A vestigial twin is a rare form of conjoined (Siamese) twinning or more accurately, of parasitic twinning, where the parasitic 'twin' is so malformed and incomplete that it typically consists of extra limbs or organs. The result is a person with – for example, an extra arm or leg attached to him or her. This phenomenon occurs when a fertilized ovum or partially formed embryo splits. If the fetus splits completely, the result is two identical twins. However, if the fetus does not split fully, the result can be anything from two whole people joined by a bit of skin (conjoined / Siamese twins), through to one person with extra body parts: the extra parts are the body of the vestigial twin.
Most vestigial limbs are non-functional, and although they may have bones, muscles and nerve endings, they are not under the control of the host. The possession of six or more digits on the hands and feet (polydactyly) usually has a genetic or chromosomal cause, and is not a case of vestigial twinning.
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