NOTOCHORD AND VERTEBRAL COLUMN. 



545 



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

 f. d. Genese d. Skelets d. Mundhohle." Archiv f. mikr. Anat., Vol. xi. Supple- 

 mentheft, 1874. 



(408) O. Hertwig. " Ueber d. Hautskelet cl. Fische." Morphol. Jahrbmh, 

 Vol. II. 1876. (Siluroiden u. Acipenseriden.) 



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

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



(410) A. Kolliker. " Allgemeine Betrachtungen iib. die Entstehung d. knocher- 

 nen Schadels d. Wirbelthiere. " Berichte r. d. kijnigl. zoot. Anstalt z. Wiirzburg, 

 1849. 



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

 Zeit.f. wiss. Zool., Vol. v. 1858. 



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

 gen, etc." Zeit. f. wiss. ZooL, 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. Zoologie, Vol. I. 



NotocJtord 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- 

 sal wall of the alimentary 

 tract, and extending from 

 the base of the front of the 



mid-brain to the end of the 

 tail. The section in the 

 region of the brain will be 

 dealt with by itself. That 

 H. HI. 



FIG. 313. HORIZONTAL SECTION THROUGH 

 THE TRUNK OF AN EMBRYO OF SCYLLIUM 

 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 

 of vertebral body; ;;//. muscle-plate; mp'. 

 portion of muscle-plate already differentiated 

 into longitudinal muscles. 



35 



