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by means of grooves on the lateral surface of the body,
above the visceral clefts ; and from the region of each arch
there projects backwards a small claw-shaped bud. These
buds lie very obliquely one behind the other, so that the
second is higher up than the first, and the third than the
second. Seen from above they are trilobate; but from
below it appears that the proximal lobes form a sort of
sheath to the median terminal lobe, so that the gill-buds
bear some resemblance to the ungual phalanx of a cat»

224. The notochord extends forwards to the upper
and posterior region of the pituitary body, tapering to a
rounded extremity, after curving a little downwards. It
underlies only the hinder two-thirds of the third vesicle; the
anterior third of that vesicle rests on a fold of membrane
which is evidently a continuation of the sheath of the noto-
chord (middle trabecula of Bathke). This membrane, where
it lies upon the infundibulum immediately behind the
second vesicle, is sharply folded upon itself to form a lower
layer, which is reflected backwards on the infundibulum and
pituitary body, and then downwards around the latter so
as to invest it entirely behind. At present there is no dis-
tiDct indication of the primary skeletal elements of the
cranium.

225. Within the rib-like elevations of the mandibular
and hyoid arches, and in the intervals between the visceral
clefts, the future visceral rods are already marked out, but
have only commenced to chondrify. Thus the rudiments
of five visceral arches are indicated, and a median section
shows the heart lying below the two hindermost of
them. Behind the heart and these arches the whole body
is full of yelk ; but above the section of the rods, a con-
siderable visceral or pharyngeal cavity exists, in the wall
of which the visceral clefts are seen. Anteriorly, above
the rudimentary mouth, and behind the pituitary body,
a considerable mass of cells still obstructs the alimentary



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94 MORPHOLOGY OF THE SKULL. [CHAP.

passage ; but where the mandibular arch is in contact with
the nasofrontal lip there is a fissure, which is all that at
present represents the mouth ; this slit however does not
yet communicate with the visceral cavity.

226. In this embryo we have, as it were, the complete
soft model of the parts to which the embryonic skeleton is
to be related. The cranial cavity is bounded by unsolidi-
fied layers of cells, and a distinct membranous cranium
can hardly be said to exist. Of definitely formed axial
structures we have only the notochord present: while the
more advanced visceral arches, five in number, represent
the ventral series of skeletal parts.

Second stage ; Larval Axohtlsjust after hatching,

227. The little creature has not undergone any very
striking external change, having increased in size about one-
fourth. The general shape of the head is more rounded
and even, the cerebral vesicles protruding less markedly
than before. Looking from above, th^ third vesicle oc-
cupies only about half the length of the head ; the second
vesicle extending over the remainder of the upper surface.
The first vesicle, though still inferior, is at the anterior
extremity of the head, immediately below the second.
The eyes are now just intermediate in position between
the nasal and auditory sacs, separated from them by a short
interval. Each eye is almost a complete globe, still show-
ing a narrow stalk below. The nasal folds are much
nearer the front wall of the face, and are more complete ;
the cells of the crater are beginning to be absorbed to form
the nasal opening. The auditory masses are globular, but a
pit remains on their supero-lateral surface.

228. The oral cavity is perfect, and communicates
with the pharjmx or cavity enclosed by the visceral arches.
It is surrounded by well-formed lips that grow from the
nasofrontal process in front, the mandibular arch behind^
and from the lateral buds described in the last stage
(§221, p. 92), which are the maxillopalatine rudiments.



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IV.] THE axolotl: second stage. 95

These lateral flaps are very considerable, overlapping the
mandibular arch; they bear a strong resemblance to
opercular folds. No cleft can be discovered between the
mandibular and hyoid arches.

Fig. 22.



Head of Axolotl, just after hatching, side yiew.

C 1, 2, 3, cerebral vesicles ; ol. Dasal sacs ; e, eyeball ; an. anditory

mass ; m. mouth ; mn. mandibular, hy, hjoid arch ; op. opercular fold ;

e,br. points to the base of the first branchial bud. The maxillopalatine

flap is discernible in front of the point to which the line from m extends.

229. The opercular fold (op.) on each lateral inferior
angle of the hyoid arch has become very distinct from the
solidified arch within. It has increased greatly in size and
covers the roots of the trilobate external gills ; moreover
it is continued upwards and backwards by growths in the
undefined upper regions of the first and secoud gill-arches.
The trilobate gill of each of the three arches behind the
hyoid has become twice as long as in the last stage, the
median finger-like lobe retaining its preeminence; but
there are no new lobes or filaments. The relative positions
of the gill-rudiments are unchanged ; the visceral clefts
are more perfect, but they only ascend as far as the con-
tiguous filament, without passing beyond.

