66 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
into close connection with the elements derived from the cartilage 
skull, in some cases replacing considerable of it. The cartilage bones 
arise by an ossification of the cartilage. Even in the sturgeons the 
chondrocranium is complete, the membrane bones being superficial 
and not intimately connected with the deeper parts. 
The names of the bones are largely based on the term- 
inology of human anatomy. In many cases what appears 
as a single bone in the human skull is represented by 
several bones in the young and in the lower vertebrates. 
In these cases the bones in the lower forms are usually 
Fic. 65.—Vomer of given names which indicate their relation to the human 
25mm. Amblystomalarva, bones or to the part or region in which they occur. 
after Hertwig, showing . 
the bone developed by the Dermal bones are apparently the older, phylogenetically, 
fusion of the bases of but for convenience the cartilage bones are considered 
teeth. first. 
The chondrocranium shows several centres of ossification, but only 
those giving rise to distinct bones are considered here.‘ The bones of 
Fic. 66.—Ventral view of schematic skull, chondrocranium dotted, cartilage bones 
with lines and dots. basioc, basioccipital; basisph, basisphenoid; als, alisphenoid; exoc, 
exoccipital; ors, orbitosphenoid; presph, presphenoid; premax, premaxilla; qu, quadrate: 
quju, quadratojugal; squamos, squamosal; zygom, zygomatic; other names in full. , 
the cartilaginous brain case may be arranged in four groups, beginning 
behind and called respectively occipitalia, sphenoidalia and ethmoi- 
? Basi- and presphenoid, for example, arise each from two centres, but in all vertebrates 
the resulting bones are unpaired. 
