After mating, inside the female, sperm swim up a tube called the oviduct, at the end of which there is an ovum, inside which resides the female sex germ. If the ovum is mature, it's already equipped with yolk, the yellow part of the future egg. The sperm may now fertilize the ovum by penetrating it and uniting the two sex-cells' genetic material. Fertilization doesn't necessarily take place soon after mating; domestic chickens and turkeys can produce fertile eggs seventy days after copulation.
After fertilization, the ovum with its yolk begins its own journey down the oviduct, a process lasting about 24 hours. During the first three or four hours, moving at about one tenth of an inch (2.3 mm) per minute, albumen (egg white) is added around the ovum and its yolk. The yellow yolk will serve as food material for the developing chick; the white will mainly keep the yolk from drying out, and will give the yolk physical support. Now the future egg slows to about 40 percent of its earlier speed, and membranes are added around the yolk and egg white. Finally the shell is put in place, taking 19 to 20 hours. The shell is mostly composed of the mineral called calcium carbonate, which has the same chemical formula as limestone. No wonder egg shells are so hard and brittle!
Technically speaking, eggs are single cells, even though we normally think of cells as too small to see with our naked eyes. In fact, eggs are the largest cells known in the animal kingdom. They range in size from tiny ones produced by hummingbirds (0.006 oz, or 0.2 gram), to nearly twenty pounds (9 kg), laid by the Elephantbird of Madagascar, which is now extinct, but was known by primitive humans. At the left you see half of an eggshell discarded by a Ruby-throated Hummingbird soon after a nestling had hatched. The coin above it is a 10-cent piece, the smallest US coin.