230. The following description of skeletal structures
is given from specimens slightly above one-third of an inch
in length (four and a half lines) \ When the cranial roof
and the whole of the brain are removed, the notochord is
seen, of great relative size, ending immediately behind the
pituitary body. On either side of the notochord, in the

1 An idea of the structures at this stage may be gathered from Fig. 23,
which represents a more advanced condition.



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96 MORPHOLOGY OF THE SKULL. [CHAP.

region of the third vesicle, are the rudiments of two seg-
mental muscular masses one behind the other, indicating
the future myotomes. Outside the first muscular segment
lies the auditory capsule, having as yet no chondrified
walls, but showing a number of shining otolithic crystals
in its interior. The semicircular canals are not yet differ-
entiated.

231. Two aggregations of nervous cells pling to the
inner wall of the ear-sac. The anterior, parallel with the
apex of the notochord and extending in front, is the first
appearance of the Gasserian ganglion : the other is at the
side of the first muscular mass and its junction with the
second ; it will become the ganglion of the glossopharyn-
geal nerve. Behind these, and parallel with the second
muscular mass, is a small ball of tissue from which the
thymus gland is developed. There is no indication of car-
tilage at the base of the cranium posteriorly.

232. The trabeculsB are distinct, though small rela-
tively to the notochord. At present they are not cartila-
ginous, but consist of very dense granular tissue. They
closely embrace the fore end of the notochord laterally,
but do not meet beyond its apex. In front of the
notochord the trabeculae diverge outwards and forwards in
a crescentic curve. Each trabecula constitutes the lateral
third of a circle whose anterior (transverse) third is defi-
cient. Within and upon this circle lie not merely the
pituitary body and infundibulum which are relatively
much smaller than in the preceding stage, but also the
basal part of the first cerebral vesicle. Thus the primary
relation of the trabeculae is to the sides and base of the
forebrain.

233. The visceral arches are already chondrified,
although very delicate. They are all continuous across the
middle line below, except the mandibular; but the median
connecting substance is not as yet true cartilage. In this
and in several succeeding stages there is no cartilage in the
maxillopalatine process.



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IV.] THE AXOLOTL: SECOND STAGE. 97

234. The mandibular arch consists of two pieces on
each side; an upper, broad and three-sided, lying on the
lateral aspect of the iauce between the eye and the ear ;
and a lower sigmoid bar, the axis of the mandible. The
upper or suspensory portion is not cartilage in this stage,
but is formed of somewhat solid indifferent tissue like the
trabeculae. The apex of this segment is growing towards
the anterior third of the corresponding trabecular bar, but
is at some distance from it and on a rather higher level.
The anterior end pf the Gasserian ganglion is wedged
in between the hinder margin of the suspensorium and
the antero-intemal face of the auditory sac.

235. The sigmoid mandibular or meckelian cartilage
is expanded at its upper end, and has a bevelled face m
relation to the antero-extemal angle of the three-cornered
suspensorium. The distal ends of the meckelian cartilages
are separated by a considerable space at the chin: they lie
above and behind the lower lip. In relation to the inner
face of each of these cartilages at their middle is an oval
mass of large globular cells. This is the stroma which
in a few days will develope into the splenial bone with its
teeth.

236. The succeeding arches, five in number, are not
markedly distinguished from each other morphologically.
They are closely connected in the middle line by granular
tissue. The hyoid arch is shorter than the first branchial,
and does not reach so high ; the branchial arches decrease
in size from before backwards, and each arch extends
higher up laterally than its successor. The hyoid is cres-
centic, and convex forwards: the branchial arches, especially
the first three, are sinuous. Each branchial arch except
the last bears a trilobate gill.

237. The skeletal elements here described consist of
the trabeculae, paired mandibular arches, each segmented
into two pieces, and simple hyoid and (4) branchial arches.
The relations of the trabeculae to the front end of the noto-
chord and to the sides and floor of the forebrain, are par-
ticularly to be noted, together with the position of the



B. M.



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98 MORPHOLOGY OF THE SKULL. [CHAP^

mandibular suspensorium with regard to the trabeculse.
The appearance of muscular segments in the floor of the
hindbrain is of considerable interest.

