106 



THE PLANT CELL. 



'Jareful focussing will now often bring to light fine refractile 

 lines radiating from points near either end of the long axis of 

 the nucleus, and passing towards the median equatorial plane of 

 the nucleus and cell; these lines are the achromatic fibrils.* 

 The fibrils soon become more obvious, and radiate from points 

 (polesj on either side of the nucleus, in the form of cone-shaped 

 bundles, which (partly) become attached by their central ends to 

 the primary chromosomes; some of them, however (the central 

 ones), pass from pole to pole without being attached to chromo- 

 somes. 



The next thing noticed is a change in position of the primary 

 chromosomes. Each of these becomes bent into the shape of a 

 U or a V, and appears to be dragged (or guided) by certain of 

 the achromatic fibrils into the median equatorial plane, where it 

 takes up a position in which the bend of the V looks towards 

 the centre of that plane, whilst the free ends of the V look away 

 from the centre towards the circumference. The achromatic 

 fibrils ha^e together, at this stage, the shape of a spindle, and 

 form what is known as the nuclear spindle or ampMaster ; and 

 the primary chromosomes form in their equatorial position what 

 is termed the monaster or single rosette, since this is the form of ; 

 the figure when seen in surface view (see Fig. 78, 6, and Fig. 85). 



Those fibrils of the spindle which have been influential in 

 pulling or guiding the chromosomes into their median position 

 are known as the mantle-fibres; they have been assumed to be 

 contractile, and they lie on the outer surface of the spindle in 

 the form of cone-shaped bundles radiating from the poles at 

 each end of the cell. This assumption is, however, partlj' 

 hypothetical, since some observers will not allow of any of the 

 fibrils being contractile, but bring forward evidence to show 

 that chemotaxis plays a rdle in the movements of the chromo- 

 somes, especially during the metaphase. It is, however, quite 

 possible in some cases to make out the cone-shaped masses of 

 fibrils at the beginning of the metaphase (see Plate I., 6 and 8). 



The prophase is completed by the time the primary chromo- 

 somes have assumed the equatorial position : no trace of nucleoli 

 can be detected at the completion of this phase, and the nuclear 

 membrane has vanished. 



* So-called because they do not stain with those dyes which the ohro 

 matin takes up. 



