NOSTRIL 



39 



the roots is secondary in the latter (Koltzoff [273]), and has taken 

 place independently in the Gnathostomes. Petromyzon, where 

 they are both separate and alternate, would then retain the 

 primitive condition found in Amphioxus. 



All the Cyclostomes differ fundamentally from the Gnathostomes 

 in possessing a single median nostril. Probably the nasal organ of 



A. m 



Fig. 24. 



Median longitudinal sections of the head of four stages in the development of Petromyzon. 

 A, the youngest, and D, the oldest stage. (After Dohrn.) /, opening leading to mouth ; h, hypo- 

 physis ; i, infundibulum ; I, lower lip ; n, nasal pit ; nt, notochord ; o, opening of nasal pit 

 and hypophysis, future median nostril ; p, pineal eye ; r, roof of brain ; s, hypophysial sac ; «, 

 upper lip; v, velum ; /;, cavity of brain ; /•;, enteron. 



the former was also originally paired (Scott), since the olfactory 

 lobes and nerves are paired, and pierce the capsule by paired 

 openings (Fig. 25). In the early embryo there is said to develop 

 a single terminal olfactory (?) plate (von Kupffer [275]); this later 

 becomes bilobed, probably incorporating two placodes, and gives 

 rise to the olfactory epithelium. The olfactory pit becomes involved 

 in the hypophysial invagination, is cai'ried some distance back, and 

 finally opens dorsally into the hypophysial or nasal canal (Fig. 24). 



