418 RALPH FAUST SHANER 



trunk of the fifth and sixth arches of the 8.4-mm. turtle (fig. 11) 

 ah'eady referred to. This branch is not an aortic arch at all. 



What, then, should be the enumeration and description of the 

 aortic arches in mammals? Rathke recognized five arches, 

 formed serially, ending with the pulmonary as the fifth. Boas 

 ('87) 'predicted' and Zimmermann described a new arch be- 

 tween the fourth and pulmonary arches, and embryologists 

 generally thereafter counted the pulmonary arch of mammals as 

 the sixth, while recognizing only four pouches together with the 

 postbranchial body, the nature of which was then uncertain. 

 Subsequently a transient fifth pouch was found, so that the 

 counting of six aortic arches became more consistent, and has 

 been generally adopted. 



Before the presence of a fifth pouch had been established, that 

 is, when four pouches and six arches were accepted, Lewis, who 

 considered that the current diagrams of the mammalian arches 

 were unjustified, wrote briefly in protest ( '06). He characterized 

 the conclusion in regard to a sixth aortic arch in mammals as 

 "a morphological speculation of much interest," and noted 

 especially that the pulmonary arch is complete before the fifth 

 arch has formed and before the fourth pouch is present, and he 

 indicated in his figures its position medial to the postbranchial 

 body. These findings seem now to be well established, and they 

 show that although physiologically this vessel is comparable 

 with the sixth arch of the turtle, morphologically it is another 

 vessel. We would conclude, therefore, that in mammals in- 

 stead of designating this vessel as the sixth arch it should be 

 named the pulmonary arch. Correlated with the increasing 

 importance of the lungs, this arch, not strictly homologous with 

 any in the turtle, has developed precociously. Its median posi- 

 tion in relation to the postbranchial body removes it from the 

 series of true branchial aortic arches. 



Having eliminated this vessel, the comparison between the 

 arches in mammals and turtles becomes very simple. In both 

 groups they are formed in succession from before backwards, in 

 mammals showing merely a reduction in the posterior members of 

 the series, the pouches and their vessels becoming vestigial 



