Part 15

We, too, grow and change like frog and butterfly. When we were, say, four or five years old, we had twenty strong little white teeth, each set firmly in the jaw bone on a stout root. By and by, these teeth, one at a time, in pretty much the order in which they first appeared, began to get loose. First we could wiggle the tooth a little; then a good deal. By and by, it hung only by the thin skin of the mouth, so that one good yank, or perhaps only the push of the tongue against it, tore it out. Or possibly we bit into an apple, and left the tooth behind.

Anyhow, the tooth hadn’t any root. The hard bone had gone, and with it the soft nerve and blood vessel that used to be inside. There was only the hollow cup that once was the crown of the tooth. The rest had been eaten up, bone, nerve and all, by our blood corpuscles. We needed larger teeth. Teeth, as I told you, once formed, cannot grow any more. So the blood corpuscles chewed up the tooth, as much as they could get at, and used the old stuff to build the new one. No doubt they would consume the entire tooth, crown and all, and not leave a shred behind, if only there were some way of holding the last bit in place while they finished it off. That on the whole would be pleasanter than having to tie a string round the tooth and yank it out.

They do act that way with growing bones. The bones, as I have already explained, grow much as a tree does, by adding on new stuff to the outside. Meantime, since all bones are hollow, the hollow also has to grow. The hollow grows by the taking down of the bone on the inside, just as a tree might decay out at the heart, about as fast as it grew under the bark. This, also, is the work of the white blood corpuscles.

Then, too, the soft gristle of little children changes into the solid bone of grown men. But the gristle never turns to bone. As before, the gristle is taken out, eaten into, devoured, by these living cells of the blood and lymph; and as fast as the gristle disappears, the bone grows into its room.

Or sometimes it happens that two little boys find themselves not in entire harmony, with the result that one little boy goes home with one eye shut up; and the next day, and all the next week, that particular eye is a most remarkable study in black and blue. That is because, during the lack of harmony, something happened to hit one little boy in the eye, and smashed up some of the little blood vessels around it, so that the blood leaked out into the flesh.

Then the white blood corpuscles, which are themselves always leaking out of the lymph spaces and blood vessels, and wandering round thru the tissues, have to set to work to clean up that spilled blood. So they actually eat up the flecks of blood; until by and by, the flesh is all clean and white again, and the black eye is black and blue no more. In general, whenever we meet with an injury or a wound of any sort, these white blood corpuscles take a hand in the healing process, eating away the damaged tissue, and allowing chance for fresh growth.

The white blood corpuscles, then, live to eat. Like certain other creatures who are always hungry, they sometimes eat things that they had better have left alone. Sometimes, people’s hair turns white suddenly—corpuscles have actually eaten up the color. Some people even go so far as to say that the reason why we grow old (since, as I have already told you, growing old is mostly growing hard) is because these white blood corpuscles have gone crazy and eaten up the softer parts of us, and left us only the hard ones to grow old with. I don’t know whether this last is so or not. But at any rate, these white blood corpuscles, which let go their hold on the places where they grow, and go wandering off all over the body eating everything in sight, would be likely to make this sort of trouble if anything would; and the man who especially says it is so, probably knows more about these matters than anybody in the world.

XLV

Nature’s Repair Shop

Of course, we get hurt in all sorts of ways—cuts, bruises, barked shins, black eyes, once in a while a bad sprain or a broken bone. Then the white corpuscles that are in our blood, and the growing life-stuff or protoplasm which is in our flesh and bones, and all this wonderful and mysterious life that is in us, take us in hand to mend us up. The same power that made us, and that keeps us alive, heals also our hurts. By and by, if we take care of ourselves, we are once more as good as new.

The small and weak and lowly creatures, which cannot take care of themselves as we can, and so are all the time getting into all sorts of trouble, are able to repair damages even better than we. The world is full of timid animals, which have neither teeth and claws to fight with, nor armor for their defence, nor speed of foot or cunning of brain with which to escape their enemies. But to make up in part for this lack, many of these simple beings seem not to mind at all such injuries as would cripple us for life; while they recover completely, and that within a few days, from accidents which would mean instant death to us.

A tiny lizard, for example, may at any moment have to scamper for his life in search of an equally tiny crack in the rock, where it may take refuge from some larger animal which wants it for breakfast. Naturally, oftentimes, the lizard so hardly escapes being “it,” that just as he whisks into safety, his pursuer snaps off his entire tail.

