204 



UROGENITAL SYSTEM 



forming glomeruli. Fluid from the ccelom and glomeruli and excreta from the 

 cells of the tubules are carried by ciliary movement into the pronephric ducts. 



The human pronephros is vestigial. It consists of seven pairs of rudimentary 

 pronephric tubules derived from the mesoderm of the nephrotomes (Fig. 198), 

 which are segmented portions of the cell mass intermediate between the primitive 

 segments and the mesodermal layers (somatic and splanchnic). Anlages of 

 pronephric tubules are formed as dorsal nodules in each segment from the seventh 

 to the fourteenth. The nodules hollow out and open into the ccelom. Dorsally 

 and laterally, the tubules of each side unite to form a longitudinal collecting duct. 



Neural tube 



Note-chord 



Cavity of gut 



Splanchnic mesoderm 



Mesoderm of yolk-sac 



Mesodermal segment 

 Cavity of segment 



Intermediate cell mass 

 Anlage of extremity 

 Ccelom 



Kill,, 



.' £, ' „V' - Somatic mesoderm 

 Umbilical vein 



Fig. 198. — Transverse section of a 2.4 mm. human embryo showing the intermediate cell mass or 

 nephrotome (Kollmann's Atlas). 



The tubules first formed in the seventh segment begin to degenerate before those 

 of the fourteenth segment have developed. Caudal to the fourteenth segment 

 no pronephric tubules are developed, but the free end of the collecting duct ap- 

 parently grows caudad, beneath the ectoderm and lateral to the nephrogenic cord, 

 until it reaches, and opens through, the lateral wall of the cloaca. Thus are 

 formed the paired primary excretory (pronephric) ducts. The pronephric tubules 

 begin to appear in embryos of 1.7 mm. (Felix in Keibel and Mall, vol. 2); in 

 2.5 mm. embryos all the tubules have developed and the primary excretory duct 

 is nearly complete. In 4.25 mm. embryos the duct has reached the wall of the 



