THE METANEPHROS 



199 



The primary excretory duct, or mesonephric duct, is solid in 4.25 mm. em- 

 bryos. A lumen is formed at 7 mm., wider opposite the openings of the tubules. 

 The duct is important, as the ureteric anlage of the permanent kidney grows out 

 from its caudal end, while the duct itself is transformed into the chief genital duct 

 of the male, and its derivatives. The mesonephros is probably a functional ex- 

 cretory organ in human embryos even though its tubules degenerate before the 

 metanephros becomes functional (Bremer, Amer. Jour. Anat., vol. 19, 1916). 

 Degeneration proceeds rapidly in embryos between 10 and 20 mm. long, begin- 

 ning cranially. New tubules are formed at the same time caudally. In all, 83 

 pairs of tubules arise, of which only 26 pairs persist at 21 mm., and these are usu- 

 ally broken at the angle between the collecting and secretory regions. They are 



Suprarenal gland- Sff -y'/;. 

 Post, cardinal vein, 



Glomerulus 

 Bowman's capsule. 



Collecting tubule 

 Secretory tubule 



■Mesonephric duct 

 Miillerian duct 

 Anlage of genital gland 



Fig. 208. — Reconstruction of the contents of the urogenital fold from transverse sections of a 12 mm. 



human embryo. X 95. 



divided into an upper group and a lower group. The collecting portions of the 

 upper group, numbering 5 to 12, unite with the rete tubules of the testis or ovary. 

 In the male they form the efferent ductules of the epididymis. In the female they 

 constitute the epoophoron. Of the lower group a few tubules persist in the male, 

 as the paradidymis with its canaliculus aberrans. In the female they form the 

 paroophoron. 



THE METANEPHROS '-" ' ^ - -^ 



The essential parts of the permanent kidney are the renal corpuscles (glom- 

 erulus with Bowman's capsule), secretory tubules, and collecting tubules. The 

 collecting tubules open into expansions of the duct, the pelvis and calyces. The 

 duct itself is the ureter, which opens into the bladder. Like the mesonephros, the 



