VERTEBRATE CEPHALOGENESIS. IV 329 



gether as a single nerve physiologically. The remarkable per- 

 sistence of these two nerves as anatomically distinct structures 

 throughout the entire vertebrate series from Amphioxus to man 

 is noteworthy and needs further attention from anatomists. 

 The concentration of the chemical sense organs, olfactive sense 

 organs, and light perceptive organs at the anterior end of the 

 neural axis is strikingly shown in Amphioxus and the anatomy of 

 the nasal chamber of man as herein described shows that the 

 morphological relations of these structures have not been much 

 disturbed throughout the evolution of the vertebrates. In 

 Amphioxus the neural lips of the anterior neuropore maintain 

 their original relations in the adult. This is indicated by the 

 position of the terminal and septal nerves which supply the tip 

 of the head and the organs connected with the ventral end of the 

 neuropore, the hypophysis, while the olfactory nerves supply the 

 organs connected with the dorsal end of the neuropore, the nose. 

 The whole neuroporic territory is concerned with testing ali- 

 mentary and respiratory supplies, i.e., food in the broad sense. 



2. AMMOCOETES AND PETROMYZON 



The head region of Ammocoetes has the shape of a truncated 

 cone with the snout truncated from above, downward, and back- 

 ward. In the larval Petromyzon the superficial territory inner- 

 vated by the terminalis and septalis which in Amphioxus forms 

 part of the body surface is withdrawn into the protection of a 

 nasal canal along with the olfactory organ and opens secondarily 

 to the outside through the nasal canal. Confining our attention 

 for the present to the morphological equivalents of the parts 

 just described in Amphioxus, we find the spear-shaped primitive 

 tip of the head withdrawn bodily into the nasal chamber to 

 form the nasal septum of Ammocoetes, each half receiving a rich 

 nerve supply from its N. terminalis (fig. 13). The characteristic 

 condition of the septum in Ammocoetes furnished the key for the 

 solution of the problem of the homologies of the nasal organ of 

 vertebrates. On either side of this septum (fig. 14) we find the 

 olfactive organs, each with its N. olfactorius. The N. septalis 



