48 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 D), 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 hemal 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 vertebre are outlined at an early stage of the embryo and their 
number is not subsequently increased. Consequently increase in 
Fic. 43.—Earlier and later stages of development of a vertebra of Amblystoma. cc, 
cartilage in centre of vertebra; ¢7, elastica interna; 7, incisure cutting through ic, intercentral 
cartilage; , notochord; ms, notochordal sheath; v, vertebra (bone) black. 
length of the vertebral column can only occur by growth of the vertebrx 
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. Asa 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, I) is produced. In some urodeles this stage is followed by 
