Morphology of a two-headed piglet.

A live-born, one-day-old diprosopic piglet was presented to the Ontario Veterinary College. The piglet had a normal body with two heads, joined in the occipital region. There were two complete snouts, four eyes and three ears. The lower jaws were immobile because of overlapping mandibular rami. Although there was only one vertebral column, the bodies of the vertebrae, but not the neural arches, were doubled from the axis to T8. There was one thyroid gland and one larynx and hyoid apparatus. The two tongues were joined at their base just rostral to the single epiglottis. The palate was completely split in the right head but only partially split in the left. the cranial nerves were normal and doubled except for IX, X and XI. The brains were fused at the pon-medulla junction. An anomalous midline tag of neural tissue resembling remnants of the medical halves of two nervous systems extended form this point to the level of T8. Possible developmental mechanisms and rates of incidence are discussed.