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DUCTIVE SYSTEM

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Originally appearing in Volume V27, Page 800 of the 1911 Encyclopedia Britannica.
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DUCTIVE See also:

SYSTEM . The metanephros or See also:hind See also:kidney begins as a diverticulum from the dorsal See also:side of the Wolffian duct See also:close to its opening into the See also:Pro. N. Pronephros. N. Nephrostome. M. N. Mesonephros. M. C. Malpighian corpuscle.

Mt. N. Metanephros. T. Testis. B. See also:

Bladder. E. Epididymis. Clo. See also:Cloaca. O.

G. See also:

Organ of Giraldes. R. Rectum. V. D. Vas deferens. M. D. Mullerian duct. U.M. Uterus masculinus.

W. D. Wolffian duct. 0. Ovary. Ur. Ureter. Ep. 0. Epoophomn. S. H.

Sessile hydatid. See also:

Par. O. ParoOphoron. P. H. Pedunculst°d hydatid. F. T. Fallopian See also:tube. S. G.

Sexual gland. U. Uterus. cloaca (see fig. 6, Mt.N.); this occurs about the See also:

fourth See also:week of See also:intra-uterine See also:life, and the diverticulum grows forward (cephalad), dorsal to the hind end of the Wolffian See also:body. In doing this it forms a duct—the metanephric duct or ureter—the cephalic end of which enlarges and divides to See also:form the calices of the kidney. From the calices numerous smaller ducts grow into the mesoderm of the hind (caudal) end of the intermediate See also:cell See also:mass and become the See also:collecting From A. F. See also:Dixon, See also:Cunningham's See also:Text-See also:Book of See also:Anatomy. Fic. 7.—Transverse See also:Section through the Body of a See also:Rat Embryo. The position where the germinal epithelium arises is indicated at a.

tubes of the kidney. While this is going on another set of tubules, probably in See also:

series with the mesonephric tubules, develop independently in the intermediate cell mass and so form all the See also:rest of the tubular system of the kidney. Toward these tubules, at one point, branches from the aorta push their way and invaginate each tube, thus forming the Malpighian corpuscles. By the eighth week the kidney is definitely formed and takes over the excretory See also:work of the mesonephros, which now atrophies; its See also:surface is distinctly lobulated, a See also:condition which persists until after See also:birth. At first, as has been stated, the ureters open into the Wolffian ducts, but later on each gains a See also:separate opening into the cloaca, and eventually these shift in a ventral direction until they reach their permanent connexion with the allantoic bladder. The bladder is See also:developed from that See also:part of the cloaca from which the allantois has grown out, and also from that part of the allantois which is nearest the cloaca. At first it is a tubular structure, but after the second See also:month becomes more pyriform, the stalk of the See also:pear corresponding to the fibrous urachus which reaches the umbilicus. Most of that part of the tubular allantois which lies between the permanent openings of the ureters and the Wolffian ducts becomes the urinary sinus and does not dilate in the same way that the permanent bladder does. This, in the See also:female, forms the whole of the urethra, and in the male the upper part of the prostatic urethra. Behind (caudad) the urinary sinus is the urogenital sinus, which is treated of in the See also:article on the REPRODUCTIVE SYSTEM. The Mullerian ducts (fig. 6, M.D.) are formed after the Wolffian ducts are fully developed.

A See also:

ridge appears in the intermediate cell mass ventral to the Wolffian duct, and into the anterior (cephalic) end of this a tubular See also:process of the coelom forces its way back-See also:ward (caudad). Before reaching the cloaca the two Mullerian ducts coalesce and open between the orifices of the two Wolffian ducts. These ducts, as is shown in the article, on the REPRODUCTIVE SYSTEM, form the oviducts, uterus and at least part of the vagina. For further details and literature see See also:Quain's Anatomy, vol. i. (See also:Longmans, See also:Green & Co., See also:London, 1908) ; J. M'Murrich, The Development of the Human Body (Rebman, London, 1906), and A. See also:Keith, Human See also:Embryology and See also:Morphology (See also:Arnold, London). See also:Comparative Anatomy. In the See also:Acrania (See also:Amphioxus) the nephridial tubules are segment-ally arranged and are only found in the pharyngeal region; each opens into the coelom by several ciliated funnels called nephrostomes, and also into the See also:atrium, which is practically the exterior of the See also:animal, by an opening called the nephridiopore. There is See also:reason to believe that we have here a pronephros of a very See also:primitive type and arranged on the same See also:plan, in many respects, as the See also:simple nephridia of such lowly forms as the See also:earthworm. There is nothing to indicate that a mesonephros is See also:present, nor are there any Malpighian corpuscles or See also:longitudinal ducts. Among the See also:Cyclostomata (lampreys and hags) the pronephros persists throughout life in Bdellostoma and probably in the See also:hag (Myxine), but a Wolffian (archinephric) duct has been evolved so that the tubules no longer open on the surface by nephridiopores.

