Page:Popular Science Monthly Volume 26.djvu/834

814 plate is formed, inclosing a narrow equatorial plane. This is the plane of cell-division. A furrow appears around the equator of the cell, which deepens, and extends inward between the equatorial plates. It continues to deepen until it finally meets in the center, and the cell is separated into two new ones.

While this is proceeding, new nuclei are forming at the nuclear poles. The fibers of the daughter-stars pass through a series of changes opposite to those above described. The ends of the loops unite until a wreath is formed. This wreath soon becomes an irregular convolution, which quickly assumes the reticular structure. Membranes form around the new nuclei. Nucleoli and granules reappear. The resting stage is regained. The original cell is replaced by two daughter-cells.

The above description, with its detailed account of the process of cell-division, is not accepted in all its particulars by other observers. There is great diversity of opinion about many points, which can not be settled without much further investigation. It is also very probable that much of the diversity of opinion arises from the fact that the cells of different organisms vary in their features of change, and that vegetable cells only distantly resemble animal cells in this particular.

Some of the unsettled questions are the following: Klein and Strasburger see little importance in the nucleolus. Klein doubts its existence. There is an open question whether it and the granules are not merely the nodes of the network. But the majority of observers speak of the nucleoli and granules as lying free in the ground-substance, in the intervals of the network. It is also a question whether or not the outer cell-substance is like the nucleus in structure. Klein holds that it is. Flemming has lately announced the discovery, in the resting-nucleus, of a very fine network, in connection with the coarser one already known. He also declares that the membrane surrounding the nucleus is really composed of minute flat plates of chromatin continuous with the fibrils of the network. These are separated by slight intervals, so that the membrane seems pierced by holes, which perhaps may be occupied by the transparent ground-substance. Others deny the existence of a nuclear membrane, and think that it is an optical illusion, caused by the arrangement of the fibers. Dr. Pfitzner has recently declared that the chromatin fibrils are not homogeneous in structure, but that they really consist of minute spherules of chromatin, held together by some other substance, probably achromatin.

Such are some of the questions to be yet settled. It would appear that the chromatin of the original nucleus becomes first regularly arranged around its center, then divides equatorially and recedes to its poles, where it forms new nuclei, while the achromatin-fibrils of the spindle may possess some chromatin, which collects upon their centers to form the equatorial plate. If Flemming's last observation