A loss like this would kill most larger animals, but not the lizard. He simply waits round for a week or two while a new tail grows, just as good as the old one, so that he is as well off as before. The same lizard has been known to lose his tail a dozen or twenty times, and each time to grow a new one. Since, therefore, a lizard’s tail is longer than his body, and nearly as large round, the animal must have grown enough new flesh to make at least five or six whole new lizards. Curiously however, the new tails, tho they look exactly like the old one, always have a rod of gristle or cartilage inside, in place of the regular backbone.

So too, with legs. A lizard that has had a leg bitten off, straightway grows a new one. He will even grow a whole new eye, when something happens to the one with which he happened to be born.

Yet oddly enough, the lizard, when he grows new legs and tails is continually liable to make the strangest mistakes. He will grow a new tail, when he hasn’t lost the old one, but only had a bite taken out of it. Then he has two tails. Sometimes he makes even a worse break than this, and grows out two new tails to replace the single one which after all he didn’t quite lose. Then he has three tails, which is at least one more than any proper lizard ought to possess.

It is the same, too, with legs. The lizard seems to get an idea that his leg has been bitten off, when it has only had a piece taken out of it. So he goes ahead to grow a new leg, and as a result, has five. In fact, it is quite possible, to manufacture a lizard, perfectly healthy and apparently happy, with eight legs and three or four tails.

Perhaps you have heard that “the early bird catches the worm.” If so, did you ever consider the transaction from the side of the worm? If the worm happens to be retiring into his hole just as the hungry robin catches sight of him, there is likely to be a tug of war between the eater and the breakfast. The worm swells out the front end of his body, and gets a grip on the sides of its hole, while the bird digs its claws into the ground. Sometimes the worm lets go, and gets eaten up. Sometimes he gets pulled in halves, and only the rear end goes down the robin’s red lane.

The worm minds being pulled in halves just about as much as a train of cars minds getting uncoupled. The front end calmly crawls away into its hole, goes on eating dirt as usual, and pretty soon grows itself a new tail as before.

But what becomes of the severed tail end, if by any chance the robin fails to eat it up? That perhaps is the queerest thing of all. If the front part that crawled away is short, say a third of the animal or less, so that two-thirds of the worm at least is left in the tail piece, then this severed tail grows a new head. So the one worm becomes two. But if the tail piece is shorter, no more say than a half the whole animal, then that tail, instead of growing a head, grows another tail. In that case, the final result is one entire worm, old head end and new tail; and two tails, one old, one new, growing end to end. But of course, two tail ends, and nothing else cannot very well make a living, and, the worm soon dies of starvation.

There is, however, a smaller creature, the fresh water hydra, which quite outdoes the earthworm, when it comes to growing on new parts. The hydra is a very small sea-anemone, about as long as a pin is thick, with a long slender stalk and a circle of long fingers or tentacles round their mouths. One finds them growing on sticks and water plants almost anywhere in ponds.

Cut off one of these fingers, and it grows again. Cut the entire animal in halves as one chops thru the trunk of a growing tree, and the root end grows a new head, and the head end grows a new; root, and there are two new hydras in place of the one old one. Cut the animal into three pieces, head end, root end, and a small bit out of the middle of the stalk. The head end forms an entire new creature. The root end forms an entire new creature. The middle piece, which is neither head end nor root end, but just a little drum-shaped snipped out of the stalk between the two, even that grows a new root end and a new head end, and becomes an entire new creature, which in time grows to be as large as the one out of which it was made.

It is, in short, as if when one ripped off the sleeve of a coat, the coat grew a new sleeve; or when one pulled off the tail of the coat, the coat grew a new tail; while the severed sleeve and tail each grew new coats—pockets, buttons, collars, tails, sleeves, and all.

Our common star-fish, that we find in the salt water pools at the sea shore after the tide goes out, also isn’t at all bad at mending itself up after an accident. The five rays which grow out from its center are continually getting broken off. So one finds often in the water, star-fish with only four rays, or with four large ones, and one smaller one just growing out to take the place of one which has been lost.

But the lost ray, if something doesn’t eat it up, will grow a new star-fish. From the broken end, four more little arms bud out like those of a tiny star just getting its start in the world. The single arm or ray, not only grows out the other five, but in addition forms a new center for them to grow to, a new mouth, a new stomach, a new nerve ring round the mouth which is the creature’s brain; and so one thing with another forms a whole new star-fish with all its parts complete. And all the while, of course, the severed arm cannot eat anything, because it hasn’t any stomach. So it has to make the new parts out of the old, as the tadpole builds its new legs out of its old tail.

Still, I don’t know why any of these things are any more remarkable than that the lizard and hydra and earthworm and star-fish should have formed in the egg in the first place. That after all, is about the most remarkable thing there is in this world.

