Motor System — Muscles and Skeleton 



10. r , 



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 segmented. 



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 vertebrae. 



