Supplementary Biological Information For Parents
( Originally Published 1923 )
The Origin of the Living Individual.—The body of every plant and of every animal consists of one or more masses of the living substance, called protoplasm, together with more or less non-living matter enclosed within or surrounding the protoplasm. The single mass of protoplasm, whether it is enclosed within a distinct wall or not, is called a cell.
Among the lowest plants and animals each individual consists of a single cell. Among the higher plants and animals each individual consists of many millions of such cells. But every animal, whether large or small, whether simple or complex, begins life as a single cell.
In many species of one-celled plants and animals the individual, on reaching full growth, divides into two equal parts, and each part starts life on its own account as an independent individual. Each of these, after a period of growth (unless destroyed in the meantime), also divides eventually into two "daughter-cells." Thus each individual originates by the breaking up of a "mother-cell," and each one ends its individual existence by dividing up into two daughter-cells. The life of the protoplasm in this way passes on from generation to generation in a continuous stream. Normally, the amount of such protoplasm doubles with each cell division and there is no death except destruction by enemies or untoward external conditions.
In plants and animals somewhat higher in the scale of life, the two cells formed by the division of the mother-cell remain attached to each other. The cells continue to grow and then to divide, the daughter-cells clinging closely together. Thus a mass consisting of many cells is soon built up. This is the process which takes place in all the higher plants and animals, including man. The individual starts out as a single cell ; this becomes two cells by the dividing of the protoplasm. Each cell again divides, so that there are four, then eight, sixteen, and so on. After a time, the rate of growth and division begins to vary in different parts of the mass. Presently the cells of certain regions become different in form and size from those of others; and various irregularities in growth appear. The parts overlap, get folded in peculiar ways, and at last the mass becomes a recognizable plant or animal. All living things, except the very simplest, go through this process of development, in the course of which a single, relatively simple mass of protoplasm be-comes changed into a complex body, made up of many different kinds of cells (muscle, skin, nerve, bone, etc.), grouped into many different kinds of organs (limbs, heart, liver, brain, etc.).
Now we know that it is not possible for any of the familiar animals to reproduce themselves by the division of the whole body into two, nor by the breaking off at random of a single cell. What is the origin of the single cell that can develop into a new individual? In mosses and ferns, in molds and mildews, mush-rooms and other fungi, as well as among many other orders of plants, and among certain low animals, special cells are produced in great numbers and discharged from the body as reproductive units. These cells are called spores, and are generally very small. Scattered by movements in the air or water, they become distributed and under favorable conditions they start to develop into new individuals.
In one-celled organisms we can readily recognize the continuity of life from generation to generation, since each individual is but a portion of its parent. And in plants and animals that reproduce by means of spores, we can see the relationship between one generation and the next, since the individual is but an elaboration of a single cell taken from the body of the parent. But the type of reproduction which we find in the highest animals and plants, the bisexual, is more involved, and raises many important practical problems.