2. The earliest and simplest kind of excretory organ in the developing vertebrate embryo; however, for humans and other mammals the structure is nonfunctional.
3. In the embryos of higher vertebrates, a vestigial structure consisting of a series of tortuous tubules emptying into the cloaca (terminal part of the gut) by way of the primary nephric duct.
In the human embryo, the archinephros is a very rudimentary and temporary structure, followed by the mesonephros (second of the three excretory organs that develop in a vertebrate embryo, becoming the functioning kidney in fish and amphibians but replaced by the metanephros in higher vertebrates) and still later by the metanephros (third and final excretory organ that develops in a vertebrate embryo).
In birds, reptiles, and mammals, the metanephros replaces the mesonephros as the functional excretory organ and develops into the adult kidney.