Phenomcna of ncurobiotaxis in thc optic system 299 



Whereas ¡n .Vmphioxus the perception-elements of smell form a 

 single placodc in the skin, from where they send out their neuritas in the 

 neuroporus wall to the central nervous system, all animáis of a higher 



Olfact. epilh. 



Fig. 26. — Growth of the olfactory bulb and olfactory stalks towards the oifac- 

 tory neuro epitheh'um in a shark. Diaf;ramniatic representation. 



range have paired olfactory placodes 1, which have a peculiar influence on 

 the form of the brain. 



Whereas in the sensory nerves the shifting in the direction of the sti- 

 mulus is confined to the cell of the first neurone, the spinal ganglion cell, 

 and in the system of hearing ofmammals, it is found in the first and se- 

 cond neurones, in the fore-brain (fig. 26) the brain zea// ifse/f mdikes a fron- 



' Sometimcs thcy have, in addition to the paired pLicodes, a media inipair 

 ene, which however mostly atrophies in the adult. 



