Siamese twins, congenitally united organisms that are complete or nearly complete individuals. They develop from a single fertilized ovum that has divided imperfectly; complete division would produce identical twins, having the same sex and general characteristics. Siamese 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; such organisms are known as monsters. Only rarely do Siamese twins survive birth. When they do, 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 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 multiple birth multiple 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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conjoined twins

or Siamese twins

Identical twins (see multiple birth) whose embryos did not separate completely. Conjoined twins are physically joined (typically along the trunk or at the front, side, or back of the head) and often share some organs. Symmetrical conjoined twins usually have no birth anomalies except at the areas of fusion and can sometimes be separated by surgery. In asymmetrical conjoined twins, one is fairly well developed, but the other is severely underdeveloped and dependent on the larger twin for nutrition. The underdeveloped twin may have to be surgically separated to save the larger twin. The term originally referred to Chang and Eng, born in 1811 in Siam, who were joined by a ligament from breastbone to navel. Widely exhibited, they married two sisters and fathered several children.

Siamese twins [′sī·ə‚mēz ′twinz]

(medicine)

Viable conjoined twins.

Siamese twins

Eng and Chang (1814–74), the original pair, were connected at the chest. [Medical Hist.: Brewer Dictionary, 828]

See : Twins