DEVELOPMENT OF TPIE SKULL OF THE COMMON' FEOG. 
191 
minal segment of the superadcled series, the “ interstapedial ” (it.st.), now lie in a deep 
fossa in the thick periotic wall ; and the “ fenestra ovalis ” only occupies the posterior 
third of the fundus. Rounded in front, the stapes fits into the concave end of the parrot- 
beak-shaped interstapedial by a cup-and-ball joint ; the piece in front, “ medio-stapedial ” 
( m.st .), is attached to the latter by the upper edge of its bevelled broad base, which 
is not entirely ossified; hut there is no joint between this narrow-topped bony bar and 
the two unossified rays in front — namely, the delicate terete “ suprastapedial ” (s.st.), 
and the large spatulate extrastapedial ( e.st .). 
With regard to the clefts that part these foremost facial bars, it may be seen that 
although they are persistent yet they have undergone very curious morphological changes. 
The first cleft, that between the trabecula and mandibular rod, began to open inside the 
angle where the two clefts diverge (fig. 11) in front of the part where the transverse con- 
nective afterwards appears (fig. 13). 
It slowly completed itself by dehiscence of the dermal tissue ; and thus an external and 
internal nostril were formed. The pterygo-palatine bar filled up the next part of the 
cleaving space ; and the remainder was filled in below by fibrous membrane, and above 
by fusion of the heads of the two arches. 
The next secondctri/ opening, the cleft between the first and second postorals, forms 
for a little while an obscure, imperfect opening through the skin, in the space in front 
of the divergence which forms the free hyoid cornu (figs. 1, 3). On the inner side, above, 
there lingers for a long while a mere membranous tract between the heads of the two 
arches; and this is in some degree persistent, for the joint-cavity between the “infrahyo- 
mandibular ” and the ear-sac (Plate IX. figs. 2 & 7) is the remnant of this space. 
But opposite the partial external opening, in the space formed by the divergence of 
the lower third of the bars (figs. 1, 2), there is no filling-in on the inner side. The an- 
gular space between the descending quadrate and the hyoid cornu (see figs. 13 & 14) 
becomes modified (Plate IX. fig. 7, eu.), and is completed into a ring by membrane ; this 
is the outer end of the “ Eustachian tube ;” and this short tube, which opens into the 
throat within, expands externally into the tympanic cavity, between the “ extrastape- 
dial, ’ the “ stylo-hyal,” and the “quadrate.” 
Thus the first postoral cleft, although showing so little on the outside, is yet largely 
developed on the inner, and, like the persistent portion of the preorcd cleft, becomes a 
structure of the highest importance ; the one helps to perfect the olfactory organ, the 
other the organ of hearing. Not only so, they are intimately related in their higher 
morphological conditions ; they both open on the same plane into the great oral chasm ; 
and the foremost pair form the bellows which supply with air the hindermost in their 
tympanic function. The process of dehiscence by which the nostrils, Eustachian tubes, 
and ear-drums were formed, was taking place in my first stage (Plate III. fig. 11), before 
the formation of the heart and liver, and a long while before the intestinal cavity. 
I need not recapitulate what has been said of the transitory arches, the branchials ; 
their clefts, although by far the most perfect, entirely disappear. 