Third Stage ; Larval Axolotls, from Jive to Jive and a half
lines in length,

238. The head has acquired its permanent perenni-
branchiate shape, being broad and flat; the cerebral
vesicles lie nearly on the same plane, and are but little
distinguishable externally. The mouth is wide and gaping,
the lower jaw having increased in length very greatly, so
as to project in front of the rest of the head. The sense
organs are rapidly becoming perfect. From being tri-
lobate the gills pass to a pinnate form by the growth of
lateral basal filaments.

239. The hinder part of the cranial notochord is
still unflanked by cartilage ; and the muscular segments
(Fig. 23, m. s.) are perfect and functional, the anterior
pair being attached to the posterior pointed ends of the
trabeculae. The auditory capsules are elegantly ovoidal,
having become cartilaginous in their floor and external
surface. The roof is unchondrified, and the semicircular
canals are not developed sufficiently to modify the exter-
nal shape of the otic mass.

240. The trabeculse (tr,) have become chondrified,
and very greatly developed. They are distinguished into
two regions, an anterior prechordal at right angles to a
posterior parachordal. The parachordal portion is thick
but flattened, and has grown back in a pointed form
at the side of the notochord, closely applied to its sheath.
The posterior extremity of this trabecular growth is de-
fined by a line joining the middle of each auditory mass
and passing through the middle of the first muscular seg-
ment and of the cranial notochord. The trabeculae do not
meet in front of the apex of the notochord. .

241. The anterior portion of each trabecula, seen from
below, appears as a rounded rod, passing directly forwards,



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IV.] THE AXOLOTL : THIRD STAGE. 99

but curved a little inwards : and the right and left bars
are widely separate from each other anteriorly, by a dis-
tance equal to the length of each prechordal part. Thus

Fig. 23.



Larval Axolotl, aboat five lines long ; upper yiew of skull, dissected.

nc. uotochord : py. pituitary body ; m, s. muscular segments ; tm. //.
thymus gland ; au. auditory capsule ; tr. trabecula, extending backwards
by the side of the notochord (the line points to part of the trabecular
crest) ; ol. nasal sac ; 5. Gasseria-i ganglion ; mk. meckelian cartilage ;
g. quadrate end, a. p. ascending process, ot. p. otic process, of thts bus-
peusorium ; pd, its pedicle, which is at a lower kvel than a.p.

Bones : pa, palatine ; v, vomer ; px, premaxillary ; d, dentary ; sp,
spleniaL

the trabeculfle altogether form three sides of a somewhat
square space, the fourth (anterior) side being deficient.

242. Viewing the trabeculae from above, a thick crest
is seen to be growing from the hinder three-fourths of
each prechordal tract. It is highest opposite the apex af
the notochord, and then suddenly diminishes behind. The
height of the crest is equal to the distance between itself
and the notochord. It is gently concave inwardly, and
sinuously convex on the outer surface, the sinuosity arising
from the fact that the temporal muscle has its origin from
a depression upon it behind the eye. The muscle extends
downwards, outwards, and fcrwards to be attached to the
proximal part of the mandible*



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100 MORPHOLOGY OF THE SKULL. [CHAP.

243. The mandibular arch in both its divisions is
now well chondrified. The suspensory cartilage, broad
above, narrower and more rounded below, has receded in
relation to the trabeculse ; its upper extremity grows to-
wards the trabecular crest, parallel with the apex of the
notochord; but it is still some distance from it The
postero-extemal part of the suspensorium is produced
behind this apex; it is concave, and is applied to the
convex antero-extemal face of the auditory cartilage. This
is the otic process (otp,), afterwards of so much import-
ance. The distal part of the suspensorium is directed for-
wards and a little outwards, reaching to the middle of the
prechordal rods. The articular condyle is a slight simple
convexity, fitting into a corresponding concavity on the
proximal end of the meckelian rod (mk). The latter is
gently arcuate, gradually tapering forwards, and ending at
a little distance from its fellow in the projecting chin.

Fig. 24.



Larval Axolotl, about five lines long; transverse vertical section of
head, through eyeballs.
C 2, midbrain; tr. trabecula; e, eyeball; pa, palatine tooth; m.
mouth ; mk, meckelian cartilage ; sp. splenial tooth.

