Egg cells are usually very small. The human ovum is about 1/175 inch in diameter, and frog eggs average about 1/10 inch in diameter. Some species, however, produce comparatively large eggs. Ostrich eggs, for example, are about 3 inches in diameter.
An egg consists of a yolk, or stored food, and protective structures, such as a hard or horny shell, a material called the white, or a jelly layer. Variations in the amount of yolk and in the number and types of protective structures are associated with the food needs and environment of the embryo. A bird embryo, which completes its development within the confines of a shell, is fed by great amounts of yolk. The frog egg develops in water, and its moderate yolk is sufficient to nourish the embryo until it hatches as a free-swimming tadpole. The egg of a mammal is almost yolkless, and the mammalian embryo, which develops within the body of the female parent, gets the bulk of its food from her.
Human egg cells grow and mature in follicles, compartments located in the woman’s ovaries. About once a month one of these follicles enlarges and ruptures. The egg is then discharged into the oviduct and begins its journey to the uterus. Embryonic development begins in the uterus if the egg is fertilized by a sperm cell. Should the egg remain unfertilized, however, it soon disintegrates.
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