Diprosopus

Diprosopus (Greek διπρόσωπος [male], "two-faced", from δι-, di-, "two" and πρόσωπο[ν], prósopo[n] [neuter], "face", "person"; with Latin ending), also known as craniofacial duplication (cranio- from Greek κρανίο, "skull", the other parts Latin), is an extremely rare congenital disorder whereby part or all of the face is duplicated on the head.

Although classically considered conjoined twinning (which it resembles), this anomaly is not normally due to the fusion or incomplete separation of two embryos. Among other things, this governs the width of facial features. In excess it leads to widening of facial features and to duplication of facial structures. The greater the widening, the more of the structures are duplicated, often in a mirror image form. This has been demonstrated in the laboratory by introducing pellets of a protein into chicken embryos, resulting in chickens with duplicate beaks. Insufficient amounts of that protein lead to opposite conditions such as cyclopia where facial features are insufficiently developed. .

Many diprosopus cases also involve other congenital disorders, particularly anencephaly, neural tube defect and cardiac malformations.

Few two-faced animals have survived due to associated internal organ abnormalities and brain abnormalities. The most famous was Ditto the pig. Ditto was raised to adulthood, but died of pneumonia caused by food inhalation when breathing through one muzzle while eating with the other. In July 2006, a 6 year old two-faced cat called Frank and Louie from Millbury, USA received publicity. In this latter case, only one oesophagus (and possibly only one trachea) was functional and this aided survival.