THE SKELETON 429 



interorbital region to the olfactory sacs. It is obvious, therefore, 

 that the parachordals and trabeciilse 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 

 trabeculae 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 trabeculae 

 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 trabeculae 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 clay; 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 clorso-lateral 

 extension (Fig. 244) fuses with the hinder portion of the otic 

 capsule, thus defining an opening that leads from the region ol 

 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 



