FIGURE 2.

The effects of angulation between the longitudinal axes of the embryonic disks in ventral conjunction. When the longitudinal axes of the embryonic disks―involved in ventral conjunction―are not entirely parallel but more or less angulated, the opposing planes of the conjunction area attain a convex and a concave aspect, respectively. The compound structures and organs that develop in the convex plane usually have a more or less normal appearance, for example, the face in cephalothoracoileopagus (top row), whereas those developing in the concave plane are subjected to interaction aplasia, the severity of which correlates with the degree of initial angulation. In cephalothoracoileopagus, this causes the compound face in this plane to present with a phenotypic spectrum that resembles that of holoprosencephaly (bottom row). Source: Specimens from the Narrenturm collection in Vienna (Austria)