THE ENDOSKELETON: VERTEBRAL COLUMN AND RIBS 65 



Turning now to the vertebra itself, we note that it consists of a central 

 circular concave portion, the centrum or body; dorsal to this an arch, the neural 

 arch, which incloses a cavity, the neural canal, in which the spinal cord, a soft 

 white body, is located; ventral to the centrum, another arch, the haemal arch, 

 which incloses two blood vessels, a dorsal caudal artery and a ventral caudal vein. 

 The neural arch terminates in a point, the neural spine, and the haemal 

 arch similarly terminates in the haemal spine. Observe the connective tissue 

 partitions which extend from the neural spine to the median dorsal line and 

 from the haemal spine to the median ventral line. These are the dorsal and 

 ventral skeletogenous septa and they, together with the horizontal skeletogenous 

 septum ahead}- mentioned, mark the chief sites of skeleton formation. 



Draw the section, labeling all parts. 



2. Sagittal section of the tail. — Obtain or make a median sagittal section 

 through a piece of the tail of a dogfish. The vertebrae form a row in the center 

 of the section. Identify the centra in the section. Each consists of two some- 

 what triangular pieces, apparently separate, the rounded apexes of the triangles 

 directed toward each other, the whole shaped somewhat like an hourglass. The 

 two ends of the centra are concave, so that diamond-shaped spaces are present 

 between successive centra. These spaces are filled with a soft, gelatinous 

 material, the notochord; which also fills the canal which runs through the center 

 of the centrum, between the apexes of the triangular halves of the centra. Centra 

 of this shape, concave at each end, are known as amphicoelous centra. Above 

 each centrum identify the neural arch, arching over the neural canal. Between 

 successive neural arches, and lying therefore dorsal to the diamond-shaped spaces 

 between the centra, observe an extra arch, inverted, however, with apex inclos- 

 ing the neural canal. This is the intercalary arch. Below the centrum is the 

 haemal canal, its sides formed by the haemal arches, rectangular in section. 

 Draw the section. 



The centrum of the dogfish vertebrae (and of the vertebrae of all the elasmobranch 

 fishes) arises within the sheath of the notochord. The cells which produce the centra migrate 

 into the sheath from the arcualia. Centra of this type are chordal centra, and vertebrae having 

 such centra are known as chordocentrous vertebrae. The neural arch is the fused basidorsals; 

 the intercalary arch, the fused interdorsals; the haemal arch is the fused basiventrals; inter- 

 ventrals are lacking (Fig. 2 2 A). The vertebrae of elasmobranchs are permanently cartilagi- 

 nous with the notochord persistent in the center of the centra. 



3. Cross-section of the trunk region.— In a cross-section of the anterior 

 part of the trunk region of the dogfish identify the following parts. The muscle 

 segments are arranged as in the tail region, their division into dorsal and ventral 

 masses being well marked by the horizontal skeletogenous septum. The dorsal 

 or epaxial muscles above the septum are thick masses, but the ventral or hypaxial 

 muscles below the septum form a thin layer inclosing a large cavity, the body 

 cavity or coelom, lined by a smooth membrane, the pleuroperitoneum. The 



