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1. The definitive excretory organ of primitive fishes. Synonym(s): head kidney
2. In the embryos of higher vertebrates, a vestigial structure consisting of a series of tortuous tubules emptying into the cloaca by way of the primary nephric duct; in the human embryo, the pronephros is a very rudimentary and temporary structure, followed by the mesonephros and still later by the metanephros.
[pro- + G. nephros, kidney]