The pronephros is the earliest stage of kidney development in vertebrates. It is a simple, tubular structure that forms in the embryo and is primarily responsible for excreting waste. The pronephros is functional in some lower vertebrates, such as fish and amphibians, but it is usually transient in higher vertebrates. In most mammals, including humans, the pronephros is replaced by the mesonephros during development. The mesonephros takes over the excretory function until the final kidney structure, the metanephros, develops. Although the pronephros is not functional in adult mammals, its formation is an important part of kidney evolution and development.