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 nerves corresponding to the sensory nerves to the skin in the 

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



IX VII 



N. thalamicus 



N. thalamicus 



N. thalamicus 



FIG. 29. Three diagrams of the head of Squalus acanthias to show the differen- 

 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. ") 

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



