Page:Popular Science Monthly Volume 49.djvu/633

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somes thus formed, one now passes toward one pole of the spindle and one toward the other. In the higher plants, each polar centrosphere 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 chromosomes formed in the mother nucleus. Each of the latter has furnished one of the former, as will be seen, to each group. The

PSM V49 D633 Plant cell in different stages of division.jpg
Fig. 4. Fig. 5. Fig. 6.
Fig. 4.—A plant cell in division, showing the nuclear spindle and the splitting of the chromosomes.
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 centrospheres 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 during 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 economy, 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.