244. In the mouth several pairs of dentigerous osseous
nuclei have appeared, arising parosteally in definite rela-
tion to the cartilaginous rods. The foremost of these are
above and behind the upper lip, and a little in front
and to the inner side of the nasal sacs; these are the
rudiments of the premaarillaries (Fig. 23, px,), and each
carries two denticles. The nasal sacs are becoming per-
forated, to open into the roof of the mouth.



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IV.] THE AXOLOTL: THIRD STAGE. 101

245. Behind and internal to the nasal sacs, and im-
mediately in front of the anterior termination of the tra-
beculae, is another pair of bony patches ; and yet further
behind these, adherent to the under surface of the rounded
ends of the trabeculse, is a third pair. These little patches
bear a number of prickly denticles, very much resembling
the groups of teeth found on the inner edges of the
branchial arches of osseous fishes, constituting their strain-
ing apparatus. The foremost pair of these patches are
the rudiments of the vomers {v.) ; the hinder, of the pala-
tines {pa,).

This stage shows the vomers in their interesting normal
relation to the primaiy inferior nares. Each of these opeuings
has one of the vomers clirectly internal to it.

• 246. The mandible has already two delicate denti-
gerous laminae of boue on each ramus. The foremost of
these, the dentary (d.), is on the outside of the distal half
of the meckelian cartilage, and even • projects somewhat
beyond it towards the middle line of the chin. The middle
of the inner face of the cartilage is covered by a shorter
and broader patch bristling with teeth, and very similar to
the vomers and palatines ; this is the splenial (sp,).

The dentigerous splenial, appearing very early in develop-
ment, is characteristic of the Urodela : the Anura, as a rule
at auy rate, have no spleuial. In this minute Axolotl, five
lines and a half in length, hatched about a week, there are no
fewer than ten bones distinguishable in relation to the mouth.

In Axolotls two or three days younger and half a line
shorter, the meckelian cartilage is not nearly so long, and in
cvtrved downwards in front. It then has only one dentigerous
plate, the splenial, bearing a few teeth ; but this splenial is on
the inside of the fore part of the cartilage. The region in
relation to which the dentary is soon develoi)ed is formed by
the elongation of the cartilage in front of the splenial.

247. The succeeding visceral arches are becoming
segmented. No part of the hyoid arch is developed into a
counterpart of the suspensory portion of the mandibular :
but the distal or ventral end has a short segment cut off



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102 MORPHOLOGY OF THE SKULL. [CHAP.

frpm it ; this is the hypohyal, and the main bar is the '
ceratohyal. The rounded upper end of the ceratohyal arr
ticulates with a scooped surface on the hinder margin of the
lower part of the suspensoriura ; and it extends no farther
upwards towards the auditory capsule.

This is a constaDt relation of the two arches in the larvse
of the Amphibia, which also have no upper hyoidean element
corresponding with the hyomandibular of Fishes in their larval
condition.

248. The two large anterior branchial arches segment
into two pieces each, the upper being twice the size of the
lower. The superior is the epibranchial element (with no
pharyngobranchial above it) ; the ipferior is the cerato^
branchial (without a hypobranchial below it). The third
and fourth branchial arches are simple ; and the median
azygous branchial elements are not yet developed.

249. In this stage the trabeculae, with the partially
chondrified auditory capsules, remain the only cartila-
ginous elements of the cranium. The trabeculae possess a
definite parachordal part, and send up lateral crests in
front of the ear masses, constituting a rudimentary sphe-
noidal wall. The mandibular suspensorium is related to
the hinder part of this wall and to the otic cartilage. The
hyoid and branchial arches are segmented. Bony par-
osteal centres have arisen in the mouth, all of them denti-
gerous : three pairs, the premaxillaries, vomers, and pala-
tines, belong to the palate : two pairs, the dentaries and
splenials, to the lower jaw. The denticles appear almost
before any true osseous deposit begins in the membrane.

Fourth Stage : Young Axoloth three-quarters of an inch in

length.

250* With very little change in outward appearance,
there has been great progress in skull-building. The pos-
terior half of the cranial notochord has now a distinct
tract of cartilage investing it on either side (Fig. 25,



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IV.] THE axolotl: fourth stage, 103

pa. ch). These are arcuate bands, pointed in frcmt where
they lie between the notochord and the auditory capsules;
and broader posteriorly where they curve outwards behind
the ampullae of the posterior semicircular canals. These
cartilages form the base of the occipital region of the
skull, but the occipital roof and side walls are still mem-
branous.

