VEETEBRATE CEPHALOGENESIS. IV 333 



3. BDELLOSTOMA 



In Bdellostoma the terminal region of the Amphioxus head has 

 been withdrawn still deeper into the head of the adult hagfish by 

 the formation of a long and capacious nasal canal (nasohypo- 

 physial canal) (figs. 21 and 22). The N. terminalis supplies a 

 well-developed sense organ having to do with the testing of the 

 respiratory water (fig. 20). About it are developed valvular 

 folds from the lining of the nasal canal which control the ad- 

 mittance of the water to the terminal organ, the septal epithe- 

 lium, and the nose and of course to the hypophysial canal. The 

 nasal respiratory mechanism can quickly close off the sense-organ 

 chamber and expel the water in the nasal canal forward as well 

 as backward. The optic nerves are long and extend outward 

 from the base of the brain to the optic cups, no lens being pres- 

 ent. The N. terminalis runs dorsad within the brain (figs. 18 

 and 19) before breaking through to the surface and then runs 

 ventrad within the median fissure between the olfactory lobes 

 to the horizontal level of the terminal organ when it runs cephalad 

 to its terminations in the tip and sides of this structure. The 

 N. septalis leaves the dorsal region of the olfactory lobe of the 



Fig. 17 Lateral view of the 'nasal' organ of Petromyzon marinus. The 

 reference letters show position of sections A to G, figured below. 17A Is a 

 dorsal view of the nasal organ of the same Petromyzon. A. Section through tip 

 of prenasal sac. B. Section through expanded portion of prenasal sac. C. 

 Section through the edge of nasal funnel, the nasal canal, the anterior ends of the 

 anterior olfactory pockets, and the naso-hypophysial canal. D. section through 

 the nasal funnel, showing the funnel valve and the anterior part of the nasal 

 sac with the free folds. E. section through the posterior part of the nasal fun- 

 nel, the body of the nasal sac with free nasal folds, the ventral half of the nasal 

 septum and the nasohypophysial canal. F. section through the nasal sac with 

 completed septum and free nasal folds in right and left nasal chambers, the 

 anterior end of the nasal gland (terminal gland) and the nasohypophysial canal. 

 G. section through the posterior part of nasal body showing posterior nasal 

 pockets, the enlarged posterior end of nasal gland, the ventral part of septum 

 and the nasohypophysial canal. 



Fig. 18 Lateral view of the right olfactory, septal, and terminal nerves of 

 Bdellostoma dombeyi, inside the median fissure separating the olfactory lobes. 

 The dissection exposes the intercerebral course of the terminal nerve for a short 

 distance. 



Fie. 19 Same view as figure 18 of another Bdellostoma brain. 



