1 20 CELL-DIVISIOX 



Every individual chromatin-j^ranule therefore contributes its quota 

 to each of the daughter-nuclei, but it is uncertain whether they are 

 persistent bodies or only temporary structures like the chromosomes 

 themselves. 



The spindle may arise from the achromatic substance of the 

 nucleus, from the cytoplasmic substance, or from both. When cen- 

 trosomes are present it is they, as a rule, that lead the way in divi- 

 sion. About the daughter-centrosomes as foci are formed the asters 

 and between them stretches the spindle, forming an auipJiiastcr 

 which is the most highly developed form of the achromatic figure. 

 When centrosomes are absent, as now appears to be the case in the 

 higher plants, the spindle is formed from fibrous protoplasmic ele- 

 ments that gradually group themselves into a spindle. 



The mechanism of mitosis is imperfectly understood. Experi- 

 mental studies give ground for the conclusion that the changes 

 undergone by the chromatic and the achromatic figures respectively 

 are parallel but in a measure independent processes, which are how- 

 ever so correlated that both must cooperate for complete cell-division. 

 Thus there is strong evidence that the fission of the chromatin-gran- 

 ules, and the splitting of the thread, is not caused by division of the 

 centrosome or the formation of the spindle, but only accompanies it 

 as a parallel phenomenon. The divergence of the daughter-chromo- 

 somes, on the other hand, is in some manner determined by the 

 spindle-fibres. There are cogent reasons for the view that some of 

 these fibres are contractile elements which, like muscle-fibres, drae: 

 the daughter-chromosomes asunder ; while other s])indle-fibres act as 

 supporting and guiding elements, and probably by their elongation 

 push the spindle-poles apart. The adequacy of this explanation is, 

 however, doubtful, and it is not improbable that the centrosome or 

 spindlc-poles are centres of chemical or other physiological activities 

 that play an essential part in the process and are correlated with 

 those taking place in the chromatin. The functions of the astral 

 rays are likewise still involved in doubt, the rays being regarded by 

 some investigators as contractile elements like muscle-fibres, by others 

 as rigid supporting fibres, or even as actively pushing elements like 

 those of the central spindle. It is generally believed further that they 

 play a definite part in division of the cell-body — a conclusion sup- 

 ported by the fact that the size of the aster is directly related to that 

 of the resulting cell. On the other hand division of the cell-body 

 may apparently occur in the absence of asters (as in amitosis, or 

 among the Infusoria). 



These facts show that mitosis is due to the coordinate play of an 

 extremely complex system of forces which are as yet scarcely com- 

 prehended. Its general significance is, however, obvious. The effect 



