DEVELOPMENT OF EXCRETORY SYSTEM 787 



are drawn into and are taken up into the walls of the expanding calyces. In 

 the fully formed kidney, about 20 of these large straight collecting ducts open 

 into the papillary ducts at the apex of the renal lobe or pyramid into a minor 

 calyx (fig. 345A, B) (Felix, '12). The outer peripheral portion of the kidney, 

 containing the glomeruli and various parts of the renal units (nephrons), 

 forms the cortex of the kidney, while the inner portion, in which lie the 

 straight collecting and papillary ducts, forms the medulla (fig. 345B). 



2) Formation of the Capsule. The metanephrogenous tissue around the 

 developing pelvis and collecting ducts of the kidney becomes divided into 

 inner and outer zones. The inner zone cells differentiate into the renal units, 

 whereas the outer zone cells form the interstitial connective tissue and outer, 

 connective-tissue capsule of the kidney. 



3) Changes in Position of the Developing Kidney. The early developing 

 kidney is located in the pelvic area at the caudal end of the mesonephric 

 kidney. As the mesonephric kidney declines in size and moves caudally, the 

 metanephric kidney pushes anteriorly and takes its final retroperitoneal posi- 

 tion at birth in the region of the first lumbar area. (Cf. figs. 3B-F; 348E-G.) 



6. Urinary Ducts and Urinary Bladders 

 a. Types of Urinary Ducts 

 The following two types of urinary ducts were mentioned above: 



( 1 ) The pronephric duct, which later becomes the mesonephric duct, is 



the functional urinary duct in the larval embryonic form of fishes, 

 amphibia, reptiles, birds, and mammals. It continues to be the main 

 urinary duct in adult fishes and amphibia, particularly in the female. 

 (See (2) below.) 



(2) A second type of urinary duct represents an outgrowth of the meso- 

 nephric duct. Examples of this type are: (a) the metanephric duct 

 and its branches in the kidneys of reptiles, birds, and mammals, 

 (b) the collecting ducts in the mesonephric kidney of all vertebrates, 

 and (c) the adult urinary ducts in the posterior kidney region of 

 certain male fishes, such as are present in the shark, Squalus acanthias, 

 and in the salamander, Triton taeniatus. 



b. Urinary Bladders 



During the development of the urinary system in the mammal, the ventral 

 portion of the cloacal area and its allantoic diverticulum become separated 

 from the dorsal cloacal or rectal area by the caudal growth of a fold of tissue, 

 known as the urorectal fold or cloacal septum. The cloacal septum even- 

 tually divides the cloaca into a ventral bladder and urogenital sinus region, 

 and a dorsal primitive rectum (fig. 348E-G). As this development proceeds, 

 the proximal portions of the mesonephric and metanephric ducts are taken up 



