DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 147 
figured the upper aspect as a perfect object, not having cut away the rim and roof of 
the chondrocranium. 
The general description just given of the third stage may serve on the whole for this, 
but the bony tracts will be especially noticed The large front paired vomers (v’.) 
protecting the recurrent cartilages (7c.c.) and JACcoBSON’s organs are still separate. The 
proper vomer (v.) is becoming very strong, and its keel is now double, in front. On 
each side of its forks, behind, there is a small bony tract ; these are the bones which 
help to unite the two sides of the nasal labyrinth to each other and to the vomers ; 
they are the hinder paired vomers (v’.) The folds of the middle turbinal (7.tb.) are 
seen to be undergoing ossification ; the hinder pair of these centres is close to the 
precurrent cartilaginous spikes, which run forwards on each side of, and above, 
the middle keeled part of the vomer. The hinder part of the nasal labyrinth is 
opened out somewhat, artificially, for display. 
The large orbitosphenoidal bones (0.s.) are running upwards towards the marginal 
band; below, they are beginning to ossify the basal tract, or presphenoidal region 
(p.s.). The posterior sphenoid is now almost completely ossified, but the bones of the 
alz and base (al.s., b.s.) are distinct. The external pterygoid process (e.pg.) is still 
unossified. The three bones, measured across the foramina ovalia (V*.), are of equal 
width ; but, further backwards, the basisphenoid is the widest of the three, having 
taken in part of the cartilaginous ale. Here the hole, right and left, for the internal 
carotid artery (7.c.) is fairly enclosed by the bony deposit of the basisphenoid. This 
shows that we are not far from the Marsupials, and this agrees with what I have just 
shown, namely, that the tympanic wings of the posterior sphenoid are basal, in the 
Hedgehog, instead of being alar, as in the Marsupials, through the special posterior 
dilatation of the basal bone in this type. The pituitary hole (py.) is now enclosed by 
bone (b.s.) ; this hole is in the fore part of an inferior groove, right and left of which 
the pterygoids (pg.), left in situ, have developed an upper dilated part, by which to 
stick, like Limpets, to the basisphenoid; their cartilaginous nucleus (pg-c.) is not yet 
all ossified. 
The ossification of the auditory capsules is progressing rapidly. The opisthotic 
centre (figs. 3 and 5, op.) reaches from the passage for the 9th and 10th nerves, 
above the inner margin of the cochlea ; it encompasses the fenestra rotunda behind 
(fig. 8, fr.), and runs up to the fenestra ovalis; above (fig. 5), it is growing round the 
meatus internus. The prootic (pr.o.) is a much smaller centre ; below (fig. 3), it 1s 
growing over the groove for the facial nerve at the inner edge of the tegmen tympani 
(t.ty.), and above (fig. 5) it is running along under the anterior canal (a.s.c.) into the 
recess for the “flocculus.” The basioccipital (b.0.) is increasing in size, but it is still 
in the midst of a wide tract of cartilage; the exoccipitals (e.0.) are just enclosing 
the condyloid foramina (XII.); the supraoccipital is widening over the foramen 
magnum (f.7m.). 
In the upper view (fig. 4), the fore part is seen to be quite unossified, but the 
U 2 
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