786 THE EXCRETORY AND REPRODUCTIVE SYSTEMS 



into the caudal end of the nephrotomic plate (nephrogenic cord). (See figure 

 348E.) The metanephric diverticulum enlarges at its distal end and thus forms 

 the rudiment of the pelvis of the kidney as in the chick (fig. 348F). As the 

 rudimentary pelvis enlarges, it sends out secondary evaginations, the rudiments 

 of the future collecting ducts of the kidney (fig. 3481). Surrounding these 

 secondary diverticula, there is the cellular substance (fig. 3481) of the 

 metanephrogenous tissue, derived from the nephrogenic cord posterior to the 

 caudal limits of the mesonephric kidney. 



In human embryos of 14 to 15 mm. (about seven weeks), four definite 

 primordia of the metanephric urinary system are established as follows (fig. 

 3481): 



( 1 ) Nephrogenous tissue is present which surrounds beginning diverticula 

 of the collecting ducts; 



(2) a system of developing collecting ducts which represents evaginations 

 from the primitive pelvis of the kidney; 



(3) from the primitive pelvis of the kidney arise the rudiments of the 

 anterior and posterior major calyces; and 



(4) the primitive ureter (metanephric duct) of which the primitive pelvis 

 is the distal enlargement. 



(The word calyx refers to a rounded, distal division of the pelvis of the 

 kidney. The plural form of calyx is calyces.) 



From each major calyx, secondary or minor calyces arise (fig. 3481), and 

 from each minor calyx, the primary or straight collecting ducts emerge into 

 the surrounding, nephrogenous, cellular mass. Each primary calyx and its 

 straight collecting-duct rudiments, together with the surrounding nephrog- 

 enous cells, form the rudiment of the future renal lobe. 



The straight collecting ducts continue to elongate and push out into the 

 surrounding nephrogenous tissue. In doing so, the distal end of each collecting 

 duct sends out several (usually three or four) smaller evaginations into the 

 surrounding nephrogenous material. These smaller terminal evaginations rep- 

 resent the rudiments of the arched collecting tubules of the collecting duct 

 system (fig. 348A). Around each of the arched-tubule rudiments, masses of 

 nephrogenous tissue condense into the S-shaped structure typical of the de- 

 veloping renal units of the mesonephric kidney of the frog, chick, and mammal 

 and in the metanephric kidney of the chick. A sigmoid-shaped concentration 

 of nephrogenous cells fuses with each arched collecting tubule and elongates 

 distally, differentiating into the parts of the typical, mammalian, metanephric 

 tubule (fig. 348A-D). 



As the kidney continues to develop, the original primary or straight col- 

 lecting ducts branch repeatedly, forming about 12 generations by the fifth 

 month of human fetal existence. As these branches arise, the pelvis of the 

 kidney and the calyces enlarge considerably, and some of the collecting ducts 



