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The pronephros is a small segment of the excretory or renal system in many types of animals that lends itself to development of the kidney. The function of the kidney in species like amphioxus or the lancelet, small marine creatures that burrow into the seabed floor, use the phronephros as their main excretory organ. In more advanced animals like mammals, it is a vestigial organ that develops first to aid in the development of the larger mesonephros renal organ of the embryo. Primitive fish such as certain species of lamprey as well as the larval stages of other fish and select amphibians also have an active phronephros. Aside from humans and other mammals, it exists but has little or no function in reptile and bird species as well.
The pronephros develops as a series of segmented, central germ layers of the embryo known as mesoderm in humans over the course of three to four weeks. It grows within the cervix for about three weeks, then starts to regress in the fourth week of embryo development, after which the attached mesonephros takes over the growth cycle of the renal system. As the mesonephros matures, it grows in a crainal-caudad manner, or from head to toe, eventually connecting to the urogenital passage. This is most important if the embryo is developing as a male, as this connection is also part of genital function. The fully-developed mesonephros will later serve as a method of urine drainage from the kidney, but, in more primitive organisms, this role is played by the pronephros in adult creatures.
The function of the kidney among different types of organisms is broken down into three divisions: the pronephros and pronephric duct, the mesonephros, and the metanephros, which develops at the five week stage in mammals. The pronephros is generally considered to be a paired organ, however, that is connected to the nephrons or nephrostomes directly above it, and, in primitive animals like hagfish, urine is filtered directly through the nephrons into the pronephros. While parts of the kidney like the nephrostomes are essential in primitive fish, they don't exist in the mesonephros, which takes over renal function in mammals.
Both primitive and advanced organisms rely on another feature of the renal system known as the glomerulus, which exists alongside the pronephros and mesonephros. The glomerulus is also known as the Malpighian tuft, and is a series of intertwined capillary blood vessels within the nephron structure that is used to remove urine and toxins from the blood. These liquids are drawn into either the pronephros or mesonephros directly from the glomerulus.