Fig. 319. Vertical section through the middle of a vertebra of Esox lucius (Pike). (From Gegenbaur.)
ch. notochord; cs. notochordal sheath; k. and k´. cartilaginous tissue of the neural and hæmal arches; h. osseous hæmal process; n. spinal canal.
A superficial osseous layer becomes very early formed in each vertebral region of the cellular sheath; while in each of the intervertebral regions, which are considerably shorter than the vertebral, there is developed a ring-like cartilaginous thickening of the sheath, which projects inwards so as to constrict the notochord. At a period before this thickening has attained considerable dimensions the notochord becomes sufficiently constricted in the centre of each vertebral region to give a biconcave form to the vertebræ for a very short period of fœtal life.
The stage with biconcave vertebræ is retained through life in the Perennibranchiata and Gymnophiona.
The chief peculiarity which distinguishes the later history of their vertebral column from that of fishes consists in the immense development of the intervertebral thickenings just mentioned, which increase to such an extent as to reduce the notochord, where it passes through them, to a mere band; while the cartilage of which they are composed becomes differentiated into two regions, one belonging to the vertebra in front, the other to that behind, the hinder one being convex, and the anterior concave. The two parts are not however absolutely separated from each other.
By these changes each vertebra comes to be composed of (1) a thin osseous somewhat hourglass-shaped cylinder with a dilated portion of the notochord in its centre, and (2 and 3) of two halves of two intervertebral cartilages, viz. an anterior convex half and a posterior concave half. The vertebræ thus come to be opisthocœlous. A longitudinal section through the vertebral column at this stage is diagrammatically shewn in [fig. 320] C.
Fig. 320. Diagram representing the mode of development of the vertebræ in the different types. (From Gegenbaur.)
A. Ideal type in which distinct vertebræ are not established.
B. Type of Pisces with vertebral constrictions of the notochord.
C. Amphibian type, with intervertebral constrictions of the notochord by the intervertebral parts of the cellular sheath.
D. Intervertebral constriction of the notochord as effected in Reptilia and Aves.
E. Vertebral constriction of the notochord as effected in Mammalia, the intervertebral parts of the cartilaginous sheath being converted into intervertebral ligaments.
c. notochord; cs. cuticular sheath of notochord; s. cartilaginous sheath; v. vertebral regions; iv. intervertebral regions; g. intervertebral joints.
To the centre of each of these vertebræ the neural arches, the origin of which was described above, become in the meantime firmly attached; and grow obliquely upwards and backwards, so as to meet and unite above the spinal cord. The transverse processes of the vertebræ would seem (Fick) to be developed independently of the arches, though they very soon fuse with them. According to Götte the transverse processes are double in the trunk, there being two pairs, one vertically above the other for each vertebra. The pair on each side eventually fuse together.