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cavities freely continuous with the rest of the tube behind,
which is thenceforth distinguished as the spinal canal.
The notochord, underlying the neural tube, stops short of
its cephalic end, terminating underneath the middle or
the hinder cerebral vesicle. The nervous tissue becoming
differentiated in the walls of these vesicles constitutes
three main divisions of the brain, viz. the fore, the mid,
and the hindbrain. The mesoblast ultimately encircles
the cranial cavity as it does the spinal canal.

12. As the cerebral vesicles enlarge in all dimensions,
their increase is not followed by any high development of
the anterior part of the lower or visceral tube. Conse-
quently, at an early period the head consists mainly of the
large neural protuberances, and these bend downwards
around the anterior end of the notochord and of the
alimentary tube. This flexure, taking place about
a point beneath the middle or hinder part of the
middle cerebral vesicle, is called the mesocephcUic
(less precisely the cranial) flexure. The first cerebral
vesicle comes to occupy an infero-anterior, the second a
supero-anterior position. In addition to the definite meso-
cephalic flexure, there is in many cases a considerable
ventral curvature of the body, and especially of the neck.
One of the most remarkable things in vertebrate develop-
ment is the way in which the cranial axis ultimately
hecomes once more approximately straight; while the
ventral appendages of the skull acquire a high degree of
development.

13. The first cerebral vesicle enlarges transvei-sely,
and the lateral protuberances after a time become con-



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6 MORPHOLOGY OF THE SKULL. [CHAP.

stricted from the axial region. Thus arise stalked optic
vesicles, which at a later period, being met by involuted
epiblast from the exterior, are pitted so that the external
wall is opposed to the inner concave wall, just as part of
a hollow indiarubber ball may be pushed in till it meets
the opposite surface of the ball. The stalks of the optic
vesicles are subsequently known as the optic nerves.
These nerves soon appear to proceed from the under part
of the brain: for the constriction by which the optic
vesicles are distinguished is much deeper above than
below, and consequently the stalk is carried downwards
towards the base of the brain.

14. The first cerebral vesicle retains its distinctive
appellation of forebrain, and gives origin to the 'cerebral
hemispheres, which are not constricted from the axis by
a stalk, and which in higher vertebrates achieve a pre-
ponderance over all other cranial structures. The hemi-
spheres themselves bud out anteriorly into smaller bulbs
constituting the pair of olfactory lobes, from which the
olfactory nerves are subsequently derived. The remains
of the primary cerebral vesicle form what is known as the
vesicle of the third ventricle, the lateral cavities or ven-
tricles of the hemispheres having been originally con-,
sidered in cerebral topography as the first two ventricles.
From the centre of the floor of the third ventricle, which
usually becomes the hindmost of the elements produced
frpm the original first vesicle, arises a funnel-shaped pro-
cess, the infundibulum, which is extended downwards to
join the pituitary body situated at the extreme fore end of
the alimentary canal.

15. The remaining principal parts of the brain, as
related to the primary vesicles, may be briefly mentioned.
The middle vesicle gives rise to the optic lobes (corpora
quadrigemina of mammals) and to the crura cerebri where
they are distinct; its cavity in higher forms becomes re-
duced to a narrow channel between the third and the
fourth ventricles, or the primary cavities of the fore and
the hindbrain. In the latter, tne roof developes into the



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l] PRELIMINARY EMBRYOLOGY. 7

cerebellum in front, the floor behind forms the medulla

oblongata.

16. The chief cranial nerves other than the olfactory
and optic, which have already been referred to, arise on
either side of the floor of the hindbrain, and in some
cases certainly by outgrowths from it. There are four
principal masses, two of which are near the anterior end
of the notochord, two near the posterior limit of the head.
The first is the rudiment of the trigeminal nerve ; it early
becomes forked peripherally, one branch passing forwards
to the eyeball, the other downwards to the mandible.
The second becomes the facial nerve; the third and fourth,
the glossopharyngeal and vagus (or pneumogastricj. All
these nerves, at or near their cerebral termination, possess
special ganglia.

17. Three pairs of pouch-like ingrowths of epiblast
originate on the outside of the primary membranous
cranial cavity, and contribute to form organs of special
sense. The mesoblast becomes distinctively aggregated
around these pouches, and more or less completely cuts
off communication between the pouch and the external
epiblast.

