part of embryo pronephros 1. The primordial kidney; a vestigial excretory structure or its rudiments developing in the embryo at four weeks, before the mesonephros. Although nonfunctional, its duct is later used by the mesonephros, which arises caudal to it. 2. The definitive excretory organ of primitive fishes. BTO:0001541 BrendaTissueOBO anterior kidney archinephron embryonic kidney head kidney pronephron