The largest of these living bricks is the yolk of an ostrich egg; since this is, of course, like all eggs before they begin to grow, a single cell. The smallest known are certain of the bacteria and germs which float about in the air, and are so minute that they cannot be made out even with the strongest microscopes. All one can see is that there is something there; something which if placed a thousand in a row, would still not reach across a grain of dust.

Few cells, however, are as small as bacteria on the one hand, nor anything like as large as the yolks of birds’ eggs on the other. Many are just comfortably visible to the unaided eye. But the great mass of cells which make up our own bodies, the bodies of other animals, and of plants are a little too small to be made out with a common pocket lens, tho an ordinary microscope shows them with ease.

While the egg yolk is dividing to form the first hundred or more living bricks out of which the little animal is to be built, the cells are all about alike, generally round except where they are flattened against one another. As soon, however, as they begin to move about into place to build the new animal, they begin themselves to change. Some remain small; others grow large. Some grow out into long strings, and become muscle fibers or nerve. At one point, many thousands together swell up with oil and become fat. At another, more thousands build themselves about with hard lime phosphate, and become bones and teeth. Those which form within them little brown granules, give the color to hair and skin. The blood is colored red by the coin-shaped cells which float in it. In certain parts of the eye, on the other hand, the cells have to remain perfectly clear and colorless, else the light could not come thru and we should never see truly.

When an animal is very young indeed, long before it is ready to leave the egg, the whole outer surface of its body is covered with a single layer of these cells. They are packed closely together, and flattened against their neighbors so that the sheet of cells is not unlike, on a small scale, the marble floor of a public building or the block pavement of a city street. Like other living cells, these grow, and divide. They cannot grow sidewise, for the space is already filled; nor inward for that way lies the entire body. So they split off a piece of their outer ends. Then they do it again, and yet again; until the outer skin of the body, from being one layer of cells in thickness has become many.

Only the original inner layer, however, grows and divides. The split off ends dry up to a roundish cracker shape, grow hard and homy, and become the thin outer skin of the body, which we run pins and needles under, and pull off or scrape off when we “bark” our shins, without hurting. This part of the skin is dead. It gets rubbed off by our clothes, or soaks off in the bath tub and has to be scrubbed off the sides. But as fast as it is removed on the outer surface, it grows again from the living bottom layer. No matter how old one gets, this lower layer of the skin continues to split off the outer ends of its cells, just as it did before there was any proper skin at all. Most parts of the body grow thruout their mass; but the skin grows only on the inner side.

On the palms of the hands and the soles of the feet the skin grows very rapidly and is especially horny. When one works with his hands more than he is accustomed, the first effect is to wear the skin thin and sore, or to pull it loose from the bottom layer and make blisters. In the end, however, the rubbing only makes the live skin work faster, until it builds great homy callouses that no work can wear thru. But when our boots do not fit and rub in one spot, this also starts up the live skin to working hard. First thing we know, we have a corn. For a corn is only an especially hard and thick callous, where the living skin made a mistake and grew too much in one little spot.

Each finger nail and toe nail is a sort of corn. It grows from a fold of skin, forming from the bottom layer like any skin, but it is especially homy, even more horny than the hardest callous. The hair, also, is a sort of corn. The skin doubles in to form a minute pocket; and at the bottom of this pocket this same living under layer of the skin grows into a narrow shaft of cells, dry and dead and homy like skin and nails.

The horns of animals, too, are only thick hard skin. Sometimes they have a core of bone inside, but the outside is just a special sort of skin. Wherever we go in the body, there we find some special sort of cell. They may be large, small, thick, thin, long, round, soft, hard. They may build this, that, or the other thing around them. They may have this, that, or the other thing inside. But in one way or another the whole body, from head to heels, is built of these cells and their products.

It is the same way with the plants. They too are built of these living bricks. Each leaf and blade of tree or grass is covered with a sheet of colorless cells one layer deep, which one can often peel off from the green pulp underneath. The green pulp, in turn, is a rather loose pile a half dozen thick, of roundish brick-shaped cells, each containing scattered grains of green coloring matter. The solid wood of a tree is only the thick walls of long slender cells, overlapping at the ends and packed tightly together. These cells lie lengthwise of the tree; that is why wood splits with the grain so much easier than it cuts across it.