Not a few animals, moreover, in addition to having this power of mending themselves up after they have been injured, have also an arrangement for getting themselves hurt in a convenient place. The ray of a star-fish, for example, almost never breaks in halves. Whenever the ray gets caught so that it has to come off, it breaks close to the center, so that the whole arm comes off at once.

The crabs, such as one finds at the sea shore in salt water, have a special place in each leg, close up to the body, where the leg is meant to break off. The hard outer shell is turned back into the flesh and makes a round plate of shell with a hole in the middle, cutting right across the leg. Thru the hole run the nerves and blood vessels, but the muscles come to the plate on each side and there stop.

So when a hungry fish “catches a crab” by one leg, the crab digs his claws into sand or sea weed and pulls. The fish backs water with his fins and he pulls. Off comes the leg. Away goes the crab in safety. The leg has pulled off at this plate of shell. No muscle has been torn. The end of the stump is nicely protected; and all the injury that has been done is to the small thread of tissue no larger than a pin, which ran thru the hole in the center of the plate.

More than this, the stump is all ready to grow out again; and actually grows out quicker, after the leg has been taken off at this “breaking joint,” than if only a small piece had been taken off lower down. Such breaking joints, where the wound is all healed over before it is made, are somewhat common among animals, especially among those which have a jointed shell on their outside.

Even here, however, there are some queer doings. Sometimes, after a five jointed leg has been pulled off, the one which grows in its place, tho just as large and just as long, has only four joints. Some crabs, and the like, which have the big claws unlike on the two sides, if they lose either one, instead of growing one like it in its place, make a new claw like the one that belongs on the other side. Also, if they lose an eye, they grow a new one; but if they lose both the eye and the eye stalk on which the eye is set, then instead of an eye, they grow a small feeler. Really, one does not know which most to marvel at—their strange power of growing new parts, or their crazy way of growing them wrong.

But of all living repair shops the most uncanny are certain little flat worms, called planarians. These are a half inch or less in length, and fairly common in some places at the sea side, tho they are likely to be mistaken for snails which they much resemble.

Cut off the head of one of these creatures, and inside a week he will grow another—eyes, brain, mouth, and all as good as before. Cut off both head and tail, and still the creature will straightway fit himself out with new ones. Split him fairly in two lengthwise of the body, and each half will grow out a new half; and there will be two planarians, each complete, where only one was before.

Or if one splits the body in halves from the neck down, then each half body grows a new half, and there is a single head with two bodies. Or on the other hand, if one splits the head in two and leaves the body, each half head will become a whole one. Then we shall get a double headed planarian, a sort of Y-shaped creature.

Sometimes, after the planarian has been split in two from the neck backwards, and formed a Y upside down with two bodies and one head, another head, or occasionally two new heads, eyes, brain and all complete, grow out facing backwards in the fork of the Y. Indeed the animal seems to have a sort of mania for growing new heads and tails in all sorts of unbecoming places. If he gets a wound or cut almost anywhere in the body, and the wound happens to open on the whole backward, out of that wound or cut will grow a new tail in addition to the one he already has. But out of such wounds as face forward, there grow out complete new heads. As many as seven different heads have thus been made to grow on a single planarian—some on the middle of the back, some underneath the body, some even on the sides of the tail, and all, no doubt, greatly to the embarrassment of their owner.

As for cut off heads and tails, one might expect them to grow new bodies and become whole animals again. They do not, however, unless the pieces are pretty large. A head, cut off close behind the eyes, grows out, not a body, but another head. So all there is to the creature is two heads, joined neck to neck and looking in opposite directions.

All the time these things are going on, while the worms are making new heads, or bodies, or the two halves of one body are filling out again, the creatures can get nothing to eat. How then do they manage to grow? They live on their own flesh. The old fragment is taken down and used to build the new. An entire worm, made from a half worm, is no larger than the half from which it came. The animal, instead of growing large, grows small.

Whether it also grows young again, is by no means clear. Apparently it does. At least, a planarian that does not get enough to eat, proceeds at once to ungrow, becoming gradually smaller and smaller, till after a few months, it is no more than a fifteenth part of its former size. Whether it really becomes a baby worm again or not, it certainly looks like one, for it is a pretty small human baby that is not more than one fifteenth as large as a grown man.

XLVI

Little Monsters

One usually thinks of monsters as large. They are always, I believe, large and horrible in the fairy stories—giants and ogres and dragons and winged horses and chimeras and three-headed dogs and I don’t know what else, all most extraordinary to imagine as well as nice and creepy to read about. Really, however, there is no reason whatever why a monster should be large. It must be horrid, or unusual, or misshapen, or quite out of the ordinary. Then it is a true and proper monster, no matter how small it is. And as a matter of fact, some monsters, as strange as any maker of fairy stories ever invented, are too small to be seen at all, unless one looks for them with a microscope.

