In mammals, three pairs of excretory organs form from the intermediate mesoderm in a cranial to caudal direction. These are the pronephroi, mesonephroi and metanephroi, respectively. The pronephroi and mesonephroi are transient organs, but their existence is required for the development of the metanephroi or permanent kidneys. The development of these three excretory organs is shown diagrammatically in Fig. 1. The pronephros is the obligatory precursor of the adult renal system. Pronephroi begin to form at 21-22 days post-coitum (dpc) in humans and 8 dpc in the mouse (Woolf et al. 2003). The pronephros develops as a nephrotome containing a nephrocoel (cavity) which is in direct communication with the adjacent intraembryonic coelom by a short peritoneal funnel. The vascularized filtration unit (glomus) projects into the nephrocoel but filters directly into the coelom. The wall of the nephrotome opposite the peritoneal funnel gives rise to a tubule which joins the nephrocoel to the pronephric duct (future Wolffian duct) via the pronephric tubules. The ciliated cells in the nephrotome move the fluid into pronephric tubules by ciliary action, from where some reabsorption can occur into a surrounding blood sinus. The pronephric duct ends in the cloaca. The number of pronephric tubules varies between different animals and species and in ungulates these tubules are represented by a giant glomerulus in the head of the mesonephros (Vize et al. 1997; reviewed in Vize et al. 2003). In general, amphibians and fish have well-developed functional pronephroi that persist throughout the life of the organism and regulate water and solute balance as well as blood pH (Drummond and Majumdar 2003). However, amniotes have rudimentary, transient pronephroi which are thought to have no renal function, although this has not been tested. Many of the same genes (Pax2, Pax8, Gata3, Lim1, FGF8, Six1, WT-1 and Wnt-4) are expressed in the pronephros as in the later forms of renal tissue (Vize et al. 1997; Hensey et al. 2002; Chan and Asshima 2006; Bouchard et al. 2002; Grote et al. 2006).