ANATOMY

Detailed explanation-1: -Kidney development proceeds in three sequential stages along the urogenital ridge in a rostro-caudal direction, forming first the pronephros, followed by the mesonephros, and finally the metanephric kidney at the caudal end of the urogenital ridge.

Detailed explanation-2: -Pronephros is the earliest nephric stage in humans, and constitutes the mature kidney in most primitive vertebrates. Mesonephros develops by the formation of mesonephric tubules from the intermediate mesoderm, it is the principal excretory organ during early embryonic life (4-8 weeks).

Detailed explanation-3: -The mesonephros is functional in adult higher fishes and amphibians, but only during embryogenesis in mammals. The metanephros is the final of the three embryonic organs; in mammals, this forms the permanent kidney.

Detailed explanation-4: -Early in development, the intermediate mesoderm in the extreme cranial end of the embryo forms a vestigial and transitory excretory system, the pronephros. Later, in the thoracic region, the mesonephros and mesonephric ducts form in the urogenital ridge.