DEVELOPMENT OE THE SKULL OF THE COMMON FKOG. 
181 
is seen the ampulla and part of the arch of the “ horizontal canal ” (, h.sc .). This section 
has been made close behind the posterior fontanelle, between the prootic and exoccipital 
(see Plate IX. fig. 6, pro., eo., p.fo cutting- through the posterior part of the parietal 
(p.), and the broad basitemporal portion of the parasphenoid ( po.s .) ; on the inner side 
the auditory nerve (7 6 ) is seen passing through the “meatus interims.” A little of the 
prootic ( pro .) is seen here, both in the cranial and in the auditory cavity ; and above, it 
encloses the crown of the anterior canal, and reaches that of the horizontal ( a.sc ., li.sc.). 
The outermost part of the projecting periotic mass is thicker here and is deeply scooped. 
If the section had been made a little further backwards, it would have passed through 
the “ fenestra ovalis ” (see Plate VIII. fig. 4 ,fs.o.) ; but in the adult the original fenestra 
is largely walled-in with cartilage, leaving only a small opening to the “ vestibule ” 
(Plate VII. fig. 16, fs.o.). The piece of cartilage seen in the fenestral pit is the stape- 
dial ( st .) ; and below the pit the unossified cartilage is “ opisthotic ” (op.), regionally con- 
sidered. This section fortunately runs through a very important articulation, namely 
that of the stylo-hyal (st.h.) with the opisthotic cartilage. 
The original ovoidal form of the periotic capsule is not entirely lost, as may be seen 
by comparing the inner view (Plate IX. fig. 5) with this section, which shows the form 
of the cavity, and the bulging inwards of the inner wall. 
Only a narrow border-land is left above between the overgrown prootic and exoccipital 
bony centres (Plate VIII. fig. 9, Plate IX. figs. 1, 3, 4, 5, 6), and the occipital arch is, 
as in all the “ Sauropsida,” confused in its ossification with the posterior face of the 
periotic mass ; nothing can more plainly bespeak caution in determining true morpho- 
logical regions from encroaching bony growths; we never see the auditory sense-capsule 
perfectly differentiated from the occipital and sphenoidal regions, as a bony mass, until 
we reach the Mammalia. 
A section made still further back passes through the epiotic eminence (Plate X. 
fig. 10, ep.), which is affected by endostosis at the surface ; this eminence is caused by 
the development of the arch of the “ posterior canal” ( p.sc .), which passes downwards 
and forwards to reunite with the anterior canal. It is crossed close above its ampulla 
by the “horizontal canal” (hs.c.), the ampulla of which was shown in the last section 
(fig. 9) ; part of the stapes (st.) is seen in the fenestral fossa behind ; and part of the 
parasphenoid (pa.s.) is seen towards the mid line of the skull. Across this part the 
skull is tolerably well ossified — not, however, by a “periotic” centre, but by the exocci- 
pital. There the bone is so thick that medullary cavities are seen in it ; it reaches to 
the narrow basioccipital region below ( h.o .), and to the wider supraoccipital region (s.o.) 
above ; this is ossified superficially, but the basioccipital region is not. 
Further explanation of the transverse sections is given by a side view, in which the 
outer surface has been partly pared away (Plate VIII. fig. 9) ; in this waypt is seen that 
the ampullae of the anterior and horizontal canals (a.sc., h.sc.) are imbedded in the large 
spreading prootic (pro.), whilst the ampulla of the posterior canal (p.sc.) is enclosed 
in the exoccipital (e.o.) Pelow the posterior ampulla is the exit for the compound 8th 
