Reproduction of Cells.—Reproduction of cells is effected either by direct or by indirect division. In reproduction by direct division the nucleus becomes constricted in its center, assuming an hour-glass shape, and then divides into two. This is followed by a cleavage or division of the whole protoplasmic mass of the cell; and thus two daughter cells are formed, each containing a nucleus. These daughter cells are at first smaller than the original mother cell; but they grow, and the process may be repeated in them, so that multiplication may take place rapidly. Indirect division or karyokinesis (karyomitosis) has been observed in all the tissues—generative cells, epithelial tissue, connective tissue, muscular tissue, and nerve tissue. It is possible that cell division may always take place by the indirect method. 9 The process of indirect cell division is characterized by a series of complex changes in the nucleus, leading to its subdivision; this is followed by cleavage of the cell protoplasm. Starting with the nucleus in the quiescent or resting stage, these changes may be briefly grouped under the four following phases (Fig. 2). 10 1. Prophase.—The nuclear network of chromatin filaments assumes the form of a twisted skein or spirem, while the nuclear membrane and nucleolus disappear. The convoluted skein of chromatin divides into a definite number of V-shaped segments or chromosomes. The number of chromosomes varies in different animals, but is constant for all the cells in an animal of any given species; in man the number is given by Flemming and Duesberg as twenty-four. 2 Coincidently with or preceding these changes the centriole, which usually lies by the side of the nucleus, undergoes subdivision, and the two resulting centrioles, each surrounded by a centrosphere, are seen to be connected by a spindle of delicate achromatic fibers the achromatic spindle. The centrioles move away from each other—one toward either extremity of the nucleus—and the fibrils of the achromatic spindle are correspondingly lengthened. A line encircling the spindle midway between its extremities or poles is named the equator, and around this the V-shaped chromosomes arrange themselves in the form of a star, thus constituting the mother star or monaster. 11 2. Metaphase.—Each V-shaped chromosome now undergoes longitudinal cleavage into two equal parts or daughter chromosomes, the cleavage commencing at the apex of the V and extending along its divergent limbs. 12 3. Anaphase.—The daughter chromosomes, thus separated, travel in opposite directions along the fibrils of the achromatic spindle toward the centrioles, around which they group themselves, and thus two star-like figures are formed, one at either pole of the achromatic spindle. This constitutes the diaster. The daughter chromosomes now arrange themselves into a skein or spirem, and eventually form the network of chromatin which is characteristic of the resting nucleus.
4. Telophase.—The cell protoplasm begins to appear constricted around the equator of the achromatic spindle, where double rows of granules are also sometimes seen. The constriction deepens and the original cell gradually becomes divided into two new cells, each with its own nucleus and centrosome, which assume the ordinary positions occupied by such structures in the resting stage. The nuclear membrane and nucleolus are also differentiated during this phase.