3i8 



CHORDATE ANATOMY 



Mesonephros. The tubules of the mesonephros are formed from 

 nephrotomes posterior to those which form the pronephros. At its 

 full development in the human embryo, the mesonephros extends from 

 the sixth cervical to the third lumbar segment. While the anterior 

 nephrotomes are segmented, the more posterior mesonephric tubules are 

 derived from a continuous unsegmented nephrogenic cord or intermediate 

 cell mass. At their first appearance, the anlagen of the mesonephric 

 tubules are budded off as solid spherical cell masses, which secondarily 

 attain connexion with the primitive duct, now become the mesonephric 

 duct. Most of the mesonephric tubules degenerate, only twenty-six pairs 

 remaining in a 20 mm. embryo. In the male, some of these are converted 

 into the efferent ductules of the testes, and in this sex the mesonephric 

 or Wolffian ducts become the ductus deferentes. 



^URETER 



Fig. 283. — A diagram illustrating the repeated branching of the collecting tubules 

 in a nine-weeks (30 mm.) human embryo. The diagram shows also the origin of 

 secretory tubules from the nephrogenic tissue. (Redrawn from Braus, after Kampmeier.) 



Metanephros. The definitive kidney of man and other amniotes, 

 the metanephros, is the last to appear in ontogenesis. Like the meso- 

 nephros, it has a double origin. The collecting tubules and the ureter 

 are derived from an outgrowth of the mesonephric duct. The cortex 

 of the kidney, on the other hand, arises from the posterior portion of 

 the nephrogenic cord in the lumbar region. In a 6 mm. human embryo, 

 the ureter appears in the form of a hollow outgrowth from the mesonephric 

 duct near its posterior end. At its anterior end, this outgrowth expands 

 into a vesicular enlargement. Growing dorsally, the vesicle comes in 

 contact with the nephrogenic cord which covers it as a cap. As the 

 ureter elongates, the nephrogenic cap is pushed anteriorly, and takes a 

 position dorsal to the posterior portion of the mesonephros. 



The vesicular enlargement of the ureter becomes the pelvis. Two 

 outgrowths from it, one anterior and one posterior, form two major 

 calyces. (Fig. 271) Two more are added later between the first two. 



