mammals provide the developing embryo with a suitable environment within the mother's womb. The other major group of reptile descendants, the birds, not only have continued the reptilian tradition, but have evolved eggs of an improved design in a wide variety of sizes, shapes, colors, and textures.

Bird eggs are virtually self-contained life-support systems. All they require for the embryo to develop properly are warmth and oxygen. Oxygen diffuses into the egg through microscopic holes formed by the imperfect packing of the calcium carbonate crystals that compose the eggshell. There are not many of these pores -- for example, they make up only about 0.02 percent of the surface of a duck egg. Carbon dioxide and water vapor diffuse outward through the same pores. Birds can lay their eggs in even drier environments than reptiles, because when the fatty yolk is broken down to provide energy for the developing embryo, water is produced as a by-product. Reptile eggs primarily use protein as a source of energy and do not produce as much "metabolic water."

The proportion of yolk differs between altricial and precocial birds. The former, which hatch so undeveloped that they require significant parental care and thus need less stored energy, generally have eggs that contain about 25 percent yolk. Precocial birds, which can walk and feed themselves shortly after hatching, have eggs with about 40 percent yolk (67 percent in megapodes, inhabitants of Australia and Pacific islands which upon hatching are virtually ready to fly). Interestingly, in spite of this difference, and