. Chordate morphology. Morphology (Animals); Chordata. Figure 6-25. Vertebral structure of Eusfhenopteron. A, lateral view of two thoracic vertebrae; B, anterior view of a thoracic; C, anterior view of a caudal vertebra showing neural and hemal arches. (After Jorvik, 1952) sclerotome material, and this is mainly in the region of what is to become the neural arch. The notochord has a distinct sheath with fibrous and external layers. The first evidence of the skeleton is the appearance of the neural arches fol- lowed by the hemal arches. The cartilaginous dorsal and the ventral arches, or arcual
. Chordate morphology. Morphology (Animals); Chordata. Figure 6-25. Vertebral structure of Eusfhenopteron. A, lateral view of two thoracic vertebrae; B, anterior view of a thoracic; C, anterior view of a caudal vertebra showing neural and hemal arches. (After Jorvik, 1952) sclerotome material, and this is mainly in the region of what is to become the neural arch. The notochord has a distinct sheath with fibrous and external layers. The first evidence of the skeleton is the appearance of the neural arches fol- lowed by the hemal arches. The cartilaginous dorsal and the ventral arches, or arcualia, lie in contact with the noto- chord. and a perichordal tube encloses their bases (Figure 6-27). These arches lie mainly in front of the myoseptum, but extend partly into the area behind the myoseptum. In some cases, a secondary procartilaginous center is observed behind the ventral arch center. This interventral center unites with the anterior arch. The vertebral body ossifies without a cartilaginous precursor in the perichordal con- nective tissue enclosing the bases of the arches. The dorsal arch, ossifying perichondrally, generally fuses to this body. The ventral arch unit forms the parapophysis of the hemal arch, ossifies perichondrally. and fuses to, or remains sepa- rate from, the body. In the process of development, the noto- chord is constricted within the body and eventually is reduced to a strand of tissue. There is a large ovoid inter- vertebral mass of connective tissue and notochord in the adjacent ends of the amphicoelous vertebrae. In teleosts, the notochord may be much constricted, or only slightly so. In some teleosts (Clupea and Lebistes), the outer part of the notochordal sheath ossifies, but this is, probably, not usually the case. The notochord is, as a rule, modified in terms of its internal structure; there are large internal spaces, or strands and patches of denser tissue. suprodorsol. Figure 6-26. Vertebral structure of a salmon (Blue-Backed Salmon)
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