CELLULAR DIVISION 15 



When the cell is ripe for division, the chromatin particles, 

 which up till then have been distributed over the whole surface 

 of the nucleus, approach each other, and unite in forming a long, 

 thin chain, which is coiled in irregular fashion (Fig. 1, B). The 

 chain seems to increase in volume, and one is able to see a little 

 later that it has been divided into a number of pieces of equal 

 length, each piece being composed, not of a continuous sub- 

 stance, but of a number of minute particles. The various pieces 

 of the chromatin chain are known as " chromosomes," and their 

 component particles as " chromomeres." The chromosomes be- 

 come clearly separate, discrete bodies (Fig. 1, C), and there 

 appears a so-called spindle — the apparatus for effecting the 

 division of the cell (sp). Of great importance is a minute 

 stainable particle (the centrosome), surrounded by a sort of 

 halo or aster (centrosphere). The centrosome is generally re- 

 garded as an indispensable element in the process of cellular 

 division — in fact, as being the dynamic centre, just as the 

 chromatin is the heredity element. When a cell is ready for 

 karyoldnesis, the centrosome enters into activity, and undergoes 

 a preHminary division, followed by that of its centrosphere. 

 During the whole karyoldnesis the centrosomes remain as centres 

 from which the centrosphere irradiates the surrounding living 

 matter like a sort of miniature sun. The two centrosomes 

 separate farther and farther from each other, until they reach 

 a position diametrically opposite one to another at the poles of 

 the nucleus (Fig. 1, D). The resulting figure is known as the 

 " karyokinetic figure," the poles being formed by the centro- 

 somes and the equatorial plane by the chromosomes. 



The significance of this karyokinetic figure is revealed by what 

 follows. The chromosomes are seen to be divided into two 

 rows, each row consisting of four chromosomes. The two rows 

 now begin very gradually to break away from each other, draw- 

 ing nearer and nearer to the two poles of the spindle (E and F). 

 They reach the respective poles, and supply the material for the 



