Urinary System

When the pronephric tubules disappear, which they do early in the embryo's development, the Wolffian duct persists as the drain for another and more important series of tubules (the mesone phros or middle kidney), formed in the intermediate cell mass behind the pronephros (fig. 4). There is some doubt whether these tubes are strictly homologous and in series with those of the pronephros; they are of later development.

By about the sixth week of intra-uterine life these tubules reach their maximum development and form the W olffian body, which projects into the coelom as the now very definite Wolffian ridge and acts as the functional excre tory organ of the embryo. When the permanent kidney is formed this organ degenerates; for its ultimate fate see REPRODUCTIVE SYSTEM.

The motanephros or hind kid ney begins as a diverticulum from the dorsal side of the Wolffian duct close to its opening into the cloaca (see fig. 4) ; this organ occurs about the fourth week of intra-uterine life, and the diverti culum 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 small er ducts grow into the mesoderm of the hind (caudal) end of the intermediate cell mass and be come the collecting tubes of the kidney. While this is going on another set of tubules, probably in series with the mesonephric tubules, develops independently in the intermediate cell mass and forms the rest of the tubular sys tem of the kidney. Toward these tubules, at one point, branches from the aorta push their way and invaginate each tube, thus forming the Malpighian cor puscles.

By the eighth week the kidney is definitely formed and takes over the excretory work of the mesonephros, which now atro phies.

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 ven trally until they reach their per manent connection with the allantoic bladder.

The bladder is developed from that part of the allantois which is nearest the cloaca. At first it is tubular, but after the second month becomes pyriform, the stalk of the pear corresponding to the fibrous urachus which reaches the umbilicus.

The Affillerian ducts (fig. 4) , are formed after the Wolf flan ducts are fully developed. A ridge appears in the intermedi ate cell mass ventral to the Wolffian duct, and into the anterior (cephalic) end of this a tubular process of the coelom forces its way backward (caudad). Before reaching the cloaca the two Milllerian ducts coalesce and open between the orifices of the two Wolffian ducts. These ducts form the oviducts, uterus and at least

part of the vagina.

Comparative Anatomy.

In the Acrania (Amphioxus) the nephridial tubules are segmental and are only found in the pharyngeal region ; each opens into the coelom by several ciliated funnels (nephrostomes) and also into the atrium, which is the exterior if the animal, by an opening called the nephridiopore. 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 nephridio pores. In the Teleostomi (bony and ganoid fish) the pronephros is aborted and the mesonephros is the functional kidney.

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.

The Dipnoi or mudfish are remarkable for having a cloacal caecum which probably functions as an urinary bladder.

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 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 reach ing the cloaca. Birds resemble reptiles very closely in their urinary system except that there is no bladder and that the ureters and vasa deferentia open independently into the cloaca.

In the Mammalia the bean shape of the kidney is fairly char acteristic. In foetal life the organ is always lobulated, and this often persists in the adult as in the ox, bear, seal and whale.

Anatomy; J. McMurrich, The Development of the Human Body (7th ed. 1923) ; A. Keith, Human Embryology and Morphology; Parker and Haswell, Text-Book of Zoology (Lon don, 1897) ; Wiedersheim's Comparative Anat. of Vertebrates, trans. W. N. Parker (London, 1907) ; Gegenbaur, Vergleich. Anat. der Wirbeltiere (Leipzig, 19o1).