neither you nor I nor the amphioxus can help ourselves. These men have, at the same time, taught us how to think hard, and how to jump so far in our thinking that we are able to see some connection between amœba and amphioxus. We may, indeed, trace this connection through the tiny, freshly laid egg of the amphioxus, for it seems that this egg is as truly a single cell as is the amœba itself. Moreover, the egg is quite like the amœba in the way that its one cell becomes many cells. In point of fact, the dividing and multiplying of the cell of the amphioxus continues in true amoeba fashion, although now it is all done within the egg itself.

Follow the history of a single amphioxus egg and see what happens. There is, as I have said, but one cell to begin with. As soon as this has been fertilized, as it is called, by coming in contact with another cell from another amphioxus, the dividing begins. That first cell now becomes two, the two become four, the four become eight. Then there are sixteen, thirty-two, sixtyfour, the number doubling each time, because each new cell is constantly multiplying itself by two. Each has its own nucleus and each is as complete as an amœba, with the remarkable difference that not one of the number makes the slightest effort to get away from its fellow. As they divide and multiply, they hold together from the very first, and the scientist who knows what is happening simply says that the young amphioxus is

growing well in the egg; that it will soon be perfectly formed.

When the cells have divided and subdivided long enough, the work of making the young amphioxus is finished. It now comes out of the egg, buries its body

ELEVEN STEPS IN THE EARLY GROWTH OF AMPHIOXUS IN THE EGG

From a photograph of a set of models showing each cell enormously
magnified. The three last models are of cross sections and

indicate how the layers are being formed

in the sand, and begins to live the life which its ancestors lived. Before long it will send off a bunch of egg cells from its own body; and, if all goes well, if those eggs are not eaten by hungry fish or destroyed in some other way, the cell of each will go through the process of

multiplying and dividing on its own account until each egg turns out its own amphioxus.

The one great difference, then, between the amoeba and the amphioxus, or between all single-celled and many-celled animals, is that in one case each separate cell leads an independent life, while in the other case the cells always cling together and each keeps its own place as a part of the living machine.

While the creature is being formed in this way within the egg, the work of each separate set of cells is definitely decided. See how it is with the amphioxus. The earliest divisions of the first cell are soon arranged in a hollow oval, one layer deep. Next, the oval has two layers, one outside, the other inside-just as would happen if you should take a hollow rubber ball and press one side of it inward until it touched the other side. From that point onward the growth of the animal is even more marvelous. For, little by little, out of those two layers, and out of a third layer which forms in the space between them, the amphioxus itself grows into shape.

No cell of the inside layer ever crosses the line and intrudes on the business of the cells of the other layers. On the contrary, each set continues busily at work multiplying and dividing within its own boundaries, doing its part in building the wonderful swimming machine called amphioxus.

When the building is done, when each cell is in place and each layer complete, then the scientist finds that the separate layers have developed into distinctly separate parts of the body, as follows:

1. The outside layer of cells has become the skin and the nervous system. It is to the study of this layer that this book is devoted.

2. The middle layer has become the muscles, the blood vessels, and the egg-making part of the machine.

3. The inside layer has become the stomach and all the part which belongs to the digestive apparatus. The special interest about these various facts is that the history of the egg of the amphioxus is precisely the history of the beginning of every other many-celled creature. Each starts with one cell and multiplies by dividing, until the animal itself is made on the pattern of the owner of the first cell. The egg cell of a hen ends by multiplying itself into the countless cells of a chick ready to hatch-claws, feathers, beak, eyes, and all. In the lizard's egg, so small and so round, the cells arrange themselves in the shape of a lizard, and when the work is done out pops a sprightly lizard, ready to live its happy life in the chinks and crannies of some stone wall. Cats and dogs and birds and you and I, all have the same history from the cell upward.

In some animals, as fishes, birds, and lizards, the cell leaves the body before the multiplying begins. It is

safely lodged in an egg that is stored with food for the growing life. In each case of this sort the cell multiplies within the egg after it is laid, until the entire animal is formed as we saw in amphioxus. The creature then comes out as a complete being - or, as we say, "it is hatched."

In other animals the cell stays within the body of its parent. There it grows and divides, even as it does in the egg outside of the body, until the animal is ready to begin life for itself. Then, "it is born," as we say. Kittens, puppies, babies, and multitudes of other animals are built up in this marvelous way; and, in every instance, the first cell decides what the completed creature shall be- whether fish, bird, beast, or man. That first cell also settles the question as to whether the creature shall have a backbone or not; and it is just here that our special interest in amphioxus comes in again.

The shadow which connects the pointed ends is the earliest form of backbone that scientists know anything about, and because of this backbone they call the little fish a vertebrate.

As it happens, the backbone of amphioxus is really no bone at all. The cells that build it up are simply stiffer than the other cells of the body, and they are packed close against each other, forming a backbone quite stout enough for any need.