184 THE GERM LAYER DERIVATIVES 



portion of the first cranial placode. It is well developed by the 

 9 mm. stage. The level is at the anterior end of the medulla. 

 This is a very large and tri-radiate ganglion, including an 

 Ophthalmic branch from the placode whose fibers pass to the 

 skin and snout, and a forked Gasserian ganglion which arises 

 from both the crest and placode elements and gives rise to the 

 maxillary (upper) and mandibular (lower) branches of the 

 fifth nerve. The entire mixed trigeminal nerve ganglion (V) 

 joins the dorso-lateral wall of the medulla oblongata. The 

 outer or non-nervous portion of the placode disappears and 

 it is believed that the non-nervous portion of the crest seg- 

 ment may contribute some of the mesenchyme to the mandib- 

 ular arch. It is certainly difficult to distinguish between them. 



VI. Abducens: This is a motor nerve arising late (11 mm. stage) 

 from the neuroblasts of the ventral side of the medulla and 

 innervating the lateral or external rectus and retractor bulbi 

 muscles of the eye. 

 VII. Facial: This is a mixed nerve, arising by the 5 mm. stage from 

 the anterior ganglionic portion of the second or acustico- 

 facialis (second cranial) crest segment and placode and is as- 

 sociated with neuroblasts coming from the medulla just poste- 

 rior to cranial nerve V. The motor (efferent) fibers are divided 

 between the hyoid and the palatine (mouth) branches, but a 

 good portion of the crest segment, from which these fibers 

 arise, is non-nervous and is presumed to give rise to mesen- 

 chyme of the hyoid arch. 

 VIII. Auditory: This is a sensory nerve arising by the 5 mm. stage 

 from the posterior ganglionic portion of the second (acustico- 

 facialis) crest ganglion, in close association with the auditory 

 placode, and innervates only the inner ear. 



IX. Glossopharyngeal: This is a mixed ganglion, arising by the 

 9 mm. stage from the ganglionic portion of the third cranial 

 crest segment and placode. Fibers of the ninth and tenth 

 cranial ganglia enter the sides of the medulla together, but 

 peripherally the ninth or glossopharyngeal nerve supplies the 

 first branchial arch, the mouth, the tongue, and the pharynx. 

 This ganglion is incompletely separated from the vagus by 

 the anterior cardinal vein. 

 X. Vagus or pneumogastric gangHon: This is mixed and arises 



