REPRODUCTION 428 
Threads of a delicate character may next be seen to 
extend from one centrosphere to the other, forming a body 
known as the nuclear spindle. The positions of the centro- 
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 dise across the spindle (fig. 168, b). This 
stage is constant in all cases of karyokinesis, though some 
d e 
Fic. 163.—Stagzs iv KaRyoxKINeTio Division oF THE NvoLEvs. 
a, resting nucleus; 5, stage of equatorial piste ; ¢, Separation of the chromosomes ; 
d, commencement of formation of cell-wall; ¢, extension of nuclear spindle 
across the cell. 
variations of the antecedent steps bave 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 
