198 SEVENTH AND AUDITORY NERVES. 



branch divides into a ramus superficialis and ramus profundus, 

 and subsequently to stage O I have no observations on it. 



By stage the fifth may be observed to have two very 

 distinct roots, and a large ganglionic mass is developed close 

 to their junction (Gasserian ganglion), PL XVI. fig. 4 a. But in 

 addition to this ganglionic enlargement, all of the branches 

 have special ganglia of their own, PI. xvi. fig. 4 h. 



Summary. The fifth nerve has almost from the beginning 

 two branches, the ophthalmic (probably the inferior ophthalmic 

 of the adult) and the inferior maxillary. The superior maxillary 

 nerve arises later than the other two as a branch from the in- 

 ferior, originating comparatively far from its root. There is at 

 first but a single root for the whole nerve, which subsequently 

 becomes divided into two. Ganglionic swellings are developed 

 on the common stem and main branches of the nerve. 



A general view of the nerve is shewn in the diagram in 

 PL XVI. fig. 1. 



Seventh and Auditory JS'erves. — There appears in my earliest 

 sections a single large rudiment in the position of the seventh 

 and auditory nerves ; but in longitudinal sections of an embryo 

 somewhat older than stage I, in which the auditory organ forms 

 a fairly deep pit, still widely open to the exterior, there are to 

 be seen immediately in front of the ear the rudiments of two 

 nerves, which come into contact where they join the brain and 

 have their roots still closely connected at the end of stage K 

 (PL XIV. fig. 10 and 15 (X and 15 h). The anterior of these 

 pursues a straight course to the hyoid arch (PL xiv. fig. 10, 

 VII.), the second of the two (PL xiv. fig. 10, au. n), which is 

 clearly the rudiment of the auditory nerve, developes a gan- 

 glionic enlargement and, turning backward, closely hugs the 

 ventral wall of the auditory involution. 



The observation just recorded appears to lead to the fol- 

 lowing conclusions with reference to the development of the 

 auditory nerve. A single rudiment arises from the brain for 

 the auditory and seventh nerves. This rudiment subsequently 

 becomes split into two parts, an anterior to form the seventh 

 nerve, and a posterior to form the auditory nerve. The gan- 

 glionic part of the auditory nerve is derived from the primitive 



