Mesonephros

The mesonephros (Greek for "middle kidney") is one of three excretory organs that develop in vertebrates. It serves as the main excretory organ of aquatic vertebrates and as a temporary kidney in reptiles, birds, and mammals. The mesonephros is included in the Wolffian body after Caspar Friedrich Wolff who described it in 1759. (The Wolffian body is composed of : mesonephros + paramesonephrotic blastema)

Structure in Human Development
In humans, the mesonephros forms many units which are similar in structure and function to nephrons of the adult kidney. Each of these consists of a glomerulus, a tuft of capillaries which arises from lateral branches of dorsal aorta and drains into the inferior cardinal vein; a Bowman's capsule, a funnel like structure which surrounds the glomerulus; and a mesonephric tubule, a tube which connects the Bowmann's capsule to the mesonephric duct. A unit consisting of a single glomerulus and the Bowman's capsule surrounding it is called renal corpuscle, and a unit consisting of single renal corpusle with its associated mesonephric tubule is called a "nephron" or "excretory mesonephric unit".

Formation of each mesonephric nephron begins when a bit of the intermediate mesoderm adjacent to the mesonephric duct differentiates to form a mesonephric vesicle. This vesicle then elongates to form the mesonephric tubule, attaching to the mesonephric duct on one side. Meanwhile, an artery from the dorsal aorta begins extending towards the mesonephric tubule. When these two structures contact each other, they form the glomerulus and the Bowman's capsule surrounding it.

The mesonephros as a whole produces urine from the 6th through the 10th week of development. Despite the similarity in structure, function, and terminology, however, the mesonephric nephrons do not form any part of the mature kidney or nephrons. As the more caudal nephrons form, the more cranial nephrons are already degenerating. In females the mesonephros degenerates entirely, though vestigial structures such as Gartner's cysts, the epoophoron, and paroophoron are common. In males, a few of the more caudal tubules will survive and give rise to the efferent ductules of the testis, the epididymis, vas deferens, seminal vesicle, as well as vestigial structures such as the appendix testis, appendix epididymis, and paradidymis.

Differences among species
The mesonephros persists and forms the anterior portion of the permanent kidneys in fishes and amphibians, but in reptiles, birds, and mammals, it atrophies and for the most part disappears rapidly as the permanent kidney (metanephros) begins to develop during the sixth or seventh week. By the beginning of the fifth month of human development, only the ducts and a few of the tubules of the mesonephros remain.