HEAD OF SQUALUS 473 



The simplest explanation then, that originating from the 

 work of Balfom', is the most suited to the facts : the third 

 somite is related to the hyoid arch and the fourth to the third 

 arch, &c. ; somites correspond to the dorsal roots lying imme- 

 diately behind them, as in the pre-otic and trunk regions, 

 and there are no discrepancies. 



In dealing with such a subject as segmentation, the essence 

 of which is a simple and orderly repetition of parts, if a simple 

 explanation can be deduced which does not produce incon- 

 sistencies or go against facts, the onus probandi must lie 

 with those who would reject such an orderly state of affairs. 



Summary. 



1. Balfour's interpretation of the somites of the head is 

 correct and free from the objections which accompany Van 

 Wijhe's. 



2. No gill-slit or arch has been lost in the neighbourhood of 

 the hyoid arch. 



3. Nine segments are included in the head of Squalus, of 

 which three are pre-otic (first, second, third) and six post-otic. 

 Of these 



One (fourth) breaks down completely into mesenchyme ; 

 One (fifth) forms muscle-fibres but later breaks down ; 

 Four (sixth, seventh, eighth, ninth) produce permanent 



myotomes ; 

 The tenth somite (first of the trunk, and second post-vagal) 



corresponds to the first mixed nerve. 



List of the Literature cited in this Paper. 



1. Balfour, F. M. — "Monograph on the Development of Elasmo branch 



Fishes ", ' Journ. Anat. and Phys.', 1876-7 and 1878. 



2. Braus, H. — " Die metotischen Urwirbel ", ' Morph. Jahrb.', vol. xxvii, 



1899. 



3. Dohrn, A.—" Die Priimandibularhohle ", ' Mitth. Zool. Sta. Neapel ', 



17, 1904. 



4. Fiirbringer, M. — " tJeber die spino-occipitalen Nerven ", ' Festschr. 



f. C. Gegenbaur ', Leipzig, 1897. 



