A, Primitive conditions as seen in any young embryo. B, Condition as it occurs in Cyclostomata, sturgeons, embryos of bony Actinopterygians. C, Condition found in Selachians and Dipnoans. D and E, Illustrating the developmental process in bony Actinopterygians and higher vertebrates. c, Centrum. d.a, Dorsal arcualia. n.a, Neural arch. nc, Notochord. nc.ep, Chordal epithelium. n.sp, Neural spine. sh.1, Primary sheath. sh.2, Secondary sheath. sk.l, Connective tissue. tr.p, Transverse process. v.a, Ventral arcualia.] A secondary modification of the centrum consists in the calcification of certain zones of the cartilaginous matrix. The precise arrangement of these calcified zones varies in different families and affords characters which are of taxonomic importance in palaeontology where only skeletal structures are available (see SELACHIANS). (b) Arcicentrous type. Already in the Selachians the vertebral column is to a certain extent strengthened by the broadening of the basis of the arcualia so as partially to surround the centra. In the Teleostomes, with the exceptions of those ganoids mentioned, the expanded bases of the arcualia undergo complete fusion to form cartilaginous centra which, unlike the chordacentrous centra, lie outside the primary sheath (figs. 13, D and E). In these forms no invasion of the secondary sheath by cartilage cells takes place. The composition of the groups of arcualia which give rise to the individual centrum is different in different groups. The end result is an amphicoelous or biconcave centrum in general appearance much like that of the Selachian. In _Lepidosteus_ the spaces between adjacent centra become filled by a secondary development of intervertebral cartilage which then splits in such a way that the definitive vertebrae are _opisthocoelous_, i.e. concave behind, convex in front. _Ribs._--In the Crossopterygians a double set of "ribs" is present on each side of the vertebral column, a ventral set lying immediately outside the splanchnocoelic lining and apparently serially homologous with the haemal arches of the caudal region, and a second set passing outwards in the thickness of the body wall at a more dorsal level. In the Teleostomes and Dipnoans only the first type is present; in the Selachians only the second. It would appear that it is the latter which is homologous with the ribs of vertebrates above fishes.