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connective. This palatine bar lies below and within the
developing temporal muscle, which is also covered by the
orhitar process, another lamina of cartilage growing from the
suspensorium in the same position (see Fig. 34). The ante-
rior or quadrate region of the suspensorium is very short;
and it bears a rounded head or condyle for the meckelian rod.

333. Only the lower region of the second or hyoid arch
has chondrified; it has become enlarged and thickened,
and diverges backwards at a considerable angle from its
articulation with the middle of the hinder edge of the sus-
pensorium. It becomes incurved towards the middle line
below, but does not yet meet its fellow. Mesially, in the
hinder part of the tongue, there is a small piece of nas-
cent cartilage, the basihyal. The four branchial arches
are simple, being curved inwards above and below,
and flattened laterally. Mesiad of the first and second
branchials there is pyriform basibranchial cartilage.

334. The upper and lower labials have now become a
pair of cartilages above and below : the upper have con-
siderable vertical as well as lateral extension; the lower
are rounded rods vertically placed in the lower lip. The
internal nares open in the roof of the mouth external to
the internasal plate and behind the trabecular cornua;
the external nares open on the upper surface of the head
in the same region,

335. The amount of development which has occurred
since the first stage constitutes a notable morphological
advance. The confluence of the trabeculse to form an
internasal plate, their union with the parachordals behind
and with the mandibular bars laterally are events having
a very high interest. Labial cartilages are well developed,

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v.] THE frog: third stage. 143

although at the same time the rudiment of the lower jaw
has been cut oflf from the at present much larger suspen-
sorium. The orbitar process is a remarkable but little
uaderstood structure. The hyoid arch has acquired a
definite articulation with the suspensorium, and basibyal
and basibranchial elements are discernible.

Third Stage : Perfect Tadpoles ovs inch long, with
hind legs appeaHng,

336. The digestive canal having. been completed in
the last stage, growth and development have been very
rapid, and great morphological advance has taken place.
The external gills have become covered by the opercular
skin-fold, and at the same time there have been de-
veloped copious tufts of internal gills which arise from
both the surfaces of the gill-arches. On the right side
the opercular fold growing backwards from the hyoid arch
unites with the skin of the thorax ; but on the left, some
way behind the last branchial arch, a vertical oval passage
for the outflowing current of water remains (Fig. 35, op.).
The coalescence of primarily distinct skeletal elements
which had commenced in the last stage, has now, after the
lapse of two or three weeks, attained its maximum. The
differing relations of the sense-capsules, each of which has
a chondrified wall, are manifest. The auditory sacs and
the eyeballs acquire their cartilaginous coat independently,
but the former coalesce with other elements, while the
latter remain distinct; and the nasal cavity has its floors,
roof, and walls, so far as they are yet formed, entirely de-
veloped as outgrowths of the trabecular plate,

337. The parachordal cartilages have united with one
another, over and under most part of the cranial noto-
chord, forming a basilar plate which coalesces on each side
with the auditory capsule (Fig. 34). Posteriorly the
parachordals curve outwards behind the ear-capsules as far
as their most prominent region, to form the two rounded
occipital condyles {oc. c.) ; and here also lateral walls and
a roof of slight longitudinal extent are developed, forming

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an occipital cincture of continuous cartilage. The roof has
coalesced with and extends between the hinder half of
the auditory masses. More anteriorly, the parachordals
have riot united; but the apex of the notochord no longer
occupies its former position. It appears to have relatively
receded, leaving a fissure occupied by membrane, the
posterior basicranial fontanelle. In front of this fissure
the parachordals are perfectly continuous with the tra-

Fig. a4.

