128 Edward Phelps Allis jr.. 



of the liyoidean arch ; in which case it would be the hj^o-operciilaris 

 of my descriptions of Amia. In this four and one-half day embrj^'o, 

 there is a w^ll-developed internal carotid artery, arising from the 

 dorsal end of the glossopharyngeal aortic arch, and hence forming 

 an anterior prolongation of the lateral dorsal aorta; but there is no 

 external carotid. In a five and one-half day embryo the external 

 carotid has appeared as a branch of the internal carotid ; and in 

 still later stages this artery becomes connected by anastomosis with 

 the ventral carotid. The latter artery then loses its connection with 

 the glossopharyngeal aortic arch, and so comes to appear as a part 

 of the external carotid. Still later, that section of the dorsal aorta 

 that lies between the dorsal ends of the glossopharyngeal and first 

 vagus aortic arches aborts, the internal carotid then appearing as a 

 direct prolongation of the glossopharyngeal aortic arch. 



The vessels in the chick would accordingly be derived from 

 those shown in my diagram of the Loricati, if the dorsal portions of 

 the mandibular and hyoidean aortic arches and a short section of 

 the dorsal aorta should completely aboi't, and the ventral portion 

 of the mandibular arch should lose its connection wdth the efferent 

 glossopharj'ugeal artery; the external carotid of the chick being the 

 equivalent of the external carotids of Amia and teleosts, plus a 

 ventral portion of the mandibular aortic arch. To call this latter 

 artery a "ventral carotid", is accordingly misleading: for it can 

 certainly contain no portion of the ventral (anterior) carotid of 

 selachians, if its connection with the glossopharyngeal aortic arch is 

 by ventral commissure, as Twining's descriptions of his earliest stage 

 would lead one to suppose. And even if the connection with the 

 glossopharyngeal arch is by intermediate commissure, as Twining's 

 figs. 2, 3 and 4 would seem to show, the artery would probably 

 still contain no portion of the anterior carotid of selachians, for 

 that artery, as usually described, includes only that portion of the 

 mandibular aortic arch that lies dorsal to the pseudobranch. 



In the Urodela, the internal carotid, as shown by Hochstetter 

 (1906) in Hertwig's Handbuch, would seem to be the equivalent of 

 the same artery in the chick; while the external carotid is the 

 ventral carotid of Twining's descriptions of young stages of the 

 chick, but with a secondary, intermediate connection with the glosso- 

 pharyngeal aortic arch. That section of the aorta that lies between 

 the dorsal ends of the glossopharyngeal and first vagus arches 

 apparently does not abort. 



