Thoracopagus parasites in two sets of twins: evidence for the fusion theory.

We report two sets of parasitic twins attached in the thoracic region. Exomphalos major in the autosite was present in both cases. While in case 1, the autosite could be salvaged after easier dissection, in case 2, the autosite could not be salvaged due to the presence of the life-threatening anomalies. The presence of a right-angled communication between the gastrointestinal tracts of the autosite and the independent tactile responses of the parasite from that in the autosite supports the 'fusion theory' to explain the fusion of the embryos that could have led to this defect. The presence of the ano-rectal malformation and common bile duct dilatation exclusively in the parasite appears to be the result of the abnormalities in the endodermal-mesodermal interactions, probably due to the relative positioning of the parasite with respect to the autosite.