ORIGI N AND STRUCTURE OF THE HEAD 149 



the myocommata, just like the rudiments of the neural 

 arches in the trunk of which they represent the direct 

 forward continuation. SewertzoFF had numbered the myo- 

 tomes in his figure as 6-11, after the system of VanWyhe 

 who counts the "praemandibular segment" as the first and 

 the hyoid segment as two segments. 1 have changed this 

 nomenclature into 4 — 9, in accordance with the conclusions 

 arrived at in the foregoing pages. To each myotome a 

 dorsal ganglion and a ventral root corresponds but under 

 the vagus ganglion two myotomes are found (4 — 5). This 

 is in accordance with Hatschek's and VAN Wyhe's opinion 

 that the vagus in Gnathostomes is a bivalent nerve and that 

 its primary ganglion has fused with the first spinal ganglion 

 behind it ("spinalartiger Vagusanhang"). According to NEAL 

 (1898, p. 238), SEWERTZOFF (1899, p. 287) and other investi- 

 gators one more spinal ganglion (that of my segment 6) 

 fuses afterwards with that of the vagus (vago-accessorius) 

 which accordingly in the adult Selachian is trivalent 

 (cf. GUTHKE, 1906, and ZlEGLER, 1908), being composed 

 of the primary vagus ganglion and two rudimentary spinal 

 ganglia following behind it. 



According to HOFFMANN (1894, p. 628) and SEWERTZOFF 

 (1899, p. 302) the cranio- vertebral limit in Acanthias lies 

 behind my somite 8, in Scyllium and Pristiurus, according 

 to Van Wyhe (1882) and others, behind my somite 7. 

 The neural arch rudiment between the somites 5 and 6 

 corresponds to the first of the four vertebral rudiments 

 described by HOFFMANN and has been compared by 

 SEWERTZOFF with the occipital arch of Amphibians and 

 with the first free arch in Petromyzon. Thus the ventral 

 root belonging to somite 5 and to the "spinalartiger Vagus- 

 anhang" must be the one that has been observed in Uro- 

 delans (cf. p. 115). It must be called jc in Amphibians 

 after FuRBRlNGER's nomenclature, not z which indicates 

 the last head segment of Scyllium. 



A faint neural arch rudiment in front of the somite 5 

 (fig. 31) evidently represents a rudimentary prae-occipital 

 arch, as described by Miss Platt (1898, p. 448) in 

 Necturus and by GOODRICH (1911, p. 104) in Siredon. 



As in Amphibians, the two anterior post-otic somites no 

 longer develop myotomes, but the second post-otic somite 

 still forms a rudimentary one. From the remaining occipital 

 somites, however, myotomes are still developed. From these 