It has been surmised that in a transitional type the tubules opened into a groove on each side of the surface of the animal and that the edges of this, coming together, formed a duct. At any See also:

rate the superficial openings of the primitive nephridia make it probable that the Wolffian duct was originally of ectodermal origin. A mesonephros has now appeared behind (caudad) the pronephros, though it is not certain whether its tubules (mesonephridia) are in series with those of the pronephros or whether they are structures on a more dorsal See also:plane; but they certainly open into the Wolffian duct, which also drains the pronephros, and so this duct is functionally simply a ureter and has nothing to do with the sexual glands. No Mullerian duct has yet been evolved. In the Teleostomi (bony and ganoid See also:fish) the pronephros is usually aborted in the adult and the mesonephros is the functional kidney. As the genital glands have See also:special coelomic relations the Wolffian duct is still merely a ureter, and in the Teleostei at least there is no true Mullerian duct. In the Elasmobranchii (sharks and rays) the pronephros is more completely and more See also:early aborted than in the last subclass, and the mesonephros is divided into an anterior or genital part, which receives the See also:vasa efferentia in the male from the testis and thus is the first See also:appearance phylogenetically of an epididymis and a posterior or renal part. The Wolffian duct therefore acts both as a vas deferens for the sperm and a ureter for the urine, though in the female it is merely a ureter. In the hindmost part of the mesonephros there are separate ducts which are called ureters and open into the See also:lower part of the Wolffian duct in the same way that the metanephric ducts of the Amniota do; it is, however, very doubtful whether they are really homologous with these ducts. The Mullerian duct (see REPRODUCTIVE SYSTEM) is present in elasmobranchs and according to See also:modern views arises as a backgrowth from the coelom as in the Amniota. The Dipnoi or mudfish are remarkable for having a cloacal caecum which probably functions as an urinary bladder. It is situated on the dorsal See also:wall of the cloaca and is not homologous with the allantoic bladder of higher forms.

A See also:

good See also:deal of the kidney (mesonephros) as it appears to the naked See also:eye is composed of lymphoid See also:tissue. In the See also:Amphibia the snake-like forms (Gymnophiona) show a very primitive arrangement of the kidney tubules, each having its nephrostome, Malpighian See also:capsule and See also:short convoluted part leading to the Wolffian duct which acts both as ureter and vas deferens. In the adult Anura (frogs and toads) the nephrostomes lose theirconnexion with the nephridia and communicate with the renal See also:veins. In the amphibians a true allantoic bladder first appears as a diverticulum from the ventral wall of the cloaca; in different forms it may be single, bilobed or even See also:double. In Reptilia the hind kidney or metanephros is developed and takes over all the excretory work; it is usually lobulated, its nephridia are never provided with nephrostomes and its duct (the ureter) opens into the Wolffian duct or vas deferens before reaching the cloaca. The allantoic bladder is present in the Lacertilia (lizards) and Chelonia (turtles), but is absent in others. Birds resemble See also:reptiles very closely in their urinary system except that there is never any bladder and that the ureters and vasa deferentia open independently into the cloaca. In the See also:Mammalia the See also:bean shape of the kidney is fairly characteristic. In foetal life the organ is always lobulated, and this sometimes persists throughout adult life as in the ox, See also:bear, See also:seal and See also:whale. More often the lobulation disappears on the surface and is only imperfectly represented, on making a section, by the pyramids; even these in some cases fuse so closely that their apices appear as a single papilla. This is the See also:case in many monkeys, carnivores and rodents. In the See also:Monotremata (Ornithorhynchus and See also:Echidna) there is an allantoic bladder, but the ureters open into the cloaca as they do in birds.

In all other mammals they have reached the bladder and open into it by valvular orifices. On comparing the embryology (ontogeny) of the urinary system with its comparative anatomy (phylogeny) the See also:

harmony of the two from a broad point of view is very striking. For further details see See also:Parker and Haswell, Text-Book of See also:Zoology (See also:Macmillan, London, 1897) ; Wiedersheim's Comparative Anat. of Vertebrates, translated by W. N. Parker (London, 1907) ; See also:Gegenbaur, Vergleich. Anat. der Wirbeltiere (See also:Leipzig, 1901). 'See also:URMIA (the name as written by the Persians is Urumieh and Urmieh; the inhabitants of the See also:place say Urmi), a See also:town in the See also:province of See also:Azerbaijan in See also:Persia, situated at an See also:elevation of 4400 ft., in an extremely fertile and highly cultivated See also:plain, 78 m. S.W. of See also:Tabriz (12o by road), 11 to 12 m. from the western See also:shore of the See also:lake of the same name, in 370 34' N. and 450 4' E. It is surrounded by a wall and deep dry ditch that can be flooded, and is encircled by orchards and gardens which extend all See also:round for See also:miles and even penetrate the See also:heart of the town. The streets are broader than is usual in See also:Persian cities, and most of them have a stream of See also:water See also:running down the See also:middle. There are a busy See also:bazaar and some old mosques. The See also:population is about 35,000, and there are See also:post and See also:telegraph offices.

The only See also:

building of importance is the See also:ark, or citadel, a walled building in the centre of the town containing an See also:arsenal and See also:barracks for a small See also:garrison. Urmia has for many years been the headquarters of various See also:missions to the See also:Nestorians of the neighbourhood: an See also:American See also:mission (since 1835) representing the " See also:Board of the See also:Foreign Missions of the Presbyterian See also:church of the See also:United States of See also:America "; the See also:French Lazarists (since 1840); See also:British, " The See also:Anglican Mission " founded by See also:Archbishop See also:Benson (1884), and a See also:Russian mission (Orthodox, since 1902). Urmia is the See also:capital of a fertile See also:district so m. See also:long and about 20 m. broad, having the same name and containing more than 300 flourishing villages. It exports See also:great quantities of dried See also:fruit and excellent tutun, See also:tobacco for chibuks, or See also:Turkish pipes.

End of Article: DUCTIVE SYSTEM

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