NOTOCHORD AND VERTEBRAL COLUMN. 449 



development. The most difficult cases of this kind are connected with the 

 shoulder-girdle. 



The possible sources of confusion in. the development of bones are 

 obviously two. (1) A cartilage bone by origin may directly ossify in 

 membrane, without the previous development of cartilage, and (2) a 

 membrane bone may in the first instance be formed in cartilage. 



The occurrence of the first of these is much more easy to admit than 

 that of the second ; and there can be little doubt that it sometimes takes 

 place. In a large number of cases it would moreover cause no serious 

 difficulty to the morphologist. 



BIBLIOGRAPHY of the origin of the Skeleton. 



(405) C. Gegeubaur. " Ueb. priruiire u. secundare Knocheubildung in it be- 

 sonderer Beziehuug auf d. Lehre von dein Primordialcranium." Jt'iuiische Zeitsclirift. 

 Vol. in. 1867. 



(406) O. Hertwig. " Ueber Bau u. Entwicklimg d. Placoidschuppen u. d. Ziihne 

 d. Selachier." Jenaisclie Zeitsclirift, Vol. TIII. 1874. 



(407) O. Hertwig. "Ueb. d. Zahusystein d. Ampliibien u. seine Bedeutuug f. d. 

 Genese d. Skelets d. Mundhohle." Archir f. mi In: Anat., Vol. xi. Supplenieutheft, 

 1874. 



(408) 0. Hertwig. " Ueber d. Hautskelet d. Fische." Morphol. Julirluch, Vol. 

 n. 1876. (Siluroideii u. Acipenseriden.) 



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

 Morph. Jahrbucli, Vol. v. 1879. 



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

 nen Scha'dels d. Wirbelthiere." Serichte v. d. kunirjl. roof. Anstalt z. Wtirzlurg, 1849. 



(411) Fr. Leydig. " Histologische Bemerkungen iib. d. Polypterus bicbir." 

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



(412) H. Miiller. "Ueber d. Eutwick. d. Kuochensubstauz nebst Bemerkungen, 

 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 iib. d. Verkuocherung u. die Kuochen d. Scha'dels d. 

 Teleostier." Niederlandisches Archiv f. Zoologie, Vol. i. 



Notochord and Vertebral column. 



The primitive axial skeleton of the Chordata consists of the noto- 

 chord and its sheath. It persists as such in the adult in Amphi- 

 oxus, and constitutes, in embryos of all Vertebrata, for a considerable 

 period of their early embryonic life, the sole representative of the 

 axial skeleton. 



The Notochord. The early formation of the notochord has 

 already been described in detail (pp. 243249). It is developed, in 

 most if not all cases, as an axial differentiation of the hypoblast, and 

 forms at first a solid cord of cells, without a sheath, placed between 

 the nervous system and the dorsal 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 in the trunk forms the basis round which the ver- 

 tebral column is moulded. 



The early histological changes in the cells of the notochord are 

 approximately the same in all the Craniata. There is formed by 

 the superficial cells of the notochord a delicate sheath, which soon 



B. E. II. 29 



