182 JOHN LEWIS BHExMER 



eDlargement to a position on its mesial border. The growth of 

 the kidney itself and the rearrangement and growth of the sur- 

 rounding parts complete the so-called migration and bring about 

 the adult relations. 



The problem thus resolves itself into the question, "What 

 thin-walled arterial vessels are present in the embryo of roughly 

 10.0 to 15.0 mm. in the area invaded by the kidney?" 



Though the mesodermal wall of the aorta or of other vessels 

 forms a barrier to new endothelial sprouts, it will naturally de- 

 velop around any branches, however small, which are still in use. 

 These smaller branches seem to have been overlooked hereto- 

 fore, but, as I hope to show, are numerous and important in the 

 present study. The aorta is usually considered to have cer- 

 tain sets of branches, called ventral, dorsal, and lateral or ventro- 

 lateral; but some confusion exists in the literature as to the distri- 

 bution of these groups. The ventral vessels, originally bilateral, 

 are derived from the vitelline plexus, and go to the intestinal 

 tract; the paired dorsal branches, which may arise by a single 

 trunk, supply the spinal cord, and usually also the body-wall; 

 the lateral branches, originally body-wall vessels, are said to join 

 the dorsal segmental arteries, and to be replaced by new lateral 

 branches to the mesonephros, irregularly arranged in several ver- 

 tical rows and not segmental. Yet later, other lateral branches 

 arise in certain regions destined for the body-wall, represented 

 by the subclavian artery. Dorso-lateral vessels may, in the tho- 

 racic region, supply the mesonephros, while in the abdominal 

 region of the same embryo similarly placed arteries go to the 

 body-wall, and the mesonephric vessels become ventro-lateral. 

 Moreover, the arrangement varies at different ages and in difTerent 

 classes of mammals at any one age. 



Such a lack of regularity implies, first, the absence of any speci- 

 ficity in the vessels and, second, some means whereby the vessels 

 may change their position on the aortic circumference, or join 

 with neighboring aortic branches. That vessels are not specific, 

 do not go to the Wolffian body because they are ventro-lateral 

 or dorso-lateral, but because they are best placed mechanicalh' 

 to supply the mesonephros, is shown strikingly in the reconstruc- 



