Neal, Problem of the Vertebrate Head. 159 



lature ; somite, "anterior" (Miss Piatt's); visceral cleft diwd arch, 

 hypothetical ; aortic ajxh, hypothetical. 



Metamere II. Neiiroincre, neuromere II (primary mid- 

 brain vesicle) ; dorsal 7ierve, ophthalmicus profundus (motor 

 fibers absent in Squalus, but present in some vertebrates); ven- 

 tral ncn'e, oculomotorius; somite vdiW V^\]h.Qs ist; visceral arch 

 and cleft, hypothetical ; aortic arch, hypothetical. 



Metamere III, Neiirovicre, neuromere III (Hinterhirn) ; 

 dorsal nerve, trigeminus ; ventral nerve, trochlearis ; somite, van 

 Wijhe's 2nd ; visceral arch, first (mandibular); visceral cleft, 

 (bounding anteriorly the ventral portion of the segment) usur- 

 ped by mouth ; aortic arch, first (mandibular). 



Metamere IV. Nenromerr, neuromere IV ; dorsal nerve, 

 hypothetical (absence correlated with the absence of a visceral 

 arch) ; somite, van Wijhe's 3rd ; ventral nerve, abducens ; visce- 

 ral cleft and arch, hypothetical ; aortic arch, hypothetical. 



Metamere V. Neuromere, neuromere V ; dorsal nei've, 

 facialis (the acusticus a specialized sensor branch); ventral nen>e, 

 abducens ; somite, van Wijhe's 4th (which together with the 3rd 

 forms in Torpedo the m. rectus posterior, Sewertzoff — rudimen- 

 tary in Squalus); visceral clef t, first (hypobranchial, spiraculum); 

 visceral arch, second (hyoid); aortic arch, second (hyoid). 



Metamere VI. Neuromere, neuromere VI ; dorsal nerve, 

 glossopharyngeus ; ventral nerve, abducens ; somite, van Wijhe's 

 5th (myotome absent in Squalus; forms first myotome of the 

 lateral trunk musculature in Petromyzon) ; visceral cleft, 2nd 

 visceral (ist branchial) ; visceral arch, third (ist branchial) ; 

 aortic arxJi, third. 



Metamere VII. Neiwonio'e, neuromere VII (the last of 

 the neuromeres having a lateral thickening. See Fig. i); dor'sal 

 nerve, vagus^ ; ventral nerve, abducens ; somite, van Wijhe's 6th 

 (myotome rudimentary in Squalus) ; visceral cleft, third (second 

 branchial) ; visceral arch, fourth ; aortic arcJi, fourth. 



Metamere VIII. Neurvrnerr, neuromere VIII ; dorsal 

 nerve, vagus^ ; ventral nerve, hypoglossus (anterior root, rudi- 



