ORIGIN OF THE RENAL ARTERY 193 



The actual origin of the renal artery is found to correspond in 

 the different groups of animals with the principles just stated. 

 In cat embryos of 6.0 to 7.0 mm. a well developed net exists, 

 opposite the four lumbar segments, beside the aorta, connecting 

 the mesonephric vessels, which have an almost lateral course, 

 with small lateral sprouts from the aorta, and those again with 

 the dorsal segmental vessels. With the increased curvature of 

 the body these dorsal connections are lost, new sprouts from the 

 lateral portion of the plexus extend toward the lateral wall, 

 and a main channel, from a mesonephric artery of the first or 

 second lumbar segment, is formed and becomes important 

 enough to assume a mesodermal coat. The lateral connections 

 of the plexus with the aorta remain small and uncoated. In a 

 10.7 mm. cat embryo (H.E.C. No. 474) other branches from the 

 lower end of this plexus have entered the superior pole of the 

 kidney, which is still without a capsule (fig. 8). The renal 

 blood supply may thus be said to be derived temporarily from the 

 mesonephric artery, since the plexus receives most of its blood 

 from this source. Meanwhile the growth of the Wolffian body, 

 aided perhaps by the advent of the kidney dorsal to it, has 

 forced the glomeruli to a more ventral position and lengthened 

 the mesonephric arteries, among them the one which sends off" 

 the main coated afferent of the plexus. The result is that a 

 sharp angle is formed between the mesonephric artery and its 

 branch to the plexus, and that the smaller, lateral roots of the 

 plexus become by far the most direct pathways to the plexus and 

 hence to the kidney. One or more of these becomes enlarged, 

 as is shown on the left side in an embryo of 11.6 mm. (H.E.C. 

 No. 1979). The lateral aortic branch is of large caliber (fig. 9), 

 and connects the coated aorta with the coated plexus, yet is it- 

 self provided with only an endothelial wall, showing its sudden 

 rise to importance. The older main channel to the plexus from 

 the mesonephric artery has become discontinuous in this case, 

 and the other lateral branches from the aorta to the plexus, 

 not seen in this section, have also lost their connections and 

 remain as vessels to the immediate periaortic tissues. On the 

 right side of this section one of these small vessels may be seen, 



THE AMERICAN JOURNAT. OF ANATOMY, VOL. 18, NO. 2 



