ORIGIN OF THE RENAL ARTERY 183 



tioii of a part of the aorta in a sheep embryo of 5.6 mm., in which 

 branches from the left aorta have become right mesonephric 

 arteries (fig. 1). The two aortae, right and left, are still partially 

 separated by one of the bars or septa of mesoderm covered by 

 endothelium frequently found where two vessels of irregular cali- 

 bre are in the process of fusing. The left aorta at this point is 

 much the larger, so that the bar is far to the right of the median 

 line, as indicated by the position of the notochord and the root 

 of the mesentery. Intestinal branches are lacking at this level, 

 and we are left in doubt as to their relations; but two vessels, 

 one large with many branches, the other much smaller, spring 

 distinctly from the left aorta and pass to the right mesonephric 

 glomeruli. If we consider the double aorta as a whole, these 

 vessels are ventro-lateral, and thus normal in their distribution; 

 but for the left aorta itself, since these branches are the nearest 

 to the aorta of the opposite side, they are actually in the posi- 

 tion of median ventral branches, ordinarily destined for the in- 

 testine. How frequently such irregularly placed vessels occur it 

 is impossible to say, as all trace of their irregularity is lost with the 

 absorption of the bars. It is evident, however, that aortic 

 branches from the medial ventral angle of each aorta are not 

 necessarily predestined to supply the intestine, but are governed 

 by mechanical convenience; and also that, at this early stage, 

 new sprouts may grow from the aortic endothelium at various 

 points. 



These new sprouts, not heretofore definitely recognized except 

 as new mesonephric arteries or subclavian arteries, are much more 

 numerous than has been supposed and afford to the already 

 present vessels the means of changing their position on the 

 aortic wall and of forming new connections with other vessels. 

 They are sent out by the aortic endothelium between the exist- 

 ing vessels, as the circumference and length of the aorta increase, 

 and like all endothelial sprouts tend to branch and anastomose, 

 both vertically and horizontally, with each other and with the 

 existing aortic branches. The intervals between the aortic roots, 

 old and new, of the resulting plexus, and the size of its mesh are 

 roughly the same as those fgured by Clark (9) in his studies 



