VERTEBRATE CEPHALOGENESIS. IV 331 



passes to the septum along with the median bundles of the olfac- 

 tory nerves. The optic nerves are well developed and leave 

 the base of the brain below and behind the N. terminalis. Owing 

 to the formation of heavy lips surrounding the mouth — which 

 in the adult became the 'sucking disk' — the nasal tube opens on 

 the top of the head in front of the brain region. The olfactory 

 organs and nerves are completely separated from each other in 

 Ammocoetes by this nasal septum, which from its origin we 

 recognize as a fundamental as well as an original landmark in 

 head anatomy. At the base of the septum is a glandular 

 structure (figs. 14, 15, and 16) which is innervated by the ter- 

 minal nerve. This paired gland is a more or less constant fea- 

 ture of the vertebrate nose from Ammocoetes up to man. It 

 has been described in many forms as the organ of Jacobson. 

 Between the Amphioxus and Ammocoetes condition of the ter- 

 minal and nasal region of the head there has occurred a trans- 

 lation of this region ventrad and caudad in the sagittal plane 

 with a concomitant enlargement of the nasal organ and the sepa- 

 ration of its right and left halves by the interposition of a sep- 

 tum formed by the spear-shaped tip of the head. In other 

 words, the olfactive organs have migrated ventrad along the 

 sides of the septum. Thus the olfactive organs come to lie laterad 

 of the terminal and septal territory, instead of dorsal, as in 

 Amphioxus. 



In Petromyzon marinus the tip of the primitive head has been 

 sunk still further below the surface of the body and surrounding 

 it the nasohypophysial canal has been complicated by the for- 

 mation of sacs and pockets with valves and folds for the recep- 

 tion and control of the water to be tested. The olfactive 

 organs have been expanded and pushed forward as well as down- 

 ward, while the terminal and septal structures occupy the ven- 

 troposterior portion of the nasal capsule. This portion of the 

 complicated nasal organ has been described as a gland. It is 

 made up of a series of pockets or tubes which open out on the 

 face of the wedge-shaped terminal organ to become continuous 

 with the folds of the septal region of the nasal chamber (figs. 

 17 and 17A). 



