each group at the poles finally fuse and two new nuclei, each similar to the original one, are constructed (Figs. 6_g_ and _h_, p. 33). In the meantime a division of the cell-body is in progress which, when completed, results in the formation of two complete new cells. As all living matter if given suitable food, can convert it into living matter of its own kind, there is no difficulty in conceiving how the new cell or the chromatin material finally attains to the same bulk that was characteristic of the parent cell. In the case of the chromatin, indeed, it seems that there is at times a precocious doubling of the ordinary amount of material before the actual division occurs. =Chromosomes Constant in Number and Appearance.--=With some minor exceptions, to be noted later, which increase rather than detract from the significance of the facts, the chromosomes are always the same in number and appearance in all individuals of a given species of plants or animals. That is, every species has a fixed number which regularly recurs in all of its cell-divisions. Thus the ordinary cells of the rat, when preparing to divide, each display sixteen chromosomes, the frog or the mouse, twenty-four, the lily twenty-four, and the maw-worm of the horse only four. The chromosomes of different kinds of animals or plants may differ very much in appearance. In some they are spherical, in others rod-like, filamentous or perhaps of other forms. In some organisms the chromosomes of the same nucleus may differ from one another in size, shape and proportions, but if such differences appear at one division they appear at others, thus showing that in such cases the differences are constant from one generation to the next.