All eggs require warmth to hatch them; the fishes know this, not as we know it, because we have read, or been told so, and can reason upon causes and consequences, and so understand why, but they know it instinctively; they possess, in common with all unreasoning creatures, what we call instinct, that is, a natural impulse to do in the right way, and at the proper time, whatever may be necessary for the maintenance of that state of existence in which God has placed them; so instinct directs the fishes when the time for spawning has arrived, to leave the deep waters, where they generally remain safe from the pursuit of man, for the shores, where the warmth of the sun can reach the eggs, and awaken the principle of life within them. So instinct teaches the bird to leave its winter home, in some far southern country, and fly hundreds of miles across land and ocean, to reach a spot suitable for the purpose of breeding and rearing its young; to collect the materials and to build its nest, and after the eggs are laid, to sit patiently on them the appointed time; to select the food proper for those little gaping bills, and to tend the fledglings carefully, until they are able to fly and provide for themselves, and then, when their wings are strong enough for the journey, and their food begins to get scarce, away they go back to the south of Europe, or Africa, straight as an arrow, and the young ones, which have never flown that way before, seem to know it as well as those which have been backwards and forwards, often and often.
But the egg, what of that? Can we describe its nature and construction in a way sufficiently clear for our readers to understand? Let us try:—it is like a series of cases or envelopes, one within the other; the outer one only, which is the last formed, being hard and unelastic, that is, it will not stretch or change its shape. Like the shells of some fish, and other testaceous animals, it is composed of carbonate of lime, which the animal has the power of secreting, as it is called, from its food. Hens sometimes lay soft eggs, without a shell; this shews a deficiency of the secreting power, or a want of the necessary material, and may generally be remedied by mixing some chalk with the food, or scattering it about the yard. Next to the shell is a skin called the membrana putaminis, that means the membrane or skin of the shell; it has also a Greek name—chorian; it is divided into two layers, which separate at the larger end, and leave a space called the vesicula aëris, that is, air vesicle, or little bladder; this contains the air necessary for the chick to breathe before it chips the shell. Enclosed in this membrane is the albumen, or white fluid, sometimes called the glair, from the Latin glarea; in the same language albus means white; and our readers who live in Albion, so called from her chalky cliffs, ought to see at once from whence we derive the word albumen; the little chords by which this bag of fluid is suspended are called chalaza; this word comes from a Greek root, and has reference to the connection between the suspending chords and the germ, or spot, in which is the vital principle.
We now come to what may be called the provision bag, because it encloses the yolk, which serves as food for the animal in embryo; it is called membrana vitelli, or the skin of life. Thus our examination of the egg has brought to view the three great necessities of all existence—protection, the shell and albumen; nutrition or food, the yolk; and the vital principle, to understand the nature of which has puzzled the greatest philosophers that the world ever saw. It is said in the Scriptures that God breathed into man the breath of life, but what this may be we can none of us tell; it is given to us and we live, it is taken away from us and we die; and so do all God's creatures, which by Him, and Him alone, live, and move, and have their being. Man can do many wonderful things, but he has not yet been, nor will he ever be, able to make an egg, much less to produce a chick from it.
Let us now fancy that we are looking upon one of these strange little elongated globes, and that instead of being opaque, that is, dark—not clear, it is transparent, so that we can see into it, and observe the changes which are taking place there. The mother bird has been sitting on it for, say twelve hours, and the warmth of her body has called into action the principle of growth, or of vitality; the little spot and ring in the centre of the yolk have become somewhat enlarged and changed in form; they are no longer round, but the outline is irregular, shewing a tendency to shoot or spread out on all sides. Four hours later shews them yet more enlarged, and getting into an oval shape, with a distinct, though somewhat broken line down the centre. When the incubation has lasted thirty-six hours, (for meaning of this word, see the first series of "Nests and Eggs,") there is a still greater increase in the germ, and a spreading of its mottled margin over a portion of the yolk; if at this stage of growth we examine it through a magnifying glass, we shall see a little body like some curious kind of caterpillar, and veins shaped like stags' horns branching out of it in every direction. On the fourth day the chick is a more strange-looking object still, with great projecting eyes with rings round them, like spectacles, and what appears to be a very blunt sort of a snout or muzzle; the whole head, and there is not much else, reminds one of that of a serpent, it might be the Cobra di Capello, or the Spectacle Snake, seen through a diminishing glass. But we cannot give a proper description of it, so we have called in the aid of our artist, who has furnished us, on the next page, with portraits of the interesting creature at four different stages of its growth. In the last we get a side view, and begin to fancy we discover some resemblance to a baby-fowl, although a very hideous one.
All this time a great change has been going on in the whole interior of the egg; the chick has, of course greatly increased, and the red veins have become more numerous, and spread over the entire surface; the yolk is scarcely distinguishable from the other portions. Now, too, the bones of the chick have begun to form, and we clearly perceive the projection of the wings and the stump of the tail, while the pupil of the eye is quite clear and distinct. Larger and larger the creature grows, until it fills all the space, and has to be doubled up in a very curious manner, with the feet and the head brought together, and the bill close to the shell, ready to be used as the instrument of liberation. Many persons suppose that the parent bird chips the shell, but this is not so, it is done by the little creature within, which has first to make its way through a thick membrane, or skin; this it does on the twenty-first day of incubation; we are speaking now of the common fowl, the periods in different species of birds vary considerably, although the process of growth in all is the same. One wonders how, with its soft bill, the little creature can make its way through the tough skin, and hard, though brittle shell; but it must be remembered that the elastic skin is stretched to its greatest extent, and when in this state, a slight prick will make a large opening; and the shell, too, is no doubt, by the pressure from within, rendered less capable of resisting the stroke, slight though it be, of the bill of the prisoner, which does sometimes fail to make its escape, and dies in confinement, if not released by some friendly hand from without.
As if to assist in the work of opening a passage to the light and air, there is found on the tip of the chick's bill a small horny scale, having at the centre a hard and sharp projecting point, which, from the position of the head, is brought into direct and constant contact with the inner surface of the shell; this scale soon loosens and comes away after the chick is hatched, there is then no further use for it. The preceding figure represents the chick as it lies closely folded up on the twenty-first day, just previous to its deliverance from bondage.
We have now finished our account of this wonderful process, and may say in the words of a recent writer, "Dull indeed of soul must the man be, (or boy either,) in whom an egg does not inspire emotions of awe and admiration, wonder and worship. The circle of life is from the adult (fully-grown animal,) to the egg. This is the vital round—the beginning and the ending—the ending and the beginning. The wheel goes round continually, life kindling sparks of life; and what is called death is the worn-out forms becoming cold and decaying away."