37 
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 reflexions the 
conclusion may not yet be drawn that all the myotomes of 
the occipital region and the corresponding ventral roots 
are of post-branchial origin and that the latter have only 
secondarily moved forward from the 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. 
The origin of the hypobranchial musculature — In Petro- 
myzon, where originally branchiomerism and mesomerism 
correspond, the hypobranchial musculature, as shown b 
NEAL (1897, p. 444) and KOLTZOFF (1902, p. 304), is derived 
exclusively from the myotomic buds of the anterior post- 
„Vag. Anhang? 
rx.sp.vent. n.lat. rcut.dors. \gnvag. gn.fac. 1my. 
My 1spgn.} Ì qn.gls. «ga.trg. 
Fig. 20. Head of Ammocoetes, hypoglossus nerve (hyp ) and 
musculature after NEAL, 1897, p. 446. 
branchial myotomes, from the 7th post-otic (the first post- 
branchial) onwards. According to NEAL probably the 
Tth — 12th, after KOLTZOFF the 7th — 14th, 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 7th — 12th post-otic myotomes 
And passing in a curve round behind the last gill-slit to the 
