The mesonephros is a temporary kidney structure that develops in embryos of vertebrates, including humans. It forms during the early stages of development and plays a crucial role in filtering waste from the blood before the permanent kidneys, known as metanephros, take over their function. In humans, the mesonephros is functional during the first trimester of pregnancy but eventually degenerates. It is replaced by the metanephros, which becomes the main excretory organ. The mesonephros also contributes to the development of certain reproductive structures, such as the mesonephric duct, which is important for the formation of male reproductive organs.