DEVELOPMENT OF THE NERVOUS SYSTEM. 59 



come enclosed as tunnel like canals and finally the successive 

 canals unite to form a continuous canal, the canal of the lateral 

 line. Meanwhile the cells of the ganglion send processes caudally 

 beneath or imbedded in the growing placode, which come into 

 relation with the sense organs as they are formed and constitute 

 the nerve of the lateral line. 



The cephalic part of the placode, in front of the auditory pit, 

 grows forward and divides into two branches which continue for- 

 ward and give rise to sense organs in the same way as does the 

 lateral line placode. One branch extends fonvard above the eye 

 and forms the supraorbital row of organs, the other goes ventral 

 to the eye and forms the infraorbital line. Ganglion cells which 

 proliferate from the placode in front of the auditory pit give rise 

 to the rami ophthalamicus superfadalis and buccalis which supply 

 these lines and form the one or two roots of these rami which 

 have been described above (p. 20, 21). 



Beneath and in front of the auditory pit a ventral projection 

 of the common placode extends downward and forward on the 

 mandibular arch. This develops in the characteristic manner 

 the hyomandibular line of sense organs. The nerve fibers which 

 supply this row of organs are also derived from the placode cells 

 and form a part of the ramus hyomandibularis. 



In this way the acustico-lateral system of sense organs and nerves 

 in most fishes is derived from an extensive thickening of ectoderm 

 on the side of the head, the dorso-lateral placode. Certain impor- 

 tant differences in other forms must now be mentioned. The anterior 

 part of the placode is not always directly continuous with the part 

 which forms the auditory pit. Sometimes this part becomes 

 invaginated to form another similar pit lying in front of the auditory 

 pit and in front of the spiracular cleft. This pit may even become 

 an enclosed sac and contain well formed sense cells (Wilson) 

 similar to those of the lateral line organs. In embryos of certain 

 amphibia (Gymnophiona) a deep pit is formed hi front of the 

 auditory pit and independently of it. This gives rise to the nerve 

 which supplies the supraorbital row of sense organs. In Amblys- 

 toma punctatum this pit appears earlier than the auditory pit. In 

 cyclostomes (Fig. 28) two separate ectodermal placodes are formed 



