120 THE STORY OF LIFE'S MECHANISM. 



the spindle fibres bear a part a fact which 

 again points to the importance of the centro- 

 somes and the forces which radiate from them. 

 Now the chromosomes in each daughter nucleus 

 unite to form a single thread, or may diffuse 

 through the nucleus to form a network, as in 

 Fig. 32. They now become surrounded by a 

 membrane, so that the new nucleus appears 

 exactly like the original one. The spindle fibres 

 disappear, and the astral fibres may either dis- 

 appear or remain visible. The centrosome may 

 apparently in some cases disappear, but more 

 commonly remains beside the daughter nucleii, 

 or it may move into the nucleus. Eventually it 

 divides into two, the division commonly occur- 

 ring at once (Fig. 32), but sometimes not until 

 the next cell division is about to begin. Thus 

 the final result shows two cells each with a 

 nucleus and two centrosomes, and this is exactly 

 the same sort of structure with which the process 

 began. 



Viewed as a whole, we may make the follow- 

 ing general summary of this process. The essen- 

 tial object of this complicated phenomena of 

 JcaryoJcinesis is to divide the chromatin into 

 equivalent halves, so that the cells resulting from 

 the cell division shall contain an exactly equiva- 

 lent chromatin content. For this purpose the 

 chromatic elements collect into threads and split 

 lengthwise. The centrosome, with its fibres, 

 brings about the separation of these two halves. 

 Plainly, we must conclude that the chromatin 

 material is something of extraordinary import- 

 ance to the cell, and the centrosome is a bit of 



