Pronephros

Pronephros the most primitive of the three excretory organs that develop in vertebrate, corresponding to the first stage of kidney development.

The pronephros develops in the anterior nephrotomes of all vertebrates. It is a paired organ, consisting of a series of nephrons filtering urine from both the pericardium fluids via openings called nephrostomes and blood from the glomerulus.

The organ is active in adult forms of some primitive fish, like lampreys or hagfish. It is present at the embryo of more advanced fish and at the larval stage of amphibians. In human beings, it is rudimentary, appears at the end of the third week (day 20) and replaced by mesonephros after 3.5 weeks.