SOMATIC CELL-DIVISION 111 



begins, the centrosome, if not already double, undergoes division, the 

 daughter centrosomes then moving apart. Each of them occupies the 

 center of a semisolid region, or aster, with conspicuous "astral rays," and 

 between them is a central spindle with fine fibrils. The centrosomes, 

 surrounded by their asters, reach opposite sides of the nucleus and 

 remain at the poles of the mitotic figure through the metaphase, anaphase, 

 and telophase. Cytokinesis is commonly brought about by a cleavage 

 furrow which grows inward from the periphery of the protoplast, rather 

 than by the differentiation of a plate-like wall as in the somatic tissues of 

 higher plants. 



Although cell-division in animals usually differs from that in plants in 

 the above two respects, the distinction is by no means a sharp one. 

 Centrosomes are regularly present in many algae and fungi, and cytokine- 

 sis by furrowing also occurs in certain cells in both the lower and the higher 

 plant groups. The point to be borne in mind is that the essential features 

 of mitosis, the general results of cell-division, and the significance of 

 these events in the life of the organism are the same in the two kingdoms. 



Duration and Periodicity of Cell-division. — The duration of the 

 process of cell-division, particularly that of the various phases of mitosis, 

 has been determined by observing living material.^ In Spirogyra, de 

 Wildeman (1891) found that mitosis was accomplished in 45 minutes at 

 12°C., but that several hours were required when the temperature was a 

 few degrees higher or lower. In the Tradescantia stamen hair mitosis 

 was carried through in 30 minutes at 45° (about the maximum tempera- 

 ture at which it would occur at all), in 75 minutes at ca. 25°, and in 135 

 minutes at ca. 10°C. In stigma cells of Arrhenatherum at 19°C. Martens 

 (1927c) finds that the prophase occupies 36 to 45 minutes, the metaphase 

 7 to 10, the anaphase 15 to 20, and the telophase 20 to 35; total, exclusive 

 of interphase, 78 to 110 minutes. In Sphacelaria fusca, a brown alga, 

 growing at 17° to 18°C., the prophase occupies 10 minutes, the meta- 

 phase 7, the anaphase 4, and the telophase 9; total, 30 minutes (W. 

 Zimmermann, 1923). W. and M. Lewis (1917) give the following figures 

 for mesenchyme cells of the chick growing in tissue cultures at 39°C. : 

 prophase, 5 to 50 minutes, usually more than 30; metaphase, 1 to 15, 

 usually 2 to 10; anaphase 1 to 5, usually 2 to 3; telophase up to cytokine- 

 sis, 2 to 13, usually 3 to 6; telophasic reconstruction of daughter nuclei, 

 30 to 120; total, 70 to 180 minutes. In similar cultures of choroidal 

 cells from the eyes of chick embryos and cartilage cells from adult fowls, 

 Strangeways (1922) finds the process to be more rapid at the same 

 temperature, complete division being accomplished in from 23 to 65 

 minutes, with the average at about 34 minutes. In a statistical study of 

 mitosis in fixed onion root tips Laughlin (1919) observes that each stage 

 shows a characteristic velocity reaction to temperature increments, and 



2 See Tischler (1921-1922), Martens (1927c), and Jaretzky (1930). 



