Archinephros

The archinephros, or holonephros, is a primitive kidney that has been retained by the larvae of hagfish and some caecilians.{{Cite book |last=Kisla |first=Seth M. |url=https://books.google.com/books?id=Hl_JvHqOwoIC |title=Vertebrates: Structures and Functions |publisher=Taylor & Francis (CRC Press) |year=2011 |isbn=978-1-4398-4052-8 |location=Boca Raton, Florida |pages=434 |url-access=limited |via=Google Books}} A recent author has referred to this structure as "the hypothetical primitive kidney of ancestral vertebrates".{{Cite book |url=https://books.google.com/books?id=9X80EAAAQBAJ |title=Recent Advancements and Research in Biological Sciences |vauthors=Shukla AK, Mani, NS |publisher=Thanuj International Publishers |year=2021 |isbn=978-81-952529-1-6 |veditors=Tripathi RB, Vijaya P, Senan VP, Sona A |location=Tamil Nadu, India |pages=70 |chapter=Comparative Anatomy: Urinogenital system of Vertebrates |url-access=limited |via=Google Books}} In the earliest vertebrates, this structure potentially extended the entire length of the body and consisted of paired segmental structures which drained via a pair of archinephrenic ducts into the cloaca. The entire structure arises from the nephric ridge, which in higher animal embryos gives rise to nephrotomes and the pronephroi at around 4 weeks gestation in humans.{{Cite book |last=Larsen |first=William J. |title=Human Embryology |publisher=Churchill Livingstone |year=1993 |isbn=0-443-08724-5 |pages=235–6}} The pronephroi are supplanted by mesonephroi and finally by definitive kidneys, the metanephroi, by around 5 weeks gestation. The archinephros is nonfunctional in humans and other mammals.{{cite web |title=Archinephros |url=http://wordinfo.info/results/archinephros |website=WordInfo |accessdate=23 September 2018}}

The three types of mature vertebrate kidneys develop from the archinephros: the pronephros from the front section, the mesonephros from the mid-section and the metanephros from the rear section.{{cite web |title=Archinephros |url=https://www.britannica.com/science/archinephros |website=ENCYCLOPÆDIA BRITANNICA |accessdate=23 September 2018}}