(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. [Pg 284] 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.