A review of over 1,800 publications concerning the embryology and pathologic anatomy of conjoined twins provides convincing evidence that they all result from the secondary union of two originally separate monovular embryonic discs. This "fusion" theory seems to be confirmed by the adjustments to union and the pattern and incidence of specific anomalies at the proposed sites of conjunction in more than 1,200 cases, all of which can be arranged in two uninterrupted series of cases, the one united dorsally (in the neural tube) and the other, ventrally (over a shared a yolk sac). No theoretical "fission" of the vertebrate embryo at any stage of development, in any plane, in any direction can explain (1) the selection of the observed sites of fusion, (2) the details of the union, or (3) the limitation to the specific areas in which the twins are found to be joined. Part I of this disquisition deals with the pertinent normal and theoretical embryology, the adjustments to union, and the parasitic cases, as well as conjoined triplets and quadruplets, and a comparison of oviparous and viviparous embryos. Part II (in a subsequent issue of this journal) will compare and correlate the abnormalities of the various organ systems involved in 1,200 cases.
Copyright 2000 Wiley-Liss, Inc.