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pronephros pro·neph·ros (prō-něf'rəs, -rŏs')
n. pl. pro·neph·roi (-roi) or pro·neph·ra (-rə)
A kidneylike organ, being either part of the most anterior pair of three pairs of organs in a vertebrate embryo, usually disappearing early in embryonic development.
most primitive of the three vertebrate kidneys, active in the adults of some primitive fish (lampreys and hagfish), the embryos of more advanced fish, and the larvae of amphibians. It is a paired organ consisting of a series of nephrons that filter urine from both the pericardial cavity fluids via openings called nephrostomes and the bloodstream from the glomerulus. Cells of the nephron tubule may secrete nitrogenous wastes into the urine and reabsorb water and nutrients. Urine passes from the nephrons into one of two long tubes, the Wolffian ducts, which run along either side of the body cavity and empty into a bladderlike urogenital sinus.