416 CHORDATA. 
or parachordals, soon fuse above and below the noto- 
chord to form the dasés craniz. The trabecule then meet 
in front under the fore-brain to form a median plate, 
called the ethmo-nasal septum. All three pairs of sense- 
organs now acquire cartilaginous sense-capsules, which, with 
the exception of that of the eye (or sclerotic) fuse on to the 
primitive cartilaginous cranium, or dasal plate, formed by 
the trabeculez and parachordals. The basal plate then 
grows up on either side to enclose the brain. The edges 
meet dorsally in the occipital region and also forwards in 
the ethmoid region. Thus is formed the cartilaginous or 
chondro-cranium. ; 
In the sharks and skates this condition remains, but 
in Amphibia a number of bones are added, and in the 
higher classes the bones almost completely replace the 
cartilage, forming a complete osteo-cranium. This osteo- 
cranium is produced partly by membrane-bones, which sink 
in, and partly by cartilage-bones. The bones of the osteo- 
cranium are arranged more or less in rings, a system which 
gave rise, in the hands of Goethe, Oken and Owen, to the 
beautiful vertebral theory of the skull. The hindmost ring 
is the occipital, with a dasioccipital, two exoccipitals and a 
supraoccipital. The second ring is the sphenoid, with a 
basisphenoid, two alisphenoids and a pair of parietals. In 
front of this is the presphenoid ring, with presphenoid (at 
base), paired ordbitosphenoids and a pair of frontals. The 
ethmoid ring completes the front-end with a mesethmoid 
and zasads. Between the occipital and sphenoid rings are 
the periotic, a bony capsule of the ear,* and the large 
sguamosal.t Connected with the orbit, and lying at the 
anterior corner of it, is the small /acryma/.{ Lastly, immedi- 
ately below the mesethmoid, in the roof of the mouth, are 
the vomers, unpaired in mammals, and the farasphenoid. 
We have already seen that the skull is, in the course of its develop- 
ment, preformed in membrane, and the greater part of it in cartilage. 
The cartilage is then gradually replaced by bone, a stronger and harder 
substance, by the process of ossification described above. If ossification 
* The periotic may be represented by as many as five separate otic bones, as in 
the cod (p. 336). 
+ The “temporal” bone of human anatomy is the fused periotic, tympanic and 
squamosal. 
{ The lacrymal is one of a series of circumorbital bones (cf cod). 
= 
