HEAD OF SQUALUS 



469 



their former existence in the form of blood-vessels, nerves, 

 skeletal elements or modilied structures. Then authors are 

 not agreed as to the exact position of this missing cleft. Whereas 

 Van Wijhe considers the hyoid arch as double, Hoffmann and 

 Piatt (14 and 15) regard the mandibular arch as representing 

 two elements fused. 



With regard to the ventral roots there is no question about 



Text-fig. 11. 



9 ; 10 



9i0f@f@ia 



Diagrammatic representation of (a) Van Wijhe's somites, {b) Bal- 

 four's somites. 



the oculomotor being the premandibular somite's nerve, and 

 the patheticus, in spite of its curious course, doubtless belongs 

 to the second and mandibular segment. The fourth and fifth 

 somites since they disintegrate have no ventral roots as such 

 (though the fifth is present in Scyllium). To the sixth somite 

 a ventral root can be seen up till about the 10 mm. stage. 



The abducens has usually been regarded as the nerve of the 

 third somite and therefore as the ventral root corresponding to 

 the facial. It certainly innervates the external rectus nmscle, 

 but Neal (12) states that in Squalus this muscle is of composite 

 origin, consisting of elements derived from the mandibular 

 as well as the hyoid somite. 