Tadpole about one inch long ; view of face and cranial floor from above,
the brain haying been removed ; the upper region of the auditoiy
capsule has been sliced off.
n.e. notochord, covered by basilar cartilage ; the pointed space formerly
occupied by tiie apex of the notochord is seen as thts posterior basicranial
fontanelle ; oc, c. occipital condyle ; a. «c , h. «c., p. se. anterior, horizontal,
and posterior semicircular canals ; g,g, Gasserian ganglion ; in front of it
the cranial side wall is seen rising from the trabecular floor ; the tbinly
chondrified median tract of the cranial floor is distinctively indicated ;
eth, anterior concave ethmoidal wall of the cranium ; e,tr, oomn trabe-
oulse; ol. nasal sac; u.l. upper labial; 1,1, lower labial; 8,0 f. snboeular
fenestra, flanked outside by ihe mandibular arch, which is united behind
with tbe cranial floor, and applied round the auditory capsule; giving off
anteriorly the orbital lamina or, p, rising over the temporal muscle ; qu.
quadrate region ; mk, meckelian cartilage, extending to the lower labial

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338. The original intertrabecular space, at first only
enclosing the pituitary body, has become much elongated ;
moreover it has acquired a thin but complete cartilaginous
floor continuous with the trabeculaB. All along the cranial
trabecular region, low side walls have arisen, which become
more prominent anteriorly, and join in front of the olfac-
tory lobes of the brain to form an anterior concave eth-
moidal wall {eth.). The first bone has appeared as a
parostosis beneath the intertrabecular space just described,
and of equal extent with it; this is the parasphenoid,

339. Instead of small trabecular horns springing from
a commissural region of no great extent, we find now a
pair of gently diverging rods, lying above the fore palate
(c. tr,). They broaden anteriorly and are stron^y decurved,
each bearing a transverse horizontal ridge articulating with
the corresponding upper labial (Fig. 34, u, L). The eth-
moidal region remains restricted, forming the front wall
of the brain-case, and uniting with the palatine bars
laterally. The nasal sacs are relatively small, lying in the
angle between each palatine bar and the corresponding
cornu, and flanked externally by the quadrate end of the

340. The auditory capsule has lost its simple external
appearance and pyriiorm shape, the development of the
ampullae and canals having distended it in various direc-
tions. The capsule is hollowed in the middle above, be-
tween the elevated portions ; and it projects outwards in
the region of the horizontal canal, forming a pterotic ridge,
the rudiment of the tegmen tympani. The fenestra ovalis,
situated infero-laterally, is elongated-oval in shape, and
filled with membrane, in which the stapes has already

341. There is a complete coalescence between the
auditory capsule and the cranial investment. The glosso-
pharyngeal nerve pierces the hinder end of the capsule
below ; and the vagus escapes between it and the occipital
cartilage. The Gasserian ganglion (gg.) lies close to the

B. M. 10

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prootic region, opposite the end of the notochord, and the
branches of the trigeminal nerve pass out of the cranium
over the pedicle of the suspensorium. The facial nerve
comes oflf from the same ganglion, cui*ves round the
fore face of the capsule, and passes to its distribution
behind the suspensorium. The auditory nerve enters the
otic mass below the junction of the anterior and posterior

Fig. 36.


Tadpole one inch long ; side view of skull dissected.

Au, anditoiy capsule ; e, eyeball; eih. ethmoidal region; ctr, comn
trabeculaB; p. pedicle; o. primary otic process of suspensorium; or.T^
orbitar process ;, palatopterygoid bar ; mck, meckelian cartilage ;
labial cartilages are indistinctly visible ; hy. hyoid bar ; 6r. 1, first bran-
chial arch ; the epibranchial connective of the arches is seen ; st, stapes ;
op. opercular opening.

342. The mandibular arch has become highly com-
plex. The suspensorium has increased in length nearly as
much as the trabeculae, and is roughly parallel with the
axis of the skull. But its hinder extremity is curved
inwards and upwards, giving oflf two forks. The anterior
and longer, the pedicle (Fig. 35, p.), is confluent with the
cranial base and side wall just in front of the auditory cap-
sule ; the posterior process (o) is closely applied to the antero-
extemal and upper face of the same capsule, arching over
the facial nerve. Between the capsule and the two forks
of the suspensorium there is a hourglass-shaped mem-
branous space.

343. The paracranial part of the suspensorium is
more than half its length, extending from the pedicle to

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the palatine connective. The suspeusorium is at a lower
level than the cranial floor, and consequently the membra-
nous svbocular fenestra (s.o.f.) is rather obliquely placed.
The eyeball lies above it, and is well seen on the upper sur-
face of the head: the temporal muscle is attached to much
of the surrounding cartili^, and covered antero-extemally
by the orbitar process {or, p.), which has become large,
elevated, and incurved over the side of the face. The
palatine bar remains a small narrow pedicle, but it also
developes on its posterior edge a small backwardly-growing
lamina of cartilage, somewhat like the orbitar process.

