 Egg-type chicken carcasses no longer appear in stores.The major milestone in 20th century poultry production was the discovery of Vitamin D (named in 1922), which made it possible to keep chickens in confinement year-round.Before this, chickens did not thrive during the winter due to lack of sunlight, and egg production, incubation, and meat production in the off-season were all very difficult, making poultry a seasonal and expensive proposition.