Microscopic study of midline determinants in janiceps twins.

: The earliest hitherto recorded developmental stage of human cephalothoracopagus (janiceps) twins, at 8 weeks of conceptional age, is described macroscopically and microscopically. The nearly perfect disymmetrical configuration of the 2 faces was compared with previously reported cases, and horizontal sectioning of the entire conceptus allowed detailed histologic description of the internal orientation of most organs and systems. The concept of conflicting facial and vertebral axes was explored to account for shared cerebral, facial, pharyngeal, and esophageal structures, and for aplasia of the pituitary and thymus glands and of the teeth. Critical points of changing axial orientation were identified at the mesencephalon, the Rathke pouch and third branchial arch, and with the hepato-cardio-respiratory complex split between the 2 axes. The caudal components conformed with previously reported cases.