Cross-sectional illustration on major types of conjoined twins.

Conjoined twins show varying degree of conjoining in either facing or side-by-side fashion. Cephalothoracopagus janiceps is a prototype of facing anomaly in which the two bodies demonstrated a cross symmetry to the midline, that is axial symmetry. Interfacial and intersternal lines crossed at a right angle and no abnormality of situs was associated. Dicephalus dipus dibrachius is a case of side-by-side union, in which the bodies facing nearly the same direction were symmetrical to the middle sagittal plane. Abnormal situs of one was always associated. Other types of conjoined twins as thoracopagus lie between the two extremes of facing and side-by-side union. The three dimensional architectures of the organs in each type would be explained using cross sectional figures of skull, thorax and pelvis. Although the facing twins share the internal organs without fusion, the organs in the side-by-side component are fused with modification of the situs. We postulate sixteen pairs of situs and four manners of division for the explanation of the midline organs and the presence of a dominant co-twin. The splenic locations in a given cardiopulmonary situs are evaluated for the appraisal and applicability of these hypotheses.