222 Journal of Comparative Neurology and Psychology. 



which invaginates and forms a vesicle in all essential respects 

 like the auditory vesicle. This is then converted into a gangli- 

 on which comes into close relation with the maxillo-mandibular 

 ganglion and gives rise to the ophthalmic nerve. A comparison 

 of this account with Coghill's (22) account of the profundus 

 and trigeminus nerves in Amblystoma indicates that the nerve 

 in this case is the ophthalmicus superficialis facialis, and the 

 placode is the homologue of that which gives rise to the R. 

 ophth. sup. VII in cyclostomes, some selachians, and ganoids. 

 That the ganoids are probably to be included here is indicated 

 by the fact that the continuity of the head lines with the audi- 

 tory placode has not been found in Amia (6). From these 

 facts the writer thinks it probable that (i) the acustico-lateral 

 system was not originally confined to a single segment ; (2) that 

 it extended forward one or two segments in front of the site of 

 the auditory pit ; (3) that it was largest, however, in that seg- 

 ment in which the auditory sac was afterward formed, and that 

 in the other segments it appropriated a greater or less part of 

 the neural crest material. 



Two further questions may now be considered: that of the 

 segmental relations of the system, and that of its relation to the 

 general cutaneous nerves. We have come in late years, and 

 rightly so, to deny segmental value to the adult lateral line sys- 

 tem ; but here is evidence that primarily it may have extended 

 through more than one segment. The fact that three separate 

 placodes are formed in the lowest forms, cyclostomes, and in 

 some other fishes and amphibia, suggests that the whole neuro- 

 mast area was not fully continuous at the start but that it was 

 early divided into three parts corresponding to the three seg- 

 ments which it occupied. (Compare 125, p. 578). It seems 

 probable that the supraorbital line, formed from the placode re- 

 lated to the profundus ; the infraorbital line, formed from a pla- 

 code related to the trigeminus ; and the hyomandibular line, 

 formed from the auditory placode, represented three segments 

 of the primitive neuromast system. The lateral line is to be 

 thought of as a late outgrowth from the auditory placode. I 

 am well aware that this is in opposition to the conclusion which 



