INTRODUCTlOlsr 



25 



of long bands or loops, but afterwards become shortened, thus 

 giving rise to short loops, or else to straight rods or rounded 

 granules. With certain exxeptions, to be mentioned later, the 

 number of chromosomes which arise in this way is constant for 

 each species of plant or animal, and also for successive series 

 of cells. By the time the process has reached this stage a special 

 mechanism appears, which has till now remained concealed in 

 the cell substance. This serves to divide the chromatin elements 

 into two equal parts, to separate the resulting halves from one 

 another, and to arrange them in a regular manner. At the 

 opposite poles of the longitudinal axis of the nucleus two clear 

 bodies — the ' centrosomes,' each surrounded by a clear zone, 

 the so-called 'sphere of attraction ' — now become visible. The 

 importance of these was first recognised by Fol, van Beneden, 

 and Boveri. They possess a great power of attraction over the 

 vital particles of the cell, so that these become arranged around 

 them like a series of rays. At a certain stage in the preparation 

 for division, the soft protoplasmic substance of the cell-body as 

 well as of the nucleus gives rise to delicate fibres or threads : these 

 fibres are motile, and, after the disappearance of the nuclear 

 membrane, seize the chromosomes — whether these have the 

 form of loops, rods, or globular bodies — with wonderful certainty 

 and regularity, and in such a way that each element is held on 

 either side by several threads from either pole. The chromatin 

 elements thus immediately become arranged in a fixed and 

 regular manner, so that they all come to lie in the equatorial 

 plane of the nucleus, which we may consider as a spherical body. 

 The chromatin elements then split longitudinally, and thus 

 become doubled, as Flemming first pointed out. It must be 

 mentioned that this splitting is not caused by a pull from the 

 pole threads (spindle threads), which attach themselves to the 

 chromatin rods on both sides ; the division arises rather from 

 forces acting in the rods themselves, as is proved by the fact 

 that they are often ready to divide, or indeed have already done 

 so, some time before their equatorial arrangement has taken 

 place by means of these threads. 



The splitting is completed by the two halves being gradually 

 drawn further apart towards the opposite poles of the nuclear 

 spindle, until they finally approach the centre of attraction or 

 centrosome, which has now fulfilled its object for the present, 

 and retires into the obscurity of the cell-substance, only to 



