Cloned chicken. Conceptual computer artwork of a two-headed (bicephalous) chicken (Gallus domesticus). Abnormalities like this could potentially result from genetic engineering experiments or from exposure to mutation-inducing chemicals in the environment, however such embryos might not all reach adulthood. Bicephaly can occur naturally, for example snapping turtles (Chelydra serpentina) and several snakes have been found with two heads. It may occur when an embryo begins to split into two but stops part way, forming Siamese twins. Alternatively it could occur due to a gene mutation or a developmental disorder.
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