204 JOHN BEARD. 



attach any weight to the reasons which Van Wijhe urged for this 

 opinion, which was based on the presence of two head cavities in the 

 hyoid arch. Van Wijhe does not appear to have attached much 

 importance to the evidence offered by the nerves, for he did not regard 

 the auditory nerve as in itself of segmental value, and he never 

 suggested the homology of the auditory organ with the branchial sense 

 organs. 



Development op the Auditory Nerve. 



In Elasmobranchii the facts of development for this segment are 

 exactly comparable to those described for the olfactory segment. The 

 arrangement is here the same. There is no gill-cleft, and of course, 

 as a consequence of the absence of that, we cannot expect to find a 

 post-branchial nerve. 



The following line of argument may, as in the case of the olfactory, 

 be used for the auditory segment. The sense organs and ganglion 

 connected with the ciliary segment are without doubt homologous with 

 the sense organs and ganglion of a cleft-bearing segment such as the 

 glossopharyngeal. The ciliary has no prse- or post-branchial nerves 

 because there is no gill-musculature or cleft. The auditory segment 

 has no prse- or post-branchial branch just as the ciliary, but its sense 

 organs, ganglion, and nerve are exactly like, and have the same structure 

 as the sense organ, ganglion, and nerve of the ciliary segment. There- 

 fore the auditory nerve, organ, and ganglion are homologous with the 

 nerve, sense organ, and ganglion of the ciliary segment, and therefore 

 are also the homologues of the nerve, sense organ, and ganglion of the 

 glossopharyngeal segment. But the sense organ and ganglion of the 

 latter are a branchial sense organ and its ganglion, therefore the 

 auditory organ is also a branchial sense organ, and the auditory nerve 

 the remnant of a segmental nerve. 



Immediately behind and somewhat overlapping the sensory thicken- 

 ing which gives rise to the facial branchial sense organ is a long and 

 broad auditory thickening (fig. 23). Behind the outgrowth of the 

 neural crest which forms the facial nerve there is at a certain stage a 

 small short outgrowth. This is the rudiment of the auditory nerve 

 (fig. 23). It soon reaches the auditory thickening, fuses with it (figs. 

 24 and 25), and the ganglion begins to be formed at the point of 

 fusion, and probably from the thickening itself as a proliferation, just 

 as in other cases. 