18. The anterior of these organs are the nasal
sacs, which are always related at first to the forebrain,
usually lying under it. Their involution is distinguished
by the prominent rim bounding it. This border is in
certain cases deficient in its lower part, and when the
process which bounds the outer side of the mouth grows
over it to unite with the fore lip, this deficiency is con-
verted into a short canal opening on the roof of the
mouth; while the orifice of the original involution be-
comes definitely located on the face. Thus two narial
orifices are formed, anterior and posterior, or superior and
inferior. The inferior openings may sometimes be carried
backwards in the mouth by supplementary plates de-
veloped beneath them, closing their access to the anterior
part of the mouth, and only allowing communication
posteriorly.



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8 MORPHOLOGY OF THE SKULL. [CHAP.

19. In its later development, each nasal pouch be-
comes a cavity lined by a sensory membrane, part of which
receives special nerves from the olfactory lobes of the
brain. Mostly, the nasal cavities are elaborated in a
labyrinthic manner; that is, the surface of the lining
membrane is extended by the projection of complex folds
into the cavity; and these are supported by corresponding
growths of hard parts. The latter are derived from or
are continuous with the cranial skeleton, and play a great
part in the morphology of the skull.

20. The second pair of involutions, the optic sacs, are
usually much larger than the nasal. Their primary posi-
tion is above and behind the olfactory sacs, at the sides of
the midbrain; but this location is frequently modified
afterwards, and they become placed at the sides of and
often below the forebrain — almost entirely anterior to it
in some cases. The optic pouches of epiblast grow in-,
wards until they indent the optic vesicles of the brain,
and, pushing their outer part inwards till it comes in
contact with their inner surface, give rise to the cup which
subsequently constitutes the retina. The epiblastic mass
gets completely cut oflf from the exterior, and forms the
crystalline lens. Mesoblastic growths form all the other
structures of the eyeballs. The shape of the eye is mainly
determined by a strong fibrous membrane, the sclerotic,
which often becomes cartilaginous, and even acquires ossi-
fications, called sclerotic plates; but inasmuch as the eye-
balls must be mobile, the sclerotic plates never enter into
combination with the cranial skeleton, and will scarcely
receive any mention in the following pages.

21. A cavity called the orbit, open externally and
bounded by skeletal elements, is in most cases formed,
to lodge and protect the mobile eyeballs. The study of
the orbit is of high interest in cranial morphology; but it is
rendered comparatively simple by the fact that the sense-
organ, with its special skeletal investment, lies complete
within the orbit; there is no interlacement of parts or
complication of regions. A primary cleft arises between



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I.] PRELIMINARY EMBRYOLOGY. 9

the optic involution and the maxillopalatine process which
bounds the mouth. This opens on the roof of the mouth
where no secondary palate is developed, and into the
nasal passage where there is such a palate. It is variously
called the lachrymal or orbitonasal canal.

22. The third pair of special sense organs, the ear-
sacs, begin as large involutions of epiblast at the sides of
the hindbrain. The epiblastic pouch speedily becomes
hollow, and the cavity early expands to large dimensions,
and gives off processes which constitute the auditory laby-
rinth. Mesoblastic tissue engirths these processes, and
forms a hard investment to the whole organ. At first this
investment is usually distinct from the proper cranium ;
but they soon coalesce, and the regions of both become
sometimes almost inextricably intertwined.

28. The main part of the cavity of the auditory
labyrinth is denominated the vestibule ; it has an upward
and backward process, the aqueductus vestibuli, towards the
situation of the primary involution and the long persistent
membranous space or fenestra in that position. Hollow pro-
cesses arise from the vestibule, anteriorly, posteriorly, and
externally ; and as they grow, mesoblast penetrates into
their concavity, leaving the periphery pervious as a tube,
which forms a curve sometimes bearing a close resem-
blance to a semicircle; hence these three tubes are called
semicircular canals of the ear. They are present in the
same relative situations in all but the lowest vertebrates.
One end of each tube dilates where it opens into the
vestibule, forming an ampulla. Another process of the
vestibule, passing forwards and downwards in the ear-
mass, is developed into the simple conical cochlea of birds,
and the spirally-twisted cochlea of mammals.

