DEVELOPMENT OE THE SKULL OF TILE COMMON FROG. 
171 
of the prootic and pterotic regions, and therefore still retains the relation of the anterior 
condyle of the hyo-mandibular. 
The remaining two-thirds of the top of the “ extrastapedial” is occupied by the com- 
mencement of a bony ray, which has been developed out of the postero-superior lobe, 
the posterior hyo-mandibular head (Plate VII. figs. 12 & 13, m.st .) ; this is the “ medio- 
stapedial” (II. & P.). 
This apparent “ columella auris” is a somewhat crooked rod of solid bone ; it is bony 
up to the extrastapedial and nearly to its base, which, however, is never entirely ossified. 
Its larger posterior end is greatly bevelled towards the skull ; and a little while since, it 
did terminate in a large snail’s-foot-like expansion of cartilage, which was growing back- 
wards towards the stapedial plate. This expansion, formed out of the posterior margin 
of the third lobe in the trifoliate “ suprahyomandibular” (Plate VII. fig. 12, slim.), has 
become a perfectly distinct segment having a cordate form, the apex of which is directed 
backwards (Plate VII. fig. 13, it.st .); this is the “interstapedial” (H. & P.) ; it now slightly 
overlaps the elegant oval valve of the “fenestra ovalis” — the “stapes” (st.). 
In the figure (Plate VII. fig. 13) the stapes is slightly dislocated from the fenestra to 
show how little this periotic segment has changed since it was evident for the first 
time (Plate V. figs. 1 & 4, st.). 
In the Frog, at any rate, there is no tendency in the stapedial plate to send a connective 
process towards the metamorphosed parts of the top of the second poststomal arch. 
In the last stage the dentary was still distinct from Meckel’s cartilage in front 
(Plate VII. fig. 5, d., ml., & Plate VIII. fig. 5); there is now' another stage of things. 
In this more advanced condition the chin-end of Meckel’s cartilage has been ossified 
into a “ mento-meckelian” rod of bone (Plate VIII. fig. 6, mk., m.mJc.) ; this is the “ lower 
intermaxillary rudiment” of Reichert. This part is very short, and is strongly united by 
a fibrous ligament to its fellow. The dentary (d.) has already coalesced with it ; but the 
ectosteal plate did not form it ; it is a proper endosteal bone, rapidly ossifying through 
the cartilage, like the prootic and exoccipital. Originally Meckel’s cartilage was much 
clubbed at its distal end ; even now the enlargement is shown, as this terminal bony 
part is thicker at its end than subterminally. 
The branchial arches have almost entirely disappeared (Plate X. fig. 1), and the 
skeletal parts of the tongue are now nearly like those of the adult (fig. 2). The “ stylo- 
cerato-hyal” (st.h., ch.) is now a narrow band of cartilage, equally attached by fibrous 
tissue to the suspensorium and to the opisthotic region. This band, still sending for- 
wards a “ hypohyal lobe ” passing into a cartilaginous plate, belongs both to the basihyal 
and basibranchial, which is now entirely composed of hyaline cartilage. The whole 
structure is apron-shaped, the hyoidean cornu forming the upper strings. Between its 
upper and broadest part it has a concave throat edge ; and below, it gives off two pairs of 
shorter strings. These are very different morphologically ; for the first, which are fee- 
ble and unossified, are the remains of the first and second branchial arches, coalesced and 
almost absorbed, whilst the hinder pair are the “ free hypobranchial horns ” (see Plate V. 
2 a 2 
