An infant with a mass of cells connected to their head
A vestigial twin is a form of parasitic twinning,[1] where the parasitic "twin" is so malformed and incomplete that it typically consists entirely of extra limbs or organs. It also can be a complete living being trapped inside the host person, however the parasitic twin is anencephalic and lacks consciousness.
Causes
This phenomenon occurs when a fertilized ovum or partially formed embryo splits incompletely. The result can be anything from two whole people joined by a bit of skin (conjoined twins), to one person with extra body parts belonging to the vestigial twin.[2] Another potential cause of this phenomenon is when the human limbs are first forming in utero. This theory states that limbs grow from start to end, and that an error in this process can cause the creation of new vestigial limbs.[3]
Description
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. The limbs can pose problems in redirecting necessary resources such as blood to the nonfunctional limbs while the body lacks these resources.[2]
