VERTEBRATE CEPHALOGENESIS. IV 325 



The N. terminalis^ is connected to the right and left side of the 

 anterior end of the ventral plate of the brain, and owing to the 

 relatively large size of this pair of nerves they appear, when 

 viewed from above, like a bifurcation of the brain. They run 

 forward to the tip of the head. The N. opticus is entirely im- 

 bedded in the front wall of the brain anterior to the ventricle. 

 In Amphioxus we have a stage of the evolution of the eye ante- 

 dating the formation of optic cups which, from Bdellostoma on, 

 presents such a prominent feature of vertebrate anatomy. The 

 N. olfactorius (figs. 2, 3, 4, and 7), being relatively small, is a 

 short nerve which arises from the anterodorsal wall of the brain 

 near to the median line and, while the right and left olfactorius 

 arise from the right and left halves of the brain respectively, 

 they are usually drawn close together into one trunk as they 

 approach the olfactive organ, although they occasionally remain 

 separate and distinct their entire length. They run dorsad and 

 cephalad and innervate the right and left halves of the olfactive 

 cup just as their homologues do in Ammocoetes and all other ver- 

 tebrates. The N. septalis arises from the dorsal wall of the brain 

 on either side of the median line above the posterior limit of the 

 ventricular cavity. The two nerves curve upward, forward, and 

 outward, and run to the sides of the anterior end of the head, 

 innervating the territory mainly caudad of the N. terminalis and 

 as far back as the hypophysis. 



In Amphioxus the body surfaces innervated by the terminal and 

 septal nerves (figs. 1 and 12) are fully exposed, except the hypo- 

 physial region which lies within the buccal cavity; with this 

 exception they form part of the body contour. The olfactory 

 organs are sunk below the body surface as a conical pit open- 

 ing directly on the surface and more or less pushed to the left 

 of the median line by the dorsal head fin fold.^ 



The distribution of the septalis nerve in Amphioxus is as fol- 

 lows: Arising from the dorsolateral territory of the brain above 

 the posterior border of the ventricle the nerve trunk soon sepa- 

 rates (figs. 1, 12 and 26) into two parts. The larger part curves 

 forward and outward, dividing as it passes to its terminal ter- 

 ritory behind the tip of the snout which is supplied by the N. 



