VERTEBRATE CEPHALOGENESIS. IV 335 



brain inside the median fissure and near its anterior end, but 

 separate from the median olfactory nerve bundle which leaves 

 the tip of the median part of the olfactory lobes. The septal 

 nerve also runs ventrad before coursing forward to supply its 

 peripheral territory. The olfactory nerves are enormously de- 

 veloped and the nerve of each side supplies three complete and 

 two half folds (plates) of the compound nasal organ, all of 

 which, heretofore, has been called the olfactory organ. The 

 N. terminalis in Bdellostoma presents a stage intermediate 

 between Amphioxus and the Selachians.^ In the former, the 

 olfactory nerves are small and in their primitive terminal posi- 

 tion on either side of the dorsal end of the neuroporic raphe, 

 while the N. terminalis is relatively large, leaving the brain nearly 

 midway between the olfactory organ and hypophysis, and strictly 

 terminal in its positions as regards the adult brain. 



In Bdellostoma the olfactory nerves have become enormously 

 increased and overshadow the N. terminalis, which, while it 

 remains terminal in its peripheral distribution leaves the brain 

 near the anterior end of the olfactory lobes within the median 

 fissure. The development of the olfactory lobes is so great that 

 the primitive anterior end of the brain is covered over and its rela- 

 tion obscured. The great increase in size of the olfactory nerves 

 causes them to enfold the forebrain in an inclosing overgrowth of 

 the olfactory lobes, which apparently forces the roots of the ter- 

 minalis upward, but they retain their relative position with ref- 

 erence to the exit of the olfactory nerve, viz., mesad and ventrad 

 of it. In Amphioxus we found branches of the septal nerve 



Fig. 20 Ventral view of Bdellostoma 'nasal' organ, to show the distribution 

 of the olfactory, terminal, and septal nerves, the median terminal organ and 

 other 'nasal' folds. 



Fig. 21 Same view of nasal organ and nasohypophysial canal in Bdellostoma. 



Fig. 22 Lateral view of nasal organ, brain, and nasohypophysial canal of 

 Bdellostoma. 



Fig. 23 A. Section through the mucosa covering terminal organ of Bdello- 

 stoma. Five layers of epithelium cells are shown above the basement mem- 

 brane beneath which lies a plexus of nerve fibrils given off from the ganglion 

 cells lying below. B. Four ganglion cells from same section. C. Surface view 

 of epithelium from same terminal organ to show relation of sensory cells to other 

 surface cells. 



