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All remnants of the pronephros disappear.


As the pronephros regresses, excretory vesicles and tubules of the second embryonic kidney, the mesonephros, develop in the intermediate mesoderm in the C-6 to L-2 segments.

One end of each excretory tubule is cup shaped and is known as the glomerular capsule. The glomerular capsule surrounds a tuft of vascular channels and together they compose a corpsule. Each tubule terminates dorsolaterally into a common longitudinal channel, the mesonephric (Wolffian) duct. The duct begins in the lower cervical segments and terminates caudally in the urogenital sinus part of the cloaca. The more primitive vesicles that, as yet, have no connections with the duct are located in the caudal part of the mesonephros.

The tubules and vesicles form a bulge in the dorsolateral part of the coelom called the mesonephric ridge.


The coelomic epithelium between the mesonephros and the dorsal mesentery thickens to form the gonadal ridge. The gonadal ridge is apparent in the C-7 to T-8 segments. The mesonephric and gonadal ridges are suspended from the dorsal body wall by a thick urogenital mesentery. The primordial germ cells are migrating from the wall of the yolk sac to the gonadal ridge. They must migrate past the gut and its mesentery before reaching the ridge.


The urogenital sinus part of the cloaca receives the mesonephric ducts and the allantois. The allantois extends as an open channel into the umbilical cord. Its blind distal segment begins to disappear.

Source: Atlas of Human Embryos.