DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 31 
galli (crg.). The diverging rows of spots, right and left, are the holes of the unossified 
cribriform plate (cr.p.), which extends some distance under the lateral masses. A 
much lesser region, shaped like a Butterfly, but with the head forwards, is seen 
behind the cribriform plate; its body or septum is low and thick, and the markings 
on each side, caused by the folds of the middle turbinal (m.tb.), diverge backwards 
and outwards, like the rows of the nerve passages. The septum, there, is the end of 
the perpendicular ethmoid (p.e.); thence the presphenoid (p.s.) begins ; it is hollow 
along the mid-line, for the roots of the huge, leafy wings are bulbous. Yet these 
swollen parts belong to the basal region, which, however, has no median bone in these 
types; they are formed by the trabecule cranii. All these winged parts may be aptly 
compared to Insects; here, the orbitosphenoids, growing outwards and forwards 
behind the nasal labyrinth, and the alisphenoids, growing out behind them, have a 
similar general form. 
Where the swollen trabecular bars have grown up into wings, there the width 
lessens a little ; it then increases, gently, and then suddenly becomes a large, dilated 
band, confluent with the aliethmoid in front, and continuous with the supra-auditory 
and supraoccipital cartilage (0.s., s.a.c., s.0.) behind. This tract has lessened very 
little since the first stage (Plate 2, fig. 1). The narrowish stem of the great lateral 
cranial band is ossified in its upper half, and this bone—the orbitosphenoid (0.s.)—- 
runs along the front part of the root ; the optic foramen (IL.) lies on the lower edge of 
this new bony centre, midway between the two margins 
The next dipterous growth has a bony median body—the body of the posterior 
sphenoid (b.s.). This ossification fits in between the swellings of the presphenoid, 
and is convex in front; it is short at present, and has a concave hinder margin 
reaching to the slightly elevated postclinoid wall. The proximal part of each wing 
is ossified by the basisphenoid ; and the alisphenoid (al.s.) is double on each side, 
for the rounded, outer free part is separately ossified from the narrow inner portion. 
This narrow part is concave at its lower margin, to enlarge the sphenoidal fissure, 
through which the Ist and 2nd branches of the 5th nerve pass (V'., V*.), besides the 
3rd, 4th and 6th nerves. The inner bone is also notched at its outer part behind, 
and the bony matter is beginning to convert the notch for the 8rd branch of the 5th 
nerve (V°.) into the foramen ovale. 
The cartilage formed by the moieties of the investing mass (parachordals) becomes 
very narrow behind the postclinoid wall, between the large cochleze (ch/.), and then 
it expands again as the threshold of the great occipital passage (fm.). The sub- 
pentagonal basioccipital (b.0.) reaches halfway to the basisphenoid (b.s.) leaving a 
long synchondrosis. The basioccipital is notched in front, and has a concave hind 
margin, of less extent than its oblique sides. The sinuous margins of the proximal 
part of the exoccipital region have their larger concavity in the front, to let out 
the glossopharyngeal and vagus nerves (IX., X.); and behind and outside these 
passages the thick square tract of cartilage is perforated for the hypoglossal (XII.). 
