Duck eggs share many traits with other vertebrate eggs. Most importantly, every egg -- whether a microscopic human egg, a frog egg, or a mallard duck egg -- provides the starting point for the development of what will become a complete organism. However, duck eggs, and bird eggs in general, are quite different in form and function from other vertebrate eggs, and these differences dramatically influence the path that development takes for the organism inside.

One of the most striking differences between bird eggs and other vertebrate eggs is the hard shell that covers bird eggs. The shell provides protection, allowing the growing embryo to develop on land outside the mother's body -- an evolutionary strategy that in some ways proves more efficient for the embryo and the mother. The strategy, of course, requires that the egg be entirely self-contained. All of the nutrients the developing embryo needs must be housed within the confines of the shell. Similarly, all gaseous metabolic waste products must be eliminated through the shell wall, and solid and liquid wastes, which accumulate inside the shell, must be stored in a way that will not harm the embryo.

Like the eggs of other organisms, a fertilized duck egg cell, or zygote, begins dividing shortly after fertilization, well before the egg is laid. Unlike most other eggs, however, a large portion of a bird egg is made up of yolk, which serves as a food source for the developing embryo. Because the yolk is simply a food source and not part of the embryo, the entire egg does not cleave or divide in half, as most vertebrate zygotes do. Instead, a flat, layered disk of dividing cells, called a gastrula, forms on the surface of the yolk, pulling nutrients from the yolk as needed. A shell of calcium, applied just before the egg is laid, completes the package; it will provide a protected environment in which the growing embryo can develop.

By the time the fertilized egg is laid it already contains thousands of cells. All of the cells in the gastrula have exactly the same genetic makeup. They differ only in their placement within the three layers of the gastrula -- the ectoderm, the mesoderm, or the endoderm. According to developmental biologists, a cell's placement within the gastrula has a lot to do with what that cell will become. The ectoderm, for example, gives rise to the skin and nervous system; cells of the endoderm become the gut and lungs; and the mesoderm becomes the muscle and bones of the embryo.

Throughout its 21-day development the embryo's cells will become increasingly specialized. Some cells will form the lining of the intestine; other cells will become neurons in the brain. Every cell, however, will remain part of an integrated whole, a complex combination of many different types of cells working in harmony with one another.