The actual causes which in the first instance gave rise to particular centres of ossification, or to the ossification of particular parts of the cartilage, are but little understood; nor have we as yet any satisfactory criterion for determining the value to be attached to the number and position of centres of ossification. In some instances such centres appear to have an important morphological significance, and in other instances they would seem to be determined by the size of the cartilage about to be ossified.
There is no doubt that the membrane bones and cartilage bones can as a rule be easily distinguished by their mode of development; but it is by no means certain that this is always the case. It is necessarily very difficult to establish the homology between bones, which develop in one type from membrane and in another type from cartilage; but there are without doubt certain instances in which the homology between two bones would be unhesitatingly admitted were it not for the difference in their 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. Gegenbaur. “Ueb. primäre u. secundäre Knochenbildung mit besonderer Beziehung auf d. Lehre von dem Primordialcranium.” Jenaische Zeitschrift, Vol. III. 1867.
(406) O. Hertwig. “Ueber Bau u. Entwicklung d. Placoidschuppen u. d. Zähne d. Selachier.” Jenaische Zeitschrift, Vol. VIII. 1874.
(407) O. Hertwig. “Ueb. d. Zahnsystem d. Amphibien u. seine Bedeutung f. d. Genese d. Skelets d. Mundhöhle.” Archiv f. mikr. Anat., Vol. XI. Supplementheft, 1874.
(408) O. Hertwig. “Ueber d. Hautskelet d. Fische.” Morphol. Jahrbuch, Vol. II. 1876. (Siluroiden u. Acipenseriden.)
(409) O. Hertwig. “Ueber d. Hautskelet d. Fische (Lepidosteus u. Polypterus).” Morph. Jahrbuch, Vol. V. 1879.
(410) A. Kölliker. “Allgemeine Betrachtungen üb. die Entstehung d. knöchernen Schädels d. Wirbelthiere.” Berichte v. d. königl. zoot. Anstalt z. Würzburg, 1849.
(411) Fr. Leydig. “Histologische Bemerkungen üb. d. Polypterus bichir.” Zeit. f. wiss. Zool., Vol. V. 1858.
(412) H. Müller. “Ueber d. Entwick. d. Knochensubstanz 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 üb. d. Verknöcherung u. die Knochen d. Schädels d. Teleostier.” Niederländisches Archiv f. Zoologie, 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 representative of the axial skeleton.
