Egg, the organized germinal body from which all animals originate, formed within the mother, and developed into the living being either before or after extrusion. In this article only those of the latter kind, chiefly of birds, will be considered; the general subject will be treated under Embryology. The eggs of many of the lower orders of animals are collected and held together in great numbers by a viscous membrane, and are called spawn. Those of birds and of many reptiles, as the tortoises and turtles, are deposited singly. Birds' eggs are contained in a calcareous shell, white or colored, formed almost wholly of carbonate of lime; the other constituents are minute quantities of animal matter, phosphate of lime, carbonate of magnesia, oxide of iron, and sulphur. Lining this hollow shell is a thin and tough membrane, composed principally of albumen. At the larger end of the egg is a space between the outer shell and this membrane, which, very small when the egg is first laid, increases with its age; it is called the vesicula aeris, and is filled with air, in which the proportion of oxygen is larger than in the atmosphere; this is said to be for the respiration of the unhatched chick.
Within the membrane is the white of the egg, or the albumen, a viscid liquid, in membranous cells, which encloses the yolk and the real germ of the animal. As this germ leaves the place of its production and passes into the egg-discharging canal, the albumen gathers around it in successive layers, a portion in very delicate membranes, called the chalazoe, which are attached to the poles of the yolk, and serve to suspend it in such a manner that the smaller and lighter half must always be uppermost. The outer layer of the albumen is less thick and viscid than that next the yolk; around it the lining membrane and calcareous shell are successively added before the egg is laid. In the hen's egg, the composition of the albumen is: water 85 parts, pure albumen 12, mucus 2.7, and saline matter 0.3, including soda with traces of sulphur; or, according to Dr. Thomson, water 80, albumen 15.5, mucus 4.5, ash 0.475. The yolk, called vitellus, is also a glairy fluid, commonly yellow, enclosed in its own membrane, and consists of a great variety of constituents, viz.: water, 41.486; a form of albumen called vitelline, 15.76; margarine and oleine, 21.304; cholesterine, 0.438; oleic and margaric acids, 7.226; phosphoglyceric acid, 1.2; muriate of ammonia, 0.034; chlorides of sodium and potassium and sulphate of potassa, 0.277; phosphates of lime and magnesia, 1.022; animal extracts, 0.4; and 0.553 of coloring matter, traces of iron, lactic acid, etc.
Upon one side of the yolk is a round spot, yellowish white, called the cicatricula, the germ of the ovum, which by the arrangement of the chalazae already referred to is always kept uppermost, and next to the source of heat supplied by the animal in sitting. As this is developed into the foetus, the albumen first furnishes nourishment to it, and when this is consumed more is supplied by the yolk. Eggs of the hen are hatched by being kept at a temperature of 104° for three weeks. Their vitality has been retained after they have been exposed to a temperature of 10° F.; and it is a remarkable fact that the freezing point of new-laid eggs is much lower than that of the water and albumen of which they principally consist, both of which congeal at about the same temperature. Eggs that have been once frozen, or have been long kept, freeze at the point their constituents would require. The specific gravity of new-laid eggs is from 1.08 to 1.09; by keeping they diminish in weight from evaporation of water, and the substitution of air through the pores of the shell. This diminution has been observed to continue for two years; an egg weighing originally 907.5 grains being reduced, as remarked by Dr. Thomson, to 363.2 grains.
When they have lost so much weight as to float upon water, they are generally unsound. The preventing of this evaporation by covering their surface with a coating of varnish, wax, gum arabic, or lard, checks their putrefaction. The Scotch sometimes drop them into boiling water for two minutes, by which the membrane within the shell is partially coagulated and rendered impervious to air. Hens' eggs vary so much in gravity, that it is a wonder they continue to be sold by numbers instead of weight. A dozen of the largest have been found to weigh 24 oz., while the same number of smaller ones of the same stock weighed only 14 1/2 oz. The fair average weight is about 22 1/2 oz. to the dozen. The relative weights of the portions of the egg as given by Dr. Thomson are: shell and membrane, 106.9; albumen, 604.2; yolk, 288.9. About one third of the entire weight may be regarded as nitrogenous and nutritious matter; a greater proportion than that of meat, which is rated at only from 25 to 28 per cent., while the nutritive portion of the oyster is only about 12 per cent. The white of the egg, from its tendency to coagulate into a hard and indigestible substance, is likely to disagree with the stomach of invalids, when the yolk may prove perfectly harmless.
Raw eggs are more wholesome than boiled, or even than those lightly poached, which are very digestible. Eggs become more difficult of digestion by being kept. - In medicine the shell is used as an antacid, its animal composition seeming to adapt it better for the stomach than chalk, the mineral form of carbonate of lime. The white is employed for clarifying liquors and sirups, which it accomplishes by entangling the small particles floating in them as it coagulates, and either rising with them to the surface, or sinking to the bottom. An astringent poultice is formed by causing it to coagulate with a piece of alum briskly stirred with it. This, under the name of alum curd, is used as an application to the eye in some forms of ophthalmia. The white is also used as an antidote to corrosive sublimate and salts of copper. The yolk is sometimes given in jaundice, and forms an excellent diet in dyspepsia; it is preferable to the white in making emulsions. - The largest eggs of which we have any account are some found in 1850 in alluvium in Madagascar. They belong to a bird which it is supposed has recently become extinct, to which M. Saint-Hilaire has given the name of epiornis maximus.
Two of the eggs are preserved in the French academy, and casts are to be seen in the principal museums of the world. One of them measures 13 1/2 in. on its longest diameter, and 8 1/2 in. on the shortest. The shell is about one eighth of an inch thick. The capacity of the egg is about 8 1/2 quarts, six times that of the ostrich's egg, and equal to 144 hens' eggs, or 50,000 eggs of the humming bird. From some of the bones of the bird which have been preserved, its height is calculated to have been about 12 ft.