148 



THE ANCESTRY OF VERTEBRATES 



Urodelans, three post-otic somites are found. Evidently 

 then three more segments have been added to the skull in 

 Acanthias where, however, the head contains one segment 

 more than in forms like Scyllium and Pristiurus on which 

 Van Wyhe and others have worked and where accordingly 

 an increase of only two segments must be assumed. In 

 forms like Hexanckus and Heptanchus still one or two 

 segments more must be added to the number found for 

 Acanthias (cf. p. 109). In Acanthias we reach a total 

 number of eight, in Pristiurus etc. of seven, head segments, 

 the latter number corresponding to that of the arches of 

 the visceral skeleton of the head. 



occ.a 

 X (q.sp.4+5) 



pr.occ. a 



Ficr. 31. 



Occipital region of an embryo of Acanthias, according 



to Sewertzoff, 1899, plate XXIX, fig. 4 (nomenclature 



altered). 



Cr. cranio-vertebral limit, g. sp. spinal ganglia, occ. a. 



occipital arch, pr. occ. a. prae-occipital arch, 4, 5, 6 etc. 



somites. 



The figure given by SEWERTZOFF (1899, fig. 4) of the 

 occipital region of an Acanthias-embryOj reproduced here as 

 fig. 31, is particularly instructive. Though SEWERTZOFF 

 could not confirm HOFFMANN'S statement that true vertebrae 

 may be distinguished here, the cartilage in the occipital 

 region representing a continuum from the beginning, yet 

 th'.' segmentation of the axial skeleton is very clearly 

 pronounced by a series of prominences corresponding to 



