CHAPTER IV ON THE INTERNAL FORM AND STRUCTURE OF THE CELL

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.
1. 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. 43ã45). Between the two centrosomes with their asters stretches a bundle of achromatic fibres, the spindle.
1. 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). Fig. 46. Fig. 47. dctr01
1. Each chromosome splits longitudinally into two: usually at this stage,ãbut it is to be noticed that the splitting may have taken place so early as the spireme stage (Fig. 48).

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