A cell consists of a mass of protoplasm, generally enclosed in a cell membrane, and containing a nucleus and nucleolus. The nucleus represents the most vital part of the cell protoplasm, and has a more granular appearance than the latter. The nucleolus is a minute solid spot in a nucleus, appearing to be more highly refractive. +Cell Division.+ When the cell is quiescent, the protoplasm appears evenly granular, but when it is stirred to active life, slender twining threads can be traced in the nucleus, perhaps consisting of one long thread twisted upon itself. On account of their readiness to take up dyes used in staining, these threads are called _chromatine threads_. When the cells are about to divide, the chromatine threads are seen to arrange themselves in a line across the center, called the _equator_ of the nucleus, forming a rosette or star shape, known as the _mother star_. Some large granules then appear in the nucleus at points on either side of this line, which are known as the _poles_ of the nucleus. The loops of the thread are directed towards the poles. Gradually these threads become arranged in radiating lines, converging at the poles, and then break away from their former connections with the equator, forming a _daughter star_ at each pole, a clear space appearing at the equator. A constriction next appears in the now clear equator, and the nucleus divides into two distinct nuclei. Simultaneously with this division, or immediately following it, the protoplasm of the cell body divides in the same place, and thus two complete cells are produced. The chromatine threads lose their rosette arrangement, and gradually become imperceptible as the new cell returns to the quiescent state. This process of cell division is known as _karyokinesis_ or _aryomitosis_. In simple cells like the leucocytes, reproduction may take place by simple fission, thus: a constriction appears in the nucleus and in the body of the cell in the same line, and the two divide without any visible protoplasmic changes. Such a simple mode of division does not occur in the more highly specialized cells of various tissues. If the karyokinetic action be not very vigorous, the nucleus may divide, but the cell body remains intact, producing the cell with two or more nuclei so commonly observed. Every cell reproduces its kind, spindle cells producing connective tissue; epithelial cells epithelium; and bone cells producing bone.