CELL. In early Christ ian usage, ( 1) a chapel erected over a tomb: or (2) a mcmastie dwelling either of a single monk or of a com munity subordinate to an abbey. artl compelled to pay tithes to it. Its occupants were obliged to present themselves at the abbey at stated times.
CELL (in animals). The morphological and pltykilogical visible unit of the body of organ isms. It is essentially the sante in plants and animals, and is usually of microscopic size. The yolk. or yellow. of a lien's (2g is, however. one cell. The name cell is something of a mis nomer. for it implies a •all-inelosed chamber. Ifut though many cells. especially among plants, are provided with title]: walls, these may be en tirely absent. and, when present, are usually,. in at least, exceedingly delicate. Conse quently. we regard the cell-wall as of less im portance than the which constitute the active parts. The cell-content is known as protoplasm. The protoplasm consists Of two main parts—a centrally placed, usually more or less spherical or ellipsoidal body called the mieleits, and the rest of the •ot/plasm. called cytoplasm. Nucleus and •ytoplasni are each de pendent on the other, and both are necessary to cell life. The nucleus is easily distinguished by the circumstanee that it contains 11111C11 sub stance which is easily stained by various dyes. This stainable substance is known as chromatin. cytoplasm usually contains relatively large water-fill441 spaces, vaned vacuoles, and small particles of food-substanee and other substances that are easily 'stained'—i.e. upon the coloring matter of dyestutis when placed in them. In its general structure protoplasm con sists typically of water and the truly vital sub stance called plasma. The plasma exists in a great variety of forms. Frequently it occurs in films surrounding little vesicles of water, as the films of foam surround air or as the wax of the honeycomb surrounds the honey. Sometimes it seems to extend in tine threads through the water-spaees. Still, again. the water Fpaces may be almost entirely absent. The liv ing. 'active' protoplasm seems to Inc. constantly in motion, as be judged by the motion of the particles which blow in the transparent current.. Besides la 1.1.yyr such as drops of oil, pigment bodies. food-gramilcs. and excretory bodies may I.e seen in the plasma or lodged in the watcr-spaces between the plasma walls.
The cell-wall. or evlbmenthratte, is produced by the cell itself. usually transformation of its own substance. It is frequently, at least. to be as made up of living t t VI'. In many cases. when the eell-wall i. thick, it (.1111 be seen to be perforated to permit of the passage of films of protoplasm by which the adjacent cells are organically connected. In plants ere a great weight has to be borne and great rigidity given, the cell-wall often gain: a thick ness and constitutes the wood of the dead plant.
The nucleus is essential to the processes of assimilation and growth, and probably controls in these operations. While its general form is subspherical, it may labcome greatly elongated. lobed, branched, form a series like a string of beads. or appear a two bodies or icon as a mass of scattered throughout the cell. In the chromatin network lie the par ticles and certain larger bodies called nucleoli. There may be many nucleoli, or only one large one. The typical nucleolus is regarded as an excretion product which is eventually east out of the nucleus. The plasm films separating the linden: front the cytoplasm are usually very evident. and, taken together. constitute the so called nuclear membrane. It is of course a. liv ing membrane. Outside of the nuclear mem brane is often found a minute particle, the centrosome.
011 Dirision.—The method by which the mul ticellular body is produced from the unicellular egg was low" misunderstood. It was thought that cells crystallized out of a homogeneous matrix, or that new cells were formed inside of the preexisting cells. It is now known that there is one process of cell-multiplieation and one only—namely. division of a cell into two equal This division involves all parts of the cell—cytoplasm. nucleus. and cell trosome. The method of cell-division varies in different cases: two main types may, however, be distinguished—mitotic. division or 'karyo kinesis,' and amitotic division. The mitotie division seems to be the more usual type, so it may be first eonsidered. For purposes of descrip tion. four series of stages or phases may be recognized: (I) The prophases, or preparatory (2) the metaphase, or acme of the di vision process: (31 the anaphases. or aggrega tion of nuclear material at the centres: (41 the telophases. or those in which the cytoplasm di vides and the two new nuclei are established.