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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
close to where the latter opens into the cloaca (Figs. 1115, 1116). The metanephros arises in the intermediate cell mass, caudal to the mesonephros, which it resembles in structure. The diverticulum from the Wolffian duct grows dorsalward and forward along the posterior abdominal wall, where its blind extremity expands and subsequently divides into several buds, which form the rudiments of the pelvis and calyces of the kidney; by continued growth and subdivision it gives rise to the collecting tubules of the kidney. The proximal portion of the diverticulum becomes the ureter. The secretory tubules are developed from the metanephros, which is moulded over the growing end of the diverticulum from the Wolffian duct. The tubules of the metanephros, unlike those of the pronephros and mesonephros, do not open into the Wolffian duct. One end expands to form a glomerulus, while the rest of the tubule rapidly elongates to form the convoluted and straight tubules, the loops of Henle, and the connecting tubules; these last join and establish communications with the collecting tubules derived from the ultimate ramifications of the diverticulum from the Wolffian duct. The mesoderm around the tubules becomes condensed to form the connective tissue of the kidney. The ureter opens at first into the hind-end of the Wolffian duct; after the sixth week it separates from the Wolffian duct, and opens independently into the part of the cloaca which ultimately becomes the bladder (Figs. 1117, 1118).


FIG. 1115– Tail end of human embryo twenty-five to twenty-nine days old. (From model by Keibel.) (See enlarged image)



FIG. 1116– Tail end of human embryo thirty-two to thirty-three days old. (From model by Keibel.) (See enlarged image)



FIG. 1117– Tail end of human embryo; from eight and a half to nine weeks old. (From model by Keibel.) (See enlarged image)

  The secretory tubules of the kidney become arranged into pyramidal masses or lobules, and the lobulated condition of the kidneys exists for some time after birth, while traces of it may be found even in the adult. The kidney of the ox and many other animals remains lobulated throughout life.

The Urinary Bladder.—The bladder is formed partly from the entodermal cloaca and partly from the ends of the Wolffian ducts; the allantois takes no share in its formation. After the separation of the rectum from the dorsal part of the cloaca (p. 1109), the ventral part becomes subdivided into three portions: (1) an anterior vesico-urethral portion, continuous with the allantois—into this portion the Wolffian ducts open; (2) an intermediate narrow channel, the pelvic portion; and

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