THE NOTOCHORD, THE VERTEBRAL COLUMN, THE
RIBS AND THE STERNUM.
INTRODUCTION.
Amongst the products of that part of the mesoblast which constitutes
the connective tissue of the body special prominence must be given to
the skeleton of the Vertebrata, from its importance in relation to
numerous phylogenetic and morphological problems.
The development of the skeleton is however so large a subject that it
cannot be satisfactorily dealt with except in a special treatise
devoted to it; and the following description must be regarded as a
mere sketch, from which detail has been as far as possible excluded.
In the lowest Chordata the sole structure present, which deserves to
be called a skeleton, is the notochord. Although the notochord often
persists as an important organ in the true Vertebrata, yet there are
always added to it various skeletal structures developed in the
mesoblast. Before entering into a systematic description of these, it
will be convenient to say a few words as to the general characters of
the skeleton.
Two elements, distinct both in their genesis and structure, are to be
recognized in the skeleton. The one, forming the true primitive
internal skeleton or endoskeleton, is imbedded within the muscles and
is originally formed in cartilage. In many instances it retains a
cartilaginous consistency through life, but in the majority of cases
it becomes gradually ossified, and converted into true bone. Bones so
formed are known as cartilage bones.
The other element is originally formed by the fusion of the ossified
bases of the dermal placoid scales already described in Chapter XIV.,
or by the fusion of the ossified bases of teeth situated in the mucous
membrane of the mouth. In both instances the plates of bone so formed
may lose the teeth or spines with which they were in the first
instance covered, either by absorption in the individual, or
phylogenetically by their gradually ceasing to be developed. The
plates of bone, which originated by the above process, become in
higher types directly developed in the connective tissue beneath the
skin; and gradually acquire a deeper situation, and are finally so
intimately interlocked with parts of the true internal skeleton, that
the two sets of elements can only be distinguished by the fact of the
one set ossifying in cartilage and the other in membrane.
It seems probable that in the Reptilia, and possibly the extinct
Amphibia, dermal bones have originated in the skin without the
intervention of superjacent spinous structures.
In cases where a membrane bone, as the dermal ossifications are
usually called, overlies a part of the cartilage, it may set up
ossification in the latter, and the cartilage bone and membrane bone
may become so intimately fused as to be quite inseparable. It seems
probable that in cases of this kind the compound bone may in the
course of further evolution entirely lose either its cartilaginous
element or its membranous element; so that cases occasionally occur
where the development of a bone ceases to be an absolutely safe guide
to its evolution.
As to the processes which take place in the ossification of cartilage
there is still much to be made out. Two processes are often
distinguished, viz. (1) a process known as ectostosis, in which the
ossification takes place in the perichondrium, and either simply
surrounds or gradually replaces the cartilage, and (2) a process known
as endostosis, where the ossification actually takes place between the
cartilage cells. It seems probable however (Gegenbaur, Vrolik) that
there is no sharp line to be drawn between these two processes; but
that the ossification almost always starts from the perichondrium. In
the higher types, as a rule, the vessels of the perichondrium extend
into the cartilage, and the ossification takes place around these
vessels within the cartilage; but in the lower types (Pisces,
Amphibia) ossification is often entirely confined to the
perichondrium; and the cartilage is simply absorbed.
The regions where ossification first sets in are known as centres of
ossification; and from these centres the ossification spreads
outwards. There may be one or more centres for a bone.
The actual causes which in the first instance gave rise to particular
centres of ossification, or to the ossification of particular parts of
the cartilage, are but little understood; nor have we as yet any
satisfactory criterion for determining the value to be attached to the
number and position of centres of ossification. In some instances such
centres appear to have an important morphological significance, and in
other instances they would seem to be determined by the size of the
cartilage about to be ossified.
There is no doubt that the membrane bones and cartilage bones can as a
rule be easily distinguished by their mode of development; but it is
by no means certain that this is always the case. It is necessarily
very difficult to establish the homology between bones, which develop
in one type from membrane and in another type from cartilage; but
there are without doubt certain instances in which the homology
between two bones would be unhesitatingly admitted were it not for the
difference in their development. The most difficult cases of this kind
are connected with the shoulder-girdle.
The possible sources of confusion in the development of bones are
obviously two. (1) A cartilage bone by origin may directly ossify in
membrane, without the previous development of cartilage, and (2) a
membrane bone may in the first instance be formed in cartilage.
The occurrence of the first of these is much more easy to admit than
that of the second; and there can be little doubt that it sometimes
takes place. In a large number of cases it would moreover cause no
serious difficulty to the morphologist.
