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Parasitic twins, a specific type of conjoined twins, occurs when one twin ceases development during gestation and becomes vestigial to the fully formed dominant twin, called the autositic twin. The underdeveloped twin is called parasitic because it is only partially formed, is not functional, or is wholly dependent on the autositic twin. In most cases, the phenotype of parasitic twins is one normal functioning individual with extra appendages or organs, leading to questions about whether or not the additional limbs and organs are in fact another person or just a mutation of the individual's body. Researchers think that parasitic twins result from mechanisms similar to those that produce Vanishing Twin Syndrome. On a developmental continuum with vanishing twin syndrome on one end and developmentally normal twins on the other, researchers propose that the patterns of conjoined twins fall in the middle.