Egg

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The egg, or ovum, is the gamete (reproductive sex cell) produced by most female animals. It is fertilized by the sperm, which is the male gamete. The term "egg" is commonly used to include the acellular structures that surround the ovum.

In "higher" animals, including arthropods (such as crabs and insects), most fish, most amphibians, many reptiles (including birds), and some mammals(the echidna and the platypus), the egg is a relatively large structure in which the young matures outside the mother's body. This form of reproduction is called oviparous, or egg-laying. In the viviparous, or live-bearing, form of reproduction used by some fish, amphibians, and reptiles, as well as most mammals, the developing egg remains within the mother's body.

The eggs of both amphibians and amniotes (reptiles and mammals) generally contain a protective covering and the cells from which the young develop. The protective covering may take on a number of different forms, depending on the animal. In amphibians the coating is gelatinous and clear. In amniotes, it may be leathery like the egg of a reptile, or hard like the egg of a bird. In live-bearing species of lizards and snakes, the shell is a thin membrane through which materials can easily pass.

In most amphibian and amniote eggs of both oviparous or viviparous species, a large yolk mass within the egg nourishes the developing embryo. In some viviparous reptiles and most mammals, nutrition comes directly from the mother through a specialized aggregation of blood vessels. In mammals, this structure is called a placenta.

The structure, size, and number of eggs produced depend on the lifestyle and environment of the animal that makes them, and there is a fair amount of variation. There are two general strategies involving the size and number of eggs: the mother's limited energy resources will either go toward making a smaller number of larger offspring or a larger number of smaller offspring.

Some species produce many offspring to increase the number that survive in a harsh environment. In this strategy, survival is largely dependent on chance, and many of the young die. Species that invest more energy into a smaller group of larger young increase the chance that each individual will survive, based on the assumption that larger young are stronger than smaller young.

Both oviparous and viviparous strategies are represented among amphibians (frogs, salamanders, and the often-hiding, wormlike caecilians). The eggs of oviparous amphibians vary greatly in size. In addition, because the gelatinous membrane that surrounds an amphibian egg is not safe from losing internal fluids, desiccation (drying out) is a serious threat.

Most species lay their eggs in or near water, although some salamanders will bury their eggs to keep them moist and cool. Because an amphibian egg is not waterproof, it exchanges fluids and gases easily with the surrounding water. This is critical for supplying the egg with oxygen and for releasing harmful waste products from the confines of the egg.

The eggs of reptiles (including birds) and mammals have features that protect them from the challenges posed by a terrestrial (land) environment. In oviparous amniotes, the eggs are protected by a sturdy shell that safeguards the embryo against desiccation, although some air and moisture can pass through the shell.

The amniote egg has several specialized compartments. The yolk sac holds the yolk that feeds the embryo. The allantois, which is an extensionof the embryo's gut, stores solid waste products. Both the yolk sac and the allantois are connected to the developing embryo.

Various other egg layers and membranes form around the ovum as it passes from the ovary through a muscular tube called the oviduct to the cloaca. The materials that make up these layers and membranes are secreted by specialized glands that line the oviduct. A clear fluid layer called the albumen—the "egg white" of a chicken egg—supplies the embryo with water and inorganic nutrients and cushions the embryo from impact if the egg is jarred or dropped.

Albumen contains a protein (also called albumen), which binds to water and inorganic nutrients and aids in their transport. These nutrients diffuse through (pass through) the walls of the blood vessels that connect the embryo (through the umbilical cord) to the mother (at the placenta, or aggregation of vessels at the wall of the uterus/oviduct).

The albumen is enveloped within the chorion, a thin membrane which also surrounds the embryo, yolk sac, and allantois. The chorion regulates the passage of gas and moisture into and out of the egg. Finally, a shell surrounds and protects the entire egg. The shell allows moisture and gas to pass through it. The shells of most lizards and snakes have a leathery consistency, whereas birds have hard, calcified shells that resist denting.

Embryonic Development.

Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology, 5th ed. Menlo Park, CA: Addison Wesley Longman Inc., 1999.

Jameson, E. W. Vertebrate Reproduction. New York: John Wiley & Sons, 1988.

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