ORIGIN AND STRUCTURE OF THE HEAD 99 



branchs (as suggested already by Gegenbaur, 1887, p. 68, 

 again) according to FiiRBRlNGER, however, do not corres- 

 pond to the hypoglossus of Amniotes. The latter is repre- 

 sented by the ventral roots of the anterior free spinal nerves 

 of Elasmobranchs. Thus there has been in Elasmobranchs a 

 primary, and in Amniotes a secondary incorporation of 

 vertebrae and nerves into the skull, accompanied each time 

 by a loss of the dorsal roots of these nerves. 



This conclusion is based mainly on FuRBRlNGER's con- 

 ception of the Amphibian skull. Here no ventral rools were 

 found to leave the cranium behind the vagus, the anterior 

 spinal nerves supply the musculature which corresponds 

 to the hypobranchial muscles in Elasmobranchs, supplied 

 by GegENBAUR's ventral vagus roots, and to the tongue- 

 musculature in Amniotes, innervated by the hypoglossus. 

 Ontogeny gives evidence of only one skeletal element in 

 the occipital region, represented by the "occipital arch." 

 (SToHR, 1879, 1881, GAUPP, 1893, SEWERTZOFF, 1895,PLATT, 

 1898, Goodrich, 1911). This is situated, in the same way as 

 the neural archs of the trunk, in the myocomma between two 

 myotomes, a little distance behind the auditory capsule 

 (fig. 21 ). and is compared by SEWERTZOFF (1895, p. 260, 

 262) to the first free neural arch of Petromyzon and to the 

 anterior one of four similar vertebral rudiments described 

 by Hoffmann (1894) in the occipital region of Acanthias. 

 (cf fig. 31) The conclusion then seems obvious, and has been 

 drawn by SEWERTZOFF, that the posterior occipital segments 

 of Elasmobranchs and Amniotes and the hypoglossus roots 

 supplying their myotomes have not yet been incorporated 

 into the skull in Amphibians. 



This, however, is deemed improbable by FiiRBRlNGER 

 (1897, p. 485), as the Amphibians would thus have a more 

 primitive position in this respect than Elasmobranchs (cf. 

 also Gegenbaur, 1887, p. 72). Though recognizing that 

 ontogeny does not reveal any indication, however transitory, 

 to support this view, he yet thinks it inevitable to assume 

 that ventral occipital nerves, corresponding to the occipital 

 nerves of Elasmobranchs, have once been present in Am- 

 phibians but have aborted, and that the occipital region 

 here too represents a multiplum of primary occipital ver- 

 tebrae. Thus FuRBRlNGER homologizes the cranium and 

 especially the occipital region of Amphibians to that of 

 Selachians, both representing a protometameric neocranium 



