There is no more fascinating sight to be seen anywhere than an incubator full of eggs just as the chickens begin to hatch out. You look through the little glass window in the side and see, at first, only rows of clean white eggs, dozens upon dozens of them, looking as if they were all ready to go into the family ice-chest or to be made into omelets for breakfast.

But they are not. First you begin to hear faint scratchy sounds. Pretty soon, here and there, a hole breaks through the broad end of an egg, and a tiny bill sticks out. The little chick is packed so tightly into the egg that it can move only its head. So it pecks and pecks; and stops to rest; and pecks again; and the hole in the shell gets larger and larger; until by and by, the egg cracks open, and a brand-new chicken draws its first long breath and looks out into the world.

After that, the chick usually takes a long rest, for it is pretty tired. When it feels better, it begins to move its legs and wings, and a half-hour or more after it first began work, it gets clear of the shell and stands up on wabbly legs, wet, bedraggled, weary, as disconsolate looking a little object as can well be imagined. Shortly, however, the feathers, which at first were plastered tight to the skin, dry off and fluff out, the legs get steady, and soon there is running about a rolypoly yellow chick, seemingly at least twice as large as the egg which held him only an hour before. Truly it is a wonderful sight, five hundred eggs turning into little chicks in an incubator, for all the world like the kernels of corn changing to pop-corn in the popper.

But wonderful as it is to see the way a chick comes out of an egg, it is still more wonderful to see the way it gets in. A fresh, new-laid egg has no chick inside. After it has been kept warm three weeks, it has—all ready to come out. The question is how the chicken got there.

Many different men have studied this question. For the most part, they have started a dozen or more eggs at once, and then taken them one by one and two or three hours apart, and cautiously broken them open to see what was inside. Sometimes, however, a student of eggs carefully cuts away the shell on one side, until he has made a hole about the size of a ten-cent piece. Over this he cements a sheet of glass as thin as paper, so that he can look through this tiny window into the egg, and see the chick grow.

This is really easier than it sounds. The yolk, as everyone must have noticed in hard-boiled eggs, does not stay in the middle of the egg, but always floats to the upper side. The chick, too, always forms on the upper side of the yolk; and when the egg gets turned over, the yolk rolls round like a barrel in the water and brings the chick to the upper side. So the chick, until it grows big enough to be a tight fit, always lies crosswise of the egg, on the upper side of the yolk just under the shell.

At first, of course, there is no chick at all, but only a round white fleck hardly larger than the head of a large pin, on the side of the yolk where the chick is by and by going to be. Before the end of the first day after the egg is laid, this little fleck has become somewhat oval in outline and an eighth of an inch across. Through its center runs a whiter line, as thick as heavy basting cotton and a sixteenth of an inch long; about half as large, that is, as an “l” or a figure “1” in the type on this page.

This is the beginning of the chick. Only it has hardly yet begun to be a chick, for it has as yet neither head, tail, wings, legs, eyes, nose, mouth, heart, stomach, brain, nor any other parts. It is in short, only a tiny line of chicken substance, which is now to begin to be made into a chicken.

Early in the second day of incubation, the little white line begins to get thicker on the end where the head is going to be. The brain and spinal cord appear first; later in the day there is the first sign of eyes and ears. At about the same time, the heart begins to form, and the minute blood vessels to grow out into the yolk like the first roots of a tiny plant. Before the end of the second day, the heart has begun to beat, and the blood vessels have begun to absorb the yolk to feed the growing chick. The yolk, in its turn, feeds on the white; for as everybody knows, the yolk and the white of an egg are stored up food, on which the little bird can live and grow until it is old enough to get out of the egg and shift for itself.

At the beginning of the third day, or a few hours before, the chick, which has been lying on the yolk face down (only it hasn’t any face yet), turns over on its left side. It is getting to be a big bird now, a quarter of an inch long and as thick as a good sized pin. Next, the brain grows rapidly; and so do the eyes, though these are not so large as the eyes of the finest needles. Now too, the nerves begin to form; also the lungs, the stomach, liver, and other organs of digestion; and there are beginnings of a tail, though without feathers.

During the fourth day, there are signs of a mouth. Legs and wings, looking just alike, begin to bud out from the body. Another day, and one can tell which is which; while now there appear beginnings of the skull and of the place where the back bone is going to be. Meantime, the little bird has become more than a half-inch long—though it does not yet look the least bit like a bird, but more like a large “?” mark. There is still no front to the body, and the heart, beating merrily away, hangs out in the yolk.

With the second week, the little chicken does begin to look something like a real bird. The bones begin to harden; while on the tip of what has been just an ordinary nose appears a speck of chalk, which will by and by harden into a bill. The claws begin to grow; and there are signs of feathers, each one still enclosed in the little transparent sac in which it forms.

At the end of two weeks, the white of the egg is all used up; and the little bird, which has been lying crosswise of the egg, now turns to bring its head toward the broad end. The yolk, too, is getting small; and on the nineteenth day, the chick pulls the last remnant into its little tummy, and begins to close over the hole. At about the same time also, he pecks through into the large air space which one sees in the broad end of an egg, when he eats it, hard-boiled, at a picnic. For a week or more, he has been breathing by means of a sort of gill, much like that of a fish, only that instead of being on the side of the head like a fish’s it grows out from the middle of the stomach on a long stalk and spreads over the inside of the shell. So the chick breathes through the shell, which is full of minute holes almost too small to be seen. But after the last bit of yolk has been taken in, this gill shrivels up and drops off, and the chick breathes with its lungs like the rest of us.

At the end of three weeks, there is nothing left of the egg but the shell and a tea-spoonful of water. The chick, which began life the size of a pin head, now fills the shell jam full, with only just room enough to peck the hole that lets him out. On the twenty-first day of his imprisonment, out he comes.