202 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



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FIG. 211. Diagram of the 

 head segments in a selachian, 

 after Neal. a, anterior so- 

 mite ; aa, aortic arch ; a/>, 

 abducens nerve ; </, dorsal 

 nerves ; /, facialis nerve ; g, 

 glossopharyngeal nerve ; gc, 

 gill clefts; h, hypoglossal 

 nerve; iv, intestinal branch 

 of vagus ; /', lateralis branch 

 of vagus; w, mediolateral 

 line; w, neuromeres ; o, otic 

 vesicle ; oc , oculomotor nerve ; 

 o/>, ophthalinicus profundus 

 nerve ;/>#, post-trematic nerve; 

 pr, pre-trematic nerve ; s, spi- 

 racular cleft ; so, mesodermic 

 somites; /, trigeminal nerve; 

 z>, vagus nerve ; /-AY, neuro- 

 meres; I n, somites of van 

 Wijhe; 1-7, functional gill 

 clefts. 



vertebrae to the neural and branchial 

 segments. With these as a basis he 

 recognized nine cranial segments. 

 Two years later Gegenbaur, using the 

 same criteria, also concluded that there 

 were nine segments in the head, al- 

 though his somites and those of Hux- 

 ley do not agree in detail. 



Both of these authors recognized 

 that the nerves behind the ear (IX- 

 XII.) were like the spinal nerves in 

 the possession of dorsal and ventral 

 roots, and that the ninth divides above 

 the first gill slit into pre- and post- 

 trematic branches (p. 63). The tenth 

 nerve, however, bears similar relations 

 in the ordinary sharks to four gill 

 clefts, and hence is a compound nerve. 

 In front of the ear the facial nerve 

 divides above the spiracular cleft, while 

 the trigeminal nerve splits in a similar 

 way on either side of the angle of the 

 mouth. This last circumstance led 

 Huxley to the view that the mouth 

 has arisen from the coalescence of a 

 pair of gill slits, a view which has re- 

 ceived a certain amount of corrobora- 

 tion from embryology. This left a 

 third division (ophthalmic) of the fifth 

 nerve out of consideration ; this was 

 supposed to represent another seg- 

 ment further indicated according to 

 Huxley's view by the orbito-nasal 

 groove, while Gegenbaur saw traces of 

 it in a pair of labial cartilages. Both 

 recognized an additional segment in 

 front of the ophthalmic, the details of 

 which are not necessary here. 



