[Illustration: FIG. 6.—Changes in nucleus and cell in indirect (mitotic) nuclear division. A, resting nucleus with two centrioles[37] in single centrosphere (_c_); B, centrosphere divided, spindle and two asters (_a_) forming; C, centrospheres separated, nuclear wall disappearing; D, resolution of nucleus into chromosomes; E, mature plasmic spindle, with longitudinal fission of chromosomes; F, chromosomes forming equatorial plate (_ep_) of spindle. (From Wilson.)] {26}When cell-division is about to take place the linin, or at least the greater part of it, assumes the character of a number of distinct threads, and the whole of the chromatin granules are distributed at even distances along these (Fig. 6, A, B, C), so as to appear like so many strings of beads. Each such thread is called a "chromosome." Then each bead divides longitudinally into two. The thread flattens into a ribbon, edged by the two lines of chromatin beads. Finally, the ribbon splits longitudinally into two single threads of beads (Fig. 6, E). During these changes the nucleole or nucleoles diminish, or even disappear, as if they had contributed their matter to the growth of the chromatin proper. In Higher Animals and Plants the nuclear wall next disappears, and certain structures become obvious, especially in the cytoplasm of Metazoa. Two minute spheres of plasm (themselves often showing a concentric structure), the "centrosomes,"[38] which hitherto lay close together at the side of the nuclear wall, now separate; but they remain connected by a spindle of clear plasmic threads (Fig. 6, B-E) which, as the centres diverge, comes to lie across the spot the nucleus occupied, and now the chromosomes lie about the equator of this spindle (Fig. 6, F). Moreover, the surrounding cytoplasm shows a radiating structure, diverging from the centrosome, so that spindle and external radiations together make up a "strain-figure," like that of the "lines of force" in relation to the poles of a magnet. Such we can demonstrate in a plane by spreading or shaking iron filings on a piece of paper above the poles of a magnet, or in space by suspending finely divided iron in a thick liquid, such as mucilage or glycerin, and bringing the vessel with the mixture into a strong magnetic field;[39] the latter mode has the advantage {27}of enabling us to watch the changes in the distribution of the lines under changing conditions or continued strain. [Illustration: FIG. 7.—Completion of mitotic cell-division. G, splitting of equatorial plate (_ep_); H, recession of daughter chromosomes; I, J, reconstitution of these into new nuclei, fission of the centrioles and of the cytoplasm. _if_, Central fibres of spindle; _n_, remains of old nucleole. (From Wilson.)]