The medial part of each mesonephric vesicle expands. Its wall becomes invaginated to form a double-sheet structure, the glomerular capsule. Blood capillaries develop from the local mesenchyme inside this capsule and join the aorta by means of the developing afferent arterioles. Efferent vessels have the structural pattern of a venula; they end in the postcardinal vein. The capillaries form a tuft (glomerulus). The glomerular capsule and the glomerulus are known collectively as the mesonephric corpuscule.
The lateral part of the mesonephric vesicle lengthens as a tube (mesonephric or secretory tubule) and soon becomes an S-shaped loop. This is the basic pattern of the formation of excretory units, or nephrons, which consist of the mesonephric corpuscle and the mesonephric tubule.
The formation and the degeneration of the mesonephros follow a cranio-caudal gradient. At the beginning of the fetal period, only a few intact excretory tubules are still available. By month 2, the mesonephros can excrete urine. From week 8 onwards, it loses its excretory function due to degeneration; this function is progressively taken over by the metanephros.