division is shown by the chromatin fibres.  During the resting stage this chromatin material may have the form of a thread, or may form a network of fibres (see Fig. 27).  But whatever be its form during the resting stage, it assumes the form of a thread as the cell prepares for division.  Almost at once this thread breaks into a number of pieces known as chromosomes (Fig. 28).  It is an extremely important fact that the number of these chromosomes in the ordinary cells of any animal or plant is always the same.  In other words, in all the cells of the body of animal or plant the chromatin material in the nucleus breaks into the same number of short threads at the time that the cell is preparing to divide.  The number is the same for all animals of the same species, and is never departed from.  For example, the number in the ox is always sixteen, while the number in the lily is always twenty-four.  During this process of the formation of the chromosomes the nucleoli disappear, sometimes being absorbed apparently in the chromosomes, and sometimes being ejected into the cell body, where they disappear.  Whether they have anything to do with further changes is not yet known.

The next step in the process of division appears in the region of the centrosomes.  Each of the two centrosomes appears to send out from itself delicate radiating fibres into the surrounding cell substance (Fig. 28).  Whether these actually arise from the centrosome or are simply a rearrangement of the fibres in the cell substance is not clear, but at all events the centrosome becomes surrounded by a mass of radiating fibres which give it a starlike appearance, or, more commonly, the appearance of a double star, since there are two centrosomes close together (Fig. 28).  These radiating fibres, whether arising from the centrosomes or not, certainly all centre in these bodies, a fact which indicates that the centrosomes contain the forces which regulate their appearance.  Between the two stars or asters a set of fibres can be seen running from one to the other (Fig. 29).  These two asters and the centrosomes within them have been spoken of as the dynamic centre of the cell since they appear to control the forces which lead to cell division.  In all the changes which follow these asters lead the way.  The two asters, with their centrosomes, now move away from each other, always connected by the spindle fibres, and finally come to lie on opposite sides of the nucleus (Figs. 29, 30).  When they reach this position they are still surrounded by the radiating fibres, and connected by the spindle fibres.  Meantime the membrane around the nucleus has disappeared, and thus the spindle fibres readily penetrate into the nuclear substance (Fig. 30).

[Illustration:  FIG. 29.—­The centrosomes are separating but are connected by fibres.]

[Illustration:  FIG. 30.—­The centrosomes are separate and the equatorial plate of chromosomes, cr, is between them.]