CHAP.XLII.] OF THE SPINAL COLUMN. 597 



system are formed; and immediately beneath it is the great 

 artery of the body, with the viscera. At the anterior and poste- 

 rior extremities of the embryo, the chorda dorsalis tapers to a 

 point. In its earliest condition, it is composed of a perfectly clear 

 gelatinous material, in the anterior extremity of which cells soon 

 make their appearance, and increase in number until the whole 

 becomes cellular. It is surrounded by a delicately fibrous sheath ; 

 external to which the blastema, which gives rise to the deve- 

 lopment of ossifying cartilage, is deposited. In this situation, 

 after a time, cartilaginous rings make their appearance, and merge 

 by insensible gradations into the fibrous sheath. The fibrous 

 structure gradually disappears, and in its place cartilage is formed, 

 while at the same time the substance of the cord is removed, to 

 give place to the developing cartilage. The cells of the cord, how- 

 ever, are not transformed into cartilage cells. Eventually only a 

 portion of the cellular substance remains between the bodies of the 

 vertebra. At a much later period, cartilaginous arches are formed 

 in the inner part of the dorsal laminae, which become converted 

 into the vertebral arches. The outer portion of the laminae dorsales 

 becomes converted into muscular tissues and integuments. 



The cranium is originally formed from an extension forwards of 

 the chorda dorsalis, and its development occurs at a much earlier 

 period than the bones of the face. In the lamprey and the stur- 

 geon, the connection between the cord and the cerebral cartilage 

 is permanent. In mammalia, those portions analogous to the bodies 

 of vertebras appear in the basis cranii ; and prolonged from these, 

 above, are portions corresponding to the neural arch of the typical 

 vertebra ; and below, parts belonging to the hamal arch. 



The body of the epencephalic or occipital vertebra is represented 

 by a distinct point of ossification, for the basilar process of the 

 occipital bone ; its neural arch, by the expanded portion of the bone 

 itself; its hamal arch, by the scapula, bones of arm, fore-arm, and 

 hand, and the coracoid processes of the scapula (coracoid bones of 

 oviparous vertebrata) . 



The body of the mesencephalic or parietal vertebra is seen in the 

 basi-sphenoid, or body of the sphenoid bone ; its neural arch is 

 formed by the mastoid portions of the temporal bones, the great icings 

 of the sphenoid and the parietal bones; its hcemal archly ihestyloid 

 process of the temporal, and by the body and greater and lesser 

 cornua of the hyoid bones. 



The prosencephalic or frontal vertebra has its body represented by 

 the anterior or spheno-orbital portion of the sphenoid; its neural arch. 



