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.]

During this time the chromosomes have been changing their position.
Whether this change in position is due