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 [Pg 285] membrane is double, and the two layers enclose an air-chamber (a). Inside the shell-membrane are the white or albumin, and the yolk. The white (alb´.) is a viscous, transparent fluid. Its innermost part (alb.), which immediately surrounds the yolk, is of thicker consistency than the rest, and is prolonged into two twisted cords called the balancers (ch.), which suspend the yolk in position. Fig. 185.—Semi-diagrammatic view of a fowl’s egg at the time of laying. a, air-space; alb, dense layer of albumin; alb´, more fluid albumin; bl, germinal disc; ch, balancers; sh, shell; sh.m, shell-membrane; sh.m 1, sh.m 2, its two layers separated to enclose air-space; yk, yolk. (After Marshall.) (× 1.) The yolk is a golden-yellow fluid enclosed in a thin, elastic membrane and hence preserving a spherical shape. On its upper surface (but under the membrane) is a small circular patch (bl.) of paler colour. This patch, called the germinal disc, is about ⅛” in diameter; it contains the living matter from which the chick will be formed; the rest of the yolk, and the white, are simply a store of inert food which is used up by the growing chick. The work of the balancers.—In order that the small living patch [Pg 286] of pale yolk, the germinal disc, may grow and develop into the chick it must be kept warm. When eggs are hatched in the natural manner, the heat is supplied by the body of the sitting hen, and the upper part of the egg is consequently kept warmer than the rest. It is important that the living part of the egg shall always lie nearest the hen’s body and thus be kept warm, and this is secured by an arrangement as effective as it is simple. The yolk is lightest in the neighbourhood of the germinal disc, and therefore always lies with this part uppermost. If the egg is slowly turned over, the yolk remains “right side up.” If it is turned over quickly, the yolk soon swings round into its original position. The twisted cords of white, the balancers, not only sling up the yolk and guard it from being thrown to one side by a sudden movement, but they also prevent it from rotating too quickly—and so possibly injuring the delicate body of the young chick—when it rights itself after the egg has been turned over.