208 ARTHUR E. SHIPLEY. 



whilst the most anterior gill-cleft — spiracle — is still present, 

 the nerve can be seen passing from the ganglia between the 

 rudimentary gill-cleft and the first persistent one — the hyo- 

 branchial. Later on the ganglion increases in size, and ex- 

 tends round the under and inner face of the auditory sac 

 towards the ganglion of the ninth nerve, but it never quite 

 reaches it, and the connection between the ganglion of the 

 seventh and of the tenth nerves must be of later origin. 

 Neither does the ganglion of the seventh fuse with that of the 

 fifth, though they are close together, and the root of the 

 seventh does not enter the ear capsule to leave it again, as is 

 the case in the adult. After the appearance of the ciliated 

 ring in the place of the first gill-cleft, the seventh nerve sup- 

 plies this structure. 



A few fibres from the brain enter the recessus labyrinthi 

 of the ear ; these arise close to the root of the seventh, and 

 constitute the eighth nerve. 



The ganglia of the ninth and tenth nerves would seem to 

 arise from a mass of cells split off" from the epiblast close 

 behind the ear. At a little later stage the ninth nerve has its 

 ganglion lying close against the posterior boundary of the ear ; 

 the nerve is continued along the posterior wall of the first 

 persistent cleft, the hyobranchial. The ganglion seems to be 

 still connected with the ganglion of the tenth nerve. This is 

 a very large structure : it lies more dorsally than the others and 

 it is in close connection with the mid-brain, having as yet deve- 

 loped no root. Behind it and connected with it lies a ganglion 

 which is situated dorsally above the second persistent gill-cleft ; 

 from this chord the main branch of the vagus is continued 

 backward, lying just external to the anterior cardinal vein 

 (fig. 42). In front of each remaining cleft the chord bears a 

 large ganglion, so that, counting the first, there are six distinct 

 ganglia borne on the vagus. I have not been able to trace the 

 fibres of this nerve beyond the last gill-cleft, but my friend 

 Mr. Ransom, of Trinity College, tells me he has traced the 

 vagus into the heart in the adult Petromyzon. Each of tKe 

 ganglia in the vagus supplies the gill-cleft behind which it lies. 



