THE HEAD AND NECK 



355 



innervated by the abducens. These first three somites will 

 become differentiated into the extrinsic eye-muscles, and they 

 all lie in front of the auditory vesicle, for which reason they are 

 called the prootic somites. 



The 4th somite is the 1st of the metotic somites, and it is 

 similar to the ones following it. It and the 5th somite in the 

 dogfish eventually disappear, being squashed underneath the 

 large developing auditory sac, and they either do not have, 

 or do not retain, any ventral nerve-roots. In Petromyzon, 

 however, no somites are lost, and the 4th becomes the most 



Fig. 167. — Reconstruction of the head of a dogfish embryo, showing the 



segmentation. 

 77/, oculomotor ; IV, trochlear ; V, trigeminal ; VI, abducens ; VII, 

 facial ; IX, glossopharyngeal ; X, vagus ; nerves ; as, auditory sac ; 

 gi to #3, first to third gill-slits ; hm, hypoglossal muscles ; hn, hypoglossal 

 nerve ; op, ophthalmicus profundus nerve ; s, spiracle ; si to s8, first to 

 eighth somite ; sn, spinal nerve ; the arrows show the position of the posterior 

 limit of the neurocranium : P, in Petromyzon ; Sc, in Scyllium ; Sq, in 

 Squalus. 



anterior of the myotomes of the body. In the dogfish, it is 

 the 6th somite which gives rise to the most anterior myotome 

 of the body. 



It is now necessary to turn to the relations which the dorsal 

 nerve-roots bear to the somites. Above the premandibular 

 somite, the cells of the neural crest group together to form the 

 ganglion of the ophthalmicus profundus nerve. This nerve 

 is lost in adult Scyllium, but it is present in Squalus, and it 

 is the dorsal root of the 1st segment, corresponding to the 

 oculomotor. 



