26.] 



CHAPTER I. THE CELL. 



119 



state (see p. 95), the first fact indicating the imminence of nuclear division is 

 that the two centrospheres separate and take up positions on opposite sides of 

 the nucleus, thus indicating the plane in which the nuclear division is to take 

 place, viz., at right angles to a straight line joining the centrospheres: the 

 change of position of the centrospheres is doubtless effected by the kinoplasm 

 in which they lie. Changes are now perceptible in the nucleus itself. The 

 fibrillar network contracts and becomes more dense, and breaks up into distinct 

 fibrils (chromosomes) consisting now of broad discs of chromatin with narrower 

 intervening discs of linin : the tangle of the somewhat V-shaped fibrils becomes 

 looser as they separate and move towards the surface of the nucleus. At this 

 stage the so-called nuclear membrane loses its definiteness, the kinoplasm 

 entering the nucleus without, however, displacing the proper ground-substance 

 of the nucleus. The kinoplasm. forms a number of threads, extending from one 

 centrosphere to the other, constituting the kinoplasmic spindle, of which the 

 centrospheres are the two poles (Fig. 77). Along these threads the fibrils move 

 till they reach the equatorial plane 



of the spindle, where they con- C 



stitute the nuclear disc, and are so 

 placed that their free ends point 

 to either one pole or the other. 

 Whilst these changes have been 

 going on, the nucleoli have dis- 

 appeared, being diffused in the 

 nuclear ground-substance, which 

 can, in consequence, be stained at 

 this stage. The fibrils now undergo 

 longitudinal splitting into two, and 

 then the nuclear disc separates 

 into two halves, in such a way that 

 one of each pair of fibrils produced 

 by the splitting of each primary 

 fibril goes to each half. The fibrils 

 constituting each half of the nu- 

 clear disc now move towards the 

 corresponding pole along the spindle-threads, changing their postion as they go, 

 so that when they reach the pole their free ends point towards the equatorial 

 plane (Fig. 78/). On reaching the pole, each group of fibrils constitutes a new 

 nucleus ; it becomes invested by a membrane, nucleoli reappear, and the fibrils 

 resume the form and structure of the resting nucleus. The two nuclei are now 

 completely formed, and are still connected by kinoplasmic spindle- threads. If 

 no cell-division is immediately to take place, no further change occurs beyond 

 the disappearance of the threads. 



Two modes of cell-formation with division may be conveniently 

 distinguished. In the one, often distinguished as free cell-for- 

 mation, several cells are formed simultaneously in the cavity of a 

 parent cell (or coenocyte) ; in the other, designated simply cell' 

 division, only two cells are, as a rule, formed as the result of the 



Fro. 77. Germinating pollen-grain of Lilium 

 Martagon with a dividing nucleus: the kino- 

 plasmic spindle is formed with a centrosphere 

 (c) at each pole; n is the nuclear disc forme;! 

 by the chromosomes. (After Guignard : x 750.) 



