430 



COMPARATIVE ANATOMY 



DEVELOPMENT OF THE UROGENITAL SYSTEM 

 The Urinary System 



The urinary organs, except the lining of the bladder, are mesodermal in 

 origin, both urinary tubules and ureter being formed from the nephrotome. 

 Strangely enough, three renal organs, pronephros, mesonephros, and 

 metanephros, develop in succession in the human embryo as well as in all 

 amniote embryos. The pronephros in man is a f unctionless rudiment. 



NAL CORD 



MVOCOEL 



^^/MYOTOME (EPIMERE) 



-PRIMITIVE DUCT 



A. 



MESOMERE 



MYOTOME 



NOTOCHORD 

 HYPOMfRE 



INNER GLOMERULUS 



AOR 



NEPHROSTOMY 



PRINCIPAL DIVISION 



PRIMITIVE DUCT 



SUPPLEMENTARY DIVISION 

 NEPHROSTOME 

 HYPOMERE 



^OUTER GLOMERULUS 

 Fig. 354. — A diagram showing three stages A-C in the development of the primitive 

 duct. The duct and each tubule connected with it arises from the mesomere. Two 

 types of glomeruli — outer and inner — become associated with the pronephric tubules. 

 (Redrawn after Felix.) 



The mesonephros probably functions during foetal life. The metanephros 

 is the definitive kidney. The occurrence of three kidneys in the embryos 

 of amniotes is best interpreted by the evolution theory. 



Pronephros. The pronephros or "head kidney" is the anterior-most 

 of the three, and in the human embryo develops from the nephrotomes of 

 segments seven to fourteen. The first of the pronephric tubules makes its 

 appearance in a 1.7 mm. embryo, and all eight tubules are formed by the 

 time the embryo has reached a length of 2.5 mm. Degeneration begins 

 soon and the anterior tubules disappear before the posterior ones are 

 differentiated. 



The development of pronephric tubules is initiated by the parietal 

 layer of the nephrotome or intermediate cell mass from which cells are 



