Part 4

Oddly enough, there does not seem to be much connection between the size of an animal and the size of its egg. Big birds, to be sure, have big eggs; and little birds have little eggs. But a great crocodile, fifteen or twenty feet long and able to bite a man in halves, is hatched from an egg no larger than that of a goose. The little salt water minnow, or killifish, which is only as long as one’s finger, has very large eggs, for a fish, almost as large as small blue berries, and quite as large as the eggs of salmon and trout which grow to be a hundred times heavier. But cod fish, which sometimes are almost as large as a man, and the great sturgeons, which are as long as three men and as heavy as a horse, have eggs not much larger than the periods on this page, smaller even than those of a tiny ant. As for the little sea creatures, star-fish, sea-urchins, and the like (which to be sure, are quite as large as a hummingbird or a wren) their eggs are but fine dust, which cloud the water and are too small to be seen at all.

However, the smaller the eggs, the more of them there are, to make up. While some birds lay only two eggs at a time, and few more than a dozen, some fishes lay a hundred or more, the cod a hundred thousand, and the sturgeon two or three million.

Sometimes, when one is poking about in the brooks in the spring of the year—as every boy and girl should do, for it is great fun—one happens upon masses of transparent jelly half as large as one’s head, full of tiny black dots. These black dots, which are just about the size of the o’s in this book, are the eggs of frogs. If instead of being in round masses, they are in long strings, a yard sometimes in length, then they are almost always the eggs of toads; but if they occur neither in masses nor in strings, but separately, then they are the eggs of newts.

It seems strange that a frog should be able to lay a mass of eggs and jelly forty or fifty times larger than the frog itself. The real egg, however, is only the dark speck; and this when it is first laid has only a thin coating of jelly, hardly thicker than paper and nearly dry. As soon, however, as it touches the water, this dry jelly begins to swell, and goes on swelling and swelling for three hours until it is a hundred times larger than it was to start with.

These balls of frog’s eggs look, then, very much like tiny hens’ eggs with black yolks, broken into a bowl ready for cooking. They really are not quite this; because the frog’s eggs have no shell and no white, being simply yolk and nothing else. In fact, the only sorts of eggs that do have white are those of birds and reptiles; while few others have shells either. The jelly of frog’s eggs is not “white,” because it is not meant for the little frog to eat, but to keep other creatures from eating him. Besides this, it helps to keep the little chap warm.

You will recall that the little chick begins as a tiny dot on one side of the yolk, and keeps growing larger and larger until it uses up both yolk and white and fills the entire shell. Not so the little frog. Always, from the very beginning, it is as large as the egg. It is the egg, in fact. You can see that the egg is dark above and light below just as the tadpole will be, and the frog after him. At first, however, the baby tadpole does not have any parts or members. He gets in proper succession, eyes, ears, backbone, brain, skin, tail, and the rest; but he does not grow any larger until he hatches out, wriggles his way thru the jelly, and begins to eat.

At first the tadpoles are very tiny, only a quarter of an inch in length; and they cling in tufts to the under side of the water plants. After that, I suppose, everybody knows what happens.

There is still another curious difference between hens’ eggs and frogs’. When a frog lays an egg, that egg is nothing else but just egg—the little frog has not begun at all to form inside it. But when a hen lays an egg, while there is no little creature in that either, still the egg has already begun to get ready to turn into a chick. Some animals go farther than this, so that when their eggs are laid, the little creature is already formed inside, and so has only the last part of his growing left to be done outside. Certain fishes, certain reptiles, and various other animals besides, actually put off laying the eggs until so late that the young is all ready for hatching. Such eggs are laid and hatched at the same time, or even hatched first and laid afterwards.

All the four-footed creatures which have fur and hair, horses, cattle, dogs, cats, monkeys, and the like, manage in this way. And because this kind of egg doesn’t get knocked about, it does not need to have either hard shell nor thick jelly to protect it, but only a thin skin. For this reason, and because the egg hatches a few moments before it is laid, people are apt to miss it entirely, and so to get the idea that these animals have no eggs at all. But they have—one egg for each little animal.

We pretend that the bunny rabbits at Easter are hatched from the colored Easter eggs. They really are hatched out of rabbits’ eggs. No one notices the remnants of the rabbits’ egg, because what little there is soon dries up to almost nothing, or else the old mother rabbit eats it. Besides, one has all one can do to look at the new bunnies. Nevertheless, all little animals come out of eggs, puppies, colts, lambs, calves, kittens, every kind of living creature that is big enough for you to see, and a good many besides that are so small that you have to look for them with a microscope.

III

Little Fishes In The Brook

Of all eggs, the most interesting, I think, are the fishes’. Nearly all of these are pretty small, little round whitish globules like sugar pills. Some, like the eggs of trout and salmon which one finds in the gravel banks of rapid streams, are as large as fair-sized beads. Many, like the eggs of sea fish which float near the surface of the ocean, would go thru the eye of a darning needle.

The point, however, which makes them especially interesting is that so many of them are like tiny glass marbles. The membrane around them is so clear, and the substance of the egg itself so transparent, that with a magnifying glass, one can look right thru the egg, and see the little white fleck inside grow from nothing at all to a real fish, long enough to reach clear round the egg and lie with its tail almost in its mouth.

