92 PLATT. [Vol. V. 



In the Teleosts, where the hyoid has no gill, its primitive 

 aortic arch does not divide, but a ventral prolongation of the 

 vein of the first branchial arch anastomoses with the common 

 root of the mandibular and hyoid arteries, which lose their con- 

 nection with the ventral aorta. 



According to Maurer (No. 12), no hyoid artery develops in 

 the Amphibians ; consequently the artery of the first branchial 

 arch follows immediately upon the mandibular artery, which is 

 then seen to arise from the base of the first branchial artery, 

 rather than independently from the aorta. When, as soon 

 happens, the mandibular artery loses its connection with the 

 ventral aorta, it still remains connected with the base of the 

 first branchial artery, which is at the same time severed from 

 the aorta. Later, the first gill vein develops from this bran- 

 chial artery. Thus it happens that the ventral prolongation of 

 the first gill vein is continued into the mandibular artery, form- 

 ing the artera carotis externa. 



To review: in the Selachians the branchial circulation be- 

 comes hyoidean by the loss of the anterior gill vein. The 

 hyoid circulation of Acanthias in the stage described becomes 

 Teleostean by the loss of the hyoid artery or becomes Amphib- 

 ian by the loss of both artery and vein. The progressive 

 change is a simple reduction by loss of parts, and not a modifi- 

 cation by the introduction of new relationships not found in the 

 Selachians. 



C. The Nerves. 



While my studies on the medulla of the chick (No. 14) and 

 the salmon confirmed the results obtained by Beraneck (Nos. 

 4, 5) and Orr (No. 13) in regard to the number, position, and 

 constancy of the "neuromeres" of the medulla, they led me to 

 believe that the primary attachment of the cranial nerves was 

 between consecutive neuromeres, rather than from the convex- 

 ity of a neuromere. I carefully examined successive constric- 

 tions of the brain in the chick, but was unable to find any 

 nerves arising from the neural crest anterior to the trigeminus. 

 Renewing the search with my present Elasmobranch material, 

 and examining the first constriction anterior to the medulla, I 

 found there a nerve, which proved to be the primitive trochlearis. 

 By comparing longitudinal sections through older embryos with 



