As in the case of supernumerary legs in abcaudal duplication, so by a somewhat similar cause we may have supernumerary arms produced in abcranial duplication. In this case the spinal column is single, one of the original divisions having disappeared, but the primordial limbs of both divisions remain. As in the former case also, the outer limbs of each division develop into the proper limbs of the individual, while the internal or adjacent ones become dwarfed appendages. The developed arms are therefore the right arm of the right division and the left arm of the left, while the appended ones are the left of the right and the right of the left. (Cleland).

Fig. 3. - Dicephalus. Abcranjal fission. (Glasgow Hunterian preparation).