. The chordates. Chordata. Motor System—Muscles and Skeleton , cell becomes greatly distended by a fluid-filled vacuole, and the entire rod is invested externally by a tough, fibrous sheath (Fig. 101). The texture of the notochord combines firmness and elasticity. Mechani- cally it resembles a length of rubber tubing with closed ends and filled with fluid. It supports but also bends readily. It is never Fig. 101. Longitudinal sections of devel- oping vertebrae of the amphibian, Amby- stoma. (Left) Earlier. {Right) Later. Cartilage and bone are forming around the notochord. (cc)
. The chordates. Chordata. Motor System—Muscles and Skeleton , cell becomes greatly distended by a fluid-filled vacuole, and the entire rod is invested externally by a tough, fibrous sheath (Fig. 101). The texture of the notochord combines firmness and elasticity. Mechani- cally it resembles a length of rubber tubing with closed ends and filled with fluid. It supports but also bends readily. It is never Fig. 101. Longitudinal sections of devel- oping vertebrae of the amphibian, Amby- stoma. (Left) Earlier. {Right) Later. Cartilage and bone are forming around the notochord. (cc) Cartilage in center of vertebra; (ei) epi- theloid internal elastic sheath of notochord; (i) incisure cutting through (ic) intercentral (intervertebral) cartilage; (n) notochord; (ns) outer notochordal sheath; (») develop- ing bone (black) of centrum of a vertebra. (Courtesy, Kingsley: "Comparative Anat- omy of Vertebrates," Philadelphia, The Blakiston Company.) The notochord persists throughout the life as the functional axial skeleton of the fishlike round-mouthed eels (cyclostomes). In its func- tional estate, it may be studied to advantage in the tail of the full- grown tadpole of frog or toad. It serves as the embryonic foundation around which a cartilaginous vertebral column may be developed (Fig. 100B), as happens in all vertebrates except the cyclostomes. In verte- brates with backbones, the bone replaces the cartilage of the em- bryonic cartilaginous vertebral column. Traces of the notochord may persist between successive Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphia : Blakiston
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