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VEGETABLE PHYSIOLOGY 



Threads of a delicate character may next be seen to 

 extend from one centrosphere to the other, forming a body 

 know as the nuclear spindle. The positions of the 

 centrospheres 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, b). This 

 stage is constant in all cases of karyokinesis, though some 



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



a, resting nucleus ; 6, stage of equatorial plate ; c, separation of the chromo- 

 somes ; d, commencement of formation of cell-wall ; e, extension of 

 nuclear spindle across the cell. 



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 recog- 

 nisable ; the halves resulting from these divisions separate 

 into two sets in such a way that half of each original 

 chromosome 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 



