ORIGIN AND STRUCTURE OF THE HEAD 145 



chondrified and the neural arch itself has been incorporated 

 intp the skull. In ontogeny it appears as the occipital arch, 

 some distance behind the auditory capsule and afterwards 

 fusing with it, leaving the foramen vagi between itself and 

 the capsule (cf. fig. 21). It is Sewertzoff 0895, p. 262) 

 who has compared the occipital arch with the first free 

 vertebral arch in Petromyzon. 



It is not easy to determine, from the somewhat diverg- 

 ing statements of different authors, the number of seg- 

 ments incorporated into the skull by this process and 

 situated between the auditory capsule and the occipital 

 arch. At any rate the glossopharyngeus and the vagus 

 have been incorporated. However, the "spinalartiger Vagus- 

 anhang", the first spinal nerve behind the primary vagus, 

 which, according to HATSCHEK, has fused with it in Gnathos- 

 tomes, represents a segment also. In Necturus Miss Platt 

 (1897, p. 448) indeed found the vagus- Anlage to have a double 

 nature and to correspond to two somites, the second and 

 the third post-otic ones. The anterior part of the rudiment 

 of the ganglion extends outwards over the second somite, 

 in the manner typical for cranial nerves, while the posterior 

 part passes directly downwards, median to the third somite, 

 in the manner of a typical spinal nerve, and becomes 

 attached to the brain by the second vagus root. In Gymno- 

 phiones MARCUS (1910, p. 378) finds close to the vagus 

 ganglion another little ganglion, sometimes fused with the 

 former and otherwise connected with it by fibrous nerve- 

 strands, and which according to MARCUS (I.e. p. 451) must 

 be homologized to the spinal ganglia and at the same 

 time be counted to the vagus complex. To this little dorsal 

 ganglion again a ventral occipital nerve, designated after 

 FiiRBRlNGER's nomenclature (and thus wrongly) as z, shows 

 close relations. The latter innervates the first myotome of 

 the musculus dorsalis which is attached to the auditory 

 capsule, in the same way as the third post-otic myotome 

 in Petromyzon (cf fig. 29) to which it evidently corres- 

 ponds The next myotome is supplied by the first spinal 

 nerve which has no dorsal ganglion. 



As to the number of somites to be observed in early 

 ontogeny between the occipital arch and the auditory capsule 

 in the head of the Amphibian, the statements of different authors 

 do not wholly agree, but from recent inves igations it becomes 

 more and more evident that in Urodelans, as in Petro- 



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