Page:Popular Science Monthly Volume 26.djvu/837

Rh the cell. Around the cell-wall, in this plane, is seen a faint ring, which pushes inward, and develops into a new wall of cellulose. It extends into the space between the two equatorial plates, and continues to grow inward until the center is reached, when it forms the dividing wall of the two new cells. As it grows, the spindle, which had previously been swelling outward, begins to contract, until it becomes a narrow series of lines, reaching from the poles to the new cell-wall. Meanwhile the polar masses secrete new membranes, and assume the condition of nuclei of the new cells. So rapid is the process that the nucleolo-nucleus often again divides ere the nucleus has completed its division, and the nucleoli again divide ere the new cells are formed. Hence the new nucleus often has two nucleoli. After complete division the lines of the nuclear spindle are still apparent. They may, by splitting, give rise to the fibers of the new cells.

Such is a recent description of the process of division in the plant cell. In the cells of some plants, however, there is a preliminary step of change which does not appear in Spirogyra. In these cases division begins with a massing of the nuclear contents in the equatorial region. The nucleus has a spindle-shape, with a dark mass in its center, and clear areas reaching to its poles. This mass splits, and its two halves retreat to the poles. The further steps of division are as above.

Thus, so far as now appears, the process of cell-division in the plant is closely analogous to, though not identical with, that of the animal. It seems, indeed, a more primitive stage of the phenomenon. The division of the nucleolus, so marked in the plant, has not been observed in the animal, and may be, in the latter, suppressed or hastened, like many of the developmental changes in the higher animals. On the other hand, the peculiar movements of the chromatin-fibrils of the animal cell have no direct counterpart in the plant. They seem to present a distinct step forward in cellular evolution, and yield the idea that the animal cell is a more advanced organism than that of the vegetable. It certainly seems to hasten or suppress embryo changes which are well marked in the latter, and to clearly display advanced stages of development which are only vaguely outlined in the latter.

There is another cell-theory extant to which some allusion must be made, as it indicates a final stage in cell-evolution in advance of that here indicated. It is known that in many cases elongations of the fibrous network extend outward from the cell. These have been seen in epithelial cells, joined so as to form a connecting link between two cells. It is well known also that numerous delicate fibrils extend beyond the walls of nerve-ganglion cells, probably as outer continuations of the internal network. It is supposed that these fibrils aggregate into bundles, and that thus the nerve-fibers, which run to all parts of the surface of the body, originate. This seems to be the cell connection of the sensory nerves, while the motor nerves leave the cells each as a single fiber. The nerve terminations of muscles present a