the cell and accompanying products from which develop animals similar to the parent which produced the egg. In the higher animals the period of egg development is long, and often the egg is retained in the body until it is hatched, and the young are partially matured, but in more numerous cases the animals lay their eggs to be hatched subsequently. A complete egg consists of four parts, the shell, the white, the yolk and the embryo. The eggs of lower animals and insects are imperfect, but all contain the embryo. The eggs of frogs and fish are deposited in large numbers in the water and are held together by a jelly-like fluid (See SPAWN). Some species of insects lay a large number of eggs and cover them with a fluid resembling varnish, which protects them from the weather and from other enemies. The eggs of birds are the most perfect and of the most general interest. The shell is composed almost wholly of carbonate of lime and has for its purpose the protection of the parts which it encloses. Just within the shell is a thin, tough membrane, which forms the lining. Next to the lining, and surrounding the yolk, is the white, which is composed almost wholly of albumen. The yolk is also inclosed in a thin membrane and is spherical. It is composed of a variety of substances, some of which contain margarine and oleine; its color is usually yellow. The germinal vesicle, or germ sport, is found within the yolk, and in the eggs of fowls it can be easily distinguished by its pearly-white appearance. It is from this that the young bird or chick is developed by incubation, the yolk and white serving for food during the process. In the large end of the egg there is a space between the lining and shell that is filled with air. As the egg grows old this increases in size. It is supposed by some that the air in this space is used by the chick while it is pecking out of the shell. The germ is developed by heat, which is supplied by the female's sitting on the nest. The eggs of fowls and most birds require a temperature of 104 deg. F. for successful incubation. The period of incubation varies with the species. The eggs of the white-eyed vireo require only seven days in which to hatch, while those of the common fowl require three weeks, and those of the turkey and most water fowl require four weeks. The number of eggs laid by different birds also varies with the species. Some birds lay only one during the year, and others, as the hen, lay a large number. The robin usually lays four, the swallow from four to six and the crow four, six or seven. In many instances the color and shape of the egg are closely associated with the habits of nesting. Birds which lay their eggs on the ground without constructing any nest, lay an egg which is rounded at one end and nearly comes to a point at the other. If blown by the wind these eggs roll round in a circle, while if they were oval, like those laid in deep nests, they could easily be blown away. (Compare in the color plate accompanying the article BIRDS, the eggs of the spotted sandpiper and the tern with those of the sparrowhawk and the crow.) Eggs laid on the ground, or in nests built on the ground, usually take the color of the pebbles or dead grass with which they are surrounded, while the bright colors belong to the eggs laid in well-constructed nests. Another peculiarity of the coloring is that the greatest variation is about the large end of the egg. (See in the color plate accompanying the article), the eggs of the wood peewee, bobolink, purple grackle, meadowlark and other birds.) The economic use of eggs is well known, and the eggs of the hen, the guinea fowl, turkey and domestic duck constitute an important item in the world's supply of food. In Labrador, the Orkney and Shetland islands and some other localities, the inhabitants collect the eggs of sea birds for food.