251. The anterior half of the cranial notochord is
closely invested by the parachordal portions of the tra-
beculae, which extend backwards to and even overlap the
occipital parachordals. The trabeculae have met just in
front of the notochord, and the apex of the latter becomes
more and more perfectly ensheathed by trabecular car-
tilage. Furthermore, the apex of the notochordal sheath
becomes distinctly ossified.

252. The anterior portions of the trabeculsB enclose
an elegant ovoid space in the cranial floor, the broad end
of the ovoid being behind. They are connected in front
by a considerable transverse tract of cartilage. This ante-
rior plate presents three regions: a median intemasal
{{, n. c), and a pair of lateral horns (c. tr,), extending out-
wards and curving somewhat backwards. The median
part projects farthest forwards, and has a convex margin
anteriorly. The lateral horns half surround the anterior
and inner face of the nasal sacs below. At present there
is no chondrification of the nasal walls. With the elonga-
tion of the trabeculae, their longitudinal crests have in-
creased in height and length, forming considerable lateral
cranial walls. Close to their hinder termination the
ascending region of each suspensorial cartilage (a. p) has
coalesced with the corresponding crest near its upper
edge.

253. The auditory masses are considerably modified
by the greatly altered condition of the enclosed laby-
rinths, and the bulgings of the semicircular canals. The
whole of the periotic wall is cartilaginous, but on the
lower surface there is a large crescentic cleft towards the



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104 MORPHOLOGY OF THE SKULL. [CHAP.

outer side. Mesiad of the fissure itself, the cartilage be-
comes extremely irregular and partially deficient, and

Fig. 25.



Larval Axolotl, three-quarters of an inch long ; upper view of basis
cranii and lower jaw, dissected.

nc, notochord; pa, eh. parachordal cartilage; ocx, occipital condyle;
tr, trabecula, pointing to its crest ; i.n.c, intemasal cartilage; c.tr. comu
trabeculaB; ol, olfactory sac; a,8C., h.sc.^ p.sc, anterior, horizontal, and
posterior semicircular canals ; mk. meckelian cartilage ; q. quadrate
condyle; otp, otic process; a,p. ascending process united with trabecular
crest ; under it the dotted lines represent the anterior branch of the
trigeminal nerve, 5'; 7', anterior branch of facial nerve; 5, Gasseriau
ganglion ; 7, main facial nerve.

Bones : px, premaxillary ; p.pg» palatopterygoid ; sq, squamosal ; ar,
articular ; sp. splenial ; d, dentary.

through its hinder part a massive otolith is visible. This
cleft is the first appearance of the fenestra ovalis.

254. The articular extremity of the suspensorium
projects further outwards than in the last stage. The
ascending process, as we have seen, has coalesced with the
trabecular crest. The postero-external or otic region has
developed a pedate otic process (pt p,)y which adheres to
the auditory wall outside the ampullae of the anterior and
horizontal semicircular canals. On the outer surface of the
suspensorium a thin rudiment of the squamosal parostosis
has appeared (sq,).

255. The meckelian rod (mk.) is twice as thick be-
liind as in front; but its posterior extremity is narrowed



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IV.] THE AXOLOTL: FOURTH STAGE. 105

to a rounded angle receiving the articular condyle. The
two halves of the mandible are closely approximated in
front; the dentary and splenial bones are much more
extensive than they were, and very largely embrace the
anterior part of the cartilage. On the inner face of the
posterior part of the mandible a thin non-dentigerous splint
of bone has appeared ; this is the rudiment of the arti-
cular (an).

256. There is one new bone in the roof of the mouth;
but those which existed before are greatly moklified. The
premaxillaries are pedate in front, with many teeth, and
have long sigmoid nasal processes. These grow back-
wards, over the junction of the internasal plate with each
comu, and extend for some distance over the frontal region
of the cranium*. They overlap the anterior ends gf a pair
of filmy bones, the frontals (/.), in the fore part of the
cranial roof, and these in their turn overlap a similar pair
behind, the parietals (Fig. 27, p,).

2o7. The vomers have increased in size, and are now
separated by a little distance from the palatines. The
latter have broad outwardly-extended dentigerous plates ;
and from each of their postero-extemal angles there pro-
jects a long flattened iagged handle, directed somewhat
backwards and outwards beneath the eyeball, to reach the
suspensorium a little behind the articular condyle. Thus
the bone has become a palatopterygoid (P'Pg)* the new



Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 9 of 31)