Mesonephros

At S11, the nephrogenic cord initially gives rise to the mesonephric duct. It develops as a solid, longitudinal cellular mass on the dorsal side of the nephrogenic cord at the level of the 9th somite. Lying directly beneath the superficial ectoderm, it separates itself from the nephrogenic cord and increases in length. It grows caudally and eventually turns ventrally, where it reaches the wall of the cloaca. At S13, the mesonephric duct forms a channel, which opens into the cloaca.
During S11, but after differentiation of the mesonephric duct commences, mesenchymal condensations develop into epithelium and form vesicles (vesiculae mesonephricae) in the nephrogenic cord at the level of the 9th (about Th1) to the 23th/24th somite (about L3). These vesicles give rise to tubules, which become connected to the mesonephric duct. Although two or more vesicles are formed at the level of each somite in the nephrogenic cord, there is no metameric segmentation analogous to the somites.