The pronephros is the earliest stage of kidney development in vertebrates. It forms during the embryonic phase and is primarily functional in some lower vertebrates, such as fish and amphibians. The pronephros consists of a series of tubules that help in the excretion of waste products. In higher vertebrates, including mammals, the pronephros is usually transient and is replaced by the mesonephros and later the metanephros, which develop into the permanent kidneys. Although the pronephros is not functional in adult mammals, its formation is crucial for understanding kidney development and evolutionary biology.