Page:Popular Science Monthly Volume 26.djvu/832

812 simplified that any skilled observer, with a good instrument and proper care, may hope to successfully employ them.

The matter of which an organic cell is composed is found to be not simply a homogeneous, or slightly granular, mass of protoplasm. On the contrary, it appears to be traversed in every direction by delicate fibers, which form an intricate network or reticulum. The interstices of this network are occupied by a fluid or semi-fluid substance of homogeneous appearance, though occasionally containing a few small granules. The reticulum occurs not only in the outer cell, but also within the nucleus, and its fibers extend to and are apparently connected with the nucleolus. Within this latter the fibrous formation has not been traced. Some observers, indeed, declare that there is no nucleolus, but that it is simply a node of the intersecting fibers. But this view is not generally entertained, and late writers ascribe to the nucleolus an important function.

In the growth and division of the cell the nucleus appears to be specially active, and the new doctrine known as karyokinesis relates principally to the peculiar metamorphoses of the nucleus during cell-division. Two phases of cell-life are now well marked. One of these is an active stage, during which transformation of the cell-contents rapidly takes place, and division follows. This is succeeded by a resting-stage, in which all activity of the nucleus ceases, the fibers grow less distinct, and a partly homogeneous condition results. This resting stage is, after a period, followed by a new period of activity.

The behavior of the cell-contents, when treated with carmine or other staining reagents, indicates that they are composed of at least two distinct substances. During the resting-stage this does not appear, for the whole cell takes the stain, though it deepens in the nucleus, and still more in the nucleolus. But during the active stage only the fibers take the stain, while the intermediate ground or basis substance remains clear and transparent. From this difference in behavior the name chromatin is proposed for the fibers, achromatin for the ground substance.

Flemming, one of the most skillful observers of these phenomena, distinguishes two forms of division—the direct and the indirect. The former—which may eventually prove to have no real existence—is a direct separation, first of the nucleolus, then of the nucleus, and finally of the cell. In the latter there is a peculiar metamorphosis of the nucleus. Flemming, from observation of the cells of Salamandra, describes the process as follows:

The resting-nucleus possesses a faintly-defined reticulum of fibrils, whose meshes hold a homogeneous ground substance, one or more nucleoli, and occasionally a few small granules. Possibly these latter are merely the nodes of the reticulum. When the active stage commences, the membrane of the nucleus disappears, as also the nucleolus and the granules. If the latter are merely nodes of the fibrillar network, we