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(Fig. 10, py,) projecting backwards from its postero-inferior
surface. There is no mesocephaUc flexure of the neural
tube. A little behind the pituitary body is the pointed
anterior end of the notochord (n. c), lying just below the
middle of the second cerebral vesicle. There is no clear
distinction between the outer layer of epiblast and that
which limits the .neural tube.

119. Almost at the anterior extremity of the embryo,
on its under surface, the olfactory sacs appear as small pits
surrounded by a circular ridge of epiblast; the extreme
open end of the neural tube is immediately above them.



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44 MORPHOLOGY OF THE SKULL. [CHAP.

The involutions for the eyeballs are much larger, and un-
closed ; they are behind the olfactory sacs, and lie quite

Fig. 9.



Embryo Salilion, about J inch long; side view of head within
chorion.

ch, chorion ; oL olfactory sac ; e, eyeball ; au. auditory mass ;
g.o.b, supraorbital band; m.v. attachment of umbilical vesicle; cL la-
chrymal cleft ; V. 1, 2, 3, 4, 6, visceral arches ; mn, first or mandibular
arch ; cl, 1, 2^ 3, 4, 5, visceral clefts.

on the inferior surface. The rudiments of the ear-sacs
(au.) are by the sides of the third cerebral vesicle, and are
consequently more dorsal than and posterior to the eye-
balls ; their line of involution is open, and longitudinally
placed. So large are the eye- and ear-sacs relatively to
the skull, that they somewhat overlap one another.

1 20. The side wall of the head arching over the eye-
ball is conspicuously thickened, forming the supraorbital
band (Fig. 9, s. o. b). Below and behind the eyeball, is a
thickened suborbital arch, which extends forwards to the
olfactory sacs of the same side. Between the anterior
extremities of these arches is a space lined with thickened
epiblast, the future mouth-cavity (Fig. 11, m.). This is
bounded in the middle line and behind by the meeting of
O-nother pair of arches, the foremost of the visceral series.



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III.] THE SALMON: FIRST STAGE. 45

The latter curve forwards to the middle line below the
auditory region, the successive arches having between them
slits or clefts (cL), which penetrate into the primitive ali-
mentary canal. It is only at a later period that the mouth
has a communication with the same cavity. The umbilical
sac is attached in front to the head, in the space between
the hindermost visceral arches; the heart (Fig. 11, A.) is
situated on the wall of the fore part of the alimentary canal,
above or within the same region.

Fig. 10.



Embryo Salmon, about | inch long ; npper view of head, dissected,
the neural tissue having been removed.
tr, trabecnla; pa.ch, parachordal cartilage; n.c, notochord; au,
auditory capsule, showing dilatations for semicircular canals ; py, pituitaiy
body; e. eyeball; mn. mandibular, hy, hyoid, br. branchial, arches.
The anterior aperture remaining in the neural tube is shown in the skin
in front of the trabecule.

121. The notochord ends pointedly at about the level
of the front end of the ear-sacs, behind the pituitary body.
On either side of it are parachordal bars {pa. ch,) cut off
squarely in front, a little behind the apex of the noto-
chord (Fig. 10). Laterally, the parachordals are flanked
by the auditory involutions, and by the mesoblastic in-
vestment which is forming a partial girdle of nascent car-
tilage round them. The inner part of the ear-cavity shows
the several dilatations for the semicircular canals (Fi^. 10).
The nascent cartilage does not appear in the floor or on



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46 MORPHOLOGY OF THE SKULL. [tJHAP.

the lower outer side of the sac below the line of invo-
lution. A little later, the auditory masses unite with
the parachordals at two points on either side, one anterior
and the other posterior ; thus a fenestra is formed in the
base of the skull (see Fig. 14, /. «. o).

122. The axial prechordal region of the cranial floor
is occupied by the trabecules (^r.), bowed outwards on either
side of the pituitary body. They are small and pointed
behind, where they curve towards the notochord, without
touching it; and more expanded in front, where they
approach one another, and lie under the fore part of
the cranial cavity, and in the roof of the mouth. The
intertrabecular space is not merely coextensive with the
pituitary body, for the trabeculae lie in the floor of the fore-
brain, and suppoil it on either side.

