122 THE ORIGIN OF VERTEBRATES 
front of this space they remain in contact, and generally unite. They 
extend forward into the nasal region (pn.). 
3. The cartilaginous capsules of the sense organs. Of these the 
auditory (aw.) and the olfactory capsules (o/.) unite more or less inti- 
mately with the cranial! walls; while the optic capsules, forming the 
usually cartilaginous sclerotics, remain distinct. 
The parachordals and notochord form together the basilar plate, 
which forms the floor for that section of the brain belonging to 
the primitive postoral part of the head, and its extent corresponds 
roughly to that of the basioccipital of the adult skull. 
The trabecule, so far as their mere anatomical relations are con- 
cerned, play the same part in forming the floor for the front cerebral 
vesicle as do the parachordals for the mid- and hind-brain. They 
differ, however, from the parachordals in one important feature, viz. 
that except at their hinder end they do not embrace the notochord. 
The notochord always terminates at the infundibulum, and the 
trabecule always enclose a pituitary space, in which lies the infun- 
dibulum (inf.) and the pituitary body (py.). 
In the majority of the lower forms the trabeculze arise quite inde- 
pendently of the parachordals, though the two sets of elements soon 
unite. 
The trabecule are usually somewhat lyre-shaped, meeting in 
front and behind, and leaving a large pituitary space between their 
middle parts. Into this space the whole base of the fore-brain 
primitively projects, but the space itself gradually becomes narrowed 
until it usually contains only the pituitary body. 
The trabecular fioor of the brain does not long remain simple. 
Its sides grow vertically upwards, forming a lateral wall for the 
brain, in which in the higher types, two regions may be distinguished, 
viz. an alisphenoidal region behind, growing out from what is known 
as the basisphenoidal region of the primitive trabecule, and an 
orbito-sphenoidal region in front, growing out from the presphenoidal 
region of the trabecule. These plates form at first a continuous lateral 
wall of the cranium. The cartilaginous wails which grow up from the 
trabecular floor of the cranium generally extend upwards so as to form 
a roof, though almost always an imperfect roof, for the cranial cavity. 
The basi-cranial cartilaginous skeleton reduces itself always into 
trabecule and parachordals with olfactory and auditory cartilaginous 
capsules. 
