The Collaborative International Dictionary of English v.0.48:

Mesonephros \Mes`o*neph"ros\, n. [NL., fr. Gr. me`sos middle +
   nefro`s kidney.] (Anat.)
   The middle one of the three pairs of embryonic renal organs
   developed in most vertebrates; the Wolffian body.
   [1913 Webster]