When the early embryo divides, but incompletely, some form of conjoined twin is formed. They are always uniovular, and have one amnion, one chorion and one placenta.
There are many different types:
The degree of shared tissue may involve only skin at one extreme or whole organs at the other extreme. Siamese twins are connected by a common liver bridge.
Conjoined twins may be separated if they have no vital parts in common.
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