Fissiparous reproduction of the amoeba.
The single-celled animal “a” is divided after different steps into two animals, “f” and “g.”
The main function of the genital glands is the production of germ-cells, the spermatozoa in the male and the ova in the female. This production is effected by means of cell-division. The division may be direct as found in the fissiparous reproduction of the amoeba. As soon as the cell reaches a certain size, it divides itself into two cells. The more complicated form of cell-division is the indirect division called “Mitosis” or “Karyokynesis.”
As shown before the cell consists of two main parts, the smaller part called the nucleus and the larger portion called the cytoplasm. The nucleus shows in its interior a linin net and deeply staining granules. Because of this quality they are called “chromatin.” The cytoplasm in the close proximity of the nucleus contains minute granules, situated either singly or in pairs which are called “centrosomes.”
In the indirect division the nucleus is divided by a complicated process. Simultaneously the cytoplasm and the membrane are also cleft in two parts. In the typical cell division two parallel series of changes occur nearly simultaneously, one affecting the nucleus, the other the cytoplasm. The chromatin which is usually (Fig. A, Cut XXVIII) in form of scattered granules, arranged along the linin network, becomes aggregated together in certain definite areas (Fig. B), forming usually a convoluted thread or skein. This skein appears either in form of a long filament or divided up into a series of segments called “chromosomes” (Fig. C). The number of chromosomes is constant for each species of plant and animal. Thus, in the common mouse these chromosomes are twenty-four in number, in the onion sixteen, in the sea-urchin eighteen, etc. The number is always an even one. By this time the nuclear membrane has disappeared, and the chromosomes appear usually as a collection of bands lying free in the cytoplasm (Fig. D).
At the same time, another series of changes has gone through the centrosome and the cytoplasm in the cell-body. The centrosome assumes an ellipsoid form, constricts transversely into a dumb-bell-shaped figure (Fig. B), and divides into two daughter centrosomes. Around each of them is gradually developed a stellate figure composed of a countless number of delicate fibrils, radiating out in all directions from the centrosome as a centre. The entire constellation is called “aster.” The two asters grow in size progressively as the two centrosomes
move apart toward the poles of the cell (Fig. C). Between the two asters a spindle-shaped system of delicate fibrils appears stretching from one aster to the other which is called “central-spindle” (Fig. D). The two asters together with the central-spindle represent the “Amphiaster”.
CUT XXVIII.
Mitosis or indirect division of a cell.
cy, cytoplasm; n, nucleus; c, centrosome; nu, nucleolus; l, linin; sk, skein; a, aster; cs, central spindle; ch, chromosome; u, U-shaped loops; mf, mantle fibres; cf, connecting fibrils.
At this point the centrosomes or the asters and the chromosomes begin to work together. A system of fibrils grows out from each aster which attach themselves to the individual chromosomes. The latter bent into U-shaped loops arrange themselves in a circle around the centre of the spindle and form the “equatorial plate” (Fig. E).
The chromosomes are now longitudinally split, and the halves move toward the poles as if drawn by the mantle fibres (Fig. F). The loops are still connected with each other by these connecting mantle fibres (Fig. G and H).