THE SKUI^L. 225 



upwards from the sides of each centrum opposite 

 its middle. 



b. The neural plates are a series of thin hexagonal 



plates of cartilage which form the sides of the 

 neural canal (figs. 48 and 51). On each side 

 there are twice as many neural plates as there 

 are vertebrae, the neural plates being placed 

 alternately opposite the middles of the vertebrae, 

 and opposite the intervertebral intervals. The 

 former or vertebral neural plates are fused in 

 the adult with the tops of the neural processes, 

 and have their posterior borders notched for the 

 passage of the ventral roots of the spinal nerves. 

 The dorsal roots of the nerves pass out through 

 notches in the posterior borders of the inter- 

 vertebral neural plates. 



c. The neural spines are a series of median nodules 



of cartilage wedged in between the dorsal ends of 

 the neural plates so as to complete the neural 

 arches above. 



Along the greater part of the length of the 

 column there are two neural spines to each 

 vertebra, but in places the arrangement is less 

 regular. 



B. The Skull. 



The skull of the dog-fish remains cartilaginous through- 

 out life, thereby escaping the complications that result from 

 the development of bone : in it, also, the several factors of 

 which the skull is made up are more easily determined than 

 in the higher forms. For these reasons it is peculiarly 

 instructive, affording an important clue to the complex con- 

 dition met with in higher vertebrates. 



The skull consists of (1) an axial tube, the cranium 

 proper, open at both ends, which lodges the brain, and is to 

 be regarded as an anterior unsegrhented -continuation of the 

 vertebral column ; and (2) the olfactory and auditory sense- 



