Johnston, Morphology of the Head. 245 



end. The nerve has the same distribution as the nerve of 

 PiNKUS and is considered by Locy to be homologous with it. 

 Locy's nerve possesses a distinct gangHon and ends centrally in 

 a mesial eminence of the infolded pallium (126). 



Until this nerve is better understood it is not worth while 

 to attempt to explain the difference in its point of attachment 

 to the brain in different species. It is important to notice, how- 

 ever, that its adult position indicates that it arises either lateral 

 or dorsal to the neuropore in selachians. Its position is there- 

 fore morphologically dorsal and both the position of its root and 

 its distribution to the extreme anterior end of the head indicate 

 that it belongs to the first segment. Since it is distributed to 

 the skin it may be supposed to be general cutaneous in char- 

 acter. Until further investigation, then, the hypothesis is sug- 

 gested that the nerve described by Pinkus and LocY represents 

 the primitive general cutaneous nerve of the first segment. 



If it is true that this nerve ends in the pallium in selachians, 

 the fact suggests the very interesting possibility that before the 

 olfactory cortex was developed a general cutaneous center was 

 located in its place. More than this can not be said until the 

 central relations of the nerve are more fully studied. 



75. Morphology of the olfactory organ. Functional and morpho- 

 logical relations to the gustatory system. 



The resemblance of the olfactory sense cells to the typical 

 sense cells of segmented worms may be taken as a starting point. 

 Since the ectoderm of these worms is provided with free nerve 

 endings (113, no, 82) comparable to the general cutaneous 

 endings in vertebrates, the sense cells probably do not have a 

 tactile function. It is more probable that they serve the func- 

 tion of taste or smell. Or perhaps it would be better to say 

 that they have an indifferent chemical sense. These cells are 

 especially numerous in the prostomium and roof of the mouth 

 in the earth-worm. In Amphioxus (24) such cells are present 

 as scattered cells in the lining of the buccal cavity, the velum and 

 tentacles, the oral cirri, and the skin of a large part of the body; 

 and in the form of cell groups or organs they are found in the 



