DEVELOPMENT OF THE NERVOUS SYSTEM. 



53 



and some selachians a complete series of segments of the neural 

 crest in this region is formed. These represent the rudiments 

 of nerv^es corresponding to the sensory nerves to the skin in the 

 trunk region. In cyclostomes these rudiments give rise to per- 



IX VII 



N. thalamicus 



-V. thalamicus 



N. thalamicus 



Fig. 29. — Three diagrams of the head of Squalus acanthias to show the diflferen- 

 ■tiation of the neural crest into the cranial ganglia. After Neal. A, i, 2, 3, somites. 



manent cutaneous nerves, while in all other vertebrates a variable 

 number of these nerves fail to develop. (Compare Chapter VI.) 

 In front of this the neural crest is larger and is divided by a slight 



