2. The strength of the shell.—Hold an egg with your thumb at one end and your first or second finger at the other, and press exactly in the line of the length of the egg. You cannot break the shell.
3. Structure.—(a) The shell.—Tap the egg gently at the middle of its broad end until the shell cracks. Then carefully remove small pieces of the shell and notice the shell-membrane, a tough skin which is closely applied to the inside of the shell. Snip through the membrane in the middle of the broad end; notice the air-chamber which lies beneath it. Observe the inner membrane which separates the air-chamber from the inside of the egg. Hold a piece of shell up to the light, and notice the small, almost transparent dots. The shell is perforated by very small pores, through which the air can pass.
(b) The white of the egg.—Tap the shell so as to crack it all round at its widest part; raise bits of shell carefully and see the membrane here. Tear through the membrane and notice that in this region there is no air-space, but the white lies just beneath the shell-membrane. Separate the halves of the shell, notice the position and shape of the yolk, and then let the contents of the egg fall gently into a basin. Observe the appearance, colour, and transparency of the white, and try to distinguish two tangled cords of firmer white—the balancers ([Fig. 185])—arising close to the yellow yolk.
(c) The yolk.—What is the shape of the yolk as it lies in the basin? How does it differ from the shape of a yolk suspended naturally in the white? What is the cause of the change of shape? Notice carefully a small paler patch in the middle of the upper surface. This is the lightest part of the yolk, so that the yolk always settles with this part uppermost after any turning of the egg, and therefore the pale patch is always more directly exposed to the heat of the hen’s body (during incubation) than is any other part of the yolk. Prick the yolk and notice that the yellow, fluid contents flow out. You have evidently pierced the thin bag which formerly preserved the shape.
(d) A hard-boiled egg.—Boil an egg in water for five minutes, and then chip round the shell in the direction of the length and, with a sharp knife, cut the whole egg into halves along this plane. Make a drawing of the section, indicating the shell, shell-membrane, air-chamber, white, and yolk in position. Observe that the white is no longer transparent and fluid, but an opaque, white, and elastic solid. Try to peel off the white in layers. They will probably break off short, but you may be able to see that the white is deposited in spiral sheets around the yolk.
The hen’s egg.—A hen’s egg bears a somewhat similar relation to the adult bird that the maize or bean seed ([Chapter I.]) bears to the adult plant; for the egg contains
(1) a speck of living matter which little by little grows and becomes marked off, by orderly arrangement, to form the various regions and organs of the adult animal; and
(2) a store of food, which enables the young chick to develop in security, without being hampered, whilst still weak and helpless, by the necessity of earning its own living. The changes by which the speck of living matter becomes the perfect chick will be considered in the next section. We must now examine the structure of the egg itself, and see what provision it contains for the nourishment and safety of the developing bird.
The egg is ovoid in shape, one end being distinctly broader than the other. This shape has the advantage of preventing the egg from rolling very far when placed upon a slightly inclined surface, and it is worthy of notice that the eggs of birds which lay on cliffs and other exposed situations are usually more elongated and pointed than others, so that when stirred they do not roll away, but simply describe a small circle and come to rest again. In the case of eggs which are laid in cup-shaped nests ([Fig. 197]) the pear-shape lends itself to close packing, and thus allows the eggs to be more easily kept warm by the parent bird.
The shell (sh., [Fig. 185]) is composed of a chalky material, and is perforated by small pores, through which the developing chick obtains fresh oxygen from the air, and gives off its surplus carbon dioxide ([p. 242]). A small piece of shell easily snaps, but the shape of the complete shell so distributes an outside pressure, especially one in the direction of the long axis, that relatively great force is required to break it. The shell is lined by a thin, parchment-like membrane (sh. m.). At the broad end of the egg this membrane is double, and the two layers enclose an air-chamber (a).