THE UROGENITAL SYSTEM 



315 



stimulation causes the erection of the penis. Pressure upon the sym- 

 pathetic centers of the hypogastric plexus also stimulates erection. In a 

 llaccid penis the arterial blood supply is reduced through occlusion of the 

 lumen of the vessels by the contraction of local thickenings of their walls. 

 When, however, the artery is dilated, a free flow of blood into the venous 

 spaces of the corpora cavernosa causes them to become engorged with 

 blood and the penis consec^uently erected. 



DEVELOPMENT OF THE UROGENITAL SYSTEM OF MAN 

 A. 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. 



INAL CORD 



::^^MYOTOME CEPIMERO 



-PRIMITIVE DUCT 



MVOCOEL 

 MYOTOME 



PRIMITIVE 

 UCT 



PRINCIPAL DIVISION 



PRIMITIVE DUCT 



SUPPLEMENTARY DIVISION 

 NEPHROSTOME 

 HY POME RE 



^OUTER GLOMERULUS 

 Fig. 279. — A diagram showing three stages, A-C, in the development of the primitive 

 duct and a pronephric tubule. The duct and each tubule connected with it arise from 

 the mesomere. Two types of glomeruli — outer and inner — become associated with 

 the pronephric tubules. (Redrawn after Felix.) 



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

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

 amnio te embryos. The pronephros in man is a functionless rudiment. 

 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 most anterior 

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



