UNITS OF LIFE Cells.—Protoplasm is necessarily a constituent of every living creature, but the organism is not entirely made up of it, and much of its bulk may be composed of substances about to be built up into protoplasm or of the accumulated dead products of its past activity, such as the woody tissue of a tree, the hard substance of bone, the skeleton of a coral. Only that part of an organism is truly alive which is protoplasmic, and the protoplasm normally exists in the form of "cells." The cell is a small, usually micro scopic, mass of protoplasm, consisting of a nucleus and surround ing cell-body. It is generally enclosed in a cell-wall formed of its own secretion. Plants and animals are made up of such cells either singly or in aggregates. Growth is due to the increase in size and multiplication of the cells. They do not grow indefinitely, but each, having reached a certain bulk, dividcs into two daughter cells. All the cells composing the body of an individual are thus derived by repeated binary fission from a single original cell. Cells may become differentiated and may acquire parts adapted for per forming various functions. Differentiation in multicellular forms is related to the progressive division of labour among the cells, comparable to the division of labour among the individuals of a community, and is accompanied by specialization for the per formance of special functions. The body of a multicellular plant or animal is thus an aggregation, not of separate units brought together, but of a multitude of related cells remaining in associa tion and building up its tissues. As a rule the tissue-cells remain in actual protoplasmic continuity by slender connecting bridges; but in animals, certain cells may become free, as, for instance, the white corpuscles of the blood. The activity of an organism is the sum of the activities of its component cells. It is probably to enable oxygen and food to reach all parts, and waste substances to escape, that the protoplasm is thus subdivided into small masses. Moreover, there is interchange of material between nu cleus and cell-body, and the relative size of the two must be kept within certain limits. The contents of the nucleus (nucleoplasm) and those of the cell-body (cytoplasm) differ considerably from each other.
Both body and nucleus are necessary for the life of a cell ; one cannot live without the other. If a cell is divided into halves, only that with the nucleus will continue to live, grow and re produce. Among true plants and animals such a cell is the smallest unit of life. But in the Bacteria, doubtfully classified as plants, there is no well-defined nucleus, the chromatin being scattered. Possibly they are the surviving representatives of a primitive stage before the typical cell structure was differentiated.
In the lowest plants, Protophyta, and the lowest animals, Pro tozoa (q.v.), the individual generally consists of a single cell, capable of performing all the functions of life and consequently often provided with highly differentiated cell-organs. Such a unicellular individual may assimilate, grow, reproduce, move and regenerate lost parts. But in the multicellular plants, Met aphyta, and multicellular animals, Metazoa, as differentiation advances, the various cells may become specialized to perform certain functions, and correspondingly unable to carry out others. More especially in animals do cells become so specialized (to form nutritive, nervous, muscular, excretory, skeletal and other tissues) that they can no longer continue to live except in asso ciation with each other, supplying each other's wants. They tend to lose the power of reproduction and usually are only capable of giving rise by division to cells like unto themselves.
A unicellular organism does not increase indefinitely in bulk; when it exceeds the size normal for the full-grown individual of the species, it tends to divide into two. The nucleus divides first, then the cell-body; the resulting two daughter-cells, each with its own nucleus, then separate and grow into adults similar to the original parent. It is the same with the cells of higher organisms. Each multicellular plant or animal starts as a single cell rich in unspecialized protoplasm, which grows and divides repeatedly, and at every division half of the nucleus passes into each daughter cell. But here, instead of the cells separating, they remain closely associated as parts of a single complex individual. Thus every cell is derived from a pre-existing cell, and every nucleus from a pre-existing nucleus.
Mitosis.—The behaviour of the nucleus and of the chromatin during cell-division is so important with regard to heredity and evolution that it must be described in further detail. It is the nucleus that initiates the division which takes place by a re markable process known as karyokinesis or mitosis (see CYTO LOGY). In the resting stage the chromatin is dispersed within the nuclear membrane in the form of granules or an irregular net work. At the start of mitosis the chromatin gathers into coiled threads which break up into definite pieces—the chromosomes. Each chromosome soon splits longitudinally into two equal halves. Meanwhile, from a small mass of denser protoplasm situated close to the nucleus and known as the "central body" or "division centre" (often containing a central granule or centro some), generally arise delicate radiating fibres (aster). By the division of this body are formed two "division centres" which move apart to the opposite poles of the nucleus, while a central spindle of fibres develops between them. The nuclear membrane having by this time disappeared, the chromosomes become arranged in a ring round the equator of the spindle. The two halves of each chromosome now separate and travel to the opposite poles of the spindle. There they again form coiled threads and break up into granules, the systems of fibres disappear, the nuclear membranes are restored, and thus two new nuclei are reconstituted similar to that of the parent-cell. Division of the cell-body then yields two nucleated daughter-cells. It is very important to notice the con tinuity of substance during this process. Cytoplasm, nucleoplasm, central bodies and chromatin are all parcelled out to the daughter cells. Above all, each daughter-nucleus receives the same number of chromosomes, and apparently exactly the same amount of chromatin.
Apart from some exceptional cases, and certain differences in detail, the process of normal cell-division and mitosis is essen tially the same in all plants and animals. The same number of chromosomes appear at every division, hence every nucleus in the body of a multicellular organism contains the same number (except the germ-cells as explained below). Moreover, the num ber of chromosomes may differ widely even among allied forms, hut is normally constant within each species.