132 THE FROG. 



VII. The facial, or seventh nerve, as noticed above, is, in its 

 early stages, continuous with the auditory nerve. Beyond the 

 auditory vesicle the facial nerve runs downwards and forwards, 

 close to the surface epiblast. Shortly before the time of hatching 

 of the tadpole, the nerve becomes connected with an ingrowth of 

 the epiblast at the level of the upper border of the notochord, 

 and at this place the ganglion is formed. Beyond the ganglion 

 the nerve divides into three principal branches : (i) a small 

 cutaneous branch which appears to develop in connection with 

 the epiblastic ingrowth, and to belong to the lateral line series 

 of nerves ; (ii) a stout post-branchial branch, which runs down- 

 wards and forwards along the hyoid arch, close to its surface ; 

 and (iii) a small palatine nerve, which runs forwards in the roof 

 of the pharynx, not far from the median plane. 



VI. The development of the sixth, or abducent nerve, has 

 not been determined in the frog. From its general relations, and 

 from what is known concerning its mode of formation in other 

 animals, it is probably comparable to a ventral root of a spinal 

 nerve. 



V. The trigeminal, or fifth cranial nerve, is the largest of the 

 whole series ; it lies immediately in front of the facial nerve, 

 with which it is in close relation from the first. 



The trigeminal nerve, like the pneumogastric, early attains a 

 large size, and in 4 mm. tadpoles (cf. Figs. 58, E, and 61) extends 

 half way down the side of the pharynx. At or shortly before 

 this stage, a thickening of the external epiblast occurs at the 

 level of the upper border of the notochord, immediately behind 

 the eye, and in front of the auditory vesicle ; this meets and fuses 

 with the nerve, the two together forming the ganglion. The 

 thickening of the epiblast extends forwards a short distance in 

 front of the ganglion, and gives rise to a cutaneous nerve, 

 similar to the lateral line nerve formed in connection with the 

 pneumogastric nerve. Shortly after hatching of the tadpole, the 

 ganglion of the trigeminal nerve recedes somewhat from the 

 surface, and becomes more deeply placed, though still remaining 

 connected with the surface by the cutaneous branch. 



Before the hatching of the tadpole, the trigeminal nerve 

 divides distally into ophthalmic and mandibular branches, of 

 which the former runs horizontally forwards, and the latter down- 

 wards and forwards, the eye lying in the fork between the two. 