123. A palatine bar is distinctly formed in the sub-
ocular band ; it is simple and arcuate, reaching in front
to the nasal sac, and behind nearly to the auditory
capsule, where it overlaps the posterior extremity of the
supraorbital ridge. The palatine rod is from the first
clubbed at the fore end, and there solidifies earliest;
it passes back into a fine point behind, much later in
solidifying. The mandibular arch is at first curved but
slightly forwards below, and may be nearly transverse,
although embryos of the same age vary greatly in this
respect. Its uppec extremity curves inwards beneath the
fore part of the ear-sac, towards the apex of the noto-
chord. . It soon becomes distinguishable into three re-
gions ; (1) the metapterygoid apex, (2) a median swelling
directed forwards, the orbital process, and (3) the lower or
meckelian, elongated forwards as a spatulate expansion,
but not yet meeting its fellow (Fig. 13, mt,pg., or, p.,mk,).

124. The hyoid arch (hy.) has an enlarged and in-
curved apex, which lies underneath the middle of the ear-
sac. Its lower end becomes curved forwards behind the
mandibular region. The two hyoids do not meet, but are
before long separated only by a small median segment.



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III.] THE SALMON: FIRST STAGE. 47

the hasihyal (Fig. 13, g, h.). There are five pairs of
branchial rods (ir.) behind the hyoid, not meeting in the

Fig. 11.



Fl



Embryo Salmon, about | inch long ; lower yiew of head, with the

arches Rhining through.
Na. nasal sacs ; e, eyeball ; au. ear-mass ; tr. trabecnla ; pl.pt.
palatine or sabocular bar; mn, mandibular, hy. hyoid, br, branchial,
arches ; pty. pituitary body ; m, mouth ; h. heart.

middle line. They become progressively smaller from the
front to the hinder one, and are all sigmoid in shape
except the last, with their apices incurved over the cavity
of the throat.

125. Most of the primary elements of the skull are
present in this early embryo, which has not yet under-
gone the mesocephalic flexure. The parachordal and tra-
becular regions are definitely marked out, and the periotic
tissue is commencing to chondrify and to unite with the
parachordals. The palatine, mandibular, hyoid, and five
branchial bars, are manifest in a simple unsegmented
condition ; but the mandibular piece is already becoming
differentiated into regions.

126. In order to simplify the description, no reference
has been made to the remarkable asymmetry of Salmon
embryos, and the great variations in the degree and kind of
asymmetry. The head is usnally twisted about its axis so that
only one eyeball, which may be either right or left, i» visible



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48 HORPHOLOGY OF THE SKULL. [CHAP.

in an upper view. The rudimentary postoral arches (which
share the general asymmetry at first) are distinct before the
clefts between them are complete. The arches differ consider-
ably in their degree of development ; the earliest distinct are
the mandibular and hyoid bars, then the branchials, next the
trabeculse, and most imperfect, the palatines. The chondri-
fication of the latter is very tardy. They enclose a soft pith
even in the third stage. Witli this may be compared their
long supprei«&ion in the Frog.

Second Stage : The metamorphosis of the hyoid arch,

127. The series of changes now to be noticed takes
place in embryos while still within the egg, or only just
hatched. The neural tube is completely closed, the cere-
bral vesicles are greatly enlarged and very prominent, and
their growth has produced a mesocephalic flexure (see
Fig. 12). Thus the second cerebral vesicle is a large boss
on the upper surface of the head, while the first vesicle
is below and in front. Behind this is the eyeball, then the
visceral arches; but the mandibular arch is produced below
(Fig. 13, mn.), forming the prominent hinder boundary
of the mouth. The nasofrontal process (n.f, p.) has grown
wide between the nasal sacs, separating the suborbital
arches, so that the oral cavity is broad and quadrate.
An opercular skin-fold is passing backwards from the upper
part of the hyoid region. Dermal papillae, the rudiments
of the gills, are noticeable on all the branchial arches
except the last.

128. The notochord remains relatively in the same
position as at first ; but the parachordals are thicker in
front, and unite with the trabeculae. Behind, they extend
farther, and are pointed; they also tend to invest the
notochord more closely. Laterally, their union with the
ear-cartilages becomes more and more marked, diminish-
ing the fenestra in the line of union with the periotic
mass (Fig. 14). The ear-capsules get much more exten-
sively chondrified ; and at the line of junction between
them and the parachordals there is a thickening and



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m.]



THE salmon: second stage.



49



upward growth, which tends to form side walls and a roof
to the cranial cavity. The semicircular canals produce

Fig. 12.



"Z. *•'




M4



Embryo Salmon^ partly hatched ; median longitudinal section of head.

