178 JOHN BEARD. 



These authors further agree in assigning a common root of origin 

 to the seventh and auditory nerves. Marshall has, however, in one 

 of his early works, drawn attention to a line of division between the 

 ganglia of the auditory and facial nerves in the chick. Now, 

 although the rudiments of the facial and auditory nerves lie very 

 closely together, I consider that at first the two are really distinct. 

 The facial grows downwards and outwards from the neural crest, and 

 just under the epiblast. When it reaches the level of the notochord 

 part of it fuses with the sensory thickening above the hyoid arch, and 

 just above the future hyoid cleft. The rest passes on (fig. 20) to the 

 lateral muscle plates of the hyoid arch. At the point of fusion with 

 the sensory thickening the ganglion is formed. Of this, one stage is 

 figured in fig. 20. In this condition the nerve is to be regarded as 

 passing through an ancestral stage. Its condition is then figured in 

 the diagram of a typical dorsal root (fig. 50), which passes from the 

 brain to the primitive branchial sense organ and its associated ganglion 

 above a gill-cleft, and from which ganglion a nerve passes along the 

 posterior side of the cleft to the muscles of the gill. 



In later stages the ganglion is still partly fused with the skin, but 

 it soon separates, leaving behind it the rudiments of several branches. 



These branches are the supra-branchial, the prce-branchial, and the 

 pharyngeal. The development of the pharyngeal branch has not yet 

 been traced. The other branches are split off from the epiblast. The 

 supra-branchial (figs. 21 and 22) is formed at the expense of the deeper 

 portion of the sensory thickening, which has begun to grow forwards 

 over the face. 



Very soon this nerve divides into tivo branches', that is, the sensory 

 thickening grows forwards as two divergent thickenings, from each of 

 which nerve-fibres are split off, and thus two branches are formed (fig. 

 51, p.b.n.). This development from the dichotomously dividing 

 rudiment has been described by Van Wijhe. 1 These two branches 

 have been described by Marshall and Spencer. 2 The upper one is the 

 portio facialis of the oph. superficialis (Marshall), the lower one the 

 ramus buccalis (Marshall and Spencer). The upper one Balfour, Mar- 

 shall, and Spencer classed as a ramus dorsalis of the seventh. As stated 

 by Van Wijhe, 8 they are concerned in the innervation of the supra- and 



1 Van Wijhe, op. cit., pp. 26, 27. 



Marshall and Spencer, " On the Cranial Nerves of Scyllium," 'Quart. Journ. of Micr. 

 Sc.,' 1881. 



3 Op. cit., p. 27. 