344*. In front of the palatine region the quadrate end
of the suspensorium is short and stout, articulating with
the meckelian rod, which is still short, and directed almost
vertically upwards and inwards, so as to come very close
to the trabecular comu when the mouth is closed. Its
proximal end is deeply and roundly notched to hinge upon
the quadrate; and it is joined to its fellow by fibrous

345. These meckelian rods do not appear to be of
much functional importance as yet : their direction alone
indicates this. The fore part of both jaws is occupied by
the upper and lower labials. The upper pair are almost
vertical falcate flaps of solid hyaline cartilage, thick and
heart-shaped in section, the upper thick edge being grooved
to embrace the decurved end of the trabecular horn. The
inferior labials are drumstick-shaped rods, with the lower
end stoutest, and the upper end attached to the anterior
face of the meckelian bar near the symphysis. They nearly
meet below.

346. The oral opening is very small, nearly vertical,
and suctorial : but a homy dentigerous plate is developed
in the front of each jaw between the labial cartilages
and the skin. The upper plate is behind the corre-
sponding labials ; the lower is below rather than behind
them. The creases of the lips are also covered with small
hooked teeth, in addition to the two principal plates.


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Somewhat behind the mouth the oral cavity widens, and
has lateral angles, as well as a deep angular fossa between
the lower labials. Further backwards, the section of the
cavity is simplified, becoming wider and less deep, with
its lateral angles downwardly produced between the
meckelian and the quadrate cartilages.

347. The hyoid arch (Fig. 35, hy.) remains completely
below the level of the suspensoriura: each moiety is a broad
and massive plate, roughly four-sided, scooped within by
an antero-inferior and a postero-superior fossa. At its most
elevated (median) part it articulates by a shallow cup-
and-l)all joint with the under surface of the suspensorium
beneath the hinder edge of the orbitar process. Its blunt
antero-inferior end articulates with the small pisiform

348. The four branchial arches are at their full
development, and are greatly modified since the last
stage. They have all coalesced together both above and
below, by the formation of a continuous thin bar of
cartilage. From the epibranchial bar (opposite the
second branchial) there project two or three small
processes. The ventral connective articulates on either
side with the rounded first and the rudimentary second
basibranchial. This hypobranchial tract, behind the basi-
branchial, overlies the pericardium; and the arches are so
much curvl(i and thrown outwards in the side wall of the
throat, that the hypobranchial and epibranchial connec-
tives lie on nearly the same level ; and the arches, which
were once immediately beneath the skin and parallel with
the sides of the face, now hang like hammocks obliquely
across the throat. The first and fourth arches are baggy
and crumpled, and extremely thin. All are covered with
papillae transversely placed, bearing the very abundant
branchial tufts*,

349. We can now review this surpassingly interest-
ing stage. If the student will make a comparison between

1 For a full description of the branchial tnfts in Bana pipiens, see
Phil. Trans, 1871, p. 166.

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v.] THE frog: fourth stage. 149

the form here described and the skull of the adult Frog,
he cannot but be astonished at the contrast ; and he may
go where he will in the broad field of the morphology of
the skull, and returning with added knowledge and
developed thought, he will find himself capable of deeper
insight attended with intensified marvel in studying this
minute skull. The figures on Plates V. and VI. in the
Philosophical Transactions for 1871 will be found fruitful
if looked at again and again. A shallow cartilaginous
boat supports the brain, and united with it behind is a
pair of ball-like masses containing the organs of hearing.
Diverging horns in front of the brain-case support large
labial cartilages. The mouth proper is entirely in front
of the cranial cavity : there are rudiments of the future
upper and lower jaws, but placed most dissimilarly to
their ultimate directions: while large transitory labials
predominate among the oral tissues. The mouth is
suctorial, but also has a pair of transitory homy dentary
plates, with other denticles. The side of the face is
traversed by a large subocular bar, confluent with cranial
cartilage in front and behind, and from its forward edge a
lamina arises to roof over the temporal muscle* The hyoid
arch has a massive and strange form, appearing to play a
very subordinate part in the structure: and the gill-arches,
framed into a complex basket-work, bound together above
and below, bent almost double in the sides of the throat,
are at the summit of their ascending development, soon to
retrograde and become obsolete.

