Bird eggs are laid by females and incubated for a variable duration depending on the species. This diagram shows a cross-section of a chicken egg on its ninth day of incubation. The embryo is surrounded by the amnion, a membrane that fills with amniotic fluid and cushions it against shock; the allantois helps the embryo obtain oxygen and handles metabolic waste; the vitellus, or yolk, is the nutrient-bearing portion of the egg, containing most of its fat, minerals, and many of its proteins and blood vessels; the chorion forms the amniotic sac and encloses the other structures; the albumen protects the yolk and embryo and provides additional nutrients; the porous shell allows oxygen to enter while keeping unwanted fluids and contaminants out.Diagram credit: KDS4444
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