THE MAMMALIAN FIFTH ARCH 495 



pendent branch." This conclusion he bases on the results of 

 previous observers and upon the other figures of Lehmann, but 

 from her fig. 10, it must be admitted that there is as much and 

 perhaps more ground for Lehmann's interpretation, because the 

 blind spurs on the aortic root in that figure are more prominent 

 than those on the pulmonic vessel. 



Lewis ('06) having investigated the conditions in the rabbit 

 and the pig, maintained that the vascular irregularities, blind 

 spurs, and anastomosing sinuses near the bases of the fourth and 

 pulmonic arches furnish no evidence that a typical arch exists 

 between them. His figures afford strong evidence to support 

 this view. He regards the attempts to demonstrate a fifth arch 

 as "morphological speculations of extreme interest" but adds 

 his belief that "the general recognition of a new arch in mammals 

 seems to be due to those considerations which led Boas to pre- 

 dict it rather than those which come from the study of mamma- 

 lian embryos themselves." 



In mole embryos of 4.7, 5, 5.5, and 6 mm., Soulie and Bonne 

 ('08) found a number of complete vessels arising from both the 

 truncus anteriosus and the fourth arch. Their 5.5 mm. embryo is 

 of interest becaue of a more typical fifth vessel. This vessel is 

 described as "entierement distinct et isole" from the fourth and 

 sixth arches. Its origin from the aortic bulb is given as 30ai 

 above the ventral lumen of the fourth arch in a plane vertical to it. 

 (From this description the ventral aorta is considered as extend- 

 ing dorsally and laterally to the point of origin of the fifth arch and 

 including the common trunk of the fourth and fifth arches.) On 

 leaving the bulbus it turns outwards, curves, and apphes itself 

 against the external face of the fifth pouch, embracing the latter iii 

 its concavity. On leaving the fifth visceral arch it turns freely to 

 unite with the aortic root, about 80^ from the lumen of the pul- 

 monic arch. Their 6 mm. embryo 'C shows remarkably distinct 

 fifth visceral arches. They maintain themselves to have demon- 

 strated perfect fifth aortic arches (regarding those connected with 

 the pulmonic as typical), but admit that the work of other obser- 

 vers may profitably be annexed to their own. 



