Eggs

A critical evolutionary development for terrestrial animals is the reptilian amniotic egg, now also characteristic of birds and some mammals. The developing embryo, protected from drying out, can survive outside of water and in a variety of habitats. The yolk provides it with food, and the albumin supplies water and nutrients. Wastes are released to the allantois, an extension of the embryonic gut. Oxygen diffuses easily through the thin outer shell of the egg; its passage to the embryo is regulated by the chorion.

From the anatomical point of view, eggs consist of a minute speck of protoplasm, the germ, tread, or cicatricle, from which the animal develops, and a much larger amount of food material, the yolk. In addition, all eggs have some form of protective coating, either a jelly mass, a shell, or a covering membrane.

The structure and development of eggs of different species vary with the condition under which the egg is produced and matures. Animals that deposit their eggs to hatch outside the body are described as oviparous; those that produce eggs with soft coverings that mature within the body (and that therefore give birth to living young) are called viviparous; and those that produce hard- or tough-shelled eggs that mature within the parent's body are said to be ovoviviparous (although they give birth to living young, they are regarded as egg layers). An egg that contains little yolk is either retained and nourished by the mother, or, if deposited outside the body, develops quickly into a self-sustaining form such as larva. The number of eggs produced varies with the likelihood of their survival. When conditions militate against the survival of eggs, as in the case of fish eggs, a large quantity of eggs is produced—as many as 14 million by the turbot. The number of individuals that can develop from a single egg varies from one to several hundred in certain chalcid flies.

In one order of mammals, the monotreme, the eggs are deposited outside the body. The platypus and the echidnas lay eggs with thin parchmentlike shells and a large yolk.

EGGS OF BIRDS

Besides the germ and yolk, the eggs of birds contain an albuminous mass, the white, which is an additional source of nourishment for the developing chick. The egg is enclosed in a tough double membrane, the membrana putaminis, which in turn is covered by a hard, three-layered shell chiefly composed of calcium carbonate. The shell protects the egg from the weight of the parents' bodies during incubation. All birds, with the exception of the mound birds, incubate their eggs by body heat.

EGGS OF REPTILES

Some species of snakes bear their young alive. Reptile eggs usually have leathery coverings that range in thickness from thin, parchmentlike layers to thick shells resembling those of birds' eggs. All have a large amount of yolk. The number of eggs laid varies from a dozen to as many as a hundred in the pythons. Pythons, unlike the related boas, incubate their eggs. Most reptilian species either bury their eggs in shallow ground, to be warmed by the sun, or lay them in underground burrows. A few, such as the crocodile, incubate their eggs in beds of fermenting vegetable matter. Some lizards lay almost mature eggs, and the young lizards break out of the soft shells as soon as the eggs are deposited. In other lizards, the eggs may hatch in the oviduct of the parent.

EGGS OF AMPHIBIA

A mass of amphibian eggs, appearing as small black spots, is contained within a gelatinous mass while they incubate in a freshwater pond. Eggs deposited in this fashion receive little or no parental protection and will soon hatch into small, wriggling tadpoles.

The eggs of frogs and toads are spherical, are usually covered with a gelatinous substance, and are laid in long strings or in masses. The eggs vary in size from a small dot, in the common toad, to the size of a pea, in a species of Malayan frog. Most species deposit their eggs in water, sometimes attaching them to water plants; others lay the eggs in damp moss or attach them to the limbs or leaves of trees overhanging the water. In several toads the eggs are carried on the back or legs of the female or male after the spawning. Water-dwelling newts and salamanders commonly lay their eggs in water; land-living species usually deposit them in holes in moist ground, but a few bear live young.

EGGS OF FISHES

Shark Egg Case with Embryos

These two dogfish egg cases show the developing embryos inside. Each egg case contains enough yolk to sustain the nutritional needs of the embryo until it hatches. The outer covering of the egg case is a tough, horny material. Each of the corners of the egg case is drawn out into a long coiled filament, or tendril, that wraps around rocks, kelps, or other materials on the sea floor, preventing the egg case from being carried away by currents and exposed to possible predation. Dorling Kindersley

Some fish in the elasmobranch group comprising sharks, rays, and skates lay eggs that resemble those of birds, containing a white as well as a yolk and covered with a horny shell. The dogfish lays eggs that have tendrils and attaches them to objects underwater. The hagfishes lay cylindrical eggs with hook-shaped processes at either end by which the eggs are linked together. The eggs of the bony fishes are usually spherical and are surrounded by a tough capsule. They may be deposited on the water bottom and covered with sand or gravel, as in salmon eggs, or laid in gelatinous masses that float. The egg mass of the anglerfish covers an area of 5.6 to 8.4 sq m (60 to 90 sq ft). Some species, such as the catfish, carry their eggs in their mouth until they hatch. A few bear live young.

EGGS OF INVERTEBRATES

The eggs of leaf insects, like their virtually invisible parents, are well camouflaged. Each of these “seeds” is actually the capsule surrounding a single egg. Abandoned by the adult, few of the developing insects will ever reach maturity. Dorling Kindersley

The eggs of mollusks, crustaceans, insects, and other invertebrate orders exhibit the widest variety in form. Mollusk eggs vary from those of the squid, which are laid in a number of cylindrical cases attached to one another like the strands of a mop, to the eggs of certain snails of the genus Bulimus, which have hard shells and resemble pigeons' eggs in size and shape. Most higher crustaceans carry their eggs attached to appendages on the underside of the body. The water flea lays two kinds, small “summer eggs” carried in a pouch on the back, and larger “winter eggs” carried in a fold of the carapace until fertilized. Normal reproduction takes place with the hatching of the summer eggs, but, in conditions of severe cold or the drying up of the pools in which the animals live, the winter eggs are fertilized and deposited. They may then lie in a dormant state for years until conditions are favorable for hatching. The eggs of insects are characterized by a great variety of egg and egg case forms as well as by the intricate markings on the eggshells of many species.

EGGS AS FOOD

The eggs most eaten by humans are those of the domestic hen, but duck eggs and the eggs of other birds are also commonly used as food. The eggs, or roe, of various fishes, especially sturgeon and salmon, are processed as a relish called caviar and considered a delicacy. Reptile and insect eggs have also been a traditional food source.

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