newt, Triturus pyrrhogaster (Kotani, 1962), no mesonephric nephrons are formed from the mesonephric mesenchyme after the removal of the Wolffian duct. At least in chicken a segment of the distal tubule is derived from the Wolffian duct (Croisille

The diversity of cell types and the complexity of the structures increase during the urogenital morphogenesis (reviewed by Saxén, 1987; Bard et al., 1996). This is exemplified by the development of the mammalian permanent kidney, the metanephros, that is preceded by two transient and primitive organs, the pronephros and the mesonephros (reviewed by Saxén, 1987). They develop along the caudally growing nephric duct, the Wolffian duct, that is initially formed in the trunk region by fusion of caudal pronephric tubules (Toivonen, 1945). Shortly before the Wolffian duct reaches the cloaca, a ureteric bud is evolved and serves as the inducer of the differentiation of the metanephros (Grobstein, 1953, 1955). Regulation of mesonephric differentiation has been studied in some mammals, but more thoroughly in amphibians and in birds. In chicken (Boyden, 1927; Gruenwald, 1937) and in the newt, Triturus pyrrhogaster (Kotani, 1962), no mesonephric nephrons are formed from the mesonephric mesenchyme after the removal of the Wolffian duct. At least in chicken a segment of the distal tubule is derived from the Wolffian duct (Croisille et al., 1974; Croisille, 1976). Mammalian mesonephric nephrons consist of a glomerulus-like body, a proximal tubule and a distal tubule (Martino and Zamboni, 1966; Schiller and Tiedemann, 1981; Tiedemann and Egerer, 1984; Smith and MacKay, 1991), but there is a great deal of variation in the size, structure, functional maturity and developmental fate of the mesonephros between species. In pig (Tiedemann and Egerer, 1984) and human (Martino and Zamboni, 1966) the mesonephros is well developed as a secretory organ during embryonic development. In murine species, however, the mesonephros is primitive and non-secretory (Zamboni and Upadhyay, 1981; Smith and MacKay, 1991). The mesonephric kidney regresses prenatally in mammals. The degradation is completed in rat by embryonic day 17 (Er17) and in mouse by day 15 (Em15) (reviewed by Saxén, 1987). The regression in mouse starts from the caudal mesonephric tubules and spreads cranially (Smith and MacKay, 1991). In females the degradation is complete but in males the remaining mesonephric tubules form the epididymal ducts and the Wolffian duct becomes the vas deferens (Moore, 1977; Orgebin-Crist, 1981; Nistal and Paniagua, 1984). The 1293 Development 124, 1293-1299 (1997) Printed in Great Britain © The Company of Biologists Limited 1997 DEV3617