Fourth Stage: Tadpoles with tails disappearing.

350. In this sta^e we consider the changes taking
place in the skull during the gradual disappearance of the
tail Before the anterior limbs are seen externally, the
lower jaw becomes much elongated, and the suspensorium,
'not increasing in size so rapidly as other structures,
appears to be carried backwards and outwards, the palatine
bar being greatly lengthened. By the time that the fore
limbs are exposed, and the tail is reduced to half its

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original size, the brain-case is complete in cartilage.
When the tail has disappeared, most of the cranial bones
are manifest; the sphenethmoid or "girdle-bone" being
a remarkable exception : the creature has become far
removed from the ichthpc tjrpe, and in many respects
specially amphibian.

351. The cranial floor is composed of almost con-
tinuous cartilage, nearly uniform in level. The inter-
trabecular region is fully chondrified, but behind the
pituitary body a triangular median fenestra remains, with
the base directed forwards; it appears to be partly a
remnant of the intertrabecular space, partly of the fissure
between the parachordals which the notochord no longer
occupies. In the basioccipital region the gelatinous re-
mainder of the notochord lies in a deep median groove
in the cartilage.

Fig. 36.

Tadpole with tail beginning to shrink; side view of skoU without
branchial arches.

n.c, notochord ; au, auditory capsule ; between it and etk. the low
cranial side wall is seen ; eth, ethmoidal region ; st. stapes ; 5, trigeminal
foramen ; 2, optic foram«n ; oL olfactory capsules, both seen owing to
slight tilting of the skull ; c. tr. comu trabeculas : u,l. upper labial, in
outline ; tu, suspensorium ; pd. its pedicle ; otpr. its otic process ; or.p.
its orbitar process; t.m, temporal muscle, indicated by dotted lines,
passing beneath the orbitar process; papg. palatopteiygoid bar; mh.
meckelian cartilage; I. Z. lower labial, in outline; c.h, ceratohyal; b.h.
basihyal. The upper outline of the head is shown by dotted lines.

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352. The side walls of the cranium are complete, and
confluent with the floor and with the auditory capsules ;
above they form a slight intumed selvedge in the roof.
The fore end of the skull is gently concave, pierced on
either side of the middle line by the olfactory nerves. The
optic nerve passes out in the side wall not far in advance
of the auditory capsules. The supraoccipitaJ cartilaginous
roof extends as far as to the fore end of the ear-capsules,
but is deficient on either side in its anterior half, leaving
two oval parietal fontanelles. The cranium and the peri-
otic cartilages constitute one broad continuous mass. The
cranial roof is membranous in its anterior two-thirds, form-
ing an oblong fontanelle.

353. In the earliest period of this stage the trabecular
cornua remain distinct, although the hinder commissural
(ethmoidal) region is more extensive than it was. The
comua become broader and more flattened, approaching
the middle line ; the nasal sacs gradually come to lie upon
them. Anteriorly the sacs are separated by soft tissue,
but behind there is a cartilaginous vertical plate, the
rudimentary nasal septum, growing from the thickening
ethmoidal region in front of the brain-case. The ethmoidal
cartilage passes directly into the palatine connective in
the antorbital region (Fig. 36).

354. Later, the septum, the comua, and the eth-
moidal region become perfectly confluent, and the original
trabecular moieties can only be distinguished antero-
externally where the extremities of the comua project
inwards as small rhinal processes from the front and lower
wall of the nasal capsule. The latter is almost completely
invested by cartilage : the septum has grown forwards to
the front of the face ; the floor is formed by the flattened
comua ; and a continuous aliseptal roof grows outwards
from the upper edge of the septum and the ethmoidal
region, curving downwards into the external nasal wall.

