A DEFINITION OF EVOLUTION 



Notochofdl 



Myotome (cut) 



Vertebra 



Figure 7. The Arcualia which Form the Vertebrae. A shows two body segments, 

 eaeh with four arcuaUa ( actually four pairs, but only the left side is shown ) . B and C 

 illustrate the separation of the anterior from the jDOsterior arcualia in each segment. 

 D shows how a \ ertebra comes to span two muscle segments because it is made up 

 of arcualia derived from two adjacent body segments. The arcualia are named inter- 

 dorsal, id; interventral, iv; basidorsal, bd; and basiventral, bv. (From Romer, "The 

 Vertebrate Body," 2nd Ed., W. B. Saunders Co., 1955. ) 



formers) form neural arches and so-called intercalary arches over the 

 spinal cord. The two ventral pairs fail to form arches in the trunk region, 

 but in the caudal region the anterior ventral pair of arcualia forms a 

 haemal arch in each segment, enclosinti the caudal arterv and vein. A 

 centrinn is formed around the notochord partly by the notochordal sheath, 

 and partly by the bases of the arcualia and the surrounding mesenchyme. 

 The articulation is strengthened by the notochord, which is still continu- 

 ous in adult sharks. Ribs are formed in conjunction with these vertebrae. 

 The entire skeleton of the adult is cartilaginous, a fact which is probably 

 pi'operly regarded as the retention of a basicallv embrvonic condition. 



l''i(;uRE S. The "Vertebrae" of the 



I.AMPHKY, CONSISTINC; OnEY OF THE 



Twt) Pairs of 1J)orsal Arcualia in 

 Each Segment, a shows interseg- 

 mental blood vessels; b, segmental 

 ner\c; c, neural tube; d, notochord; 

 e and g, dorsal arcualia; and o, lon- 

 gitudinal blood vessels. (From Hy- 

 inan, "Comparative Vertebrate Anat- 

 omy," 2nd Ed., University of Chicago 

 Tress, 1942.) 



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