82 
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 Mexanchus and Heptanchus still one or two 
segments more must be added to the number found for 
Acanthias (l. c.p. 29). 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. 
Gr 
0CC.à ë 
. q. Sp. 6 ‚Sp.7 k 
X (9.sp4re) ne en g.sp.s gespe 
en een 
proce. 4 
\ 
\ ' 1 
| EA 
4 
Fig. 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. 4. 
oecipital arch, pr. occ. a. prae-occipital arch, 4, 5, 6 etc. 
somites 
Especially the figure given by SEWERTZOFF (1899, fig. 4) 
of the occipital region of an Acanthias-embryo, reproduced 
here as fig. 31, is very instructive. Though SEWERTZOFF 
could not confirm HOFFMANN’s statement that true vertebraë 
may be distinguished here, the cartilage in the occipital 
region representing a continuum from the beginning, Yêt 
the segmentation of the axial skeleton is very clearly 
pronounced by a series of prominences corresponding fo 
