22 



BIOLOGY 



distinct, and that the chromatin of the nucleus is in the 

 form of a network of irregularly-disposed threads. But 

 when the cell prepares for division, the chromatin 

 assumes the form of a continuous coiled thread the 

 spireme or it breaks up into a definite number of frag- 

 ments which are either straight or loop-shaped. These 

 fragments or loops gradually give rise to the chromo- 

 somes, and during this stage the nuclear membrane 



Stages in Karyokinesis. 



disappears. Meanwhile the centrosome divides into 

 two, and the two parts take up their positions at oppo- 

 site poles. Between the two centrosomes a number of 

 delicate fibres stretch, forming what is called a spindle, 

 and radiating out from each centrosome are numerous 

 fibres which stretch into the protoplasm. The chromo- 

 somes are then arranged in an equatorial plate at the 

 centre of the spindle fibres. Then each chromosome 

 splits lengthways into two similar portions, forming a 

 double set, which are pulled, pushed, or otherwise travel 

 along the spindle fibres towards the centrosomes. When 



