64 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



ventrally by the cartilaginous arcualia. On the dorsal side are the large basi- 

 dorsals, united to form a neural arch topped by a neural spine (supradorsal) . 

 Between the bases of the basidorsals are the small interdorsals. The ventro- 

 lateral regions of the notochord are covered on each side by the large basiventrah 

 each of which bears a projecting process. Note that the basiventrals of the two 

 sides do not meet below the notochord. Between the ventral portions of the 

 basiventrals are the small interventrals. 



2. The tail vertebrae of the bowfin.— Examine a demonstration specimen 

 of the tail of the bowfin. Compare with Figure 21C. Note that the notochord 

 is not visible, being completely inclosed by the vertebrae. In the latter there 

 is an alternation of a vertebra bearing dorsal and ventral arches, with one devoid 

 of arches. The former consists of the fused basidorsals and basiventrals, form- 

 ing neural and haemal arches and a central centrum. The latter consists of the 

 fused interdorsals and interventrals. There are thus two vertebrae to each 

 segment of the body, a condition not uncommon in primitive vertebrates. 



3. Vertebrae of Stegocephala— Study Figure 21D, or K, Figure47-B, page 53. 

 These are representations of the vertebrae of extinct Amphibia. Each vertebrae 

 consists of six pieces of which four are visible from each side. There is a dorsal 

 arch composed of the fused basidorsals, a large ventral piece composed of the 

 fused basiventrals, and two small posterior pieces on each side, the dorsal one 

 being the interdorsal, the ventral one, the interventral. The large notochord 

 is inclosed by these arcualia. 



D. THE VERTEBRAL COLUMN OF THE DOGFISH 



i. Cross-section of the tail. — Obtain a cross-section through the tail of the 

 dogfish and study the cut surface. Be sure that you have a section passing 

 through the junction of successive vertebrae and not through the center of a 

 vertebra. 1 The center of the section contains the vertebra, composed of carti- 

 lage — a clear, relatively soft material. Outside of the vertebra is the layer of 

 voluntary muscles, consisting of a number of leaves, each separated from its 

 neighbor by a plate of connective tissue, the myoseptum or myocomma. Each 

 leaf is a myotome or muscle segment. The myotomes appear in whorls in the 

 section because they are zigzag in form like those of Amphioxus. Each cross- 

 section will consequently cut a number of myotomes. The muscle segments are 

 somewhat indistinctly divided into dorsal and ventral portions by a partition 

 which extends from the central region of the vertebra to the skin where it meets 

 a line, the lateral fine, running along the outer surface of the skin. This connec- 

 tive tissue partition is the horizontal skeletogenous septum. The muscles above 

 it are the dorsal or epaxial muscles, those below it, the ventral or hypaxial 

 muscles. 



1 When the section passes through the center of the vertebra, four rays forming a sort of cross will 

 be seen in the vertebra. These are areas of calcification. 