I have already said that at the time of year when the tree is growing rapidly, these woody cells are large; but when the tree is growing slowly, they are small. So each year there is a change from large cells formed in the spring to smaller ones grown in the fall. The next year, the living substance of the cell moves off to the growing region next the bark, and leaves the old wood cells empty. These, therefore, never change; and because the large cells and the small ones do not look quite alike, we see the annual rings of wood in the tree trunk, as thick as card board, which give us the light and dark lines in our furniture and our hard wood floors. From these one can tell, not only how old the tree is, but also what were its good years when it grew rapidly, and what its poor seasons when it hardly grew at all. If a drought came along any summer, or if insects one year ate off all the leaves, that too shows in the wood. But trees which grow in the tropics, where they keep growing the whole year thru, do not have annual rings.

While some cells of the tree form wood and some green pigment, others in the bark produce cork, as one can see nicely in the thin layers of cork in the bark of an elm. The cells of juicy fruits swell up with water, and form sugar and various flavoring matters and pleasant acids. Where the animal cells swell up with oil and become fat, the plant cells swell up with starch grains and become a potato or the thick seed-leaves of a bean. But other cells form gum, rosin, turpentine, pitch, and the various oils and the like, pleasant or bitter, which we use for food and medicines.

So the plant, like the animal, is just a great mass of different sorts of these living bricks, and of the various substances which they form within and around them.

Naturally it takes millions upon millions of these living bricks to build up the body of a man or an apple tree, still more of a whale or one of the giant redwood trees of California. Many humbler creatures, on the other hand, both animals and plants, contain comparatively few. Our common green pond scums, for example, which tho they are plants, have neither leaves nor stems nor roots, are like single long lines of tiny green barrels set end to end. Our common sea-lettuce is a sheet of cells only one layer thick; while other sea-weeds and water plants are but bundles of a score or more. Often the fewer such bricks there are, the larger they are; even at times, to a half-inch in thickness and an inch or more in length.

A vast number of plants and animals, moreover, are single cells. Such among plants are the yeasts with which most of us make our bread, and a few of us brew our beer. Such also are the hundreds of different sorts of bacteria, which tho some of them are the germs of various catching diseases, are for the most part useful enough. But of these we shall learn more by and by. The green spots and patches on the bark of old trees and fences, and sometimes even on damp earth, are due to enormous numbers of minute plants, green with the same green pigment as the leaves of the largest tree; while the green tint of the gray lichens on rocks and tree trunks is caused by similar single-celled plants which grow among the white fibers of the lichen proper. Besides these, there are many like plants which float about in fresh water, each a single cell.

The diatoms which one finds in the mud at the bottom of ditches and mud-puddles, tho they have shells and move about, are usually counted among plants; but the water of most ditches and puddles swarms with amoebas, infusoria, animalcules of various sorts, most of them large enough to be made out with the unaided eye when seen in a tumbler against the light, and each a single cell.

Many animals, then, and many plants are just one single cell and no more. Many others, like pond scums and sea-lettuces,—which are plants,—and sponges and jelly-fishes,—which are animals,—are composed of many cells, but all pretty much alike. But the animals and plants which we know best, kittens and oak trees and horses and grass, and the creatures we know best of all which are ourselves, are made up of many cells, and many different sorts—skin and bark and wood, flesh and fat and leaves and hair and all the rest, so many that it would take half an hour merely to write them all down.

These “cells” then, are the living part of every plant and animal. Each of them became by the splitting in halves of an older cell; each of these in turn by the splitting in halves of a still older cell, until we get back to the egg which is the great-great-great-grandfather of them all. But the egg itself arose by the splitting of still another cell, which, of course, was part of the parent’s body. This came from yet another, and so on back to the beginning of life on this earth, tho nobody knows how long ago that was.

So the living flesh of us has always been alive. Most of it will die; but some of it will live on in our children and our children’s children, until the and of the world.