BIBLIOGRAPHY _of the origin of the Skeleton_.
(405) C. Gegenbaur. "Ueb. primäre u. secundäre Knochenbildung mit
besonderer Beziehung auf d. Lehre von dem Primordialcranium."
_Jenaische Zeitschrift_, Vol. III. 1867.
(406) O. Hertwig. "Ueber Bau u. Entwicklung d. Placoidschuppen u. d.
Zähne d. Selachier." _Jenaische Zeitschrift_, Vol. VIII. 1874.
(407) O. Hertwig. "Ueb. d. Zahnsystem d. Amphibien u. seine Bedeutung
f. d. Genese d. Skelets d. Mundhöhle." _Archiv f. mikr. Anat._, Vol.
XI. Supplementheft, 1874.
(408) O. Hertwig. "Ueber d. Hautskelet d. Fische." _Morphol.
Jahrbuch_, Vol. II. 1876. (Siluroiden u. Acipenseriden.)
(409) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u.
Polypterus)." _Morph. Jahrbuch_, Vol. V. 1879.
(410) A. Kölliker. "Allgemeine Betrachtungen üb. die Entstehung d.
knöchernen Schädels d. Wirbelthiere." _Berichte v. d. königl. zoot.
Anstalt z. Würzburg_, 1849.
(411) Fr. Leydig. "Histologische Bemerkungen üb. d. Polypterus
bichir." _Zeit. f. wiss. Zool._, Vol. V. 1858.
(412) H. Müller. "Ueber d. Entwick. d. Knochensubstanz nebst
Bemerkungen, etc." _Zeit. f. wiss. Zool._, Vol. IX. 1859.
(413) Williamson. "On the structure and development of the Scales and
Bones of Fishes." _Phil. Trans._, 1851.
(414) Vrolik. "Studien üb. d. Verknöcherung u. die Knochen d. Schädels
d. Teleostier." _Niederländisches Archiv f. Zoologie_, Vol. I.
_Notochord and Vertebral column._
The primitive axial skeleton of the Chordata consists of the notochord
and its sheath. It persists as such in the adult in Amphioxus, and
constitutes, in embryos of all Vertebrata, for a considerable period
of their early embryonic life, the sole representative of the axial
skeleton.
[FIG. 313. HORIZONTAL SECTION THROUGH THE TRUNK OF AN EMBRYO OF
SCYLLIUM CONSIDERABLY YOUNGER THAN F IN FIG. 28.
The section is taken at the level of the notochord, and shews the
separation of the cells to form the vertebral bodies from the
muscle-plates.
_ch._ notochord; _ep._ epiblast; _Vr._ rudiment of vertebral body;
_mp._ muscle-plate; _mp´._ portion of muscle-plate already
differentiated into longitudinal muscles.]
The Notochord. The early formation of the notochord has already been
described in detail (pp. 292-300). It is developed, in most if not all
cases, as an axial differentiation of the hypoblast, and forms at
first a solid cord of cells, without a sheath, placed between the
nervous system and the dorsal wall of the alimentary tract, and
extending from the base of the front of the mid-brain to the end of
the tail. The section in the region of the brain will be dealt with by
itself. That in the trunk forms the basis round which the vertebral
column is moulded.
The early histological changes in the cells of the notochord are
approximately the same in all the Craniata. There is formed by the
superficial cells of the notochord a delicate sheath, which soon
thickens, and becomes a well-defined structure. Vacuoles (one or more
to each cell) are formed in the cells of the notochord, which enlarge
till the whole notochord becomes almost entirely formed of large
vacuoles separated by membranous septa which form a complete
sponge-like reticulum (fig. 313). In the Ichthyopsida most of the
protoplasm with the nuclei is carried to the periphery, where it forms
a special nucleated layer sometimes divided into definite
epithelial-like cells (fig. 314), while in the meshes of the reticulum
a few nuclei surrounded by a little protoplasm still remain. In the
Amniotic Vertebrata, probably owing to the early atrophy of the
notochord, the distribution of the nuclei in the spaces of the
mesh-work remains fairly uniform.
[FIG. 314. SECTION THROUGH THE SPINAL COLUMN OF A YOUNG SALMON.
(From Gegenbaur.)
_cs._ sheath of notochord; _k._ neural arch; _k´._ hæmal arch; _m._
spinal cord; _a._ dorsal aorta; _v._ cardinal veins.]
