PHARYNX AND AORTIC ARCHES 417 



In mammals, on the contrary, the so-called fifth arch arises 

 after the pulmonary arch is well established. In many of the 

 reported cases of it, e.g., the original fifth arch of Zimmerman 

 and in many of those of Tandler and Lehmann, the fifth arch 

 is more closely related to the fourth, beginning or ending in the 

 fourth, or both, and otherwise pursuing a highly variable and 

 erratic course. A vessel that appears after the pulmonary arch 

 is completed and has no real relation to it is hardly a fifth arch 

 in the sense that obtains in lower vertebrates. 



The position and relation of the sixth arch of the turtle are 

 also of considerable morphological significance. The sixth arch 

 (and the resultant pulmonary arch) lies lateral to the post- 

 branchial body (fig. 11, 8.4 mm.). In this particular embryo 

 there is, in addition, a transient vessel (fig. '11, x) which arises 

 from the distal common trunk of the fifth and sixth arches, and 

 winds around the medial side of the postbranchial body. 



The sixth or pulmonary arches of many other reptiles similarly 

 occupy the lateral position. Such is the case in Lacerta (5 mm., 

 H.E. C. 731), Sphenodon (7.9 mm., H. E. C. 1491), Aristelliger— 

 a geckonid lizard — (4.9 mm. H. E. C. 1884), Iguana (10 mm.), and 

 probably in the alligator. Snakes (Eutaenia and Oxybalus) 

 are possible exceptions; before the relation of their arches can 

 be established, the nature of the posterior part of the pharynx 

 of these forms must be investigated. 



In mammals the pulmonary arch lies medial to the post- 

 branchial body. The normal mammalian condition is well 

 shown in Tarsius (fig. 12 after Hafferl) and in the mole (fig. 13 

 after Soulie and Bonne). The rabbit and pig are other examples 

 (Lewis) . 



The postbranchial body of mammals is now generally con- 

 sidered to be a prolongation of the pouch-forming area of the 

 pharynx, out of which a fifth pouch occasionally develops as 

 a lateral outgrowth, just as in the turtle. If the mammahan 

 pulmonary arch is a true sixth arch, it should also lie lateral to 

 the postbranchial body in order to be in proper relation to the 

 fifth pouch. Instead, it lies medial to the postbranchial body 

 and takes the course of the transient branch of the common 



