434 



COMPARATIVE ANATOMY 



or Wolffian ducts become the ductus deferentes. It is generally assumed, 

 but not demonstrated, that the mesonephros functions as an excretory 

 organ in the embryo. 



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 



neural tube 



notochord 



dorsal aorta 

 afferent vessel 

 glomerulus 



efferent vessel 



somite 



posterior 



cardinal vein 



collecting vein 

 connecting 

 sub- and post, 

 cardinals 



capsule 



mesonephric 

 duct 

 subcardinal 

 vein 



A, B. 



Fig. 359. — Diagrams showing the relations of the blood vessels to a mesonephric tubule. 

 (From Patten's "Embryology of the Pig," based on figures by McCallum.) 



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. (Fig. 360) 



The vesicular enlargement of the ureter becomes the pelvis. Two 

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

 calyces. Two more are added later between the first two. Minor caly- 

 ces arise by the continued subdivision and branching of the major 

 calyces. Further branching to the twelfth generation produces the 

 collecting tubules and the medullary portion of the kidney. Those of the 

 fifth generation to the number of twenty to eighty for each renal calyx 



