Balut ( , ; also spelled as balot) is a fertilized developing egg embryo that is boiled or steamed and eaten from the shell. It is commonly sold as street food in South China and Southeast Asian countries, notably the Philippines, Cambodia (, ) and Vietnam (). The term comes from the Filipino language. The length of incubation before the egg is cooked is a matter of local preference, but generally ranges between 14 and 21 days. Description A balut is a fertilized bird egg (usually a duck) which is incubated for a period of 14 to 21 days, depending on the local culture, and then steamed. The contents are eaten directly from the shell. Balut that is incubated for longer periods have a well-developed embryo and the features of the duckling are recognizable. The partially-developed embryo bones are soft enough to chew and swallow as a whole. The mallard duck (Anas platyrhynchus), also known as the "Pateros duck", is often used to make balut. Balut is common street food in the Philippines and other localities, and is also sold in stores and malls. It is a relatively cheap source of protein and calcium. Balut was introduced to the Philippines by the Chinese in 1565 or around 1885 and since then, balut has been included as a traditional part of the culture. Wherever Filipinos migrated for work, a large market for balut would develop. Controversies arose as knowledge of the food spread around the Southeast Asian countries and then globally. People questioned the ethics of eating balut. Preparation Traditionally, the fertilized eggs are incubated in the sun or buried in sand, and stored in baskets to retain warmth. In order for the embryo to develop normally, it must be exposed to heat for the correct period of time, while ensuring that the temperature is not too hot to harm the eggs or too cold to permit growth. The embryo is super sensitive to high temperatures, and is easily killed upon cooking in the sun. After nine days, the eggs are held to a light, in a process called candling, to reveal the embryo inside. The production of balut depends on egg maturation cycles, where the egg begins developing and changes in texture. Throughout these various maturation periods, different temperatures are required to accentuate the specific egg and embryo characteristics. Within the first few stages of maturation, balut is known as