24. In almost all vertebrates the mesoblastic investr
m^t of the otic capsule becomes at least cartilaginous,
veiy often bony : but the cartilage is scarcely ever com-
plete on all aspects. It may be incomplete internally
on the cranial side; or fenestrae may be formed in it



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10 MORPHOLOGY OF THE SKULL. [CHAP.

externally, where the labyrinth abuts on certain secondary
structures found in higher vertebrates.

25. Another organ of special sense, that of taste, is
formed by epiblastic involution in the head. It does not
present a manifest paired aspect, its involution being part
of that of the mouth-cavity or alimentary vestibule, which
discharges m^iny functions besides that of taste. The oral
involution early takes place in the angle between the,
downbent cranial vesicle and the fore part of the aliment-
ary tube. The mesoblast outside the latter grows for-
ward ventral ly and laterally, surrounding a hollow on the
inferior surface of the head, into which the, copious in-
growing epiblast is received. The hinder limit of this
involution is somewhere about the level of the fore end of
the hindbrain. It is at a comparatively late period that
a slit places the alimentary canal in communication with
the oral cavity.

26. At about the point of junction of these two
cavities, a diverticulum of epiblast is given off, passing
upwards into the floor of the cranium: this forms the
pituitary body already spoken of (§ 14, p. 6). The stalk -
of connection with the cavity beneath is obliterated, and
the pituitary body always occupies a definite position in
the cranial floor in front of the end of the notochord.

27. The mouth, throat, and neck are parts little
differentiated in the embryo when already the neural
vesicles have attained a high development. Subsequently
they increase very greatly in size, growing both forwards
and backwards. The mesoblast does not split at its ex-
treme anterior end to form a visceral wall within the
body-wall. A transverse section beneath the fore end of
the notochord shows a simple tubular cavity, which is the
commencement of the digestive canal. In this region, as
it grows more extensive, a series of thickenings arises on
each side in a vertical plane, constituting what are called
visceral folds. In general they are curved somewhat for-
wards towards the middle line below, and the foremost of



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l] PEETJMINARY EMBRTOLOGY. 11

them grows especially forwards to envelope the oral invo-
lution of epiblast.

28. The greatest number of these folds which occurs
in vertebrates is nine; frequently no more than six
appear. The first pair of folds receives the name of
mandibular ; the second, that of ht/oid ; the remainder are
named branchial. The valleys between the folds usually
become perforated in more or less of their extent; thus
there is established a series of visceral clefU, which may
remain open throughout life, affording a channel of com-
munication between the cavity of the throat and the
exterior ; or some or all of the cleft* may become closed
at a later stage of development, and so remain during the
life of the animal.

29. The boundaries of the mouth are primarily con-
stituted, (1) anteriorly, by the nasofrontal process which
is the termination of the investment of the head be-
neath the forebrain, and between the olfactory sacs;
(2) superiorly, by the floor of the cranial vesicles, forming
the pnma.r J palate : (3) laterally and somewhat superiorly,
by the muxillopalatine processes which grow forwards
on each side along the line of the primary palate from the
Upper end of the mandibular folds ; and (4) inferiorly by
these mandibular folds.

30. The visceral folds, and especially those named
branchial, become the basis for the development of gills
in varied forms, which may persist throughout life, or only
during a portion of embryonic existence. In the higher
forma, where no gills are developed, the appearance of the
branchial folds is to a large extent evanescent, though not
less certain and definite than in the other cases.

31. A brief reference to the vascular system will
close this general account, and enable us to proceed to
the history of the skeletal elements. The heart is formed
in the mesoblast at the anterior region of the pleuro-
peritoneal cavity. A tube proceeds from its fore end in
the mid- ventral mesoblast : this is the bulbus aortve. It



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12 MORPHOLOGY OP THE SKULL. [CHAP.

passes along the ventral line into the neighbourhood of
the visceral folds, and gives off a series of branches, which
surround the fore end of the alimentary canal, one passing
outwards and upwards along each visceral fold to gain the
dorsal side of the throat. These channels or aortic arches
unite to form a pair of tubes corresponding to that from
which they originated ventrally ; and these dorsal tubes
pass backwards right and left of the notochord at some
distance from it, uniting sooner or later to constitute the
primitive dorsal aorta.

