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 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.
The balancers are rendered necessary by the hen’s habit of turning her eggs two or three times a day. It is often supposed that the eggs are turned in order to keep them equally warm on all sides, but this is unnecessary. Most probably the egg is turned in order to alter slightly the young chick’s position from time to time, and allow its parts to grow naturally, unimpeded by the other contents of the egg.
53. THE DEVELOPMENT OF THE CHICK.
1. A simple incubator.—Eggs are best incubated in the natural manner, that is, by the warmth of the hen’s body; but if a sitting hen cannot be obtained, an ordinary water-oven, such as is used in chemical laboratories, may be made to answer. It should be heated by a self-regulating burner, and kept at a temperature of about 40° C. The eggs should be turned two or three times a day, and the air of the oven should be kept moist by sprinkling water upon pieces of cloth, blotting paper, or hay, kept with the eggs. Spring is the most favourable time of the year for making the observations, as eggs laid at other seasons are not always in a condition to produce chicks.
2. How to mark the eggs.—The most instructive changes take place during the first five days of incubation. If all the stages of the first five days are to form the subject of one lesson, an egg should be marked “5” with pencil, and then put into the incubator or under the hen five days before the lesson; a day later, an egg numbered “4” should be put in, and so on. The numbers will then indicate the length of incubation at the time of the lesson, and the eggs should be examined in order, from 1 to 5. If one egg is to be examined each day, five should be put in the incubator at the same time; no numbering will then be required.