CELL-LIFE AND CELL-MULTIPLICATION. 257 



loses its identity. Be this as it may, however, the essential 

 fact is that during the growth-period the chromatin sub- 

 stance is widely distributed, and concentration of it is one of 

 the chief steps towards a division of the nucleus and pre- 

 sently of the cell. 



During this process of mitosis or karyokinesis, the dis- 

 persed chromatin having passed through the coil-stage, 

 reaches presently the star-stage, in which the chromosomes 

 are arranged symmetrically about the equatorial plane of the 

 nucleus. Meanwhile in each of them there has been a pre- 

 paration for splitting longitudinally in such way that the 

 halves when separated contain (or are assumed to contain) 

 equal numbers of the granules or chromomeres, which some 

 think are the ultimate morphological units of the chromo- 

 somes. A simultaneous change has occurred : there has been 

 in course of formation a structure known as the amphiaster. 

 The two centrosomes which, as before said, place themselves 

 on opposite sides of the nucleus, become the terminal poles 

 of a spindle-shaped arrangement of fibres, arising mainly 

 from the groundwork of the nucleus, now continuous with 

 the groundwork of the cytoplasm. A conception of this 

 structure may be formed by supposing that the radiating 

 fibres of the respective asters, meeting one another and 

 uniting in the intermediate space, thereafter exercise a trac- 

 tive force; since it is clear that, while the central fibres of 

 the bundle will form straight lines, the outer ones, pulling 

 against one another not in straight lines, will form curved 

 lines, becoming more pronounced in their curvatures as the 

 distance from the axis increases. That a tractive force is at 

 work seems inferable from the results. For the separated 

 halves of the split chromosomes, which now form clusters on 

 the two sides of the equatorial pUme, gradually part com- 

 pany, and are apparently drawn as clusters towards the op- 

 posing centrosomes. As this change progresses the original 

 nucleus loses its individuality. The new chromosomes, 

 halves of the previous chromosomes, concentrate to founcj. 