32. The aortic arches never quite maintain their
primitive arrangement in adult life. They most nearly re-
tain it in the permanently branchiate forms. But one or
two of the anterior arches always become disconnected with
the dorsal tube, and by means of secondary ramifications
supply the head and brain with blood. One pair of these
vessels, the internal carotid arteries, will be mentioned
in succeeding pages, as passing into the cranial cavity in
a definite position. Other portions of the aortic arches
are differentiated in abranchiate conditions or forms of
higher vertebrates, and furnish blood-vessels to internal
respiratory organs : and finally but two pairs, or one pair,
or one aortic arch, may be left in the adult in continuity
with the dorsal aorta.

33. The skeletal elements with which we have to do
arise entirely as developments of the mesoblast. Very
early the mesoblast at the sides of the neural tube behind,
the head is divided by transverse partitions into a longi-
tudinal series of more or less definite segments, the proto^
vertebrce. These extend backwards as far as the end of
the neural canal, but stop short anteriorly at the level -
of the posterior end of the third cerebral vesicle. The
slightly differentiated mesoblast grows upwards so as to
surround the neural canal, and also invests the notochord.

34. When chondrification takes place, a continuous
cartilaginous investment surrounds the notochord. This
cartilage is ultimately segmented into vertebral pieces, so



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I.] PRELIMINAKY EMBRYOLOGY. 13

that the lines between adjacent vertebrae are intenne-
diate between the dividing lines of the protovertebral
segments ; thus each vertebral body corresponds with the
contiguous parts of two protovertebrae. Lateral vertebral
cartilages become gradually extended round the spinal
canal on each side so as to form neural arches, which at a
later period coalesce on the dorsal aspect of the canaL

35. By the time that the vertebra is complete in
cartilage the notochord usually begins to degenerate,
especially where it is encircled by the vertebral bodies or
centra. In these "situations it may become perfectly ob-
literated. Very frequently remnants of the notochord
persist in the intervertebral spaces.

36. Ossification commences in the vertebrae in the
region of the centrum immediately surrounding the noto-
chord. Other centres of ossification arise, one on either
side of the neural canal. Where ribs are developed they
grow outwards from the region of the vertebral centra.
Borne of the ribs may extend completely round the body-
wall so that the corresponding paii-s meet ventrally ; and
a junction is effected between several succeeding medio-
ventral elements, to form a caililaginous sternum. The
costal arches ossify by one or two centres on each side ;
the sternum often by several paired centres.

37. No protovertebrae are formed by the side of the
notochord in the cranial region. Much of the mesoblast
investing the brain and constituting the visceral folds and
other processes belonging to the head, undergoes a gradual
but early transformation into cartilage, forming definite
skeletal tracts before intercellular substance has appeared.
It is the object of the following pages to trace in several
types of vertebrates the rise and history of these carti-
laginous tracts and of the bones developed within or in
proximity to thqm.



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CHAPTER II.

THE SKULLS OF THE DOGFISH AND THE SKATE.

38. The eggs of Elasmob ranch fishes, of which the
principal are known as Sharks and Rays, when deposited,
are enclosed in a strong horny capsule or "purse," secreted
from the oviduct. This capsule is pillow- shaped, the
corners being pointed in the Rays, and produced into long
tendril-like processes in the Shark and Dogfish. The
embryo remains enclosed in the purse until about six
months after oviposition, and during this period the most
important metamorphoses take place.

First Stage.

39. In embryos of the lesser Spotted Dogfish {ScylHum
canicvda), from eight lines to an inch in length*, the head
and the branchial region are proportionately large and
conspicuous, and external gills are present; the body is
slender, tapering to a long thread-like tail. The embryo
is extremely active, and has attached to it a yelk-sac of
about three-quarters of an inch in diameter.

40. The neural tube in the head is completely closed,
but the covering of the third or posterior cerebral vesicle
is very thin. The latter vesicle (Figs. 1 and 2, G 3) lies
directly above the anterior end of the notochord : the
second vesicle (C 2) is immediately in front of the noto-
chord, forming the foremost rounded ^ p^,rt of the head ;
while the first vesicle {G 1) is beneath the second, and is

^ These embryos correspond in most respects with stages M and N,
described and figured by Mr Balfour {Journ. Anat. Vol. x. p. 568, and
Plate XXV.).



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CHAP. II.] THE DOGFISH: FIRST STAGE. 15

totally below the level of the notochord as well as ante-
rior to it. Thus the mesocephalic or cranial flexure is
already fully established.

