2O2 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 ; ab, 

 .abducens nerve ; dn, dorsal 

 nerves; f, facialis nerve; g, 

 glossopharyngeal nerve ; gc, 

 gill clefts; h, hypoglossal 

 nerve ; iv, intestinal branch 

 of vagus ; h', lateralis branch 

 of vagus; w, mediolateral 

 line ; , neuromeres ; o, otic 

 vesicle ; oc, oculomotor nerve ; 

 -a/>, ophthalmicus profundus 

 Jierve ;po, post-trematic nerve; 

 //, pre-trematic nerve; s, spi- 

 Tacular cleft ; so, mesodermic 

 somites; /, trigeminal nerve; 

 z>, vagus nerve ; I-XI, neuro- 

 meres; I II, 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. 



