ORIGIN OF THE RENAL ARTERY 181 



causes the segmental arrangement of certain vessels. Equally, 

 the condensation of mesenchyma around a vessel to form its coat 

 should prevent any further outgrowth of endothelium to form new 

 branches. Such a mesodermal wall, in a layer three or four cells 

 thick, is found in human embryos at 10.0 mm. and in the other 

 mammals examined at about the same stage. The renal arteries, 

 then, should be present before this stage, or, if later, must arise 

 from the smaller and as yet uncoated aortic branches. 



Following further this idea of the inability of endothelial sprouts 

 to pierce condensed mesenchyma, we must recognize that the 

 presence of a very shghtly differentiated capsule around an organ 

 will protect it from future ingrowth of blood vessels. Many 

 vessels may become incorporated in the further development of 

 the capsule as vessels to the capsule, not supplying the organ 

 itself, but the fact that a vessel enters the organ proper, whether 

 at the hilus or at some point on the periphery, indicates that the 

 vessel was present and actively in use before the capsule was dif- 

 ferentiated, and that the differentiation took place around the 

 vessel. Wilson (8), in his study of hypernephromata, states that 

 the renal capsule is distinct in man at 13.0 mm., though not com- 

 pletely formed until much later, and my own observations agree 

 with his on this subject. From the point of view of the ability 

 of the kidney to receive, as well as of the aorta to send out new 

 branches, we must, then, look for the renal vessels earlier than has 

 been usually supposed. 



We should not be deceived by the generally accepted idea of 

 the great migration of the kidney and its relatively late arrival 

 at its adult position. If we use the dorsal segmental arteries as a 

 guide and the bifurcation of the aorta as a fixed point, and realize 

 that the adult renal arteries arise opposite the third or fourth 

 pair of dorsal arteries above this point (that is, the second or first 

 pair of lumbar arteries in man), we find that the kidney has 

 completed its migration with respect to its blood supply very 

 early, at about 15.0 mm. in man and pig, and at corresponding 

 ages in the other mammals examined. Moreover, the perma- 

 nent renal arteries are found often to enter at first the anterior 

 pole of the organ, and secondarily to be moved by its actual 



