DEVELOPMENT OF THE SKULL OF THE COMMON FOWL. 
797 
yet appeared ; but above the facet the bone spreads equally forwards and backwards, in 
front overlapping the postfrontal and frontal, and behind covering the upper part of the 
mastoid and exoccipital regions; the hinder lobe overlies the root of the hyoid arch. 
When the coalescing roof-bones are removed by a saw, there is displayed v^iat is seen 
in Plate LXXXVII. fig. 2 : anteriorly the ethmoidal ossification (eth.) has advanced 
halfway to the isthmus, and backwards along the upper and middle bars ; behind the 
latter is seen the small presphenoid (p.s.), having the hinder orbito-sphenoids ( o.s . 2) 
on each side ; in front of these is seen the smaller pair (o.s. 1). The alisphenoids ( a.s .) 
are attached by a fibrous band to the posterior orbito-sphenoids ; the large oblique groove 
for the 5th nerve (5) is seen to burrow them behind. 
Behind the deep perforate “ sella” is still seen the primordial fissure (p.b.f.) or fonta- 
nelle ; and at some distance behind this is the ridge formed by the notochordal shaft-bone. 
The saw has passed through the periotic territory, so that the whole of the horizontal 
semicircular canal with its ampulla (li.s.c.) is seen — also its connexion behind with the 
ascending posterior canal, the ampulla of which ( p.s.c .) and that of the great anterior 
canal ( a.s.c .) are seen lying in the same plane ; outside the ampulla of the horizontal 
canal the upper tympanic recess ( u.t.r .) is exposed. 
The under view of a similar skull with the lower part of the basitemporal removed, 
is shown in Plate LXXXV. fig. 9 : here the common opening of the eustachian tubes 
(< eu .) is laid bare, the posterior pterygoid processes (p.r.p.) are cut through, and the 
carotid canals (i.c.) and cochlea (c.l.) are exposed. The tympanic cavity (ty.) is seen 
growing forwards into the anterior recess (a.t.r.), and the opening into the upper recess 
(u.t.r.) is seen behind the double quadrate condyle (pc.). 
Another section, more highly magnified, is shown in Plate LXXXV. fig. 8 ; it is an 
upper view T , and the saw has passed through a lower plane than in Plate LXXXVII. 
fig. 2. Here the stapes is seen in situ , and the membrana tympani (m.t.) is seen passing 
over its triradiate end to be attached, anteriorly, to the great posterior pterygoid process. 
Behind the vestibule (v.b.) is seen the base of the ampulla of the posterior canal (p.s.c.), 
and a section of the lower part of its arch. Mesiad of the ampulla is seen the opening 
for the vagus (8), and further inwards still that for the 9th (9). A bristle is figured 
as traversing the right drum-cavity and eustachian tube (eu.) ; and on each side of the 
eustachian opening the internal carotids (i.c.) come up. The part seen in shade, as the 
floor of the drum-cavity, is the basitemporal (b.t.). 
The right stapes is shown in Plate LXXXVII. fig. 3 (6 diam.), from above, and at its 
base. The base is irregularly oval ; the shaft, which with the base is ossified by ectostosis, 
is very slender ; it widens, and then breaks out into a complex triradiate cartilage. The 
antero-inferior bar of this cartilage (“ infrastapedial,” i.st.) is slender and arcuate; the 
middle part (“extrastapedial,” e.st.) is much broader and is spatulate; it curves forwards 
and downwards, and touches the postero-internal face of the quadrate. The postero- 
superior bar (“ supra-stapedial,” s.st.) is bifurcate at its free part, and from its lower 
edge there is a broad thin lamina which is continuous with the free end of the “ extra- 
