VERTEBRATE CEPHALOGENESIS. IV 339 



nerves to their central as well as peripheral terminations. The 

 presence of coordinating sympathetic fibers in the terminal and 

 septal nerves seems to be definitely proved in a number of the 

 species studied. The diagrams shown in figure 26 cover four 

 stages in the transformation of the apical head region of Am- 

 phioxus into the nasal chamber and septum of the higher forms. 



Stage A is pre-Amphioxus condition. The hypophysial organ 

 (pre-oral pit) is shown on the outside of the head in the position 

 it reaches during larval life just before it enters the buccal 

 chamber. Here it is in the surface territory innervated by the 

 septal nerve. 



Stage B represents the relation of the parts in the adult Am- 

 phioxus. The hypophysial organ is housed in the buccal cavity. 



Stage C is a pre-Bdellostoma condition antedating the forma- 

 tion of a nasobuccal partition. The entire head territory of the 

 Amphioxus stage is now housed in the buccal chamber and pro- 

 jects into it along the sagittal plane, forming a partial septum 

 which partly separates the nasal portion of the common naso- 

 buccal chamber into right and left nasal chambers. 



Stage D represents the Bdellostoma condition. The primitive 

 apical territory is now enclosed in a 'nasal capsule/ open below, 

 which is divided into right and left halves by the terminal organ, 

 which forms a complete septum for structures of the capsule. 

 The nasal chamber is further separated from the buccal cavity by 

 a horizontal partition. The nasohypophysial canal is an exten- 

 sion forward and backward of the nasal chamber. 



The cranial nerves in man are not, therefore, twelve in number, 

 but fourteen. In the order of their connection with the brain 

 we may tabulate them as follows: 



Nerve 1, terminalis Nerve 8, abducens 



Nerve 2, opticus Nerve 9, facialis 



Nerve 3, olfactorius Nerve 10, acusticus 



Nerve 4, septalis Nerve 11, glossopharyngeus 



Nerve 5, oculomotorius Nerve 12, vagus 



Nerve 6, trochlearis Nerve 13, accessorius 



Nerve 7, trigeminus Nerve 14, hypoglossus 



