Fig.167. Head of a shark embryo. Fig. 167—Head of a shark embryo (_Pristiurus_), one-third of an inch long, magnified. (From _Parker._) Seen from the ventral side.
The innermost median part of the primitive segment plates, which lies immediately on the chorda (Fig. 145 _ch_) and the medullary tube (_m_), forms the vertebral column in all the higher vertebrates (it is wanting in the lowest); hence it may be called the _skeleton_ plate. In each of the provertebræ it is called the “sclerotome” (in opposition to the outlying muscular plate, the “myotome”). From the phylogenetic point of view the myotomes are much older than the sclerotomes. The lower or ventral part of each sclerotome (the inner and lower edge of the cube-shaped provertebra) divides into two plates, which grow round the chorda, and thus form the foundation of the body of the vertebra (_wh_). The upper plate presses between the chorda and the medullary tube, the lower between the chorda and the alimentary canal (Fig. 137 _C_). As the plates of two opposite provertebral pieces unite from the right and left, a circular sheath is formed round this part of the chorda. From this develops the _body_ of a vertebra—that is to say, the massive lower or ventral half of the bony ring, which is called the “vertebra” proper and surrounds the medullary tube (Figs. 164–166). The upper or dorsal half of this bony ring, the vertebral arch (Fig. 145 _wb_), arises in just the same way from the upper part of the skeletal plate, and therefore from the inner and upper edge of the cube-shaped primitive vertebra. As the upper edges of two opposing somites grow together over the medullary tube from right and left, the vertebra-arch becomes closed.
The whole of the secondary vertebra, which is thus formed from the union of the skeletal plates of two provertebral pieces and encloses a part of the chorda in its body, consists at first of a rather soft mass of cells; this afterwards passes into a firmer, cartilaginous stage, and finally into a third, permanent, bony stage. These three stages can generally be distinguished in the greater part of the skeleton of the higher vertebrates; at first most parts of the skeleton are soft, tender, and membranous; they then become cartilaginous in the course of their development, and finally bony.