SOME MODERN VIEWS OF THE CELL. 



611 



somes thus formed, one now passes toward one pole of the spindle 

 and one toward the other. In the higher plants, each polar cen- 

 trosphere divides into a pair at about the time of the splitting of 

 the chromosomes. Thus finally there are accumulated at each 

 pole as many daughter chromosomes as there were mother chro- 

 mosomes formed in the mother nucleus. Each of the latter has 

 furnished one of the former, as will be seen, to each group. The 



Fig. 4. A plant cell in division, showing the nuclear spindle and the splitting of the chromo- 

 somes. 



Fig. 5. A plant cell in a late stage of division, the daughter chromosomes collecting at the 

 poles of the spindle and the new cell wall beginning to appear. 



Fig. 6. Two daughter cells with nuclei which have nearly reached the resting stage, and each 

 with a pair of centrosomes, the result of division of a mother cell like Fig. 1. 



chromosomes of each group now fuse by their ends into a thread, 

 and this gradually thins out until, by an inverse process to that 

 observed at the beginning of division, it passes into the condition 

 of a nuclear network. Meantime new nuclear membranes have 

 been formed and two daughter nuclei with accompanying centro- 

 spheres replace the original one. Just what the mechanics of 

 karyokinesis is has not been determined with certainty; and 

 students of the cell are not yet agreed whether the centrospheres 

 exert an attractive influence on the chromosomes, or are mere 

 passive points of attachment for the fibers of the spindle. 



There remains one constituent of the nucleus whose fate dur- 

 ing nuclear division has not been discussed. This is the substance 

 which forms the nucleoli. These bodies usually disappear slowly 

 while the chromosomes are becoming individualized, and very 

 commonly have quite disappeared before the disappearance of 

 the nuclear membrane. Nothing more is then seen of them until 

 after the constitution of the daughter nuclei and the formation of 

 their membranes. Then nucleoli reappear within these nuclei. 

 To what parts of the cell their substance is distributed while they 

 are unrecognizable, and what purpose they serve in the cell econ- 

 omy, we do not yet know ; but they are probably composed of a 

 reserve substance which furnishes material for some formative 

 process, perhaps for the spindle fibers, as Strasburger now thinks. 



