CHAPTER IV 



The Centrosomes , Spindle and Asters 



ALTHOUGH the behaviour of the chromosomes in 

 nuclear division has especially attracted the attention 

 of cytologists, in consequence of the probability that in it 

 is to be found the solution of some of the fundamental 

 problems of heredity, yet the nature and mechanism of the 

 achromatic spindle offers problems hardly less fascinating, 

 and perhaps equally far-reaching as regards the nature of 

 vital processes. From the short account of nuclear division 

 given in the preceding chapter, it will be seen that the pro- 

 cess consists in the accurate longitudinal division of the 

 chromosomes, which are the essential part of the nucleus, 

 and the separation of the halves into the two daughter cells. 

 The mechanism by which this process is carried out is the 

 achromatic spindle with the centrosomes at its poles, and 

 although the spindle is such a familiar object, there has 

 been, and still is, the utmost diversity of opinion about its 

 nature and origin. 



The spindle arises as a sheaf of fibres between the two 

 centrosomes formed by the division of the original centro- 

 some which appears shortly before the nucleus divides. 

 With regard to the origin of the centrosome itself there is 

 no complete agreement. When cells are rapidly dividing, 

 there is no doubt that the centrosome may persist from one 

 cell-generation to the next; the daughter-nucleus after a 

 division is formed close to the centrosome at the pole of the 

 spindle, this centrosome then divides, a new spindle begins 

 to be formed between the halves at right angles to the axis 



