In all the Craniotes the soft, indifferent cells of the mesoderm, which originally compose the skeletal plate, are afterwards converted for the most part into cartilaginous cells, and these secrete a firm and elastic intercellular substance between them, and form cartilaginous tissue. Like most of the other parts of the skeleton, the membranous rudiments of the vertebrae soon pass into a cartilaginous state, and in the higher Vertebrates this is afterwards replaced by the hard osseous tissue with its characteristic stellate cells (Figure 1.6). The primary axial skeleton remains a simple chorda throughout life in the Acrania, the Cyclostomes, and the lowest fishes. In most of the other Vertebrates the chorda is more or less replaced by the cartilaginous tissue of the secondary perichorda that grows round it. In the lower Craniotes (especially the fishes) a more or less considerable part of the chorda is preserved in the bodies of the vertebrae. In the mammals it disappears for the most part. By the end of the second month in the human embryo the chorda is merely a slender thread, running through the axis of the thick, cartilaginous vertebral column (Figures 1.182 ch and 2.329 ch). In the cartilaginous vertebral bodies themselves, which afterwards ossify, the slender remnant of the chorda presently disappears (Figure 2.330 ch). But in the elastic inter-vertebral disks, which develop from the skeletal plate between each pair of vertebral bodies (Figure 2.329 li), a relic of the chorda remains permanently. In the new-born child there is a large pear-shaped cavity in each intervertebral disk, filled with a gelatinous mass of cells (Figure 2.331 a). Though less sharply defined, this gelatinous nucleus of the elastic cartilaginous disks persists throughout life in the mammals, but in the birds and most reptiles the last trace of the chorda disappears. In the subsequent ossification of the cartilaginous vertebra the first deposit of bony matter ("first osseous nucleus") takes place in the vertebral body immediately round the remainder of the chorda, and soon displaces it altogether. Then there is a special osseous nucleus formed in each half of the vertebral arch. The ossification does not reach the point at which the three nuclei are joined until after birth. In the first year the two osseous halves of the arches unite; but it is much later--in the second to the eighth year--that they connect with the osseous vertebral bodies. (FIGURE 2.336. Skeleton of the breast-fin of Ceratodus (biserial feathered skeleton). A, B, cartilaginous series of the fin-stem. rr cartilaginous fin-radii. (From Gunther.) FIGURE 2.337. Skeleton of the breast-fin of an early Selachius (Acanthias). The radii of the median fin-border (B) have disappeared for the most part; a few only (R) are left. R, R, radii of the lateral fin-border, mt metapterygium, ms mesopterygium, p propterygium. (From Gegenbaur.)