302 MR. W. K. PARKER ON THE STRUCTURE AND 
tensor tympani muscle, and the rest of the neck with the rounded head is the 
‘“‘manubrium mallei” (mé.). The dark jagged space is the tympanic cavity, a development 
of the first postoral cleft, and which runs forwards into the mouth-cleft as the Eusta- 
chian tube. Now it is easy to see how the membrana tympani is formed; for the 
inhooked apex of the mandibular rod, creeping like a tendril toward the auditory sac, 
necessarily carries with it the liming membrane of the cleft wrapped over its head. 
The shaft is not shown here, because it has been severed with the fore part of the 
shoulder or “ tubercular” portion. 
On the left side of the larger figure (Plate XXX. fig. 5) the fellow of this is seen, but 
cut away further backward. Below the “ manubrium” is seen the shaft of the next arch 
(now to be called stylohyal); its direction is downwards and forwards to the root of 
the tongue; a good distance must be supposed between this and the section through the 
ceratohyal already described (Plate XXX. fig. 2, e.hy.). 
The outer ear or concha (ea.) is fast passing into cartilage ; it is curiously folded upon 
itself, and runs round the external orifice of the cleft; it is much modified already 
from its Batrachian and Plagiostomous prototypes, the “annulus tympanicus” of the 
former, the “ principal opercular” of the latter. ‘The interest attached to the vegetative 
gemmation of the membrana tympani is more than rivalled in the metamorphic 
changes that take place in the succeeding arch and in the neighbouring territory of the 
ear-sac. In the first stage we saw that the simply oval primordial ear-pouch was deve- 
loping into a lobular form, and that there were three bulgings on the outer side of the 
sac (Plate XXVIII. figs. 5 & 8, au.). ‘The middle of these, by a process of gemmation, has 
freed itself to a great extent from the wall of the ear-capsule, thus forming a “ fenestra” 
in that wall, which, however, is closed by the separated nodule of cartilage. This twin 
bud (Plate XXX. fig. 6, st.) (it has two papular elevations which look forward and out- 
ward from its free surface) is covered externally by delicate indifferent tissue, ready 
to become cartilage. Being in the posterior wall of the first postoral cleft, the second 
arch (hyoid), whilst sending its orbicular head inwards, does not become infolded in the 
mucous membrane lining the cleft, but is free to creep, tendrilwise, to the surface of 
the ear-sac; this it does, and conjugation takes place between its orbicular “ capitulum” 
and the freed auditory bud. But in the first stage we saw that a curious kind 
of segmentation was taking place through the shoulder of the second postoral bar 
(Plate XXIX. fig. 9, hy.); now that process is much more complete, and the simple bar 
has undergone a process the exact counterpart of that by which the blade of the orange- 
leaf articulates with its petiole: whilst this has been going on, a rounded “ tuberculum,” 
distinct as that in the rib of a bird, has been developed on the detached upper segment 
(Plate XXX. fig. 6, s.c.7); this is the “ short crus of the incus ;” the neck growing towards 
the ear-sac is the ‘‘ long crus” (/.¢.7.); its expanded, conjugating end the nidus of the “ os 
orbiculare;” the ha/f-shoulder above is the body of the incus, which articulates with the 
shoulder of the arch in front (Plate XXX. fig. 3, 7.,m.); and the bigeminal segment of the 
auditory sac is the young stapes (st.). The other half of the shoulder, or tubercular 
