THE PLANT-CELL 



61 



The Resting Nucleus. The resting nucleus (Fig. 44, A) contains a 

 complicated network, made up of linin-threads, in which are imbedded 

 more or less numerous chromatin-granules. One or more nucleoli are 

 also usually present. 



Prophases. The first 

 sign of approaching divi- 

 sion is a shortening and 

 thickening of the linin- 

 filaments, which sometimes 

 may be shown to constitute 

 a single long and very 

 much tangled thread. This 

 is accompanied by an in- 

 crease in the amount of 

 chromatin, which forms 

 a series of disks arranged 

 along the linin-thread, like 

 beads, separated by short 

 intervals (Fig. 44, D). The 

 spaces between the chro- 

 matin disks may almost 

 completely disappear as 

 the thread shortens, so that 

 the thread appears almost 

 homogeneous. There next 

 follows a longitudinal splitting of the nuclear filament, which thus 

 forms two threads, lying close together and often hard to distinguish. 



Chromosomes. Each filament divides transversely into a definite 

 number of pieces nuclear segments, or Chromosomes, which are in 

 pairs, one segment of each pair belonging to each half into which the 

 original nuclear filament splits longitudinally. The two chromosomes 

 of each pair sometimes fuse more or less completely together. The 

 chromosomes appear homogeneous, and stain very strongly with the 

 usual nuclear stains. Their form varies a good deal, from almost 

 globular to elongated, straight, or bent rods. 



While these changes are taking place in the nuclear filament, the 

 nucleolus usually shows signs of disorganization, and finally is no 

 longer visible. Just what becomes of its substance is still doubtful. 



Spindle-fibres. In the cytoplasm immediately surrounding the 

 nuclear cavity, there may be detected extremely fine filaments, which 

 sometimes form a thick tangled layer about the nucleus, but later 

 show a more or less distinct radiation (Fig. 44, B, C). These begin to 

 penetrate into the nuclear cavity, whose wall becomes less evident, 

 and finally quite unrecognizable. 



Metaphases. As the nuclear membrane disappears, the chromo- 



FIG. 43. Cells from the root-tip of an Onion, 

 dividing by mitosis, or karyokinesis (X525). 