The planarian worms that I have just been telling about are monsters. If a two-headed calf is a monster, that people who go to the circus will pay to see, then surely a planarian is a still greater monster, with one extra head in the small of its back, another on the side of its tail, and four or five more hanging on at various places anywhere over its body; and this to say nothing of as many superfluous tails stuck on anywhere between. You know already that these monstrous planarians are formed because the worm, instead of healing up a cut as he should, seems possessed to grow a new head or tail out of it. I am going to tell you now how certain other strange monsters come to be.

You remember that earlier in this book, almost in fact, at the very first, I told you about how the eggs of all animals are, to begin with, single cells; and how afterwards, when they begin to grow, they split, first into two cells, then into four, then into eight, and so on, until finally, the single cell of the egg has become the hundreds and thousands which build the young animal.

Some of these eggs, especially those of certain of the small sea creatures such as star-fish and sea-urchins and the like, are extraordinarily tough. Indeed they have to be, else few of them would ever live to grow up at all. It is quite possible to take these eggs when the single cell has divided into two, and shake these two apart into two separate half eggs. You might think that each half egg would form a half animal. Instead, much like the planarian cut in halves, it forms a whole animal, half size.

Or we can wait till there are four cells, and shake these apart. Then we get four complete creatures, each quarter size. Or we can in the same way make eight. Beyond that we cannot go. Eight animals out of what was intended for one is all that egg can manage.

On the other hand, it is possible to take a half dozen eggs, which ought properly to have made as many separate creatures, and make them stick together into one gigantic egg, from which will hatch out a single gigantic animal, as large as the six or eight together which ought to have come from six or eight eggs.

Other strange things happen when the eggs, instead of growing in ordinary sea water, are put into water which has just a little bit more or less of something in it. For common sea water contains some half dozen different kinds of salt besides the one kind that people get out and use on the table and for cooking; and more or less of any one of these makes a salt water which is not quite the same as the salt water of the ocean. In fact, you must have noticed that salt water made with ordinary table salt does not taste by any means the same as ocean water.

A different kind of salt water (not very different either, I doubt if anybody could tell them apart by the taste,) sometimes makes a lot of difference with the creatures that grow in it. A little more of one thing, for example, makes little star-fish with their stomachs hanging out of their mouths, instead of inside their bodies where all proper stomachs belong. A little more of another, makes the sea minnows which live in it have only one eye, and that right in the middle of their foreheads like the single eye of the giant cyclops that I trust you have all read about long ago in the story of Ulysses. If you haven’t you certainly had better right away, for it is one of the great stories of the world, and has been told to children, and to grown men and women as well, for at least three thousand years, and nobody knows how much more.

Then again, by making things a little different in another way, the baby fishes grow like other proper fishes except that they do not have any hearts. Naturally, however, these particular monsters do not live to grow up. People make all sorts of strange creatures now-a-days by doing something to the young eggs when the new animal is forming.

Curiously too, all this sort of thing may happen to almost any egg by accident. If the two halves of the young egg get separated entirely, then the egg brings forth twin creatures so nearly alike that it is almost impossible to tell them apart at all. But if the two halves get only partly separated, the result is some sort of double monster.

So there are two-headed chickens, and two-headed snakes, and two-headed turtles, and two-headed calves. Sometimes only the tip of the nose is double. Sometimes it is the whole head. Sometimes there are two heads, four front legs, two hind legs, and one tail. Occasionally there is one body with eight legs, because the legs doubled and the body did not.

Just about as often, the doubling begins on the other end. Then there are two tails; or four hind legs and two front ones. Occasionally, two complete bodies are joined at one small region only They just missed being a pair of common twins It all depends on how well separated the first two cells of the egg happened to get.

The famous “Siamese Twins” were about like any other twins, except that they were fastened together side by side by a band of flesh under the arm. They lived to grow up, married, and travelled about the country exhibiting themselves for years. Finally one died; thereupon the other died almost immediately after. But as for that, any twins, if they are markedly alike, are likely to die at about the same time.

Often, too, only the buds which are forming the limbs get divided. Then there are extra fingers or toes or claws or thumbs. Cats, for some unknown reason, are especially apt to have double paws or extra toes. Oddly enough, tho nobody knows why, this happens more commonly on the front paws than on the hind ones.

So you see, the strange power which a few creatures have of making new parts, or extra parts, like the tails of lizards or the heads of planarians, belongs also to all creatures when they are very young. Most of them lose the power as they grow old. But in some animals, like crabs and earth worms and planarians, it lasts up to old age.

XLVII

How The Animals Keep Their Tools Sharp