DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 145 
The multiperforate meatus internus, further forwards, and the swollen proximal part 
of the cochlea (ch/.), are well seen in this view, over the long spheno-occipital syno- 
chondrosis. Between these tracts the bony and cartilaginous parts of the occipital 
arch (s.0., ¢.0., b.o.), the passages for the postauditory nerves, and the occipital 
condyle (oc.c.) are seen. 
Visceral arches of the Second Stage. 
The compound mandible has developed considerably since the last stage (Plate 22, 
fig. 4), the dentary bone (d.) having grown round MrcKev’s cartilage (mk.) for some 
distance, in its middle part, and the upper edge of the bone is now hollowing out to 
form the tooth sockets. In front, the basimandibular (b.mn.) is less, and it is now 
well defended by bone on its outside. Behind, the trilobate, semicartilaginous 
“ramus” has assumed its permanent form, although the coronoid and angular processes 
are equally cartilaginous with the condyloid or articular part. Under the latter, 
MECKEL’s cartilage is being let into the bony plate ; behind this part it arches upwards, 
and below the arched part the ectosteal malleai centre (m/. = articulare externum) 
is enlarging. The malleal part of this primary mandible shows a small but distinct 
posterior angular process (p.ag.), as the elbow of the manubrium (mb.), or internal 
angular process.. The incus (7.) and the stapes (sf.) are still quite unossified, but they 
are assuming their permanent form, 
The annulus tympanicus (a.ty.) is a ecrescentic band of not very solid cartilage, 
which is becoming bony along the middle; this bony tract will use up all the 
cartilage, not leaving any to form a “ bulla.” 
Third Stage of the Skull of Evinaceus europeus ; new-born young, 23 inches long. 
In the “endocranium” at this stage we find a considerable advancement in growth ; 
seen from below (Plate 19, fig. 1), and from above (fig. 2), it is roughly pyriform in out- 
line, gradually enlarging up to the auditory region, and then suddenly lessening. The 
short snout (fig. 1, a/.n.) has the nostrils (en.) lateral; this region has a definite 
bracket-shaped selvedge where it comes in contact with the premaxillaries, and from 
the submesial part of this hind edge the recurrent cartilages (7c.c.) are given off ; 
these are large tongue-like tracts, convex infero-laterally and concave on the upper and 
inner face, where, for a short distance, they form a perfect tube round each J ACOBSON’S 
organ. They are supported on their inner face by the dagger-shaped front paired 
vomers (v”.), whilst the vomer, proper (v.), runs in between them in front. That bone, 
overlapping these parts in their hinder third, runs backward to the end of the 
right and left subcranial recesses of the nasal labyrinth ; it is widely forked in its 
hinder, and strongly carinate in its middle, third. 
The aliseptal region (a/.sp.) narrows in at its middle, and then expands again to 
become aliethmoidal (c/.e.). 
