ioo PISCES 



inside and outside the elastica externa the mesoblastic tissue may 

 develop into cartilage, continuous with the arches, and forming 

 hour-glass- shaped centra constricting the notochord segmentally. 

 The remains of the ruptured elastica may thus become buried in 

 the centrum. 



Such centra, formed at all events partly inside the sheaths, are 

 called chordal centra, in contrast to those of the first type, the 

 perichordal centra, which develop outside the sheaths. 



The structure of the vertebral column is by no means uniform. 

 A varying number of elements may serve to arch over the neural 

 and haemal canals, and the centra themselves may show traces of 

 a complex build. Attempts have been made to reduce the various 

 types of structure of the vertebral column, not only of the Pisces, 

 but also of the Tetrapoda, to a single scheme of homologous parts 

 (Gadow and Abbott [146], Schauinsland [384]). But it is doubtful 

 whether such a proceeding is really justifiable, since the different 

 types may well have been independently developed. Moreover, 

 when comparing different regions of the vertebral column 

 together, authors often try to interpret the structure in agreement 

 with some artificial scheme uniform throughout ; but it should be 

 remembered that there is no reason to believe that the arrangement 

 of the dorsal elements was ever exactly like that of the ventral, or 

 that of an anterior segment ever exactly like that of a posterior. 



The elements entering into the composition of a single vertebral 

 segment are very variable, both in number, in size, and in shape. 

 Yet it appears to be possible to reduce them to the following 

 common plan of four chief paired elements. The neural canal is 

 arched over by two elements: the neural arch (basidorsal [140], 

 caudal arch [384]) behind, and the intercalary arch (interdorsal [146], 

 cranial arch [384]) in front. Below are found corresponding elements 

 enclosing the haemal canal: a haemal arch (basiventral [146], 

 caudal arch [384]), and an interhaemal arch (interventral [446], 

 cranial arch [384]). The interdorsals, and especially the inter- 

 ventrals, are liable to reduction. It is very difficult in ontogeny 

 to follow the exact limits between consecutive segments, since 

 the myocommata spread out when they reach the vertebral 

 column, and the myotomes become very much bent. But the 

 blood-vessels passing upwards from the aorta and cardinal veins 

 may be taken to indicate the original anterior limit of a segment. 

 The spinal nerves pass out in front of the vessels to reach the 

 myocommata (Fig. 45). The neural arch lies between the nerve 

 and the vessel. The basidorsals and interdorsals vary greatly in 

 relative development; they frequently surround the nerve-roots. 

 The roof of the neural canal may be closed above (as in Elasmo- 

 branchs) by a row of median or paired elements, the supradorsals. 

 On the top of these runs the longitudinal ligament. Above this 



