The intricate physiological processes which accompany this "mitosis"
have been very closely studied of late years. The inquiry has led to
the detection of certain laws of evolution which are of extreme
importance in connection with heredity. As a rule, two very different
parts of the nucleus play an important part in these changes. They
are: the chromatin, or coloured nuclear substance, which has a
peculiar property of tingeing itself deeply with certain colouring
matters (carmine, haematoxylin, etc.), and the achromin (or linin, or
achromatin), a colourless nuclear substance that lacks this property.
The latter generally forms in the dividing cell a sort of spindle, at
the poles of which there is a very small particle, also colourless,
called the "central body" (centrosoma). This acts as the centre or
focus in a "sphere of attraction" for the granules of protoplasm in
the surrounding cell-body, and assumes a star-like appearance (the
cell-star, or monaster). The two central bodies, standing opposed to
each other at the poles of the nuclear spindle, form "the double-star"
(or amphiaster, Figure 1.11, BC). The chromatin often forms a long,
irregularly-wound thread--"the coil" (spirema, Figure A). At the
commencement of the cleavage it gathers at the equator of the cell,
between the stellar poles, and forms a crown of U-shaped loops
(generally four or eight, or some other definite number). The loops
split lengthwise into two halves (B), and these back away from each
other towards the poles of the spindle (C). Here each group forms a
crown once more, and this, with the corresponding half of the divided
spindle, forms a fresh nucleus (D). Then the protoplasm of the
cell-body begins to contract in the middle, and gather about the new
daughter-nuclei, and at last the two daughter-cells become independent
beings.