In the past, several theories have been proposed to explain the occurrence of anorectal malformations. Most investigators believe that these malformations are the result of an impaired process of septation. However, in 1986 vd Putte challenged all theories that tried to explain anorectal malformations by a faulty fusion of lateral ridges of the cloaca. To elucidate the principles of normal and abnormal cloacal development, the authors studied the morphology of this region in normal embryos of rats and abnormal embryos of SD mice, which often have abnormal cloacas. Using scanning electron microscopy (SEM), 245 normal rat embryos and 80 abnormal SD-mice embryos were observed. The results were as follows. (1) In normal embryos the region of the future anal opening can be identified soon after the establishment of the cloacal membrane. This part is a fixed point in cloacal development. (2) In abnormal embryos the cloacal membrane is too short. The region of the future anal opening is missing. (3) In abnormal embryos a spectrum of malformed cloacas can be observed. This is in accordance with the spectrum of anorectal malformations clinically observed in humans. (4) The authors' observations support recent findings that the "fistula" in anorectal malformations resembles a normal anus at an ectopic position.