306 



MANUAL OP BOTANY 



the nucleus consists essentially of a delicate network of fibrils 

 of chromatin, embedded in a hyaline substance, and surrounded 

 by a more or less well-defined outline derived from the cell- 

 protoplasm and known as the nuclear membrane. Associated 

 with it in some cases are two small centrospheres. Karyokinesis 

 begins by the network of fibrils becoming coarser and gradually 

 separating to form a long coiled fibre. The nucleoli disappear, 

 and the nuclear membrane can no longer be -distinguished. At 

 the same time, in those eases in which centrospheres have been 

 seen, they shift their position and come to lie on opposite sides 

 of the nucleus at some little distance from it. The long 

 coiled fibre of chromatin breaks up into a number of pieces, 

 often V-shaped, which point towards the centre of the nucleus. 

 The number of these varies in different cells, but is constant in 



Pig- 661. stages in karyokinetic division of tlie nucleus, a. Besting nucleus, 

 t. Stage of equatorial plate, c. Separation of the cbroniosomes. d. Com- 

 menceibent of formation of cell-wall, e. Extension of nuclear spindle 

 across the cell. 



those of the same tissue. These pieces of the fibre are known as 

 chromosomes. The chromatin in the fibres is broken up into 

 thin discs, which are separated from each other by smaller 

 discs of unstainable substance. Threads of delicate character 

 may next be seen to extend from one oentrosphere to the other, 

 forming a body known as the micleiir spindle. The ends of the 

 spindle are known as the poles of the nucleus. The nuclear 

 fibrils, or chromosomes, travel along these threads, with which 

 their points are in contact, till they form a disc across the 

 spindle. This stage is constant in all cases of karyokinesis, 

 though the details of its formation vary in different eases. The 

 body is 'sometimes called the equatorial plate (fig. 661, 6). 

 After this stage is reached, each chromosome splits longitu- 

 dinally into two, and the ecjuatorial plate divides into two in 



