1. The 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 (embryonic organ of excretion in reptiles, birds, and mammals that develops into the kidney) in higher vertebrates.
2. The second type of excretory organ to develop in the vertebrate embryo.

It consists of a series of twisting tubules which arise from the mephrogenic cord caudal to the pronephros and that at one end from the glomerulus and at the other connect with the excretory mesonephric duct.

The organ is the permanent kidney in lower animals, but in human and various other mammals it is functional only during early embryonic development and is later replaced by the metanephros, although the duct system is retained and incorporated into the male reproductive system.