In the early stages of development the spaces in the notochordal
sponge-work, each containing a nucleus and protoplasm, probably
represent cells. In the types in which the notochord persists in the
adult the mesh-work becomes highly complicated, and then forms a
peculiar reticulum filled with gelatinous material, the spaces in
which do not indicate the outlines of definite cells (figs. 315 and
318).
Around the sheath of the notochord there is formed in the
Cyclostomata, Ganoidei, Elasmobranchii and Teleostei an elastic
membrane usually known as the membrana elastica externa.
In most Vertebrates the notochord and its sheath either atrophy
completely or become a relatively unimportant part of the axial
skeleton; but in the Cyclostomata (fig. 315) and in the Selachioidean
Ganoids (Acipenser, etc.) they persist as the _sole representative of
the true vertebral axis_. The sheath becomes very much thickened; and
on the membrana elastica covering it the vertebral arches directly
rest. In Elasmobranchii the sheath of the notochord undergoes a more
complicated series of changes, which result first of all in the
formation of a definite _unsegmented_ cartilaginous tube[199] round
the notochord, and subsequently (in most forms) in the formation of
true vertebral bodies.
[199] This tube consists of a peculiar form of fibrous tissue
rather than true cartilage, though part of it subsequently
becomes hyaline cartilage.
[FIG. 315. SECTION THROUGH THE VERTEBRAL COLUMN OF AMMOCOETES. (From
Gegenbaur.)
_Ch._ notochord; _cs._ notochordal sheath; _m._ spinal cord; _a._
aorta; _v._ cardinal veins.]
Between the membrana elastica externa and the sheath of the notochord
a layer of cells becomes interposed (fig. 316, _n_), which lie in a
matrix not sharply separated from the sheath of the notochord. The
cells which form this layer appear to be derived from a special
investment of the notochord, and to have penetrated through the
membrana elastica externa to reach their final situation. The layer
with these cells soon increases in thickness, and forms a continuous
unsegmented tube of fibrous tissue with flattened concentrically
arranged nuclei (fig. 317, _Vb_). Externally is placed the membrana
elastica externa (_mel_), while within is the cuticular sheath of the
notochord. This tube is the cartilaginous tube spoken of above and is
known as the cartilaginous sheath of the notochord.
[FIG. 316. LONGITUDINAL SECTION THROUGH A SMALL PART OF THE
NOTOCHORD AND ADJOINING PARTS OF A SCYLLIUM EMBRYO, AT THE TIME OF
THE FIRST FORMATION OF THE CARTILAGINOUS SHEATH.
_ch._ notochord; _sc._ sheath of notochord; _n._ nuclei of
cartilaginous sheath; _me.e._ membrana elastica externa.]
The exact origin of the cartilaginous tube just described is a
question of fundamental importance with reference to the origin of the
vertebral column and the homologies of its constituent parts; but is
by no means easy to settle. In the account of the subject in my memoir
on _Elasmobranch Fishes_ I held with Gegenbaur that it arose from a
layer of cells _outside_ the sheath of the notochord, on the exterior
of which the membrana elastica externa was subsequently formed. To
this view Götte (No. 419) also gave his adhesion. Schneider has since
(No. 429) stated that this is not the case, but that, as described
above, the membrana elastica externa is formed before the layer of
cartilage. I have since worked over this subject again, and am on the
whole inclined to adopt Schneider's correction.
It follows from the above description that the cartilaginous tube in
question is an essential part of the sheath of the notochord, and that
it is to some extent homologous with the notochordal sheath of the
Sturgeon and the Lamprey, and not an entirely new formation.
[FIG. 317. TRANSVERSE SECTION THROUGH THE VENTRAL PART OF THE
NOTOCHORD AND ADJOINING STRUCTURES OF AN ADVANCED SCYLLIUM EMBRYO AT
THE ROOT OF THE TAIL.
_Vb._ cartilaginous sheath of the notochord; _ha._ hæmal arch; _vp._
process to which the rib is articulated; _mel._ membrana elastica
externa; _ch._ notochord; _ao._ aorta; _V.cau._ caudal vein.]
This sheath forms the basis of the centra of the future vertebræ. In a
few adult forms, _i.e._ Chimæra and the Dipnoi, it retains its
primitive condition, except that in Chimæra there are present delicate
ossified rings more numerous than the arches; while in the Notidani,
Læmargi and Echinorhini the indications of vertebræ are imperfectly
marked out. The further history of this sheath in the forms in which
true vertebræ are formed can only be dealt with in connection with the
formation of the vertebral arches.
