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ME. W. K. PAEKEE ON THE STEUGTUEE AND 



tensor tympani muscle, and the rest of the neck with the rounded head is the 

 " manubrium mallei" (mb.). 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 lining 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. 3) 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, c.hy.). 



The outer ear or concha [ca.) 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 tlie shoulder of the second postoral bar 

 (Plate XXIX. fig. 9, liy.) ; 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 

 (Plato XXX. fig. 6, s.c.i) ; this is the " short crus of the incus ;" the neck growing towards 

 the ear-sac is the " long crus " {l.c.i.) ; its expanded, conjugating end the nidus of the " os 

 orbiculare;" the half-shoulder above is the body of the incus, which articulates with the 

 shoulder of the arch in front (Plate XXX. fig. 3, m.) ; and the bigeminal segment of the 

 auditory sac is the young stapes {st.). The other half of the shoulder, or tubercular 



