Cells may increase in number by direct (amitotic) or indirect (mitotic or karyokinetic) division. There is no doubt that mitosis occurs, but not all investigators are convinced that cells ever divide amitotically. Direct division was once considered the only method of cell multiplication. It was described as a simple division of the nucleus into two parts (Fig. 2), preceded by a division of the nucleolus into two, and succeeded by a constriction of the entire cell ; the result was two daughter cells each with one nucleus containing one-half of the nucleolus. As we shall see later, amitosis has been described in cells of the germ-cell cycle, and must therefore be reckoned with in any discus sion of the phys ical basis of heredity.
Mitosis or karyokinesis in volves a rather complicated series of pro cesses which cannot be fully discussed here but will be out lined very briefly with the aid of Fig.3.
FIG. 2. Amitosis. A. Division of blood-cells, in the embryo chick, illustrating Remak's the
scheme, ae = successive stages of division . J. i. _ (From Wilson, 1900.) B. Amitotic nuclear
division in the follicle cells of a cricket's egg. Granules which (From Dahlgren and Kejmer, 1908.)
Are scattered through the nucleus in the resting cell (A) become arranged in the form of a long thread or spireme (B). At the same time the centrosomes move apart (A, c; B, a), and a spindle arises between them (C). While this is going on, the nuclear membrane generally disintegrates and the spireme segments into a number of bodies called chromosomes ((7) ; these take a position at the equator of the spindle, halfway between the centrosomes (D, ep) . The stage shown in Fig. 3, D, is known as the amphiaster ; at this time
FIG. 3. Mitosis. Diagrams illustrating mitotic cell division. (From Wilson.) A, resting cell; B, prophase showing spireme and nucle olus within the nucleus and the formation of spindle and asters (a) ; C, later prophase showing disintegration of nuclear membrane, and breaking up of spireme into chromosomes; D, end of prophases, showing complete spindle and asters with chromosomes in equatorial plate (ep); E, metaphase each chromosome splits in two; F, ana phase the chromosomes are drawn toward the asters, if = inter zonal fibers; G, telophase, showing reconstruction of nuclei; H, later telophase, showing division of the cell into two.
All of the mechanism concerned in mitosis is present. There are two asters, each consisting of a centrosome surrounded by a number of radiating astral rays, and a spindle which lies between them. The chromo somes lie in the equatorial plate (ep).
(6) During the second stage, the metaphase, the chromosomes split in such a way that each of their parts contains an equal amount of chromatin (E, ep) . As we shall see later, this is one of the most significant events that takes place during mitosis.
(c) During the anaphase (F) the chromosomes formed by splitting move along the spindle fibers to the centrosomes. As a result every chromosome present at the end of the prophase (Z)) sends half of its chromatin to either end of the spindle. The mechan ism that brings about this migration is as yet some what in question. Fibers are usually left between the separating chromosomes; these are known as interzonal fibers (F, if).
(d) The telophase (G, H) is a stage of reconstruction from which the nuclei emerge in a resting condition ; the chromatin becomes scattered through the nucleus, which is again enveloped by a definite membrane
(77) ; the centrosome divides and, with the centre sphere, takes a position near the nucleus. Finally the cycle is completed by the constriction of the cell into two daughter cells.
There are a number of differences between the sort of mitosis just described and that which occurs during the maturation of the egg and spermatozoon ; these and certain other phases of cell division will be considered in their appropriate places in other parts of this site.