SKULL ii 



metaotic somite in Elasmobranchs (van Wijhe [495], Neal [308a]) ; 

 from the 7th to the 14th in Petr&myzon (Koltzoff [272]); from 

 the 3rd to the 5th in Necturus (Platt [33 la])). The processes from 

 these somites grow down behind the last gill-slit, and then pass 

 forwards again -below to form the hypoglossal musculature, and it 

 is the ventral nerve -roots corresponding to them which form the 

 spino-occipital nerves of fish, representing the compound hypoglossal 

 nerve (Xllth cranial nerve) of higher forms (Fiirbringer [143]). 

 Thus we get a very variable number of hypoglossal constituents in 

 the different groups of Craniate vertebrates (Figs. 3, 44). 



After Huxley's brilliant attack on the Vertebral theory of the 

 skull, came Gegenbaur's epoch-making work on the Elasmobranch 

 skeleton (1872). He showed that though the chondrocranium of 

 the adult Selachian forms a continuous box, yet it exhibits many 

 signs of an original segmental structure, at all events behind the 

 infundibulum and the exit of the oculo-motor nerve. This part of 

 the skull, including the occipital, auditory, and part of the orbital 

 regions, is traversed below by the notochord, gives exit to segmental 

 nerves, and is connected with segmental gill-arches. It was there- 

 fore called the c vertebral ' region as distinct from the more 

 anterior part of the skull, including a part of the orbital and the 

 ethmoid regions. The latter was called the ' prevertebral ' region ; 

 it is chiefly formed by a forward growth round the nasal organs 

 (Gaupp [150-3]). 



Following Kathke, Huxley had pointed out that the basis 

 cranii of all Craniata arises in the embryo as a basal or parachordal 

 plate embracing the extremity of the notochord, below the brain, 

 and two trabeculae cranii in front, one on either side of the 

 infundibulum. Stohr [426] showed that, in the Urodela, the 

 'vertebral region' is developed from three distinct centres 

 the parachordal, the mesotic cartilage of the auditory capsule, and 

 an occipital segment resembling a vertebra. Subsequently Froriep 

 found, in the occipital region of birds and matnmals, some three 

 metameres with distinct somites, skeletal segments, and nerves 

 (hypoglossal). Eosenberg [374.], Sagemehl [379], and Gegenbaur 

 [160] also studied the process of assimilation of vertebral segments 

 (Fig. 6). This gradual inclusion of skeletal segments, scleromeres, 

 in the occipital region of the skull of fish has lately been followed 

 in great detail from embryo to adult by Sewertzoff [407], 

 Hoffmann, Brans [47], and Schreiner [390a]. It has now been 

 conclusively shown that in the Gnathostomes skeletal segments 

 of vertebral nature do, in the course of ontogeny, and doubtless 

 also in that of phylogeny, become attached to, fused with, and 

 finally completely embodied in the occipital region, carrying with 

 them their corresponding nerves. Originally spinal nerves thus 



