416 RALPH FAUST SHANER 



thus constituted, differs from the preceding ones in beginning 

 and ending in the arch anterior to it. For some time the proxi- 

 mal and distal parts of the fifth arch serve as common trunks 

 for the fifth and sixth arches. 



The middle segment of the fifth arch now withers away. The 

 proximal and distal segments are then taken over by the sixth 

 arch to form with it the composite pulmonary arch of the 9.5-mm. 

 embryo. The pulmonary arch is not exactly synonymous 

 with the sixth; it is rather the entire sixth arch plus the proximal 

 and distal segments of the vanished fifth. 



The pulmonary artery is already present in the 8.4-mm. 

 embryo (fig. 11). It arises from the sixth arch. 



In all non-mammalian vertebrates, the fifth and sixth arches 

 are closely related, the two quite generally beginning and ending 

 in a common trunk. Such is the case in birds (Locy), Lacerta 

 (Peter), the frog, salamander, and siren (Boas, '82), and Polyp- 

 terus, Ceratodus, and Lepidosteus (Boas, '80). The develop- 

 ment of few forms is known in detail sufficient to determine the 

 exact nature of the relation. In Lepidosteus, Miiller finds the 

 fifth and sixth arches appearing simultaneously from a common 

 ventral trunk. In the frog (Marshall and Bles, '90), the fifth 

 arch is well developed before the sixth appears; the latter begins 

 and quite often ends in the fifth arch in the younger tadpoles. 

 In older tadpoles the fifth arch atrophies, and the sixth, on be- 

 coming the pulmocutaneous arch, seems to take over the proxi- 

 mal segment of the fifth arch, much as the pulmonary arch does 

 in the turtle. 



Taking non-mammalian vertebrates as a whole, the fifth 

 arch is a vessel which extends from the truncus arteriosus to the 

 dorsal aorta, between the fourth and fifth pouches. It appears 

 after the fourth arch and before the sixth. When the fifth 

 arch is a complete and functioning vessel, the sixth appears, 

 and begins or ends, or both, in the fifth, so that the proximal and 

 distal segments of the latter serve as common trunks for both 

 arches. In reptiles and birds, at least, the middle part of the 

 fifth arch then degenerates, leaving the proximal and distal 

 segments to constitute, with the sixth arch, the pulmonary 

 arch of the older embryos. 



