Cell, a microscopic anatomical form, very abundant in most vegetable and many animal tissues. It has received its name from the fact that in its simplest form it consists of a closed membranous sac or utricle, more or less globular in shape, enclosing a cavity which is filled with a fluid or semi-fluid material. This is the variety of cell which is found in many loose and succulent vegetable tissues. The cell wall usually has upon some part of its inner surface a well defined rounded or oval spot, termed the nucleus; and the nucleus exhibits also a smaller rounded spot of a darker color than the rest, termed the nucleolus. In tissues where the component cells are very abundant and closely set, they are often polyhedral in form, being flattened against each other by mutual compression. Sometimes they are elongated and tubular or prismatic. The elongation of the vegetable cell is sometimes so great, as in certain of the alga) or cryptogamic water plants, that they form, by being connected with each T)ther end to end, long and slender filaments. This also occurs in some of the higher vegetable structures. Other cells send out radiating processes or prolongations, so as to present a stellated figure.
The internal cavity is sometimes partly occupied by a solid deposit upon the inner surface of the cell wall; sometimes it contains a gelatinous liquid, granules of chlo-rophyl or the green coloring matter, starch grains, and sometimes perfectly formed crystals. There are three great groups of the cryptogamic vegetables, namely, algse, lichens, and fungi, which consist exclusively of cells; in the higher vegetable forms we find also fibres, air tubes, and vessels. The simpler vegetable cells often multiply very rapidly by a process of budding, or by spontaneous division, or by both combined. They have the power of absorbing nutritious material from the exterior, and converting it into the substance of their own material. - The cells of animal tissues are not precisely similar to those of vegetables. They are usually much smaller, and surrounded by a larger proportion of intercellular substance. They do not, as a rule, present a well marked cell wall enclosing a distinct cavity, but consist rather of a mass of soft animal matter, the consistency of which is nearly or quite the same throughout. Very frequently a nucleus and nucleolus, similar to those seen in vegetable cells, are imbedded in its substance.
Thus the red globules of the blood in birds, reptiles, and fish contain a well marked nucleus, though it is absent in those of the mammalia. The nucleus and nucleolus are both very distinct in the cells of the gray nervous matter, and the various kinds of epithelial cells have always a nucleus and usually also a nucleolus. Animal cells also vary in form. The red globule of human blood is flattened and circular; in birds, reptiles, and fishes, it is flattened and oval. The epithelial cell covering the surface of mucous membranes is thin, membranous, and pentagonal or hexagonal in the mouth, fauces, and oesophagus; columnar or prismatic in the rest of the alimentary canal; furnished with vibratile cilia in the air passages, the oviducts, the Eustachian tube, and the ventricles of the brain. There are also hexagonal pigment cells in the choroid coat of the eye, and stellated pigment cells in the web of the frog's foot. The glandular cells of the liver always contain one or more minute oil drops imbedded in their substance.
It has been a favorite theory with many microscopists that all the anatomical elements of the animal tissues are directly produced by the development or transformation of simple cells; but this view has never been universally adopted, and the evidence in its favor does not become more convincing with the progress of microscopic discovery.