Some eggs are much clearer than others. The clearest are, at first, like clear glass, so that they can not be seen at all under water. Soon, however, a tiny vague white spot begins to form on the lower side. Then one can make out that the egg is covered with a rather thick membrane, that within this is a narrow, clear space filled with water, while within this and still smaller, floats the tiny yolk which is the real egg that is going to become the little fish.

The white spot on the yolk is not itself fish, but only fish stuff, which is being made ready to turn into fish by and by. The spot grows larger and thicker, until it looks like a round dab of putty stuck on the side of a marble. When this cap has grown until it is about half as much in diameter as the egg itself, it thins in the middle and thickens at the edges, until it forms a ring. A very strange thing, thereupon, happens to this ring. It begins to grow; and as it grows, it keeps slipping farther and farther round the egg. Soon it has become a band round the middle of the egg. Then as it moves along still farther to the other side of the egg, it has, of course, to grow smaller in order to fit. So it does, and the extra length taken in at one point in the ring, forms the body of the little fish. The head has already begun to form from a thickening at one side of the ring before it passed the middle of the egg. The two sides of the ring form the two halves of the body. But the tail being easier to make, does not grow out until much later.

Now there is a head and a body; but the only difference is that the head is bigger. Neither has any parts. There are no eyes, ears, nose, or mouth in one; nor any fins, backbone, stomach, nor scales in the other. These all appear later, much as in the chick—eyes, ears, brain, and heart early; fins and tail, scales and the whole front of the body not until long afterwards.

Many learned men have spent their entire lives in studying the way in which all these various parts form in the young animal, and a most strange and fascinating study it is, quite worth any man’s spending his life on. If I were to tell all that is known about the least part of one fish, the tale would fill up this entire book and leave no room for anything else. I shall, therefore, tell about the eyes only—partly because they are interesting and important organs; but more because they happen to be parts of the body which form in the same manner in all animals that have a backbone, whether they are fishes, frogs, birds, four-footed beasts, or human beings. The eyes with which you, my reader, are reading this page, grew in the way I shall describe, as I have myself seen it in the egg of cod and sea-bass.

In general, the part of the body to form earliest is the brain. Next after that come the eyes. These begin as two buds which grow out one on each side of the brain where the head is going to be. Each is a hollow, bubble-like affair on a short stalk; as much as anything, except for size, like a hollow rubber ball stuck on a pencil stub. One would think that this hollow ball would simply change into an eyeball; but it doesn’t, for Nature rarely does things simply. Instead, one side of the eye-bud folds in, as you might push in a hollow rubber ball with your finger, until it forms a cup. This cup is the eyeball. The sides grow out until the hole narrows down to the dark opening in the middle of the eye which we call the pupil. Various kinds of eye-stuff grow over the edge and form the interior parts of the eye; other tissues on the outside thicken the walls and form the transparent cornea in front; and while the pupil is still large, a portion of the substance which is later to become the skin, buds into the eyeball to form the lens of the eye. The reason, then, for this round-about process, this doubling in of the original eye bud to make a cup, which afterwards closes down to the eyeball we finally use, is to get various substances inside the eye, and finally to leave a pupil for the light to enter.

Thus far, like the little chick, the little fish has had no front to its body. It lies on the yolk, curled round it like a child with the stomach ache hugging a pillow. By and by the tail grows out free of the yolk. The head also lifts clear, and the lower jaw has room to form. Last of all, the sides of the body grow completely round the yolk, and put it where it will do the most good.

Now the fish is ready to hatch. For some time it has been giving occasional wiggles inside the egg membrane; finally it breaks thru and floats out. It is a tiny helpless creature, still more than half yolk. It cannot swim, but floats, belly up, and mouth wide open, not yet able so much as to close its jaws.

From this time on, the fish grows rapidly, living on the yolk, which grows smaller and smaller. At first the little creature floating on its back can only give an occasional wiggle. As the yolk becomes more manageable, the fish wiggles more. Soon it turns for a moment on its side, then clear over; and by the time the last of the yolk has disappeared, it is swimming right side up and has begun to eat the still tinier water creatures which are its food. At this stage, if it is a fresh water fish, it begins to be visible in the shallows in schools of minute, but veritable, fishes a quarter inch long and mostly eyes.

Of Plants’ Eggs

The plant’s egg is, of course, the seed. We commonly say that the plant grows from the seed. And so it does. Yet this is not exactly true either, because the ripe seed is already a little plant, folded up tight and packed away in a hard case, like a chick inside its shell.

If one takes, for example, an ordinary bean or a peanut, peels off the shell and opens it carefully, it separates into two halves, held together by a little nodule at one end. These two halves, which together form pretty much the entire bean, are really two fat leaves. They are the yolk of the bean egg, on which the new bean plant is going to feed until it has grown leaves and root, so that it can pick up a living for itself out of the earth and air.

The rest of the new plant is the little nodule which lies between these seed-leaves. Curled up against the outside of the seed, like a puppy’s tail between its legs, is a short fat root; while hidden away between the seed-leaves is the next pair, tiny leaves almost too small to see, but real leaves nevertheless.

So the bean is an egg. Not a new-laid egg, but an egg with a little plant inside, all ready to hatch out and grow.

If instead of cooking and eating the bean, we plant it in the ground, or in wet sawdust or blotting paper, it soon hatches out. The shell drops off, the seed-leaves first take in water and swell and then shrink away to nothing as the growing plant eats them up. The little root grows down, the little leaves grow up, the whole plant turns green and begins to climb the bean pole.