1. pronephros
2. mesonephros
3. metanephros
1. Pronephros: The pronephros is the most primitive of the three embryonic kidney structures. It develops in the earliest stages of vertebrate embryogenesis but is non-functional and regresses in most vertebrates. In some organisms, such as lampreys, the pronephros functions as the major excretory organ during early development.
2. Mesonephros: The mesonephros is the intermediate kidney structure that develops after the pronephros regresses. It functions as the primary excretory organ in fish and amphibians, and also plays a role in waste excretion and maintenance of fluid balance in reptiles and birds. The mesonephros eventually regresses in most mammals but contributes to the development of the male reproductive system.
3. Metanephros: The metanephros is the final and most advanced kidney structure that develops in vertebrate embryos. It serves as the functional kidney in adult mammals, including humans. The metanephros forms from the ureteric bud, an outgrowth of the mesonephric duct, and the metanephric mesenchyme, which differentiates into nephrons. The metanephros is responsible for filtering waste products from the blood, regulating water and electrolyte balance, and maintaining acid-base homeostasis.