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 

 given names which indicate their relation to the human 

 bones or to the part or region in which they occur. 

 Dermal bones are apparently the older, phylogenetically, 

 but for convenience the cartilage bones are considered 

 first. 



FIG. 65. Vomer of 

 2 5 mm. A mblystoma larva, 

 after Hertwig, showing 

 the bone developed by the 

 fusion of the bases of 

 teeth. 



The chondrocranium shows several centres of ossification, but only 

 those giving rise to distinct bones are considered here. 1 The bones of 



FIG. 66. Ventral view of schematic skull, chondrocranium dotted, cartilage bones 

 with lines and dots, b&sioc, basioccipital; basisph, basisphenoid ; als, alisphenoid; exoc, 

 exoccipital; ors, orbitosphenoid; presph, presphenoid; premax, premaxilla; #w,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- 



1 Basi- and presphenoid, for example, arise each from two centres, but in all vertebrates 

 the resulting bones are unpaired. 



