NOTOCHORD AND VERTEP.RAL COLUMN. 



545 



(407) O. Hertwig. " Ueb. d. Zahnsystem d. Amphibien u. seine Becleutung 

 f. d. Genese d. Skelets d. Mundhohle.'' Archiv f. niikr. Anat., Vol. XI. Supple- 

 nientlieft, 1874. 



(408) O. Ilevlwig. " Ueber d. Hautskelet d. Fische." MorpJwL Jahrbuch, 

 Vol. n. 1876. (Siluroidcn u. Acipenseriden.) 



(409) O. Hertwig. " Ueber d. Hautskelet d. Fische (Lepidosteus u. Polyp- 

 terus)." Morph. Jahrbitch, Vol. v. 1879. 



(410) A. Kb'lliker. " Allgemeine Betrachtungen lib. die Entstehung d. knocher- 

 nen Schadels d. Wirbelthiere. " Bcrichte T. d. konigl. zoot. Aastalt :. Wiirzburg, 

 1849. 



(411) Fr. Leydig. " Histologische Remerkungen lib. d. Polypterus bichir." 

 Zeit.f. wiss. Zoo/., Vol. V. 1858. 



(412) H. Miiller. " Ueber d. Entwick. d, Knochensubstanz nebst Bemerkun- 

 gen, etc." Zcit. f. iviss. Zoo!., Vol. ix. 1859. 



(413) Williamson. "On the structure and development of the Scales and 

 Bones of Fishes." Phil. Trans., 1851. 



(414) Vrolik. " Studien lib. d. Verknocherung u. die Knochen d. Schadels d. 

 Teleostier." Niederliindisches Archiv f. Zoologic, Vol. i. 



Notochord and Vertebral column. 



The primitive axial skeleton of the Chordata consists of the 

 notochord and its sheath. It persists as such in the adult in 

 Amphioxus, and constitutes, in embryos of all Vertebrata, for a 

 considerable period of their early embryonic life, the sole repre- 

 sentative of the axial skeleton. 



The Notochord. The early formation of the notochord 

 has already been described in 

 detail (pp. 292 300). It is 

 developed, in most if not all 

 cases, as an axial differ- 

 entiation of the hypoblast, 

 and forms at first a solid 

 cord of cells, without a 

 sheath, placed between the 



nervous system and the dor- 



' FIG. 31=5. HORIZONTAL SECTION THROUGH 



sal wall ol the alimentary THE TRUNK OF AN EMBRYO OF SCYLLIUM 



tract, and extending from 



CONSIDERABLY YOUNGER THAN F IN FIG. 28. 



The section is taken at the level of the 

 notochord, and shews the separation of the 

 cells to form the vertebral bodies from the 

 muscle-plates. 



ch. notochord; ep. epiblast ; Vr. rudiment 

 region of the brain will be of vertebral body; mp. muscle-plate; mf. 



portion of muscle-plate already differentiated 

 dealt with by itself. That into longitudinal muscles. 



the base of the front of the 

 mid-brain to the end of the 

 tail. The section in the 



H. III. 



35 



