The following is a brief epitome of the visible changes undergone by a typical cell, leading up to the act of segmentation, and constituting the phenomenon of mitosis or caryokinetic division. In the egg of a sea-urchin, we see with almost diagrammatic completeness what is set forth here[231]. [Illustration: Fig. 44.] [Illustration: Fig. 45.] 1. The chromatin, which to begin with was distributed in granules on the otherwise achromatic reticulum (Fig. 42), concentrates to form a skein or _spireme_, which may be a continuous thread from the first (Figs. 43, 44), or from the first segmented. In any case it divides transversely sooner or later into a number of _chromosomes_ (Fig. 45), which as a rule have the shape of little rods, straight or curved, often bent into a V, but which may also be ovoid, or round, or even annular. Certain deeply staining masses, the nucleoli, which may be present in the resting nucleus, do not take part in the process of chromosome formation; they are either cast out of the nucleus and are dissolved in the cytoplasm, or fade away _in situ_. 2. Meanwhile, the deeply staining granule (here extra-nuclear), known as the _centrosome_, has divided in two. The two resulting granules travel to opposite poles of the nucleus, and {174} there each becomes surrounded by a system of radiating lines, the _asters_; immediately around the centrosome is a clear space, the _centrosphere_ (Figs. 43–45). Between the two centrosomes with their asters stretches a bundle of achromatic fibres, the _spindle_. 3. The surface-film bounding the nucleus has broken down, the definite nuclear boundaries are lost, and the spindle now stretches through the nuclear material, in which lie the chromosomes (Figs. 45, 46). These chromosomes now arrange themselves midway between the poles of the spindle, where they form what is called the _equatorial plate_ (Fig. 47). [Illustration: Fig. 46.] [Illustration: Fig. 47.] 4. Each chromosome splits longitudinally into two: usually at this stage,—but it is to be noticed that the splitting may have taken place so early as the spireme stage (Fig. 48). 5. The halves of the split chromosomes now separate from one another, and travel in opposite directions towards the two poles (Fig. 49). As they move, it becomes apparent that the spindle consists of a median bundle of “fibres,” the central spindle, running from pole to pole, and a more superficial sheath of “mantle-fibres,” to which the chromosomes seem to be attached, and by which they seem to be drawn towards the asters. 6. The daughter chromosomes, arranged now in two groups, become closely crowded in a mass near the centre of each aster {175} (Fig. 50). They fuse together and form once more an alveolar reticulum and may occasionally at this stage form another spireme. [Illustration: Fig. 48.] [Illustration: Fig. 49.]