DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
21 
sphenoid (o.s.), which appears in the figure to be cut off from its base; this is due, 
however, to the fact that this section is through the sphenoidal fissure. Here the 
basal part ( p.s .) is spindle-shaped in section ; this is due to the fact that the trabeculae 
are here adding their thickness to the solid intertrabecula, and that they are giving 
off the orbitosphenoids, which, here, are not continuous with their root. The 
frontal (f.) is broad, and curls round the orbitosphenoid, into the orbit. Here the 
pterygoids (pp.) are very solid, and protect a transversely elliptical space, the naso¬ 
palatine canal ( n.p.c .). Above and outside each pterygoid, a small V _s ^ a P e( i section 
of bone is seen; this is the foremost part of the lower alisphenoidal centre (see Plate 2, 
fig. 6, al.s.). The jugal (j.), dentary (d.), and Meckelian rod ( mk .), are very similar in 
this to the last section. 
15 th Section (Plate 4, fig. 3).—Here the presphenoid (p.s.) is far separated from 
the large upper band of the orbitosphenoid (see Plate 2, fig. 1, o.s.) ; its triple form 
answers to its compound nature, formed as it is from a bulbous middle, and two 
wedge-shaped, lateral, pieces—the intertrabecula and the trabeculae. The Mammalian 
type of skull is seen here, for the alisphenoid (al.s.) overlaps the lower part of the 
orbitosphenoidal region, and lies a good distance outside it. There is a thick core of 
cartilage at this part, with an ectosteal deposit on its upper edge, the foremost part 
of which was seen in the last section. The naso-palatine canal (n.p.c ) is kidney¬ 
shaped in section here, the “ lrilus ” being above. On each side of this passage the 
pterygoids (pg.) are seen to be very thick in this their hinder part. Laterally, the 
point of the zygomatic process of the squamosal (sq.) is seen riding over the hind 
part of the jugal (j ), and, inside this, the ramus of the mandible is seen to be 
composed of a mass of hyaline cartilage, besides the bony dentary (d.) which protects 
Meckel’s rod (mk.). 
1 6th Section (Plate 5, fig. 4).—-We are here in the widest part of the hemispheres, 
and the fore part of the pituitary body (py.) is cut through. Beneath, the basal 
cartilage is composed of the beginning of the trabeculae (tr.), and it is seen to be 
double not triple.* 
This prepituitary part of the basisphenoid (b.s.) is cut across close in front of the 
narrow pedicle of the alisphenoid (see Plate 5, fig. 1, al.s., b.s.), in the innermost part 
of the sphenoidal fissure, so that the cartilage appears in two pieces on each side of 
the basal mass. The alisphenoid is not merely calcifying; it is being rapidly 
converted into bone, within, and, externally, has two thick ectosteal plates on it, an 
upper and a lower alisphenoidal bone (see Plate 2, fig. 6 , and Plate 5 , fig. 1 , al.s.', 
al.s.). The parts of the mandible and face are like what are seen in the last section ; 
but the naso-palatine canal (n.p.c.) has now opened into the fauces. The upper band of 
* In these decalcified sections the lime-salts were removed, but the cartilage cells were becoming 
calcified (see Plate 2, fig. 1 , b.s.) as the basisphenoidal bone. In the 19th and 20th Sections (Plate 4, 
figs. 8, 9) the same is true of the middle of the basioccipital region. 
