We have already seen how the conclusion was reached that cells can arise only from pre-existing cells, and that this occurs usually by division. Under favorable circumstances, when a growing cell has reached a certain rather indefinite limit of size, it proceeds to divide into two cells. But each of the new cells must have a nucleus and centrosphere; and we know that these can only arise from already existing ones, and by division. The process of nuclear division, as before remarked, is usually a very elaborate one, commonly known to students of the cell by the name karyokinesis. Indeed, so fundamentally important does this process appear, that the simpler method sometimes observed is believed by many biologists to have a pathological significance. The first sign of approaching karyokinetic division in a nucleus is the thickening of the threads of the nuclear network. This thickening continues, and at the same time the power of the threads to take up certain staining substances, now much used in their study, rapidly increases. Gradually the network resolves itself into a loose skein or coil, and at last this breaks up into a number, varying much in different species, but pretty constant in the same one, of separate rods or loops. These individualized portions of the original nuclear network have received the name of chromosomes, from their marked capacity for staining. Meanwhile the centrosphere, if previously single, has divided, and one of the pair has moved to each of the poles of the nucleus. At least in the higher plants, this division has occurred long before.
|Fig. 1.||Fig. 2.||Fig. 3.|
|Fig. 1.—A plant cell, with its nucleus showing the chromatic network and nucleoli, and with a pair of centrospheres.|
|Fig. 2.—A plant cell whose nucleus is preparing for division.|
|Fig. 3.—A plant cell in an early stage of division, its twelve chromosomes separated.|
The nuclear membrane now disappears and there is formed, perhaps from the homogeneous protoplasm of the centrospheres, or from the substance of the nucleoli, or from both, a spindle-shaped framework of delicate fibers, about whose equator the chromosomes become arranged in a circle. Then is completed a process which may have begun much earlier, and each chromosome is split longitudinally into two. Of each pair of daughter chromo-