41. Each sense capsule is seen in a very rudimentary
condition as an infolding of the epiblast of the side of the
head. These infoldings are at present subequal in size.
The nasal sacs (Fig. 1, Na) are situated upon the infero-
lateral surface of the head, external to the first cerebral
vesicle. The young eyeballs {E) are almost vertically
over the nasal sacs : above each eyeball is a noteworthy
elevation of the skin, the supraorbital band. The ear-sacs
(Au) are on a higher level and more posterior, flanking
the hinder part of the third cerebral vesicle.

Fig. 1.



Head of embryo Dogfish, 11 lines long. Tr, trabecula; PL Pt,
palatopterygoid ; M.Pt^ metapterygoid region; A/n, mandibular cartilage;
Hy, byoid arch ; Br. 1, first branchial arch ; S'p, mandibulo-hyoid cleft ;
Cil, first branchial clef t ; Lch, so-called lachrymal cleft ; Na, olfactory
radiment; E, eyeball; Au, auditory mass; CI, 2,3, cerebral vesicles;
Hm, hemispheres ; /.n.p, nasofrontal process.

42. On the under surface of the head is a large
square mouth, bounded in front by the median antero-
inferior projection of the head, called the nasofrontal
process (/. n. p.), lying between the nasal sacs ; behind, by
the rudimentary lower jaw (Mn) ; and laterally by a pro-
cess passing forward from the upper end of each half of
the mandible (PL Pt). Behind the mouth on each side
are seen six clefts {Sp.y CI.) curving downwards from the
neural towards the haemal region of the neck roughly



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16 MORPHOLOGY OF THE SKULL. [CHAP.

parallel to one another. The clefts on each side corre-
spond, but they do not meet below.

43. Thick and somewhat prominent ridges intervene
between the visceral clefts; and from the surface of each
ridge or arch there arises a series of long filamentous
external gills. The heart is plainly to be seen through
the transparent skin in the middle line beneath, between
and below the posterior arches just described. Behind
the heart is the umbilicus, and on either side of the umr
bilical region the rudiment of a pectoral fin projects.

44. The notochord in the cephalic region (Fig. 2, n.c.)
lies beneath the neural axis, and is curved somewhat down-
wards*, at the same time slightly tapering. It does not
extend so far forwards as the third cerebral vesicle does,
but ends above the middle of the mouth. In embryos of
the Dogfish at this stage and also in Pristiurus embryos
the anterior end of the notochord has been seen subnio-
niliform in outline, presenting from five to seven head-
ings in the distance between its anterior extremity and
the middle of the auditory region.

45. The pituitary body (py), lying behind the first
or inferior cerebral vesicle, and connected with it by
the infundibular process, is closely in front of and a
little below the notochord ; it is the lower boundary of a
space formed by the curvature of the neural axis, re-
sembling the concavity of the hook of a crozier. This
space is filled with delicate gelatinous tissue; it is the
transitory "middle trabecula" of Kathke.

46. Flat bars of nascent cartilage, the parachordals^ ,
are found in the hinder part of the cranial floor on either
side of the notochord, extending backwards into the neck
for three or four times the length of their intracranial
portion, without showing any vertebral segmentation.
The inner edge of each parachordal is grooved to embrace

1 This downward curvature is considerably greater at an earlier
stage ; see Balfour, Joum, Anat. Vol. x. PI. xxiv. G, H, and /.

* Fig. 5, p. 24, which shows these structures in the next stage, may be
consulted for many points in this description.



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n.] THE dogfish: first staoel 17

the notochord partially ; but they are Dot identified with
its sheath, which is already cartilaginous. The para-
chordals extend further forwards internally, where they
reach the middle of the beaded region of the notochord ;
externally, they pass nearly to the fore end of the audi-
tory masses. In front they abut unconformably, at an
angle of about 120", upon the hinder part of the trabeculae.
The parachordals are crescentically scooped and bevelled
where they lie between the auditory capsules, and the
latter rest upon the bevelled edge : they are wider l>ehind
this region, having a straight external edge.

Fig. 2.



Head of embryo Dogfish, 11 lines long, median longitudinal section.
CI, 2, 3, cerebral vesicles; nc. notochord; //y. pituitary body; m,tr, middle
trabecnla; i.n.p. intemasal plate; m.»p. mean la spinalis ; m. mouth; ph,
pharynx; sp. spiracle; el. visceral clefts; mn. mandibular, hy, hyoid, br. 1,



Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 2 of 31)