i i .1 BOOK OI 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. 'I he 

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

 history, hut 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 - 



IMadder 

 FiR. 351 



cal membrane 



Metanephric blastema 



Metanephric M 

 (inner zone) 



Primitive renal pelvis 



Ureter 



IMC. .< \(). 1'ic.m a reconstruction of the anlage of the kidney (metanephros), etc., of a hum.m 

 embryo at the beginning of the 5th week. Sr.hrrincr. 



by a process of evagi nation. The convoluted renal tubules and glpmeruli arc 

 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 evaginalion continues to grow dorsally in the mesenrliyme 

 Inward the vertebral column, and at the same time becomes <.,!T'-n-ntiatcd 

 into two parts, a narrow stalk and a dilated terminal portion. The sf.ilk 12 

 I lie Inreninncr of I he itrrlrr, the dilated end is the primitive reno 

 .)]() and 351). When the dilated end reaches the ventral side of the unrbraJ 



