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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 SYSTEM. The metanephros or hind kidney begins as a diverticulum from the dorsal side of the Wolffian duct close to its opening into the Pro. N. Pronephros. N. Nephrostome. M. N. Mesonephros. M. C. Malpighian corpuscle. Mt. N. Metanephros. T. Testis. B. Bladder. E. Epididymis. Clo. Cloaca. O. G. 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. Par. O. ParoOphoron. P. H. Pedunculst°d hydatid. F. T. Fallopian tube. S. G. Sexual gland. U. Uterus. cloaca (see fig. 6, Mt.N.); this occurs about the fourth week of intra-uterine life, and the diverticulum grows forward (cephalad), dorsal to the hind end of the Wolffian body. In doing this it forms a duct—the metanephric duct or ureter—the cephalic end of which enlarges and divides to form the calices of the kidney. From the calices numerous smaller ducts grow into the mesoderm of the hind (caudal) end of the intermediate cell mass and become the collecting From A. F. Dixon, Cunningham's Text-Book of Anatomy. Fic. 7.—Transverse Section through the Body of a 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 series with the mesonephric tubules, develop independently in the intermediate cell mass and so form all the 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 work of the mesonephros, which now atrophies; its surface is distinctly lobulated, a condition which persists until after birth. At first, as has been stated, the ureters open into the Wolffian ducts, but later on each gains a 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 developed from that 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 month becomes more pyriform, the stalk of the 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 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 article on the REPRODUCTIVE SYSTEM. The Mullerian ducts (fig. 6, M.D.) are formed after the Wolffian ducts are fully developed. A ridge appears in the intermediate cell mass ventral to the Wolffian duct, and into the anterior (cephalic) end of this a tubular process of the coelom forces its way back-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 Quain's Anatomy, vol. i. (Longmans, Green & Co., London, 1908) ; J. M'Murrich, The Development of the Human Body (Rebman, London, 1906), and A. Keith, Human Embryology and Morphology (Arnold, London). Comparative Anatomy. In the Acrania (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 atrium, which is practically the exterior of the animal, by an opening called the nephridiopore. There is reason to believe that we have here a pronephros of a very primitive type and arranged on the same plan, in many respects, as the simple nephridia of such lowly forms as the earthworm. There is nothing to indicate that a mesonephros is present, nor are there any Malpighian corpuscles or longitudinal ducts. Among the Cyclostomata (lampreys and hags) the pronephros persists throughout life in Bdellostoma and probably in the 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 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 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 fish) the pronephros is usually aborted in the adult and the mesonephros is the functional kidney. As the genital glands have 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 early aborted than in the last subclass, and the mesonephros is divided into an anterior or genital part, which receives the vasa efferentia in the male from the testis and thus is the first 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 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 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 wall of the cloaca and is not homologous with the allantoic bladder of higher forms. A good deal of the kidney (mesonephros) as it appears to the naked eye is composed of lymphoid tissue. In the Amphibia the snake-like forms (Gymnophiona) show a very primitive arrangement of the kidney tubules, each having its nephrostome, Malpighian capsule and 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 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 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 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 Mammalia the 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, bear, seal and 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 case in many monkeys, carnivores and rodents. In the Monotremata (Ornithorhynchus and 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 harmony of the two from a broad point of view is very striking. For further details see Parker and Haswell, Text-Book of Zoology (Macmillan, London, 1897) ; Wiedersheim's Comparative Anat. of Vertebrates, translated by W. N. Parker (London, 1907) ; Gegenbaur, Vergleich. Anat. der Wirbeltiere (Leipzig, 1901). 'URMIA (the name as written by the Persians is Urumieh and Urmieh; the inhabitants of the place say Urmi), a town in the province of Azerbaijan in Persia, situated at an elevation of 4400 ft., in an extremely fertile and highly cultivated plain, 78 m. S.W. of Tabriz (12o by road), 11 to 12 m. from the western shore of the 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 round for miles and even penetrate the heart of the town. The streets are broader than is usual in Persian cities, and most of them have a stream of water running down the middle. There are a busy bazaar and some old mosques. The population is about 35,000, and there are post and telegraph offices. The only building of importance is the ark, or citadel, a walled building in the centre of the town containing an arsenal and barracks for a small garrison. Urmia has for many years been the headquarters of various missions to the Nestorians of the neighbourhood: an American mission (since 1835) representing the " Board of the Foreign Missions of the Presbyterian church of the United States of America "; the French Lazarists (since 1840); British, " The Anglican Mission " founded by Archbishop Benson (1884), and a Russian mission (Orthodox, since 1902). Urmia is the capital of a fertile district so m. long and about 20 m. broad, having the same name and containing more than 300 flourishing villages. It exports great quantities of dried fruit and excellent tutun, tobacco for chibuks, or Turkish pipes.
End of Article: DUCTIVE SYSTEM
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