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 [Pg 287] 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. 3. How to examine the eggs.—Have ready a basin of water, heated slightly above the temperature of the hand (i.e. to about 40° C.), and dissolve table-salt in it in the proportion of a level teaspoonful of salt to a pint of water. The young chicks will keep alive longer in this solution than in ordinary water. Tap the shell in the middle of its broad end, and open the air-chamber (a, Fig. 185 ) completely. Then crack the shell in the middle of the length and, keeping the length of the egg horizontal, cut transversely round the middle of the shell with scissors in a vertical plane, until the halves are on the point of coming apart. Then lower the egg into the warm saline solution, pull the halves of the shell apart, and float out the contents. Examine the embryo carefully, making out as much as possible, and then snip round it with a pair of fine scissors to remove it from the yolk; float it into a watch-glass and cover it with weak alcohol (equal parts of water and spirits of wine). Examine it with a lens. After it has remained for a day in weak alcohol, put the embryo into strong alcohol in a small bottle (writing the age on a label), and preserve it.