SKULL OF VERTEBEATA. 447 



concrescence may also obtain in tlie vertebral column. Modifi- 

 cations in tlie region, wliich. tbus became continuous, were brought 

 about partly by influences wliich aiiected it directly from with- 

 out, and partly by influences which affected it from within (the 

 development of the brain). 



6) Inasmuch as those nerves only can be seen to have any 

 likeness to spinal nerves which are found in that division of the 

 cranium which is traversed by the notochord, it follows that this 

 portion only can be dei-ived from vertebi-ae ; the branchial skeleton 

 also belongs to this portion. We must therefore distinguish between 

 this portion of the cranium which is vertebral, and the anterior, 

 or evertebral, portion, which does not exhibit any relations to the 

 vertebrae. This latter is rather a secondary formation, although it 

 has had its origin in the vertebral portion. 



The number of vertebrte, which enter into the composition of the 

 cranium, are, so far as we yet know, nine at least. But this is by 

 no means saying that the number may not have been much larg-er. 

 Such a supposition is borne out by a large number of facts which are 

 known to us with, regard to the distribution and mode of origin of 

 the nerves in the Selachii ; and these point also to the atrophy of 

 visceral arches. The arrangement in Amphioxus is just as much to 

 the point, for in it a large number of branchial arches are persistent. 

 All that portion of the primitive spinal column (notochord and 

 perichordal tissue), which extends along the branchial framework, of 

 Amphioxus would then be homologous with that portion of the axial 

 skeleton which, in the Craniota, has passed into the cranium. 



Gegenbaue, C, Uutersuchungen ziir vergl. Anat. der Wirbelthiere. III. Das 

 Kopfskelefc der Selacliier als Grundlage zui- Beurtheilung dor Genese des 

 Kopfskeletes der Wirbelthiere. Leipzig, 1872. 



Skull. 



§ 341. 



The skulls of tbe Craniota are divided into two very different 

 groups. In the one the above-mentioned internal branchial skeleton 

 is well developed, and its most anterior segments take on the form 

 of maxillary organs, and affect the form of the cranium by being 

 either directly, or indirectly, connected witli it. This arrangement 

 obtains in the Gnathostomata. The other form is represented in the 

 Cyclostomata, in which it is possible to make out indications of the 

 same arrangements in the cephalic skeleton as those which are seen 

 in the Gnathostomata ; but the differences are so great, that it is 

 impossible to compare all the parts with anything like exactness. 



The notocbord is continued into the hase of a capsule which 

 encloses the brain, and which is, in comparison with other parts of the 

 skeleton which are to be regarded as belonging to the skull, of a 

 remai'kably small size. In Petromyzon two enlargements which. 



