DEVELOPMENT OF THE NERVOUS SYSTEM. 



57 



sense cells have come to join the ganglia. In fact very little is 

 certainly known as to the nature of these ectodermal thickenings. 

 The presence of the placodes in man and the separation between 

 the root ganglia and those formed in connection with the placodes 

 suggest that the placodes probably have the same history and 

 function in all vertebrates. It is of the highest importance for 

 the morphology of the nervous system that the exact history and 

 fate of the placodes should be traced in one or more classes of 



OP* - 



FIG. 33. Three stages in the development of the acustico-lateral system in the 

 sea bass. From H. V. Wilson, a. s., auditory sac; a. s. t., anterior sensory tract; 

 B. s. o., preauditory pit; g. s., gill slit; /. /., lateral line anlage; In., lens; m. con., 

 furrow between midbrain and hindbrain: med., medulla oblongata; n. s., nasal 

 sac; op. n., optic tract; op. s., optic vesicle; s. /., common sensory furrow. 



vertebrates and that the distribution, central connections and 

 functions of the fibers formed from ectodermal cells, if any, should 

 be ascertained. This can doubtless, be done by extirpation 

 experiments on embryos. 



In front of the anlage for the glossopharyngeus nerve there is a 

 segment of the neural crest which gives rise to the sensory root 

 of the VII nerve. This is deeply constricted from the segment 

 for the IX nerve and is wholly separated from the portion belong- 

 ing to the trigeminus group. While the ganglion of the IX nerve 

 grows down wholly behind the auditory pit, that of the VII nerve 

 comes into contact with the pit, in lower vertebrates in contact 

 with its caudal surface, in higher vertebrates with its mesial wall. 



