188 JOHN LEWIS BREMER 



If this takes place after the main aortic branches are coated, 

 no new sprouts will arise from them to form new anastomoses, 

 and an area near the aorta will be left which receives no blood 

 supply from the larger aortic branches. Into this area grow 

 sprouts from some of the discarded roots of the original plexus, 

 already protruding beyond the aortic wall, but not strictly to be 

 considered new aortic branches, because they were present before 

 the aortic coat developed. Such rejuvenated vessels become the 

 small arteries to the Ij^mph glands and sympathetic ganglia near 

 the aorta, the bronchial and oesophageal arteries, and by recur- 

 rent branches the vasa vasorum of the aorta itself. They, or the 

 plexus of which they were a part, also send twigs to the organs 

 developing in or migrating into the area immediately lateral to 

 the aorta, namely the suprarenal gland and the kidney. 



Which part of the plexus between the mesonephric arteries 

 and the dorsal segmental arteries will remain, and what its con- 

 nections will be, depends apparently on mechanical forces 

 brought into play by idiosyncrasies of development in the differ- 

 ent classes of mammals. Of the embryos examined in this study, 



Fig. 2 Rabbit embryo, 10.0 mm., 14 days, H.E.C. No. loo. Sect. 457. To 

 show small aortic branches between the larger ones, and part of the periaortic 

 plexus. The mesodermal aortic wall is present, and has partly buried the smaller 

 branches. At 'x' an obliterated branch, d.ari., dorsal segmental artery; m.ari., 

 mesonephric artery. X 165 diam. 



Fig. 8 Cat embryo, 10.7 mm., H.E.C. No. 474, Sect. 434-452. Reconstruc- 

 tion of the aorta at second lumbar segment; lateral plexus, pi., with mesodermal 

 coat connected, at an acute angle, with a mesonephric artery, m.art., by a thick 

 walled vessel, con., and with the aorta by three slender vessels of endothelium 

 only, arranged in two vertical rows. Branches to body-wall and to kidney, k. 

 Note portions of ventral plexus toward .superior mesenteric artery, s. in. art. 

 The left side of the drawing is incomplete (cf. figs. 6 and 9). X 130 diam. 



Fig. 9 Cat embryo, 11.6 mm., H.E.C. No. 1979, Sect. 308. Drawing of the 

 aorta and the left permanent renal artery, r.art., just enlarged and with simple 

 endothelial wall, from the aorta to the plexus, pi. The plexus is coated; its con- 

 nection, con., with the mesonephric artery, m.art., is obliterated and can be fol- 

 lowed no further in neighboring sections. The renal branch of the plexus, 

 r.hr., enters the kidney, k., at a lower level. On the right side is seen a lateral 

 branch, lat.br., not now continued to the right plexus, and dorsal to it the remains 

 of another, re., veins; sijm., sympathetic nerves; b.w., branch to body-wall. 

 X 165 diam. 



