VERTEBRAE OF COMMON COD. 99 



1 8. VERTEBRAE OF COMMON COD (Gadus morrkua). 



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, in- 

 dicating 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 



ymtral vesicular cells with a thin superficial epitheliomorphic layer produced 

 r the peripheral aggregation of the protoplasm of certain of the cells, each 

 >parent cell containing a nucleus. A cuticula comes next : and most externally 

 ., 