C 1, 2, 3, fore-, mid*, and hindbrain ; w, pituitary body ; n,c, note-
chord; tr. trabecula; m. mouth; mn. man<ubalar, hy, hyoid, 6r. bran-
chial arches.

bulgings in the ear-cartilage, and a small space remains
unchondrified over the posterior canal (epiotic fenestra),

129. The mesocephalic flexure causes the trabeculae
to run downwards as well as forwards ; anteriorly, how-
ever, they curve upwards again, below the fore end^
the cranial cavity (Fig. 12, tr,). Their hinder-hooked
extremities bend inwards more and more towards the
parachordals, and finally are completely fused with them.
The time of this union is very variable; sometimes it
occurs before hatching, in other cases it is not perfect
in the second week after that event.

130. In front of the pituitary region the trabeculse
become approximated along their whole length, and then
gradually Aised ; and at the same time a median longitu-
dinal ridge arises. This ridge increases in prominence
anteriorly, where the trabeculae lie in the nasofrontal,
process, and are laterally expanded, growing towards the
subocular or palatine arches ; thus the ethmopalatine
connection is being formed (Fig. 14). The fore extremi-

B. M. 4



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50 MORPHOLOGY OF THE SKULL. [CHAP.

ties of the trabeciilse develope a transverse ridge beneath,
in the roof of the mouth; above, they diverge from
the median crest, and form a flat spatulate lamina on
each side, partially overarching the olfactory sac. The
remainder of the cranial investment is membranous; para-
chordals, trabeculae, and auditory cartilages constitute the
whole of the chondrocranium*

Fig. 13. ^



P9

Ml.



Embryo Salmon, not long before hatching ; under view of head, with
arches seen through.
N.f.'p. nasofrontal process ; oZ. olfactory sac ; e. eyeball ; m, mouth ;
ir. trabecula ; 'p-'pg* palatine cartilage ; m, mandibular arch ; mk, mecke-
lian region ; g. quadrate region ; mt.pg. metapterygoid region ; or,p.
orbital process; g.h. glosso- or basihyal; h.h. hypohyal; c,h, oeratohyal;
h.m. hyomandibular.

131. The subocular or palatine bar remains little
developed in this stage, forming the lateral wall of the
mouth ; anteriorly it is in contact with the ethmopalatine
process of the trabecula. Behind, there is a cleft between
it and the eyeball, passing down into the palate. The
mandibular arch undergoes more modification; the inferior
meckelian part becomes bent at a large angle with the
proximal portion, and meets its fellow in the middle line
(Fig. 13). . .



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m.] THE SALMON: SECOND STAGE. 61

This flexure generally takes place forwards, but it may
be directed backwards, so that very diverse appearances may
be presented by the facial arches of different embryos, owing
to this cause and also to the asymmetry of the creatures ; but
the morphological result is the same.

132. The proximal region of the mandibular arch
becomes broadened, and an upper segment is cut off iu a
peculiar manner so as to form a ball-and-socket articu-
lation. The ball is on the upper, the socket in the
lower piece ; and the junction is defended by an angular
process of the latter, running up behind. Furthermore,
the whole arch travels considerably downwards from the
axial parts and becomes less closely attached to the cranial
wall. .

133. Before the segmentation of the mandibular car-
tilage, the most striking division takes place in the hyoid
arch. In addition to the median basihyal mentioned in
§ 124 (p. 46) a small globular piece, the hypohyal, is cut
off from the inferior end of each hyoid bar ; while the rest
of the arch is split into two along its entire length, so that
one bar lies directly behind the other and in contact with
it. The anterior of these is the hyoniandibular, the poste-
rior the ceratohyal (Fig. 13, hm., c.h).

134. At a later period, concurrently with the tra-
velling downwards of the mandibular arch from the cranial
mass, the hyomandibular diverges from the ceratohyal
below, remaining in contact with it above. The hyoman-
dibular being thus turned forwards, reaches the mandibular
arch, and becomes applied to its upper segment and the
proximal part of its lower segment (Fig. 14).

135. A further change of relation is subsequently
effected. As the throat increases in size, the hyoman-
dibular remains closely apposed to the auditory cartilage,
while the ceratohyal is found lower and lower in position,
as if it had slid down half way along the hinder edge
of the hyomandibular (see next stage. Fig. 17). At this
point the upper end of the ceratohyal becomes fixed by an

4—2

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52 MORPHOLOGY OF THE SKULL. [CHAP.

inferkyal ligament, in which cartilage is subsequently de-
veloped. The hyomandibular broadens above and developes

Fig.U.



