Eggs

Eggs are generally round and smooth, but those of insects vary in form, and are often spiny or tubercuiate. Turtles have nearly spherical; crocodiles and snakes, oval eggs. Most birds' eggs are smaller at one end than at the other; but those of fowls are spheroidal, those of ducks, oval; and those of most sea-fowl, pear-shaped.

Impregnation may take place subsequently to the extrusion of the egg, when, of course, the whole development of the young goes on outside the body of the parent; or it may occur before extrusion. In the latter case hatching may take place outside, only a slight development, such as the formation of the "tread" or cicatricula in the hen's egg, taking place beforehand. Both these cases are termed oviparous. On the other hand, the eggs may be retained in the oviduct until the young are ready to leave, when the animal is termed ovoviviparous; or, the young leaves the egg and is nourished for a considerable time within the womb of the female by the close apposition of certain vascular appendages of its body to the walls of that cavity, when the animal is termed viviparous.

Eggs also differ considerably in the characters of their outer coverings. Many molluscs have their eggs enclosed many together in a leathery cup, which is either single, as in Purpura, or one of a compound mass of cups, as in the common whelk. Sharks and rays have their eggs enclosed in a four-cornered leathery pouch with mooring tendrils at its corners, familiar, when empty, on the beach, as "mermaids' purses." The eggs of frogs and toads are surrounded by a tough albuminous layer, which becomes gelatinous in water: those of snakes and lizards are parchment-like; and those of turtles emcl crocodiles have a calcareous shell like those of birds. Birds'-eggs, though often white, are coloured in many kinds, rarely uniformly, as in the dark green of that of the emu, but commonly spotted or lined with black, brown, green, or red. The mottling often serves a protective purpose, as in the eggs of plovers and those of various seabirds, which are left exposed among pebbles, and closely resemble them.

Cod's roe is used as food and as bait; that of the sturgeon is made into caviare. The eggs of the crocodile and turtle are esteemed articles of food, as are those of many birds, especially the ostrich, plover, turkey, duck, and fowl. The importation of hen's eggs from France and Belgium is an important and rapidly growing industry.