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conjoined twins,congenitally united organisms that are complete or nearly complete individuals, historically known as Siamese twins. They develop from a single fertilized ovum that has divided imperfectly; complete division would produce identical twins, having the same sex and general characteristics. Conjoined twins remain attached at the abdomen, chest, back, or top of the head, depending on where the division of the ovum has failed. In some instances the individuals are joined only by a band of musculofibrous tissue and can be separated surgically, but in other instances they share vital organs and separation may not be possible. Sometimes an ovum divides in such a way that an organism develops having one body and two heads, or one head and two sets of limbs. About half of all conjoined twins are stillborn, and many die within a week or so of birth. When they do survive, fatal illness in one dooms the other unless separation is possible.
The name Siamese twins derives from the most famous of conjoined male twins, Chang and Eng, born in Siam (Thailand) of Chinese parents in 1811. They were exhibited in traveling shows worldwide for many years beginning in 1929. Retiring from touring in 1839, they settled in rural North Carolina, adopted the surname Bunker, and became U.S. citizens. Styling themselves as Southern gentlemen, they became prosperous slaveholders, married two sisters and fathered at least 21 children, and supported the Confederacy in the Civil War. They toured again for a while after the war. Never surgically separated, they died within several hours of each other in 1874.
See also multiple birthmultiple birth,
bringing forth of more than one offspring at birth. Although many smaller mammals bear several young at a time, multiple births are relatively uncommon in humans and other primates.
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See J. A. Orser, The Lives of Chang and Eng (2014) and Y. Huang, Inseparable (2018).