494 FRANK REAGAN 



All workers are agreed that the transitory vessel representing 

 a new arch is to be looked for between the fourth and pulmonic 

 arches. 



Although the results of previous observers have been rather 

 exhaustively discussed, mention may profitably be made of points 

 which have direct bearing on the present work. 



The first evidence in favor of the predicted existence of a new 

 arch in mammals, was adduced by Zimmermann ('89), when he 

 found in a 7 ram. human embryo, a small vessel arising from and 

 re-entering the posterior side of the fourth arch. He also 

 described a complete and typical fifth arch in an eleven day rab- 

 bit. The arch arose from the ventral aorta and emptied into the 

 aortic root, not far from the dorsal lumen of the pulmonic arch. 



Tandler ('02) found, in a rat embryo, a broad connection 

 between the fourth and pulmonic arches, parallel to the dorsal 

 aorta; he identified it as a fifth arch, despite the fact that it was 

 without connection with either aorta, and that there was no 

 corresponding pharyngeal pouch. In two human embryos he 

 found vessels connecting the ventral aortae with dorsal portions 

 of the pulmonic arches. 



Lehmann ('05) has figured a complete arch for the pig, and has 

 described it as follows: 



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 downwards, follows along 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 back from the rudi- 

 mentary vessel, a branch which joins the sixth arch immediately ven- 

 tral to its union with the dorsal aorta. 



From this description we are to interpret the portion of the 

 vessel joining the aorta as the distal portion of the arch proper, and 

 the vessel to the pulmonic as a branch of it. These indeed would be 

 conditions requisite to a typical arch; but Locy's interpretation of 

 the same vessel is not in accord with the above view when he de- 

 scribes it as "passing dorsad and caudad to unite with the pulmonic 

 arch near the union of the latter with the aortic root." He 

 also states that "it is connected with the aortic root by an inde- 



