Page:Popular Science Monthly Volume 26.djvu/836

816 more deeply. The density of the chromatin appeared to increase inwardly.

From the outer cell-wall fibers pass inward to the nuclear membrane, which they probably penetrate, and become continuous with the fibers of the nucleus. These, in their turn, seem to connect with what appears to be a membrane of the nucleolus. Inside the latter there is no evidence of fibrous structure.

In cell-division the first stage is the aggregation, on opposite sides of the nucleus, in a line with the long axis of the cell, of a quantity of pale, slightly granular protoplasm. This is perhaps derived from the peripheral layer, and travels inward along the fibers, since minute thickenings, of similar appearance, show themselves upon these fibers. The nucleolus swells out into two protuberances, in the same axial line, joined by a bridge of denser matter. This change is, perhaps, connected with the division of the nucleolo-nucleus, since subsequently two of the latter are visible, while the nucleolus returns to its former state. At the same time it is found to have considerably increased in size. The next visible change occurs in the nucleus, whose contents aggregate at the nuclear poles, push through the membrane, and combine with the outer aggregation of protoplasm to form two dark amœboid lumps. From these polar masses fibers run inward and outward, though the external fibers have become loose and flaccid.

In the next stage the nuclear membrane disappears. The spindle of fibers which runs inward to the nucleolus is bordered by two darker strands, possibly the remnants of the membrane. This composes the nuclear barrel. The nucleolus divides by a dumb-bell-shaped constriction, similar to what appears in the division of amœbæ. It resembles what Flemming calls "direct division." The two halves of the divided nucleolus—each containing one of the nucleolo-nuclei—now move outward toward the poles, a new line of fibers forming between them as they separate. These bodies almost seem to have a repulsive energy, for the polar masses recede before them. The connecting lines also spread outward centrally, so that the nuclear barrel becomes considerably elongated and widened. It resembles a barrel with thick, narrow ends and widely swelled-out middle. Eventually the nucleoli reach the polar masses, into which they penetrate, while the substance of the latter spreads inward so as to inclose them. The rudiments of new nuclei are thus formed, between which extend rows of fine fibrillar lines, much separated centrally. The "nuclear barrel," with the nucleolus in its center, has thus been succeeded by the "nuclear spindle," with no central mass.

There now appears a row of dots, stretching across the equator of the spindle. This quickly separates into a double row. At the same time the lines of the spindle are sundered centrally, and the dots seem to be minute disks at their extremities. In this manner a double "equatorial plate" is formed, inclosing the circular equatorial plane of