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 Barnum's circus for many years; although never separated, they married and fathered a total of 22 children. They died within 2 hours of each other in 1874. See also multiple birth.