v. 8 



PRO-OTIC SOMITES 



149 



view cannot be maintained in detail and the theory was brought to 

 ridicule by T. H. Huxley and others. The segmental value of the skull 

 floor and sides is not at all easy to determine; the parachordals arise 

 as a pair of unsegmented rods on either side of the notochord. 



ma: ha 



Fig. 99. Diagram of the segmentation of the head of a dogfish. 



cr. limit of neurocranium; vr. limit of visceral arch skeleton; a. auditory nerve; aa.i, pre- 

 occipital arch; aa.2, occipital arch; ab. abducens nerve; ac. auditory capsule; ah. anterior 

 head cavity; c. coelom;/. facial nerve; gl. glossopharyngeal nerve; ha. hyoid arch; hm. hypo- 

 glossal muscles; hy. hypoglossal nerve; la. pila antotica; m. mouth; m.2-6, myomere 2-6; 

 ma. mandibular arch; tnb. muscle-bud; nc. nasal capsule; om. oculomotor nerve; prf. pro- 

 fundus nerve; scl. sclerotome of segment 10; sp.1-2, ganglion of spiral nerve 1-2; t. trochlear 

 nerve; tr. trigeminal nerve; v. vagus nerve; vgl. vestigial ganglion of segment 7; vc. ventral 

 coelom; vr. ventral root of segment 6. (From Goodrich.) 



8. The pro-otic somites and eye-muscles 



Ideas about the segmentation of the head were first correctly for- 

 mulated by F. Balfour. In his studies of the development of elasmo- 

 branchs (1875) he showed that three myotomes, the pro-otic somites, 

 can be recognized during development in front of the auditory capsule 

 (Fig. 99). The auditory sac, pushing inwards and becoming sur- 

 rounded by cartilage, then breaks the series of myotomes, so that 

 several are missing in the adult, though the series is complete in the 

 embryo. 



If this analysis is correct we should be able to recognize that the 

 nerves of the head belong to a series of dorsal and ventral roots, 

 similar to that in the trunk, the ventral roots being those for the 

 myotomes and the dorsal roots, running between the myotomes, 

 carrying sensory fibres for the segment and motor-fibres for any non- 

 myotomal musculature present (p. 36). In the spinal region the 



