ORIGIN OF THE RENAL ARTERY 185 



fixing fluid, leaving only slight irregularities in the endothelial 

 wall. In many stained and sectioned embryos the endothelium 

 of the aorta is in festoons with few points of contact with the 

 mesodermal layers; such points of contact correspond to the posi- 

 tions of the roots of this plexus, as found in better preserved 

 specimens. 



The aortic wall of condensed mesenchyma will, as has been 

 said, naturally form around such of the vessels of this plexus as 

 are still in use, and thus frequently portions of the horizontal 

 and vertical vessels are buried deep in the wall. The parts of the 

 net not in use will retract or otherwise disintegrate, leaving per- 

 haps no trace of their former position and connections. Figure 

 2 shows several of these smaller aortic branches, two between the 

 dorsal segmental artery and the mesonephric artery, others ven- 

 tral to the latter. A portion of the net remains as anastomoses 

 between some of the more ventral, and together with the roots is 

 partially buried in the mesodermal coat of the aorta. The more 

 dorsal of the roots have been carried further apart by the growth 

 of the aortic wall, have lost their connections with the net, and one 

 of them has become practically obliterated, leaving only a point 

 of attachment of the endothelium to mark its position. The tips 

 of two of the roots lie in the loose connective tissue and can 

 send out further sprouts. Figures 3 and 4 show a portion of the 

 periaortic net in an older embryo; the persisting vessels in this 

 case are the anastomoses near the aorta, and the capillaries from 

 them peripherally. Though the intermediate roots from the aorta 

 are absent, the position of these peripheral capillaries indicates, 

 to my mind, that the roots were originally three in number be- 

 tween the dorsal segmental artery and the mesonephric artery. 

 Figure 5 shows the wealth of partly buried vessels found in the 

 dorsal region of the aorta, representing anastomoses, both verti- 

 cal and horizontal, between dorsal segmental and lateral aortic 

 branches, and indicating that two sets of roots per segment in 

 the vertical direction is the general arrangement in this case. 



These are only a few of the many instances in which I have 

 found remains of this periaortic plexus. Similar connections 

 between mesonephric arteries and ventral arteries are very com- 



