The mesonephros arises in the unsegmented intermediate mesoderm (nephrogenic cord) late in the 4th week, caudal to the rudimentary pronephros. The nephrogenic cord, from the first thoracic to the third lumbar segment, differentiates into mesonephric vesicles which elongate to form an S-shaped loops (tubules). At their medial end, Bowman's capsule forms a mesonephric (renal) corpuscle together with the glomerulus. At the opposite end, the tubules enter the mesonephric (Wolffian) duct. Some parts of the mesonephric (Wolffian) duct do not completely disappear, but they persist in the male as the epididymis duct, ductus deferens, seminal vesicle and ejaculatory duct. Also, some of the mesonephric tubules near the gonad persists in the male as the efferent ductules of the testis.

1. Gut
2. Gonad
3. Bowman's capsule
4. Mesonephric duct
5. Excretory tubule of mesonephros
6. Glomerulus
7. Mesentery

In the 6th week, the mesonephros forms a large ovoid structure just lateral to the midline (nephrogenic ridge). This structure, together with gonadal primordium (genital ridge), makes a major contribution to the urogenital ridge.

1. Gonad
2. Interstinal loop
3. Mesonephric duct
4. Neural tube
5. Mesonephros