. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. CELL NUCLEUS. 29 CELLS AND CELL TISSUES. The constituent parts of which an organ is made up are known as tissues. They possess a definite structure, visible with the help of a microscope, and have either the form of cells or of structures derived from cells. Tissues have a function corresponding to their special structure, and this function determines the whole function of the organ. They may, therefore, be regarded as organs of a lower order. The ultimate unit, the organ of the lowest orde
. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. CELL NUCLEUS. 29 CELLS AND CELL TISSUES. The constituent parts of which an organ is made up are known as tissues. They possess a definite structure, visible with the help of a microscope, and have either the form of cells or of structures derived from cells. Tissues have a function corresponding to their special structure, and this function determines the whole function of the organ. They may, therefore, be regarded as organs of a lower order. The ultimate unit, the organ of the lowest order, or ele- mentary organ,* from which all tissues are derived, is the cell. The essential part of a cell is not, as we have already seen, the membrane or the nucleus, but the protoplasm, with its special molecular arrangement, in which reside the functions of independent movement, of metabolism and of reproduction (fig. 1). The nucleus of a cell is either a solid mass of protoplasm or a more fluid structure enclosed by a firm membrane, and may con- tain one or more solid bodies (nucleolus). Different as are the forms which the nucleus may take, it always contains a fluid sub- stance, the nuclear fluid, and a pro- toplasmic substance, the nuclear substance of a special importance for the functions of the nucleus (fig. 17). An important and very general property of protoplasm is its power of contractility. The living mass presents, in connection with metabolism, phenomena of move- ment. These movements are not merely confined to the currents of solid particles suspended in the viscous contents of the cell, but are shown also in the change of form of the whole cell. If the outer part of the protoplasm has condensed so as to give rise to a cell membrane, , if the cell has acquired a distinct wall, the changes in its form are very much restricted. In other cases the movement shows itself in a quick or slow change in the outer form. The cell in this case manifests * Th. Schwann,
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