KEPKODUCTION 



428 



Threads of a delicate character may next be seen to 

 extend from one centrosphere to the other, forming a hody 

 known as the nuclear spindle. The positions of the centre- 

 spheres are called the poles of the nucleus. When no 

 centrospheres can be detected, the threads of the spindle 

 nevertheless converge to two similarly situated poles. Some 

 of the spindle fibres stretch uninterruptedly from pole to 

 pole, while others become in some way attached to, or 

 entangled with, the chromosomes. The latter travel 

 along these threads, with which their points are in contact, 

 till they form a disc across the spindle (fig. 163, &). This 

 stage is constant in all cases of karyokinesis, though some 



FIG. 163. STAGES ix KARYOKINETIC DIVISION OF THE NUCLEUS. 



a, resting nucleus; 6, stage of 



d, commencement of formation of 

 across the cell. 



plate ; c, separation of the chromosomes 

 11-wall ; e, extension of nuclear spindle 



variations of the antecedent steps have been observed, the 

 details of the formation of the disc not being always iden- 

 tical. This body is sometimes called the equatorial plate. 

 At some time during this preliminary period each chromo- 

 some splits longitudinally into two, though the fission is 

 generally not observable till the equatorial plate is recognis- 

 able ; the halves resulting from these divisions separate into 

 two sets in such a way that half of each original chromo- 

 some makes its way towards one pole, and the other half 

 towards the other. The two sets of chromosomes so formed 

 travel back along the spindle fibres, each going to one of 

 the two poles of the nucleus, their positions as they go being 

 such that their convex sides point towards the pole which 