In Teleostei there is present, as in Elasmobranchii, an elastica
externa, and an inner notochordal sheath. The elastica externa
contains, according to Götte, cells. These cells, if present, are
however very difficult to make out, but in any case the so-called
elastica externa appears to correspond with the cartilaginous sheath
of Elasmobranchii together with its enveloping elastica, since
ossification, when it sets in, occurs in this layer. The sheath within
becomes unusually thick.
In the Amphibia and in the Amniota no membrane is present which can be
identified with the membrana elastica externa of the Elasmobranchii,
Teleostei, etc. In Amphibia (Götte) there is formed round the
notochord a cellular sheath, which has very much the relations of the
cartilaginous tube around the notochord of Elasmobranchii, and is
developed in the same way from the perichordal connective tissue
cells. It is only necessary to suppose that the membrana elastica
externa has ceased to be developed (which in view of its extreme
delicacy and unimportant function in Elasmobranchii is not difficult
to do) and this cellular sheath would then obviously be homologous
with the cartilaginous tube in question. In the Amniota an external
sheath of the notochord cannot be traced as a distinct structure, but
the connective tissue surrounding the notochord and spinal cord is
simply differentiated into the vertebral bodies and vertebral arches.
_Vertebral arches and Vertebral bodies._
Cyclostomata. The Cyclostomata are the most primitive forms in which
true vertebral arches are present. Their ontogeny in this group has
not been satisfactorily worked out. It is however noticeable in
connection with them that they form for the most part isolated pieces
of cartilage, the segmental arrangement of which is only imperfect.
Elasmobranchii. In the Elasmobranchii the cells forming the vertebral
arches are derived from the splanchnic layer of the mesoblastic
somites. They have at first the same segmentation as the somites (fig.
313, _Vr_), but this segmentation is soon lost, and there is formed
round the notochord a continuous sheath of embryonic connective tissue
cells, which gives rise to the arches of the vertebræ, the tissue
forming the dura mater, the perichondrium, and the general investing
connective tissue.
The changes which next follow result in what has been known since
Remak as the secondary segmentation of the vertebral column. This
segmentation, which occurs in all Vertebrata with true vertebræ, is
essentially the segmentation of the continuous investment of the
notochord and spinal cord into vertebral bodies and vertebral arches.
It does not however follow the lines of the segmentation of the
muscle-plates, but is so effected that the centres of the vertebral
bodies are opposite the septa between the muscle-plates.
The explanation of this character in the segmentation is not difficult
to find. The primary segmentation of the body is that of the
muscle-plates, which were present in the primitive forms in which
vertebræ had not appeared. As soon however as the notochordal sheath
was required to be strong as well as flexible, it necessarily became
divided into a series of segments.
The condition under which the lateral muscles can best cause the
flexure of the vertebral column is clearly that each myotome shall be
capable of acting on two vertebræ; and this condition can only be
fulfilled when the myotomes are opposite the intervals between the
vertebræ. For this reason, when the vertebræ became formed, their
centres were opposite not the middle of the myotomes but the
intermuscular septa.
These considerations fully explain the characters of the secondary
segmentation of the vertebral column. On the other hand the primary
segmentation (fig. 313) of the vertebral rudiments is clearly a
remnant of a condition when no vertebral bodies were present; and has
no greater morphological significance than the fact that the cells of
the vertebræ were derived from the segmented muscle-plates, and then
became fused into a continuous sheath around the notochord and nervous
axis; till finally they became in still higher forms differentiated
into vertebræ and their arches.
During the stage represented in fig. 28 _g_, and somewhat before the
cartilaginous sheath of the notochord is formed, there appear four
special concentrations of the mesoblastic tissue adjoining the
notochord, two of them dorsal (neural) and two of them ventral
(hæmal). They are not segmented, and form four ridges, seated on the
sides of the notochord. They are united with each other by a delicate
layer of tissue, and constitute the substance in which the neural and
hæmal arches subsequently become differentiated.
At about the time when the first traces of the cartilaginous sheath of
the notochord arise, differentiations take place in the neural and
hæmal ridges. In the neural ridge two sets of arches are formed for
each myotome, and resting on the cartilaginous sheath of the notochord
in the region which will afterwards form the centrum of a vertebra,
and constituting a true neural arch; and a second separate from the
cartilaginous sheath, forming an intercalated piece[200]. Both of them
soon become hyaline cartilage.
[200] The presence of intercalated pieces in the neural arch
system of Elasmobranchii, Chimæra, etc. is probably not the
indication of an highly differentiated type of neural arch, but
of a transitional type between an imperfect investment of the
spinal cord by isolated cartilaginous bars, and a complete system
of neural arches like that in the higher Vertebrata.
