What are Siamese Twins?
Siamese twins are formed from a single egg which develop into two almost separate balls of cells, this is not a regular twin. Each ball becomes an identical twin. As a result cells in Siamese twins become confused about where they are in the body - they don't know which of the two Siamese twins they are actually in.
In normal embryo and fetus development every cell that knows where it is in the body because their neighbors produce chemical messages. So a skin cell knows not only it's skin, but that it is - say - nose skin, rather than chin or ear or lip skin. In Siamese twins these chemical messages don't work properly - how can they? The end results can be very confusion, a single organism with two heads, two hearts, four legs and arms are Siamese twins.