S55. The extemal nostrils are on the upper surface
of the face, very near its fore end. They are bounded in

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front by a distinct but small prorhinal cartilage, derived
from the upper labial, the lower part of which is sepa-
rated, and covered by the premaxillary ; while the aliseptal
lamina curves round behind, and bounds them infero-
externally. In this, external boundary an ectosteal rhinal
bone arises, "^vhich extends in the outer wall down to the
nasal floor* The roof of the cavity is mostly behind the
external nostril, forming an alinasal pouch directed back-
wards and outwards, distinct from the ethmoidal cranial
wall and the palatine bar. The cartilage curves down-
wards infero-externally as a falcate process to bound the
inferior or internal nostril (which is more posterior than
the external), becoming continuous with the main part of
the nasal floor. The internal nostrils are separated by a
comparatively wide basinasal plate.

356* In previous stages we have seen the mandibular
suspensorium as a subocular bar parallel with the crauial
wall, and connected with it in front by a short palatine
pedicle. The latter gets gradually elongated backwards
and outwards, so that the original subocular bar is at
length situated entirely behind the eyeball : first passing
through a stage in which it forms a V with the palatine,
the angle being subocular (Fig. 36). Finally, the palatine
(or palatopterygoid as it may now be called) itself becomes
a subocular bar, nearly parallel with the cranial wall, but
descending to a lower level behind. The suspensorium,
considerably shorter than the palatopterygoid, is directed
forwards, outwards, and downwards, at an angle of about
50^ with the cranial axis (Fig. 37).

357. The anterior or palatine part of the subocular
bar remains connected with the lateral ethmoidal region
of the skull by a short transverse process beneath the
alinasal cartilage; it also gives off antero-externally a short
prepalatine spur. The palatine region soon becomes
marked off from the pterygoid by the appearance of con-
striction at the anterior third of the bar. Posteriorly the
pterygoid is continuous with the suspensorium at the
junction of its middle and lower thirds.

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358. The orbitar process, formerly so large, does not
advance ia size, as the suspensorium increases. The latter
is lengthened in front of the orbitar process, which con-
sequently, instead of being anterior to the eye, comes to
be placed quite behind it, covering part of the hinder
surface of the temporal muscle (Fig. 36, or. p,). It finally
becomes an inconspicuous thin lamina on the fore edge
of the suspensorium.

359. The upper or metapterygoid region of the sus-
pensory cartilage is more and more differentiated into
regions, though its size diminishes relatively to the qua-
drate or lower tract. The attachment to the cranial floor
remains, but the primary posterior process is first enlarged
as a triradiate lamina, and then segmented off, to be trans-
formed into the tympanic annulics in a later stage (a. t
Fig. 38). The upper posterior angle from which this
cartilage has been cut off forms a rounded otic process
which is apposed to the outer half of the front part of
the auditory capsule. The hinder edge of the main part
of the suspensorium becomes thin and shell-like, convex
internally and concave externally.

360. The distal extremity of the suspensiorum (su.) is
concavo-convex, fitting accurately upon the condyle of the
greatly elongated sigmoid mandible (mk.). The latter is
thicker behind, and pointed in front; its course is forwards
and inwards, subparallel with the cranial floor. The lower
labial becomes identified with the symphysial end of the
meckelian cartilage. The facial nerve passes out of the
cranium in front of the periotic cartilage, and runs
downwards and forwards on the inside of the posterior
margin of the suspensorium, where it divides into a
chorda tympani or mandibular branch and two lesser
hyoid branches.

361. The hyoid arch loses its lozenge shape, and elon-
gates, articulating with the middle and upper part of the
hinder edge of the suspensorium instead of near its distal
extremity. It is expanded above and below, and constricted
in the middle. ' Furthermore, it coalesces early with the

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soft azygous basihyal ; so that the arches of opposite sides
are continuous below. Confluence also takes place between
the basihyal and the basibranchial, thus fusing the hyoid
and the branchials into one hyobranchial system. The
hypobranchial regions have also lost their distinction in
this broad medio-ventral plate of cartilage. The first and
fourth branchial arches lose their flattened expanded form,
and all become very slender (Fig. 37). But each epibranchial
connective sends upwards a series of six tooth-like lobes.
Later, these same spurs degenerate and pass into fibrous
tissue ; while a process developed from the hypobranchial
region of the first and second arches elongates, diverging
from the middle line.

362. Ossification is just commencing in the fibrous
tissues of the cranial roof when the tail begins to dimi-
nish in size : it advances very far before the end of this
stage. Bones aflfecting the cartilaginous cranium arise
some weeks later than the earliest membrane bones, but

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 13 of 31)