414 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-dise across the spindle (fig. 168, 6). This 
stage is constant in all cases of karyokinesis, though some 
a b c _ @ e 
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Fic. 163.—Stacrs 1n Karyoxinetic Division oF THE NucLEvs. 
a, resting nucleus ; b, 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 
