Mass production of chicken meat and eggs began in the early 20th century, but by the middle of that century meat production had outstripped egg production as a specialized industry. The market for chicken meat has grown dramatically since then, with worldwide exports reaching nearly 12.5 million metric tons (about 13.8 million tons) by the early 21st century. The breeds of chickens are generally classified as American, Mediterranean, English, and Asiatic. While there are hundreds of breeds in existence, commercial facilities rely on only a select few that meet the rigorous demands of industrial production. The single-comb White Leghorn a Mediterranean breed widely used throughout the global egg industry is a prolific layer that quickly reaches sexual maturity. The Cornish Cross, a hybrid of Cornish and White Rock is one of the most-common breeds for industrial meat production and is esteemed for its compact size and rapid, efficient growth.