THE MESONEPHROS 



207 



its derivatives. The mesonephros is probably not a functional excretory organ 

 in human embryos for its tubules degenerate before the metanephros becomes 

 functional. It may have some other function and produce an internal secretion. 

 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 in- 

 terrupted at the connecting points between the collecting and secretory regions. 

 They are divided into an upper group and a lower group. The upper group, 

 numbering 5 to 12, unites with the rete tubules of the testis or ovary. In the 

 male they form the efferent ductuli of the epididymis. In the female they con- 



Supra renal gland 



Uowman's 

 Capsule 



Collecting tubule 

 ■Secretory tubule 



Mesonephn'c. 

 duct 



M.ueljerCan 



-flnlage genital gland 



Fig. 201. — Reconstruction of a mesonephric tubule, glomerulus and mesonephric duct from a 12 mm. 

 human embryo as seen in transverse section. X 95. Postcard, v., posterior cardinal vein. 



stitute part of 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 metanephros is of double origin. The ureter, pelvis, calyces and collecting 

 tubules are outgrowths of the mesonephric duct. The secretory tubules and the 

 capsules of the renal corpuscles are differentiated from the caudal end of the 

 nephrogenic cord and thus have the same origin as the mesonephric tubules. 



