DEVELOPMENT OP THE SKULL IN THE MAMMALIA. 
145 
sphenoids, but it goes further backwards, yet there is a considerable tract of cartilage 
separating it from the basioccipital. 
A groove in its lower face leads to the still unclosed pituitary space (see fig. 2, py.) ; 
whilst right and left it has a deeper groove filled by the terete pterygoid (pg-) with its 
cartilaginous terminal nucleus ( pg.c .). Behind the pterygoids, and in front of the 
sinuous wedge-like growth of bone in the middle, there is a definite rudiment, as an 
oblique ridge, of the future tympanic wing. 
The broadest part of the skull is behind the posterior sphenoid ; here the huge 
cochleae (chi.) display their coils, and outside them the broad tegmen tympani has 
on its inside the groove for the facial nerve (VII.), which escapes from the skull 
behind the edge of the alisphenoid. This groove is protected behind by the confluence 
of the epihyal ( e.hy ) with the auditory capsule; the nerve escapes behind it, through 
the stylomastoid foramen. In front of the epihyal, the fenestra ovalis (fs.o.) is 
seen obliquely, and the fenestra rotunda (fr.) mesiacl of it. Behind the fenestra 
rotunda, and further inwards, the large passage for the 9th and 10th nerves (IX., X.) is 
seen, and behind that passage the lesser hole for the 12th nerve (XII.) The basi¬ 
occipital ( b.o .) is roughly hexagonal; the exoccipitals ( c.o.) are developing in the 
hollow between the paroccipital convexities and the condyles ( oc.c .). 
The upper view of the endocranium of this stage (Plate 19, fig. 2) is shown with the 
upper part cut away for the better display of the fundus cranii. The crenate hinder 
margin of the nasal capsules is almost transverse, only a little concave, and is so 
thoroughly pre-cranial as to display nearly all the large cribriform plate ( cr.p .). 
The top of the septum nasi (s.n.) and perpendicular ethmoid (p.e.) is seen at its 
junction with the alee or roof (al.sp., al.e ); the middle part of the continuous partition 
is thicker than the end. The upper pai't of the capsule is constricted twice, so as to 
form a front, a middle, and a hind, enlargement. The lateral part is constricted, gently, 
once. There is no definite cartilaginous crista galli ( cr.g .), but merely a gentle rising 
of the wall in front, directly behind the end of the roof. 
The cribriform plate has floored the whole fossa, and this tract, as well as the 
lateral walls, are confluent with the fore edge of the anterior sphenoid. The broad short 
presphenoid ( p.s .) is unossified, and it has in its middle a hollow, showing the double 
nature of the bar. The orbitosphenoids ( o.s .), are ossified, proximally, close to the base, 
in their hinder margin; the fore part and the whole of the main wing still remain soft. 
This pyriform centre is perforated by the optic nerve (II.), which escapes near the 
hind margin, half-way up the ascending bony tract. The inturned upper edge has 
been cut away, but the band is shown to run backward to, and to be confluent with, 
the auditory capsule. 
The hind margin of the orbitosphenoid (o.s.) is sinuously concave and hides the 
more distinct alisphenoid (al.s.) at its fore edge. The basisphenoid (b.s.) occupies the 
hind half of the small open pituitary space (py.) ; it is separated by a large tract 
of cartilage, as yet, from the basioccipital (b.o.). Both before and behind, that centre is 
MDCCCLXXXV. U 