Embryo Salmon, not long before hatching; lower view of skull dis-
sected, the branchial arches having been removed.
Tr, trabecnla, with transverse ridge in roof of palate ; p.pg. palatine
bar, near ethmopalatine conjunction ; py. pituitary space ; n.c, notoohord ;
pa.ch. parachordal cartilage which has coalesced with au. auditory
capsule, except where a fenestra /.8.o. is left ; mk. meckeUan cartilage ;
q. quadrate region ; mt.pg, metapterygoid region ; the line should be pro-
longed to the tract inside the meckeUan bar ; h.m, hyomandibular turned
forwards at its ventral end, and applied by its symplectic tract «y,' to the
quadrate region of the mandibular arch; chy, ceratohyal; ft./L hypo-
hyal ; gX basi- or glossohyal.

a backward spur; below, it remains styliform. The
mandibulo-hyoid cleft becomes gradually obliterated.

136. The branchial arches are not as yet segmented
like the arches in front. They fuse in the middle line
below, all but the last pair ; and then four small median
azygous ventral pieces, the basibranchials, are cut out. The
upper end ^of these arches curve inwards most elegantly
over the sides and roof of the throat. The hinder ^fifth)
branchial pair remains much smaller than the rest.



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in.] THE SALMON: THIRD STAGE. 53

137. The important period of development included
in this second stage witnesses many significant changes.
The mesocephalic flexure being fully established, the
trabeculse unite with the parachordals, and the latter, in
close union with the otic cartilages, form lateral and superior
occipital growths. The anterior portions of the trabecute
unite and send up a median crest, the rudiment of the
interorbital and nasal septa ; while lateral and anterior
trabecular growths establish the ethmopalatine and the
cornual regions. But the most intense iiiterest attaches to
the modifications by which the mandibular and hyoid arches
are segmented, and their segments take up the relative
positions essentially characteristic of Osseous Fishes. The
preponderance of growth in particular regions etfects all
that to outward observation looks like the shifting of parts.
It is, we hope, made plain how the mandibular elements,
removed to a distance from the axial parts, derive their
suspensormm from the arch behind, which is remarkably
segmented for that purpose. The formation of definite
hypo- and basibranchial pieces is to be noted at this period :
the principal branchials do not yet present any segmenta-
tion.

Third Stage: Salmon-fry of the second week after
hatching,

138. The head has become much larger, and the
mesocephalic flexure is lost ; a longitudinal section shows
the parts of the brain lying approximately in a straight
line on the flattened cranial floor. The well-developed
brain fills the proportionally large cranial cavity; the
auditory protuberances are very marked at the sides. The
articulation of the meckelian rod with the proximal man-
dibular element is still farther removed beneath the level
of the cranial floor, and the mandible has grown forwards
underneath the fore part of the head. No ossifications in
cartilage appear as yet; but parostoses* have arisen in

1 By parostosis is meant an ossification in subontaneons fibroas tissne ;
it may reach to and involve the perichondrium of a cartilaginous tract ;



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54 MORPHOLOGY OF THE SKULL. [CHAP.

more than one region, the basicranial splint, or para-
aphenoidy being especially noticeable. In many points there
are agreements between the cartilaginous cranium of the
Salmon at this stage and the cartilaginous and osseous
cranium of the adult Polypterus; see Traquair On the
Cranial Osteology of Polypterus [Joum. AnatYohv, p. 106).

139. The occipital region of the chondrocranium has
progressed, but is yet far from perfect. The notochord

Fig. 15.



Salmon fry, second week after hatching ; upper view of skuU dissected,
the brain being removed, and the inferior parts somewhat displayed.

Pa.ch. marks the anterior limit of the parachordal cartilage ; nc. noto-
chord, lying in the posterior basicranial fontanelle produced by the
recession of the parachordals ; o.wi. the orbital muscles diverging from
beneath the cranial floor in the notochordal region; so, supraoccipital
tract; a.sc, anterior, h.s.c. horizontal or external, p.s.c. posterior
semicircular canals ; tr. trabeculae ; t.cr. tegmen cranii, or cartilaginous
roof of the cranium; fo, superior fontanelle, heart-shaped; «.n. nasal
septum ; s.n.l. subnasal lamina ; mn. mesonasal cavity ; u.l. upper
labial cartilage; p.pg, palatopterygoid bar; q. quadrate condyle; mt.pg,
metapterygoid region of mandibular arch.

Buoh a bone may be denominated a membrane bone. Ectostosis sig-
nifies ossification arising between perichondrium and the superficial cells
of cartilage, which gradually becomes absorbed and replaced by bone.
Endostosis is the direct calcification of cartilage.



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nr.] THE SALMON: THIRD STAGE. 55

is retracted so as to occupy only the hinder half of the
postpituitary (basilar) region, and the parachordal masses
have receded from one another, leaving a considerable
oval space (posterior bdsicrarnal fantaneUe) between them
aDteriorly; while posteriorly they do not yet unite with
each oiher in any portion of their length, although they
invest the notochord closely behind (Fig. 15).

