[Illustration: FIG. 112.—A, transverse section of a precaudal vertebra; B, similar section of a caudal vertebra. _h.a_, Haemal arch (basi-ventrals); _h.c_, haemal canal; _h.s_, haemal spine; _n.c_, neural canal. Other reference letters as in Fig. 111.] The different vertebral components are by no means of equal morphological value. The basi-dorsals and basi-ventrals, and the inter-dorsals and inter-ventrals, are the primary elements and the most important. The supra-dorsals are merely cartilages segmented off from the basi-dorsals and inter-dorsals, while the ribs and the infra-ventrals are similarly derived from the basi-ventrals and inter-ventrals respectively. As to the vertebral elements which collectively form a vertebra in the Dog-Fish, it would seem from evidence afforded by the neuromeres,[175] and more especially by the facts of development, that each complete skeletal segment or vertebra consists of a pair of basi-dorsals with the preceding pair of inter-dorsals, and of a pair of basi-ventrals with the next succeeding pair of inter-ventrals. It must be emphasised, however, that, considered as a joint or segment in a flexible back-bone, a vertebra is a physiological unit, the morphological value of which may differ widely in different Fishes. Hence, in other Fishes, the grouping of vertebral components to {196}form individual vertebrae may be quite different to that which takes place in the Dog-Fish, and may even be accompanied by their more or less complete fusion.