4 8 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



This is the rhachitomous or temnospondylous vertebra. Still farther expansion 

 of hypo- and pleurocentra causes the former to unite with the neural arch, while 

 the two pleurocentra meet below the notochord (fig. 42 Z>), the result being two rings 

 in each somite, the embolomerous vertebra, which occurs in some stegocephali, 

 some fossil ganoids and in the tail of Amia. Lastly these two rings (often called 

 centrum and intercentrum) may fuse, giving the typical centrum. 



The neural and haemal spines which complete the arches are formed by seg- 

 mental chondrifications of the interspinous ligament which runs the length of the 

 body above and between the halves of the neural arches. 



The vertebrae are outlined at an early stage of the embryo and their 

 number is not subsequently increased. Consequently increase in 



I 



FIG. 43. Earlier and later stages of development of a vertebra of Amblystoma. cc, 

 cartilage in centre of vertebra; ei, elastica interna; i, incisure cutting through ic, intercentral 

 cartilage; , notochord; ns, notochordal sheath; v, vertebra (bone) black. 



length of the vertebral column can only occur by growth of the vertebrae 

 themselves. When first formed each centrum encircles the notochord 

 and prevents its increase in diameter at this point, while between the 

 centra it can expand. As a result the notochord soon resembles a string 

 of beads (moniliform) with intervertebral enlargements. Then, as 

 additions are made to the centra to increase their length, the new parts 

 must form around the intervertebral enlargements and in this way the 

 ends of the centra become cup-shaped and the; amphiccelous condition 

 (fig. 43, /) is produced. In some urodeles this stage is followed by 



