A treatise on zoology . ached correspond to the lower partof the mandibular arch (Meckels cartilage), and perhaps to thehyoid arch as well (Ayers and Jackson). Development supportsthe view that the tongue cartilages correspond to the mandibulararch (Stockard). Some of the more interesting points in the development of themuscles and nerves of the Cyclostomes ha^-e already been dealtwith above (p. 5, etc.). The permanent separation of the ventralfrom the dorsal roots of the spinal nerves in the Petromyzontia,and their junction in Myxinoidea, is very difficult to explain ([341],Figs. 18, 20, 22).
A treatise on zoology . ached correspond to the lower partof the mandibular arch (Meckels cartilage), and perhaps to thehyoid arch as well (Ayers and Jackson). Development supportsthe view that the tongue cartilages correspond to the mandibulararch (Stockard). Some of the more interesting points in the development of themuscles and nerves of the Cyclostomes ha^-e already been dealtwith above (p. 5, etc.). The permanent separation of the ventralfrom the dorsal roots of the spinal nerves in the Petromyzontia,and their junction in Myxinoidea, is very difficult to explain ([341],Figs. 18, 20, 22). On the whole, it seems probable that the fusion of NOSTRIL 39 the roots is second;iry in the hitter (Koltzoff [273]), and has takenplace independently in the Gnathostomes. Pctromyzon, wherethey are both separate and altei-natc, wonld then retain theprimitive condition found in ;. All the Cyclostomes ditier fundamentally from the Gnathostomesin possessing a single median nostril. Probably the nasal organ of. Fig. 24. Median longitudinal sections of tlie head of four stages in the development oi , the youngest, and D, tlie oldest stage. (After Dohrn.) /, opening leading to mouth; h, hypo-physis ; i, infundibuluiu ; I, lower lip ; n, nasal pit; nt, notocliord ; o, opening of nasal pitand hypophysis, future median nostril ; p, pineal eye ; r, roof of brain ; ,«, hypophysial sac ; k,Tipper lip ; v, velum ; B, cavity of brain ; £, enteron. the former was also originally paired (Scott), since the olfactorylobes and nerves are paired, and pierce the capsule by pairedopenings (Fig. 25). In the early embryo there is said to developa single terminal olfactory (?) plate (von Kupffer [275]); this laterbecomes bilobed, probably incorporating two placodes, and givesrise to the olfactory epithelium. The olfactory pit becomes involvedin the hypophysial invagination, is carried some distance back, andfinally opens dorsally into the hypophysial or nasal canal (Fig. 24)
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