THE SKELETON 429 
interorbital region to the olfactory sacs. It is obvious, therefore, 
that the parachordals and trabeculae must form with relation to 
one another the angle defined by the cranial flexure. 
The parachordals appear in fishes as paired structures on 
either side of the notochord, uniting secondarily around the 
latter; but in the chick the perichordal portion is formed at the 
same time as the thicker lateral portions, so that the parachordals 
exist in the form of an unpaired basilar plate from the first. The 
trabecule are at first paired (in the earliest membranous condi- 
tion), but soon fuse in front, while the posterior ends form a pair 
of curved limbs (fenestra hypophyseos) that surrounds the infun- 
dibulum and hypophysis, and joins the basilar plate behind the 
latter. At the same time that the parachordals and trabecule 
are formed by condensations of mesenchyme, the latter con- 
denses also around the auditory sacs and olfactory pits in direct 
continuity with the parachordals and trabecule respectively; so 
that the auditory and olfactory capsules are in direct continuity 
with the base of the neurocranium from the beginning. 
Chondrification begins in the primordial cranium about the 
sixth day; it appears first near the middle line on each side, and 
extends out laterally. Somewhat distinct centers corresponding 
to the occipital sclerotomes may be found in some birds, but 
they soon run together, and the entire neurocranium forms a 
continuous mass of cartilage (sixth, seventh, and eighth days). 
During this process the trabecular region increases greatly in 
length simultaneously with the outgrowth of the facial region, 
and the angle defined by the cranial flexure becomes thus appar- 
ently reduced. The posterior border of the fenestra hypophyseos 
marks the boundary between the basilar plate and trabecular 
region. 
In the region of the basilar plate the following changes take 
place: (1) in the post-otic or occipital region a dorso-lateral 
extension (Fig. 244) fuses with the hinder portion of the otic 
capsule, thus defining an opening that leads from the region of 
the cavity of the middle ear into the cranial cavity (fissure met- 
otica). This expansion is pierced by the foramina of the ninth 
tenth and eleventh nerves. (2) The otic region becomes greatly 
expanded by the enlargement of the membranous labyrinth. The 
cochlear process grows ventrally and towards the middle line and 
thus invades the original parachordal region (Fig. 168). The 
