30 



VERTEBRATE SKELETON 



sive neurapophyses are closed by roof plates, possibly modified spin- 

 ous processes. As neurals and haemals do not meet on the sides of 

 the notochord, no centra are formed, but there are non-metameric 

 calcifications of the chordal sheath in Chimcera. In the adult the 

 various parts fuse with little regularity, while just behind the head 

 a large number of vertebral segments unite so that the centra are 

 composed of a cranial half followed by a caudal half, each entirely 

 enclosing the notochord. 



In more normal Elasmobranchs centra arise by chondrification 

 within the sheath (p. 19), these corresponding rather closely to the 

 neurals and ha^mals of the early stage and situate at about the level 

 of the myosepta. These centra restrict the chorda, which therefore 



Fig. 31. Fig. 32. D 



Fig. 31. — Longitudinal section of vertebras of Squatina (Hasse. '79-82). /. inter- 

 vertebral line; w, notochord; v, vertebral centra; heavily calcified parts of amphicoelous 

 centra darkest, around these (stippled) are rings of calcified cartilage. Unmodified 

 notochordal tissue light. 



Fig. 32. — Diagrammatic sections of vertebrse. A, B, cyclospondylous Elasmo- 

 branchs; C, asterospondylous Elasmobranch; D, cross section of Teleost vertebra; bone 

 black, cartilage stippled. 



can increase in diameter only at the level of the middle of the parent 

 myotomes. Thus the notochord becomes a series of intervertebral 

 enlargements and vertebral constrictions, and, as the centra increase 

 in length, they adapt themselves to this condition, extending 

 over the intervertebral enlargements, thus becoming shaped like 

 an hour-glass, hollow at either end (amphicoelous, fig. 31). The 

 successive centra are connected by the ligamentous parts of the 

 sheath in which no cartilage is ever developed. Later, the centra 

 increase in thickness and the successive layers of cartilage are calci- 

 fied. In some sharks these layers form complete rings so that a 

 transverse section shows concentric layers of hard and softer parts 

 (cyclospondylous vertebrae, fig. 32, yl, .S). In others no calcifica- 