140. Laterally, the parachordals do not become broad,
as in the Dogfish and Skate ; they form a floor for the
hindbrain, and externally are fused with the periotic
cartilages. Nevertheless, there is a distinct notch in
front between the otic and parachordal elements ; and the
latter also project backwards behind the ear-cartilages.
Each of these hinder processes is divided into two lips ;
these are the primordial articulating surfaces of the
cranium with the first vertebra. Lateral occipital carti-
lage now exists, largely fused with the ear-capsules, but
rising free on the backward projections of the parachordals.
Above, they have passed inwards on each side, and coa-
lesced to form the supraoccipital plate {s.o. Fig. 15).

141. The ear-cartilages are not greatly modified ; the
semicircular canals are large proportionately, and may be
distinctly seen through the cartilage. On the under surface
the fenestra formerly mentioned (§ 121 p. 46) in the line of
junction between the periotic and the parachordal masses
is much smaller relatively, being carried outwards by
the latter growing underneath the former. The cartilage
beneath the bulging vestibule is very thin, and a large
otolith within it is well seen from outside. The upper
and outer eminence of the ear-capsule (the pterotic
ridge) is prominent, overhanging the articulation of the
hyomandibular.

142. The hinder division of the trigeminal nerve
escapes from the cranium by a foramen where the fore
part of the parachordal is fused with the auditory carti-
lage; its anterior (ophthalmic) division passes over the
notch between these two elements. The foramen for the



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56 MORPHOLOGY OF THE SKULL. [CHAP.

facial nerve (Fig. 18, 7 a.) is behind that for the trigeminal
(5) on the inferior aspect of the skull ; while the glosso-
pharyngeal and vagus nerves (8) emerge in the same line
near the posterior part of the basis cranii. The orbital
muscles (o.m.) pass forwards from their basicranial
attachment to the orbit on each side of the pituitary body.

143. The hinder portion of the trabeculse, in the
region of the pituitary body, does not show much change :
slender arcuate rods enclose a somewhat triangular inter-
trabecular space, much larger than the pituitary body;
and posteriorly they lie above and unite with the fore
extremity of the parachordals. Anteriorly there is great
change ; the brain-case has become elevated, and the
forebrain no longer lies directly on the trabeculae, but on
a membranous floor which forms the upper and inner
boundary of each orbit, while a median vertical inter-




Salmon fry, second week after hatching ; transverse section of head,

through forebrain and eyeballs.

T. cr. tegmen cranii; m.cr. membranous cranial investment; Clb.

forebrain ; i.o.a. interorbital septum ; 1, olfactory crus ; tr, coalesced

trabeculse, with superior ridge ; pa, palatine cartilage ; mk. meckelian

cartilage ; m. mouth ; e. eyeball.

Bones : /. frontal ; pa.8. parasphenoid ; ms.pg, mesopterygoid ; inx»
maxillary ; dr, dentary ; or. articular.



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III.] THE SALMON: THIRD STAGE. 57

orbital septum connects this floor with the trabeculae (Fig.
16, i.0.8.). The latter are not horizontally placed, but
arched upwards so as to unite at rather more than a right
angle, and with a sharp edge upwards (Fig. 16, tr,) The
trabeculae become broader where they coalesce, and at
the front of the orbit they curve suddenly outwards
on each side, forming the ethmopalatine processes, to
which the fore ends of the palatine rods are connected
(Fig. 15, p. pg,). The trabecular plate in its most anterior
portion is broad and quadrate below, flooring the nasal
sacs and ending with a broad edgo, truncated at the
external angles, and showing very little distinction of free
trabecular comua.

144. The anterior part of the trabecular crest fthe
nasal septum) has increased in height; its moieties diverge
in front and behind, so as to form crescents. The anterior
fork becomes shallow towards its termination; the
posterior is more elevated, and unites with the ethmo-
palatine process and with the cranial roof, to be presently
described. In the diverging angle between the posterior
forks of the internasal cartilage there is a median blind
membranous pouch (Fig. 15, m.n.c), beneath which is the
trabecular plate. The olfactory crura pass out of the brain-
case under the hinder part of the forebrain, and run in
the <)t-bit horizontally on either side of the interorbital
septum, finally gaining a level a little above the coalesced
trabeculaB in the ethmoidal region, where they perforate
the postnasal walL

145. The fore part of the cranial roof has become
chondrified. This cranial roof, or tegmen cranii {t cr ), is
broad and quadrate, and convex above, covering the fore-
brain and the anterior part of the midbrain. It is con-



Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 5 of 31)