394 



TEXT-BOOK OF EMBRYOLOGY. 



in the embryos of higher forms. Beginning in the Reptiles and continuing up through the 

 series of Birds and Mammals, another set of urinary organs — the kidneys — develops. The 

 mesonephroi also develop in these forms, even to a high degree, thus repeating the ancestral 

 history, but retain their original function only in the earlier embryonic stages. 



THE KIDNEY (METANEPHROS) . 



The kidneys are the third set of urinary organs to develop. They assume 

 the function of the mesonephroi as the latter atrophy, and constitute the per- 

 manent urinary apparatus. Each kidney is derived from two separate anlagen 

 which unite secondarily. The epithelium of the ureter, renal pelvis, and 

 straight renal tubules (collecting tubules) is derived from the mesonephric duct 



Mesonephros 



Mesonephric duct 



Metanephric blastema 



Metanephric blastema 

 (inner zone) 



Primitive renal pelvis 



Cloacal membrane 



Urete 



Fig. 349. — From a reconstruction of the anlage of the kidngy (metanephros), etc., of a human 

 embryo at the beginning of the 5th week. Sclireiner. 



by a process of evagination. The convoluted renal tubules and glomeruli are 

 derived directly from the mesenchyme, and in this respect resemble the meso- 

 nephric tubules and glomeruli. 



The Ureter, Renal Pelvis and Straight Renal Tubules. — During the 

 fourth week (in embryos of about 5 mm.) a small, hollow, bud-like evagination 

 appears on the dorsal side of each mesonephric duct near its opening into the 

 cloaca. The evagination continues to grow dorsally in the mesenchyme 

 toward the vertebral column, and at the same time becomes differentiated 

 into two parts, a narrow stalk and a dilated terminal portion. The stalk is 

 the forerunner of the ureter, the dilated end is the primitive renal pelvis (Figs. 

 349 and 351). When, the dilated end reaches the ventral side of the vertebral 



