Two contradicting theories exist to explain the origins of conjoined twins. The older and most generally accepted theory isfission, in which the fertilized egg splits partially. The second theory is fusion, in which a fertilized egg completely separates, but stem cells (which search forsimilar cells) find like-stem cells on the other twin and fuse the twins together. Conjoined twins share a single common chorion, placenta, and amniotic sac, althoughthese characteristics are not exclusive to conjoined twins as there are some monozygotic but non-conjoined twins that also share these structures in utero.
The mostfamous pair of conjoined twins was Chang and Eng Bunker (Thai: อิน-จัน, In-Chan) (1811–1874), Thai brothers born in Siam, now Thailand. They traveled with P.T.Barnum's circus for many years and were billed as the Siamese Twins. Chang and Eng were joined by a band of flesh, cartilage, and their fused livers at the torso. In moderntimes, they could have been easily separated. Due to the brothers' fame and the rarity of the condition, the term came to be used as a synonym for conjoined twins.