424 THE DOGFISH chap. 



faces converted into bone. The biconcave intervals 

 between them are filled with a soft intervertebral substance 

 (Fig. 104 ntc), which is also present between the first 

 vertebra and the skull and which represents part of the 

 embryonic notochord (see pp. 203 and 404). The centra 

 are united by ligament, so that the whole vertebral chain 

 is very flexible. 



In the frog it will be remembered that the completely ossified centra 

 are procalotis, and are artimlated with one another (pp. 36 and 57). 



Connected with the dorsal aspect of the series of centra 

 is a cartilaginous tunnel, consisting of the neural arches 

 (Figs. 104 and 108, n. a) enclosing the spinal cord : it is 

 divided into segments, corresponding with, but usually twice 

 as numerous as the centra owing to the presence of inter- 

 calary pieces. Arising from each centrum on either side is 

 a neural process, which in the adult fuses with a neural plate 

 (Fig. 104, «./) and is perforated posteriorly for the exit of 

 the ventral root of a spinal nerve {for") ; and fitting between 

 two consecutive neural plates is an intercalary piece, the 

 interneural plate (/. «./), perforated by an aperture for the 

 dorsal root of a spinal nerve {^for). The arch is completed 

 above by the neural spines (n. sp), which fit- in between the 

 neural and interneural plates respectively, and are thus, like 

 the lateral elements of the arch, twice as numerous as the 

 centra. The first vertebra has facets for articulation 

 with the condyles of the skull. 



In the anterior part of the vertebral column the centra 

 give off paired outstanding transverse processes, (Fig. 104 

 B, tr. pr), to the end of each of which is articulated a 

 short, cartilaginous rod, the rib (r). Further back the 

 transverse processes are directed downwards, instead of 

 outwards, and in the whole caudal region they unite 



