The Wolffian duct (WD, Nephric duct) develops under the influence of signals from the lateral plate and the paraxial mesoderm from the intermediate mesoderm (IM). The WD elongates caudally in the growing embryo. The WD starts out as a ‘‘cord’’ of loosely associated cells without clear epithelial properties. It converts into a tube of polarized epithelial cells in a rostral–caudal sequence, in response to signals from the overlying surface ectoderm, including BMP4. The caudal growing end of the WD remains a cord until it completes its growth and fuses with the cloaca at ∼E9.5.