Embryonic history of the aortic arches in mammals. 419 



smaller than in the preceding stage. The body flexure is less marked 

 than in the younger embryos. 



A reconstruction of the aortic arches of the left side is shown 

 in Fig. 24, Plate 25. Short spurs on the aortic roots are present in 

 the mandibular and hyoidean visceral arches, which correspond in 

 position to the dorsal remnants of the first and second arches in 

 the two preceding stages. The more anterior branch bends outward 

 and upward, while the other passes outward for a short distance, 

 and then downward. 



A long slender vessel runs forward from the ventral end of 

 the third arch of either side leaving this artery slightly above the 

 union of the third pair of arches. Its course lies for the most part, 

 parallel to the anterior prolongation of the dorsal aorta, and just 

 back of the cleft between the mandibular and hyoidean visceral 

 arches, it divides into three small branches. The courses of these 

 vessels upon either side are symmetrical except near their union 

 with the third pair of arches. Here the vessel from the left side 

 approaches the median line, while the one from the right joins the 

 third arch of that side at some distance from the median plane, its 

 point of connection corresponding to that of the small spur passing 

 forward from the ventral end of the right third arch in Fig. 22, 

 Plate 24. 



The third arch is of about the size of the continuation of the 

 dorsal aorta in front of it and appears to be continuous with the 

 latter account of the marked decrease in the diameter of the dorsal 

 aorta between the third and fourth arches. This is especially notice- 

 able in the region near its union with the third arch, where its 

 channel is greatly reduced. 



The left fourth arch in this embryo is large and of especial 

 interest on account of its connection with a complete fifth aortic 

 arch. A short distance from its union with the truncus arteriosus 

 the fourth arch increases greatly in width and there is given off 

 from its posterior side, near the middle of the arch, a smaller but 

 perfectly distinct vessel, which bending slightly dowuAvard, follows 

 closely the course of the fourth arch, and joins the dorsal aorta 

 immediately beneath it. Just ventral to its union with the aortic 

 root there passes backwards from the rudimentary fifth vessel, a 

 branch which lies close to the dorsal aorta and joins the sixth arch 

 immediately ventral to its union with the dorsal aorta. This vessel, 

 upon the left side, is moderately large, and well defined. A vessel 



Zool. Jahrb. XXII. Abt. f. Anat. 28 



