Incubation refers to the process by which certain oviparous (egg-laying) animals hatch their eggs, and to the development of the embryo within the egg. The most vital factor of incubation is the constant temperature required for its development over a specific period. Especially in domestic fowl, the act of sitting on eggs to incubate them is called brooding. The action or behavioral tendency to sit on a clutch of eggs is also called broody, and most egg-laying breeds of poultry have had this behavior selectively bred out of them to increase production.