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the excretory organ in the embryos of lower vertebrates; in higher vertebrates, including man, the pronephros is vestigial and nonfunctional. During embryonic development, the pronephros is replaced by the mesonephros. In most taxonomic groups the excretory ducts of the pronephros have a single filtering apparatus—a vascular glomerulus located near the funnel (nephrostome) through which each tubule of the pronephros opens into the coelom. The other ends of the tubules merge and form the rudiment of the pronephrotic canal, which grows toward the posterior end and terminates in the cloaca.