[History of classification systems of conjoined twins by specific consideration of the classification systems of Louis Bolk (1866-1930)].

Classification systems concerning conjoined twins have been developed in the nineteenth century by many authors, a.o. J. F. Meckel (1816), I. Geoffroy Saint-Hilaire (1832), E. F. Gurlt (1831), Fr. Ahlfeld (1880), C. Taruffi (1881), and in the beginning of the twentieth century, a.o. J. W. Ballantyne (1902) and E. Schwalbe (1907). However, these classification systems were very complex and mainly based on the external morphology of the conjoined twins. In contrast to many other classification systems the professor in Anatomy at the University of Amsterdam Louis Bolk divided conjoined twins in only three main groups: 1 greater than diplopagi simplex caudad; 2 greater than diplopagi simplex craniad; 3 greater than diplopagi simplex mesad. The last group was divided into ventrad and laterad conjuncti. For the study of conjoined twins the Vrolik Collection and many other specimens of the Museum of the Department of Anatomy and Embryology of the University of Amsterdam were very important for Bolk. Three factors were the main reason that the concept of the classification of double monsters of Bolk has not been cited often in the international literature: 1 greater than the publications of the classification in a national journal in Dutch; 2 greater than the coincidence of the publication of E. Schwalbes famous handbook "Die Morphologie der Missbildungen des Menschen und der Tiere II. Die Doppelbildungen" in the same period and 3 greater than the problem that the verification of the classification, mainly based upon morphogenesis, has not been possible because of technical problems in performing these experiments in mammals.(ABSTRACT TRUNCATED AT 250 WORDS)