156 FOOT-NOTES TO EVOLUTION. 



each segment receives a series of fibrils from each pole 

 of the amphiaster, the two series being attached along 

 opposite sides of the chromosomes. Under the influence 

 of these fibres, probably by direct pulling, the chromo- 

 somes, now bent into V- or U-shaped loops, tend to place 

 themselves in a circle around the centre of the spindle, 

 transversely to its long axis, and form the " equatorial 

 plate" (Fig. 5, ). 



The changes thus far constitute the " prophases " of 

 the division. The " metaphases " following these con- 

 sist primarily in the longitudinal splitting 

 Splitting of the of each chromosome and the moving 

 chromosomes. 



apart of the halves. This longitudinal 



splitting of the chromosome into two equivalent parts 

 forms the most important act of the whole cell division, 

 and is of the greatest theoretical significance. By it 

 the chromatin substance of the original nucleus is equally 

 distributed between the two daughter nuclei, so that 

 each receives a half of each original chromosome. The 

 elaborate mechanism and consequent expenditure of 

 energy involved in this careful longitudinal division of 

 each chromosome, rather than a simple mass division, 

 such as might be brought about by far less complicated 

 means, indicates clearly that the distribution of the defi- 

 nite organization of the chromatin to the daughter cells 

 is of primary importance, a conclusion which is further 

 strengthened by much evidence too extended to be en- 

 tered upon here. 



In the " anaphases " and " telophases," which in- 

 clude the closing stages of division, the daughter chro- 

 mosomes migrate along the fibres of the central spindle 

 toward its poles, perhaps through the direct contraction 

 of the mantle fibres under the influence of the centro- 

 some, though this and many other points regarding the 

 forces at work must be left for future investigation 



