196 



THE UROGENITAL SYSTEM 



Anlages of pronephric 

 duct 



Nephrotome 

 Somatic mesoderm 



-Splanchnic mesoderm 

 Pronephric tubule Cwlom Noiochord Entoderm 



Fig. 204. — Diagrams showing the development of the pronephric duct and pronephric tubules (modi- 

 fied from Felix). A shows a later stage than B. 



Neural tube. 



Notochord- 



Cavity of gut- 



Splanchnic mesoderm. 



Mesoderm of yolk sac. 



Mesodermal segment 

 Cavity of segment 



Intermediate cell mass 



Anlage of extremity 



Ccelom 



■Somatic mesoderm 



'sLMS — Umbilical vein 



Fig. 205. — ^Transverse section of a 2,4 mm. human embryo showing tlie intermediate cell mass or 



nephrotome (KoUmann). 



reaches, and perforates, the lateral wall of the cloaca. Thus are formed the 

 paired primary excretory (pronephric) ducts. The pronephric tubules begin to 



