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 {an.) and the olfactory capsules (ol.) 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 trabecular, 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 

 trabecular 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 trabecular arise quite inde- 

 pendently of the parachordals, though the two sets of elements soon 

 unite. 



The trabecular 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 floor 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 maybe distinguished, 

 viz. an alisphenoidal region behind, growing out from what is known 

 as the basisphenoidal region of the primitive trabecular, and an 

 orbito-sphenoidal region in front, growing out from the presphenoidal 

 region of the trabecular. 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 

 trabecular and parachordals with olfactory and auditory cartilaginous 

 capsules. , 



