Egg
a mass or speck of protoplasm developed in the females of all but the lowest animals and when impregnated with the corresponding substance of the opposite sex capable of producing organisms like the parents. The egg throughout the animal kingdom is , in the last analysis, one single perfect cell, in which there are four parts, the cell wall, the cell substance, the nucleus and the nucleolus. The eggs of animals lower than the reptile have usually only three parts, viz., the ger the germinal spot or dot, the germinal vesicle, and the vitellus or yolk; the first being contained in the vesicle, and that again in the yolk.
Such eggs are usually of microscopic size and before being impregnated do not differ from any other cell or from the whole of a single-celled animal. The parts of an egg named are in general terms the same as those used for cells, but each part has its special name. Thus the nucleolus, the smallest recognizable constituent, is called the germinal spot or spot of Wagner because it was discovered by Wagner in 1836. The nucleus is called the germinal vesicle or vesicle of Purkinje because it was discovered by Purkinje in 1825. The common cell substance or protoplasm is called the vitellus or yolk and the cell wall is called the vitelline membrane.
Some eggs have other regular constituents as, for instance, a quantity of colored albumen or food yolk, like that constituting the yellow of a hen's egg. Secondly, a quantity of colorless albumen, called the white of the egg, which usualwhite of the egg, which usual ly coagulates when warm and is used in photography for preparing the paper .
Thirdly, the egg-shell which, especially in birds, consists of a membrane coated with carbonate of lime and in the eggs of reptiles possessses the appearance and consistency of parchment. The white and the yellow of eggs are inclosed in the shell wall which, as it increases in size, attains a special thickness and toughness and is called the egg-pod or putamen.
Under the term egg is included the ovum of every kind of mammal; but in general the English term egg is used only of those animals that do not produce their young alive. All animals differentiated by sex lay eggs. Those in which the egg passes out of the body and those in which the egg remains inside the body to hatch are called ovoviviparous; those whose eggs are retained in connection with the parent by means of a placenta and an umbilical cord so that the young are brought- forth alive are called viviparous. Through repeated division of the germinal vesicle a multitude of cells is formed out of which the embryo is developed. In the case of birds, reptiles, and the majority of insects, the young is nourished in the egg by means of the albumen there stored SO that after the egg is laid the development continues till the animal hatches out. Eggs the whole of whose yolk makes up into the body of the embryo are called holoblastic. Others with food yolk which does not undergo segmentation are meroblastic. Birds' eggs are meroblastic. All birds lay eggs and so also do most reptiles, amphibians fishes; insects, crustaceans, and mollusks are oviparous. In this class are Jusk included the ornithorhynchus echidna. The eggs of amphibians are usually found in floating glutinous masses. The eggs of fishes are popularly known as roe or spawn.
Eggs vary vastly in size. The ova of mammals are usually spherical and microscopic. The human ovum is among the smallest known, being a minute spherical body from 1-120 to 1-125 of an inch in diameter, while the largest known egg is that of the extinct elephant bird Epyornis Maximus of Madagascar, the shell of which had a capacity of about two gallons and was six times the size of the egg of the ostrich.
The eggs of birds, especially of fowls and some reptiles, as turtles, are commonly used for food. A hen's egg of good size weighs about 1,000 grains, of which the white constitutes 600, the yolk 300, and the shell 100. There are generally 10.7 parts shell, 11.9 parts albumen, 12.8 parts fat, .7 parts salt, and 63.9 water. Besides their use as a food, hen's eggs are used in the technical arts , the albumen in which they are so rich serving in dying, manufacture of leather, and various other purposes.
The science of birds' eggs is called oölogy.