References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 Subject Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 1 Introduction The developmental steps which lead to the formation of the human perineum seem firmly established (Arey 1965; Hamilton and Mossman1972; Moore and Persaud 1998; Wartenberg 1993; Sadler 1995; Larsen 1997). They form the base for the evaluation of the pathogenesis of a great variety of complicated and often serious malformations which occur in this region. This concept has, however, been challenged by the results of an investigation into the n- mal and abnormal development of the anorectum in pig (van der Putte and Neeteson 1983, 1984; van der Putte 1986). Observations revealed that at least in pig, a major element in current ideas about the early development of the perineum, namely the process by which the original simple cloaca is sub- vided into a urogenital and anal part is incorrect, while additional obser- tions strongly suggested that the same may be true for ideas about female and male sexual transformation. A preliminary investigation in human - bryos gave similar indications (van der Putte 1986). The data supported e- lier critical findings (Politzer 1931, 1932; Wijnen 1964; Ludwig 1965) which have apparently been ignored, possibly because they seemed to hinder the understanding of the pathogenesis of congenital malformations such as - perforate anus and hypospadias.