Embryonic history of the aortic arches in mammals. 407 



plete arch, and except for the short union of the dorsal rudiment 

 with the sixth arch, its position is similar with relation to the 

 aorta, to Zimmermann's description of the fifth arch in the rabbit. 

 Tandler found the fifth and sixth arches in the human embryo 

 uniting to form a common stem, a short distance ventral to their 

 union with the dorsal aorta. That variations occur in the fifth 

 arch in the same species, is evidenced by the differences which 

 Zimmermann and Tandler found in the course of this vessel in the 

 human embryo. Such individual variations are naturally to be 

 expected in rudimentary structures. The considerable distance 

 between the fourth and sixth arches in Fig-. 10, Plate 23, is signi- 

 ficant. I am unable however, to find a distinct fifth entodermal 

 pouch. It may also be noted that the truncus arteriosus is still 

 undivided. 



Embryo of twelve and one-half days. Fig. 11, Plate 22, 

 shows a surface view of an embryo twelve and one-half days old. 

 The mandibular and hyoid visceral arches are still prominent, but 

 the boundaries of the more posterior branchial arches are becoming 

 les distinct. The maxillary process of the mandibular arch now 

 extends to the nasal pit, and the cleft forming its dorsal boundary 

 is nearly vertical. 



Fig. 12, Plate 23 shows a reconstruction of the aortic arches 

 of the left side of the same embryo. There are present in this 

 stage three pairs of complete aortic arches, the third, fourth and 

 sixth, and also a pair of extremely small spurs from the dorsal 

 aortae in the hyoid visceral arch. The complete vessels are of 

 nearly uniform size, and about equally distant from one another. 

 The truncus arteriosus is still undivided. No dorsal remnant of the 

 first arch could be found. The laterally directed dorsal spur shown 

 at II in Fig. 12, Plate 23, is probably a dorsal remnant of the 

 second. A vessel passing forward from the ventral end of the third 

 arch, requires description. This vessel divides shortly after leaving 

 the third arch, into two branches, one of which runs directly forward, 

 and another, not possible to show in the reconstruction, which bends 

 toward the exterior of the hyoid arch, and near the cleft between 

 this visceral arch and the mandibular divides into two branches, 

 passing into either of the above mentioned visceral arches. The 

 diameter- of the dorsal aorta between the third and fourth arches 

 is reduced and scarcely more than half its size behind its union 

 with the fourth arch. It is also smaller than the dorsal aorta above 



