THE anterior and posterior surfaces of the centra of these vertebrae, as in all fishes with distinct vertebrae, except Lepidosteus, are concave, the anterior surface being less so than the posterior. They are therefore amphicoelous. The two concavities communicate by a fine central canal. Their surfaces are marked by two sets of lines, radial and concentric, indicating the directions in which ossification has taken place. More or less wedge-shaped cavities are observable around the periphery of the centrum. There are two of especial depth on the ventral surface immediately internal to the bases of the haemal arches in the more anterior vertebrae of the series, while two shallower cavities occupy the median line. In the posterior vertebrae the lateral cavities become more and more shallow, and finally disappear, while the median cavities coalesce and deepen. These cavities in the living animal are filled by connective or fatty tissue. The neural arches are continuous with the centra and coalesce to form a median spine. At the base of each arch there are two processes on either side, an anterior broad process, with a slight groove to its outer side, and a posterior process, pointed and adapted to fit more or less closely into the groove just mentioned.
The anterior process becomes more and more pronounced in the anterior dorsal vertebrae, and in the first vertebra is of huge size and articulates with the skull. The vertebrae carry well-developed inferior or haemal arches projecting outwards and downwards, and continuous with the centrum. In the median vertebrae they are broad and flat, and serve to protect the air-bladder, which is largely developed in the Gadidae, and devoid of an air-duct. In these same vertebrae a smaller process is developed behind, but parallel with the haemal arches. It is probably of no significance beyond assisting in the protection of the air-bladder. The haemal arches fail to be developed only in the four first vertebrae. They carry the ribs. In the region of the tail they are united with the ribs and form a caudal canal, as in the Perch.
The bones of fish are the poorest in inorganic constituents of all the five classes of Vertebrata, and the bone-cells, the characteristic microscopic structures in true bone, may in the fully formed bone be absent altogether or very rare, e. g. in the Perch or Pike. According to Kolliker, they are to be found in all Ganoidei and in the majority of Teleostei which possess an air-duct to the air-bladder.
The formation of the vertebral centra in Teleostei appears to take place as follows: - The embryonic chorda dorsalis, when fully formed, is composed of central vesicular cells with a thin superficial epitheliomorphic layer produced by the peripheral aggregation of the protoplasm of certain of the cells, each apparent cell containing a nucleus. A cuticula comes next: and most externally a layer which, according to Grassi, is an elastica externa, but, according to Gotte, is composed of flat nucleated cells in a single layer, with an abundant intercellular substance. In this substance Gotte states that the first ossifications forming the primary centrum start. The chorda grows intervertebrally, and shrinks to a fibrous cord in the middle of the vertebra. Hence the amphicoelous centra. The cuticula thickens and becomes fibrous intervertebrally, as does the external layer, forming the ligaments which unite the centra peripherally. The neural and haemal arches rest with their bases on the chordal sheaths, but do not grow round the chorda as they do in the Elasmobranch. The main, i. e. secondary osseous substance, is periosteal, formed from connective tissue, which developes between the bases of the arches and imbeds them.
There are four principal types of structure in the vertebrae of adult Teleos-teans. (1) Concentric laminae are laid down round the primary centrum. The bases of the arches ossify and fuse indistinguishable with the laminae; but isolated masses of cartilage, remnants of them, may persist here and there temporarily, e. g. the Cyprinoids. (2) Delicate bony radial laminae with intervening clear connective tissue, appear first of all; finally the bony matter becomes spongy, and the connective tissue is converted into a fatty medulla. The bases of the arches persist as cartilage, and form the well-known cross, e.g. Pike. The majority of Teleostei belong to this type. (3) The bony matter forms a delicate spongy mass; the intervening substance consists of small round cells with a clear, firm matrix; the bases of the arches may retain isolated masses of cartilage (Cyclopterus lumpus) or may ossify completely (Chironectes sp. ?). (4) There are delicate radial united by a few concentric, bony laminae. The intervening substance is hyaline cartilage: Monacanthus penicilligerus, Diodon.
The differences depend upon (1) the persistence of cartilage in the bases of the arches or the total conversion of the latter into bone, and (2) either the total or partial conversion of the intervening connective tissue into bone; and in the latter case the persistence of the remnant as connective tissue or its conversion into either medulla or cartilage.
The neural or superior appear before the haemal or inferior arches, and both structures appear earlier in the anterior than in the posterior region of the column. In Cyprinoid fishes certain of the arches may ossify without previous chon-drification. In the anterior region of the column the haemal arches may become entirely imbedded within the centrum, e. g. in the Perch; and in the posterior region they become not only more and more ventral in position, but they bend more and more towards the ventral median line. Finally they appear to unite, inclosing the caudal canal. The most posterior, however, attached to the terminal vertebrae fuse into solid knobs. It may be observed that in the dorsal region ribs are articulated to their ends. These ribs are developed continuously with the arches in Elasmobranchii, Ganodei so far as is known, and in some Teleostei, whereas in others they are developed independently, but yet in close apposition to their ends. In Amia and Lepidosteus the caudal canal is formed by the haemal arches plus ribs which are cut off from them, but which articulate with them, as in the dorsal region. And it is these ribs that unite distally, completing the canal.