Theoretical and analytical embryology of conjoined twins: part II: adjustments to union.

A previous report in this journal (Spencer, 2000) discussed the probable embryologic etiology of conjoined twins, along with a system of classification based on the embryological structures postulated to be involved in the union. Part II correlates and compares the variations in the abnormal development of the individual organ systems in more than 1,200 actual cases, revealing details of embryogenesis not considered in previous publications. The site, incidence, and range of anomalies in the conjoined structures, as well as the associated malformations, follow a definite pattern as the union proceeds from one area to another; many can be explained in relation to the proposed embryologic adjustments to union, including both temporal and spatial influences. In addition, six currently inexplicable or unclassifiable cases are briefly described (including one with 12 feet), as well as two examples of early abnormal conjoined twins. Copyright 2000 Wiley-Liss, Inc.