So we are not built like a cement or a wooden house, but like a brick one. We are made of little living bricks. When we grow it is because these living bricks divide into half bricks, and then grow into whole ones again. But how they find out when and where to grow fast, and when and where to grow slowly, and when and where not to grow at all, is precisely what nobody has yet made the smallest beginning at finding out.
VI
More About Living Bricks
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.