General physiology; an outline of the science of life . i (76), Flemming (82),Strasburger, (80, 88), O. Hertwig (76, 77, 78, 92), van Beneden(87), Boveri (87, 88, 90), and others, who have found objectsbest fitted for this purpose in the cells of young larvae of sala-manders, in the pollen-cells of lilies, and in the transparenteggs of the sea-urchin and the round-worm of the horse. a. Direct Cell-division The simplest form of cell-division is the direct or amitotic cell-division, which, however, is comparatively rare and, beyond certain 192 GENERAL PHYSIOLOGY unicellular organisms and leucocy
General physiology; an outline of the science of life . i (76), Flemming (82),Strasburger, (80, 88), O. Hertwig (76, 77, 78, 92), van Beneden(87), Boveri (87, 88, 90), and others, who have found objectsbest fitted for this purpose in the cells of young larvae of sala-manders, in the pollen-cells of lilies, and in the transparenteggs of the sea-urchin and the round-worm of the horse. a. Direct Cell-division The simplest form of cell-division is the direct or amitotic cell-division, which, however, is comparatively rare and, beyond certain 192 GENERAL PHYSIOLOGY unicellular organisms and leucocytes, has been met with onlyin very few forms of cells. The division of Amoeba can serve as atype (Fig. 71). While the Amoeba is creeping, the originalspherical nucleus becomes gradually lengthened, then biscuit-shaped, then constricted through the middle; the connecting-piece becomes constantly slenderer and finally breaks; and thustwo new nuclei result, which immediately assume the sphericalform. Then the division of the protoplasm begins; the Amoeba. Fig. 71.—Amoeba polypodia in six successive stages of division The dark body surrounded by aclear area in the interior is the nucleus, the pale body the contractile vacuole. (After P. ) becomes constricted in a similar manner between the two nucleilike a dumb-bell and creeps towards the two sides, until only athin thread of protoplasm unites the two halves, this finally breaksso that two new Amoebae, each with one nucleus, result from thedivision. The process requires a long time, usually several hours,and does not always proceed smoothly: the protoplasm often flowstogether into one mass after a considerable constriction hastaken place, and then flows apart again, until, finally, the unitingbridge is torn through. ELEMENTARY VITAL PHENOMENA 193 b. Indirect Cell-division By far the great majority of all animal- and plant-cells follow themode of the so-called indirect or mitotic cell-division, in which theprotoplasm is
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