Embryonic history of the aortic arches in mammals. 421 



Fig. 25. Plate 22 shows a drawing of the exterior of this embryo. 

 The branchial arches are undergoing modifications, the two posterior 

 ones being now indefinite in outline. The two lobes of the first 

 arch are nearly equal in length and size. 



Fig. 26, Plate 25 shows a reconstruction of the aortic arches 

 for the right side of the same embryo. This does not, however, 

 include the anterior branches of the carotids. 



The third arch is the most anterior complete arch present, and 

 with the forward prolongation of the dorsal aorta forms the internal 

 carotid. A short common carotid is present upon either side of the 

 two uniting at c in Fig. 26, Plate 25. It should also be noted that 

 the union of the arches from either side is some distance below the 

 origin of the external carotids. The external carotid near its union 

 with the common carotid gives off two small branches {1 in Fig. 26, 

 Plate 24) directed outward, and then passes forward through the 

 hyoid arch. In this same arch, a small branch (not figured) leaves 

 the internal carotid, in a manner similar to that described for the 

 dorsal remnant of the second arch in the earlier stages. The dorsal 

 aorta between the third and fourth arches has become greatly 

 lengthened and attenuated, and while still complete, has only a very 

 small lumen. The fourth arch is still a large vessel upon either 

 side, but that of the left side is of greater size than the right. A 

 fifth arch probably in the process of degeneration, is in close con- 

 nection with the fourth and sixth arches. About midway in the 

 course of the fourth arch, a vessel leaves directed toward the sixth 

 arch, and ends blindly a little more than half way across the inter- 

 vening space. Its channel is irregular and is partly divided by 

 strands of tissue. An extremely small vessel connects the posterior 

 end of this artery with the fourth arch. Midway between the systemic 

 and pulmonary arches, a branch proceeds from the dorsal aorta 

 toward the blind end of the vessel above described and ends a short 

 distance dorsal to its termination. This branch has two connections 

 in its course, with the sixth arch. The one from its dorsal end is 

 extremely small, and can be followed only with difficulty, but the 

 second one, directed downward from near the ventral end of the 

 main branch, is of about the same calibre as this vessel. A fifth 

 branchial pouch also exists in this embryo. 



The sixth arches unite at a in Fig. 26, Plate 25, and are in 

 such close proximity near their union that the origins of the pul- 

 monary arteries lie close to each other. In the middle part of their 



28* 



