The middle one of the three pairs of embryonic renal organs developed in most vertebrates; the Wolffian body.

the primitive excretory (urinary) apparatus of the embryo (cf. metanephros or definitive kidney).

the second temporary stage of kidney development (pro-, meso-, meta-). The intermediate mesonephros develops and disappears with the exception of its duct, the mesonephric duct, which will form the male reproductive duct system. In males, the mesonephric tubules go on to form the ducts of the testis. In females, these degenerate. A few mesonephric tubules remain as efferent ductules in the male and vestigial remnants in the female.( More? Urogenital Notes)