104 THE CELL 



bent chromatin rods of equal length, called chromosomes 

 (Fig. 8, b), the nucleolus and nuclear membrane ' dis- 

 appearing. A spindle-shaped structure, consisting of 

 exceedingly deHcate protoplasmic threads, appears in 

 the cell, frequently stretching across almost its entire 

 width (Fig. 8, c). This spindle is called the achromatic 

 spindle, because its threads do not take up stains and 

 become deeply coloured as do the chromosomes. The 

 threads converge on two spots (poles). In some plant 

 cells and in most animal cells each pole is occupied by 

 a staining granule called a centrosome to which the 

 threads are attached. The chromosomes now become 

 arranged on the spindle at its equator. Each chromo- 

 some commonly has the form of a rod bent in the middle 

 rather like a hairpin, the free ends projecting outwards, 

 perpendicular to the long axis of the spindle, the bend 

 directed towards the centre of the spindle (Fig. 8, d). 

 At this time each chromosome is seen to have a double 

 structure, the bent rod being split longitudinally, but 

 this splitting may occur earlier, so that the double 

 structure is visible as soon as the chromosomes appear 

 from the resting nucleus (Fig. 8, c). Spindle fibres 

 running from the poles towards the equator (and distinct 

 from the fibres on which the chromosomes rest, which 

 run continuously from pole to pole) now become 

 attached to the chromosomes at or near the inwardly 

 directed bend (Fig. 8, A). These fibres contract and 

 thus pull the longitudinal halves of the chromosomes 

 (daughter chromosomes) apart, the daughter chromo- 

 somes remaining in contact longest at the free ends 

 (Fig. 8, e and B). By continued contraction of the 



• In some animal cells the nuclear membrane remains intact 

 during karyokinesis, the whole of the processes described taking place 

 inside it, 



