DEVELOPMENT OF THE NERVOUS SYSTEM. 6l 



tion of the rami maxillaris and mandibularis trigemini. The 

 anterior arilage loses its connection with the brain and the fibers 

 which grow centrally from its ganglion cells follow caudally over 

 the trigeminal ganglion and enter the brain along with the root 

 of the trigeminus. The peripheral fibers from this ganglion run 

 cephalad through the dorsal part of the orbit and form the N. 

 ophthalamicus profundus. Figure 29 shows the early stages in 

 the differentiation of these two ganglia. While the above descrip- 

 tion applies to selachians it may be taken in a general way as typical 

 for vertebrates. In cyclostomes the two nerves are more indepen- 

 dent in their origin and adult relations, and in all vertebrate embryos 

 the two ganglia can be recognized. 



In certain selachians an anlage of a sensory nerve is formed 



FIG. 34. A transverse section through the nasal sac of an embryo of a bony fish 

 at about the time of hatching, to show the origin of the fibers of the olfactory nerve. 



from the neural crest between the mesencephalon and diencephalon. 

 It is known as the nervus thalamicus (Fig. 29). The anlage is 

 small, comes to be closely related to the profundus trunk and may 

 furnish material for the ciliary ganglion. In the chick the ciliary 

 ganglion is formed in part of cells which migrate out from the 

 neural tube along the fibers of the oculomotor nerve and in part 

 of cells which come from the ophthalamicus profundus ganglion. 



In front of this point the neural crest does not give rise to any 

 nerve which is found constantly in vertebrates. In selachians, 

 however, the N. terminalis (see p. 31) possesses a ganglion and in 

 Squalus acanthias develops in much the same way as do other 

 sensory nerves. In the chick ganglion cells are found in connec- 

 tion with the olfactory nerve during its development which may 

 bear some relation to this nerve in fishes. 



The olfactory nerve differs from all other nerves in vertebrates 



