The Segmentation of the Head in Squalus 



acanthias. 



By 



G. Rylands de Beer, B.A., B.Sc, 



Christopher Welch Scholar, Demy of Magdalen College, Demonstrator 



in the Department of Zoology and Comparative Anatomy, 



University Museum, Oxford. 



With 13 Text-figures. 



There are two views with regard to the segmentation of the 

 head. One has arisen out of Balfour's (1) pioneer work, the 

 other is due to Van Wijhe (18). They both agree on many 

 points, and the difference between them lies in the interpreta- 

 tion of the numerical relations of the different segmented 

 structures, as pointed out by Goodrich (8). In order that they 

 may be compared, a short account of these views will now be 

 given. 



Balfour, in his classical ' Development of Elasmobranchs ', 

 expressed the opinion that the six visceral clefts are related 

 to six consecutive somites situated dorsally and posteriorly to 

 each respective cleft, the clefts being intersomitic. In front 

 of the spiracle he recognized two somites (premandibular and 

 mandibular), so that in all, from the anterior extremity to 

 behind the last gill-slit, there are eight somites, of which the 

 six posterior are simply and harmoniously related to six visceral 

 arches and clefts. 



The recognition of the nature of the cranial nerves is due to 

 the work of Marshall and Van Wijhe. Five nerves are regarded 

 as dorsal roots, viz. ramus ophthalmicus profundus, trigeminal, 

 facial, glossopharyngeal, and vagus, the latter being really 

 compound and probably representing four segments. There 

 are then eight dorsal nerve elements, and if each is related to 



