414 Habriet Lehmann, 



the hyoid arch, A short distance from its origin it divides into 

 two branches which diverge, and finally terminate at about the 

 level of the ventral end of the complete portion of the first arch. 

 A few small sinuses extend between the end of the more anterior 

 branch, and the chain of sinuses described in connection mth the 

 first arch. The truncus arteriosus projects forward for a short 

 distance, beyond the ventral ends of the third pair of arches. At 

 the union of the truncus and third arch upon either side, and from 

 the more anterior wall, a small and short branch is given off which 

 is directed toward the chain of sinuses described above, but with 

 which no definite connection can be found. This pair of vessels 

 joins the truncus a short distance from the median plane. 



The third arch in this stage, is somewhat smaller than the 

 fourth arch, a reversal of the condition in the two preceding embryos. 

 The dorsal aorta between the third and fourth arches is slightly 

 reduced in diameter. 



Tlie fourth arches are large vessels of about equal size on both 

 sides of the embryo. A structure connected with the dorsal end of 

 the fourth arch on either side (Fig. 18 F, Plate 24) is notewoi'thy. 

 It consists of a chain of small sinuses extending from the posterior 

 wall of the fourth arch, near its dorsal end, to the dorsal aorta, the 

 last sinus definitely joining the latter vessel behind its union with 

 the fourth arch. Upon the right side the connection is clear between 

 the most anterior sinus and the fourth arch. In fact, these structures 

 upon the right side form a practically continuous, though very slender 

 vessel. 



The sixth arch is at a considerable distance from the fourth. 

 It is similar in size and form to that described for the preceding 

 embryo. This arch increases in diameter near its dorsal end, and 

 shows two unequal connections with the aortic root. It unites with 

 the ventral wall of the latter vessel somewhat towards the median 

 plane, so that in a side view this end of the sixth arch is behind 

 the aortic root. Small ingrowths of the surrounding tissue cross the 

 lumen of this arch at its dorsal end, but have not been shown in 

 the reconstruction on account of the already complicated appearance 

 of this region. The general character of this portion of the sixth 

 arch indicates that it is undergoing changes of some kind. A small 

 pulmonary artery is present, but can be traced as a continuous vessel 

 only a short distance posterior to the sixth arch. 



