DEVELOPMENT OF THE SKULL IN THE BATEACHIA. 189 



cartilages are of full size at present. Besides the odd outer bone below, two pairs 

 more are now jiresent above, and the post- and pre-auditory endoskeletal centres have 

 appeared {pa.s.,/.}^., 2^^-> ^-O-, ^^^ ^^^e patch behind V.) ; three pairs of new cartilages, 

 also, have appeared — the spiracular and the liervianent upper labials {sp.c., u.P.k.P.). 



The notochord (nc.) is ftist shrinking ; the condyles (oc.c.) are weU formed ; and a 

 crescentic bony tract appears on each side, reaching to the foramen for the 9th and 

 10th nerves (IX., X.) ; these are the ex-occipitals (e.o.) ; they are climbing up behind, 

 and flanking the auditory capsules in the opisthotic region ; the prootics are small. 

 The whole basis cranii is now well cliondrified, and behind, has completely coalesced with 

 the auditory capsules ; they are obliquely placed, the horizontal canal {h.s.c.) and its 

 fringing tegmen (t.tij.) pushing outwards. Outside the oblique, oval, vestibular con- 

 vexity, below, we see a perfect oval stapes (fig. 11, vb., st.) ; and between the otic process 

 of the suspensorium (ot.j).) and the capsule there is a thick biconvex mass of cartilage 

 wedged in ; this is the distinct spiracular cartilage (sp-c.) — the "annulus tympanicus" 

 that is to be. The interorbital region, or mid skull, lessens considerably forwards, and 

 bulges gently ; its floor is perfect and so are its walls, but the roof is membranous from 

 the narrow occipital arch (lettered /r/i.) to the perpendicular ethmoid (the hind part of 

 s.n.) : this is the great fontauelle {fo.). The rudiment of an ethmo-nasal wall seen in 

 the last stage (fig. 8, p.e.) is now complete, and yet has quite a clear outhne above and 

 below (figs. 10, 11, ft.n.) ; this is the liigh fore part of the intertrabecula, which often, 

 in this "Family" (the Hylidae), grows forwards as a free prenasal rostrum. The broad 

 bulbous hmder end is directly in front of the parasphenoid below, and of the fontanelle 

 above ; its fore end is narrow and is stiE growing, for it will run forwards between, and 

 even in front of, the cornua trabecula3 (dr.). Here we see the distinction between the 

 nasal roof-cartilage (n.r.), or ah-septals, and that growth of the skull itself at its closing 

 in, which belongs to the true ethmoidal region. Here the hind part of the roof is the 

 ali-ethmoid (al.e.) — an ox-horn-shaped spreading and curving growth of cartilaginous 

 wings, enfolding the sides and hind part of the nasal sacs. The nasal roofs (n.r.) 

 are broad lunules of newer cartilage, their deep concavity, or notch, margming the 

 nostrils (e.n.). 



The cornua trabeculse (c.tr.) are quite like those of the last stage (figs. 8 and 9), but 

 they have dwindled down to half their proper size. In front of these, but more than 

 twice as lai-ge, are the temporary upper labials (it.l.) ; these have a deep cleft behind, 

 more than half dividing each, and a long roimd outer angle ; the outer part con-esponds 

 to the augulo-labial of the Lamprey, and the inner moiety to half its anterior dorsal 

 cartilage, or main upper labial. 



Beliind this inner part, running obliquely backwards and outwards, there is a half- 

 tube of thin bone, rather dilated at its base, in front ; this is the premaxillary (px.), 

 which thus develops its nasal process, first. 



Each bone reaches the front of the outer nostril {e.n.), and has under it two little 

 beads of uew cartilage ; the inner lobule lies under the bone and is discoid ; the 



