Part 27

Where has the tail gone? The white blood corpuscles have eaten it up; and with it, a lot of other things, gills among the number, which the tadpole has to have, but the frog has no use for.

The tadpole, as you know, has no legs. When they start growing (you can see them under the skin, almost the first sign of the coming change) these growing legs turn something into the blood which is a signal to the blood cells to get to work eating up the tail. So the blood cells do get to work. They devour the flesh, cutting into it much as certain insects gnaw into wood, or a woodchuck digs a hole in the ground. Then as fast as the blood cells take down the flesh that is no longer useful, it is used again to build up the new legs. So really the tadpole’s tail is used over to make the frog’s legs.

Much the same thing happens when a caterpillar turns to a butterfly. The caterpillar, as you have no doubt often seen, shuts himself up in a hard shell. Inside this coffin, or chrysalis, the body of the caterpillar is taken to pieces, and changed over into a sort of thick milk. Then out of this thick milk, the butterfly is formed.

So the caterpillar does not become a butterfly simply by growing wings. It is really almost a new creature, built up out of the material of the old, as one might rip up an old coat and have it woven into a rug; and we may be very sure that no butterfly ever has the slightest recollection of the time when it crawled on a dozen legs, and chewed leaves for its food.

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.