Many glomeruli and excretory tubules differentiate in the mesonephros causing it to project as a ridge from the dorsal body wall in the C-7 to L-3 segments.
The mesonephros and the more medial gonad are attached to the dorsal body wall by a short, thick urogenital mesentery.
The excretory tubules drain into an enlarged mesonephric duct coursing caudally on the lateral surface of the mesonephros.
The mesonephric duct on each side terminates in its respective horn of the urogenital sinus.
METANEPHROS
A minute outpouching called the metanephric diverticulum (ureteric bud) develops on the urogenital sinus horn and grows dorsocranially. Its blind termination dilates to form the primitive renal pelvis, which is surrounded by a cellular condensation called the metanephrogenic mass (blastema).
The metanephrogenic mass is at the level of the L-3 and L-4 segments.
GONAD
The gonadal ridge is very prominent medial to the mesonephros in the T-3 to L-2 segments.
Cells begin to condense adjacent to the surface epithelium
UROGENITAL SINUS
As the urogenital sinus is separated from the rectum, it dilates and becomes a large chamber with two horns, each of which is continuous with its respective mesonephric duct.
A constriction develops at the junction of the sinus with the allantois and gradually obliterates the lumen, thus producing an apex in the sinus.
EXTERNAL GENITALIA
A midline mesenchymal condensation raises the ectoderm into the genital tubercle that lies between the umbilical cord and the cloacal membrane.
A midline urogenital groove extends caudally from the tubercle and is flanked on each side by a urogenital fold. The cloacal membrane in the floor of the groove separates the cloaca from the exterior. Lateral to the urogenital fold is another mesenchymal condensation, the labioscrotal swelling.
Source: Atlas of Human Embryos.