ORIGIN AND STRUCTURE OF THE HEAD 



103 



This result at first sight seems to be irreconciliable with 

 the conclusions of VAN Wyhe (1882) and others, that the 

 occipital myotomes belong to the branchial region and 

 alternate regularly with the gill-slits. A closer examination, 

 however, seems to me to show the way to get out of this 

 difficulty and to prove that from the above reflections the 

 conclusion may not yet be drawn that all the myotomes of 

 the occipital regio'i and the corresponding ventral roots 

 are of post-branchial origin and that the latter have only 

 secondarily moved forward from Ihe trunk to beneath the 

 vago-accessorius, to which they originally would have had 

 no relation at all -To this end we shall not restrict our 

 considerations to the Elasmobranchs but first examine 

 other groups of Vertebrates (cf. Plate I) 



The origin of the hypobranchial musculature - In Petro- 

 myzon, where originally branchiomerism and mesomerism 

 correspond, the hypobranchial musculature, as shown by 

 Neal (1897, p. 444) and KOLTZOFF (1902. p. 304t, is derived 

 exclusively from the myotomic buds of the anterior post- 



„Vag.Anhanq. 

 rx.sp.vent. n.Iat. r.cut.dors. jgn.vag. gn.fac. imy. 



ji^my. ' ! isp-gn.;; gn.gis. jgn.trg. ,' 



15 ID ^0 <D r 



•• \lp i^- 'vl \\ \ \ \\ \\ \\\! 



Fig. 20. Head of Ammocoetes, hypoglossus nerve (hyp ) and 

 musculature after neal, 1897. p. 446. 



branchial myotomes, from the Z**" post-otic (the first post- 

 branchial) onwards. According to NEAL probably the T^ — \2^^, 

 according to KOLTZOFF the 7*^ — l4*^ post-otic myotome 

 contribute to its formation. This hypobranchial musculature is 

 innervated according to NEAL by a hypoglossus nerve compo- 

 sed of the ventral roots of the 7**' — \2^^ post-otic myotomes 

 and passing in a curve round behind the last gill-sHt to the 



