314 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



leaving one after a few hours, and as forming a 

 fresh one, when they again settle down. 



In the greater number of the Vertebrata the 

 notochord is very profoundly modified in character, 

 and in the higher forms it disappears altogether from 

 the adult, its place being taken by the jointed 

 vertebral coltiaam. In the lowest stages, such 



O ' 



as are found in the Cyciostomata, Chimera 



among the Elasmobranchs, and the Dipnoi, the noto- 

 chord remains imconstricted, but cartilaginous or 

 calcareous deposits become aggregated around it, 

 while above, and sometimes also below it, there 

 appear arches of cartilage, which protect the over- 

 lying nerve-cord and the subjacent blood-vessel. 



In more complete Vertebrae it is possible to 

 distinguish a basal portion, or centrum, which 

 grows round the notochord, from the overlying 

 neural and the underlying Biaemal arches, which 

 enter into more or less close union with it. In many 

 Fishes the notochord remains well developed between 

 the separate vertebral centra, and these are, in the 

 simplest cases, excavated both in front and behind, 

 whence they are known as aniphiccelous ver- 

 tebrae ; in better developed forms a smaller amount 

 of notochord is persistent, and we then get either 

 proccelous or opisthoccelous vertebrae, accord- 

 ing as the excavation is on the anterior or the pos- 

 terior face of the centrum ; sometimes, as in the frog, 

 the invasion of the notochord by cartilage or bone is 

 never complete, and in such cases a cross-section 

 of the centrum of the vertebra reveals the presence 

 of a central notochord (Fig. 129; ch). In the frog, 

 as in the Amphibia generally, the neural arch 

 and the centrum become firmly connected with one 

 another, and from the centrum there are given off 

 horizontal pieces of bone, which form the so-called 

 transverse processes. The separate vertebrae 



