200 



THE UROGENITAL SYSTEM. 



T-he primary excretory (Wolffian) duct, or mesonephric duct, is solid 

 in 4.25 mm. embryos. 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 niale, and its derivatives. 



That the human mesonephros is a functional excretory organ is 

 plausible (Bremer, 1916), but not proved. Degeneration proceeds rapidly 

 in embryos between 10 and 20 mm. long, beginning 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 usually 

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

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



Suprarenal gland' y. 



?w 



Post, cardinal vein. 



Glomerulus 

 Bowman's capsule- 



'ollecting tubule 

 Secretory tubule 



Mesonephric duel 

 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. 



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 epididy- 

 mis. In the female they constitute the epoophoron. Of the lower group 

 a few tubules persist in the male, as the paradidymis. In the female they 

 %>rm the paroophoron. 



THE METANEPHROS 



The essential parts of the permanent kidney are the renal corpuscles 

 (glomerulus 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 metanephros is of double origin. 

 The ureter, pelvis, calyces, and collecting tubules are outgrowths of the 



