We will primarily discuss the development and function of the kidneys in the excretory system. The kidneys originate from lateral plate mesoderm that differentiates into the nephric ridge, which buds into blocks of tissue called nephrotomes. Blood flow to the glomerulus comes from the afferent arteriole and leaves through the efferent arteriole. If the renal capsule encloses the glomerulus, it is called an internal glomerulus. If into the coelom, it is called an external glomerulus.
Internal glomeruli are common among vertebrates.
The holonephros is currently found only in the larvae of hagfishes, where it develops from the entire nephric ridge. The remaining kidney types differ in their placement in the body and the extent to which they run through the body. The pronephros is the first formed kidney of a vertebrate embryo, which lies dorsal to the pericardial cavity and forms the archinephric duct. The pronephros is found in primitive fishes and in the larval and embryonic stages of some vertebrates. This is also called the head kidney.