There is a considerable portion of the original tissue of the neural
ridge, especially in the immediate neighbourhood of the notochord,
which is not employed in the formation of the neural arches. This
tissue has a fibrous character and becomes converted into the
perichondrium and other parts.
[FIG. 318. SECTION THROUGH THE VERTEBRAL COLUMN OF AN ADVANCED
EMBRYO OF SCYLLIUM IN THE REGION OF THE TAIL.
_na._ neural arch; _ha._ hæmal arch; _ch._ notochord; _sh._ inner
sheath of notochord; _ne._ membrana elastic externa.]
The hæmal arches are formed from the hæmal ridge in precisely the same
way as the neural arches, but interhæmal intercalated pieces are often
present. In the region of the tail the hæmal arches are continued into
ventral processes which meet below, enclosing the aorta and caudal
veins.
Since primitively the postanal gut was placed between the aorta and
the caudal vein, the hæmal arches potentially invest a caudal section
of the body cavity. In the trunk region they do not meet ventrally,
but give support to the ribs. The structures just described are shewn
in section in fig. 318, in which the neural (_na_) and hæmal (_ha_)
arches are shewn resting upon the cartilaginous sheath of the
notochord.
While these changes are being effected in the arches the cartilaginous
sheath of the notochord undergoes important differentiations. In the
_vertebral_ regions opposite the origin of the neural and hæmal arches
(fig. 318) its outer part becomes hyaline cartilage, while the inner
parts adjoining the notochord undergo a somewhat different
development, the notochord in this part becomes at the same time
somewhat constricted. In the _intervertebral_ regions the
cartilaginous sheath of the notochord becomes more definitely fibrous,
while the notochord is in no way constricted. A diagrammatic
longitudinal section through the vertebral column, while these changes
are being effected, is shewn in fig. 320 B.
These processes are soon carried further. The notochord within the
vertebral body becomes gradually constricted, especially in the median
plane, till it is here reduced to a fibrous band, which gradually
enlarges in either direction till it reaches its maximum thickness in
the median plane of the intervertebral region. The hyaline cartilage
of the vertebral region forms a vertebral body in which calcification
may to some extent take place. The cartilage of the base of the arches
gradually spreads over it, and on the absorption of the membrana
elastica externa, which usually takes place long before the adult
state is reached, the arch tissue becomes indistinguishably fused with
that of the vertebral bodies, so that the latter are compound
structures, partly formed of the primitive cartilaginous sheath, and
partly of the tissue of the bases of the neural and hæmal arches.
Owing to the beaded structure of the notochord the vertebral bodies
take of necessity a biconcave hourglass-shaped form.
The intervertebral regions of the primitive sheath of the notochord
form fibrous intervertebral ligaments enclosing the unconstricted
intervertebral sections of the notochord.
A peculiar fact may here be noticed with reference to the formation of
the vertebral bodies in the tail of Scyllium, Raja, and possibly other
forms, viz. that _there are double as many vertebral bodies as there
are myotomes and spinal nerves_. This is not due to a secondary
segmentation of the vertebræ but, as I have satisfied myself by a
study of the development, takes place when the vertebral bodies first
become differentiated. The possibility of such a relation of parts is
probably to be explained by the fact that the segmentation of the
vertebral column arose subsequently to that of the nerves and
myotomes.
Ganoidei. In Acipenser and other cartilaginous Ganoids the hæmal and
neural arches are formed as in Elasmobranchii, and rest upon the outer
sheath of the notochord. Since however the sheath of the notochord is
never differentiated into distinct vertebræ, this primitive condition
is retained through life.
Teleostei. In Teleostei the formation of the vertebral arches and
bodies takes place in a manner, which can be reduced, except in
certain minor points, to the same type as that of Elasmobranchii.
There are early formed (fig. 314 _k_ and _k´_) neural and hæmal arches
resting upon the outer sheath of the notochord. The latter structure,
which, as mentioned on p. 549, corresponds to the cartilaginous sheath
of the notochord of Elasmobranchii, soon becomes divided into
vertebral and intervertebral regions. In the former ossification
directly sets in without the sheath acquiring the character of hyaline
cartilage (Götte, 419). The latter forms the fibrous intervertebral
ligaments. The notochord exhibits vertebral constrictions.
The ossified outer sheath of the notochord forms but a small part of
the permanent vertebræ. The remainder is derived partly from an
ossification of the connective tissue surrounding the sheath, and
partly from the bases of the arches, which do not spread round the
primitive vertebral bodies as in Elasmobranchii. The ossifications in
the tissue surrounding the sheath usually (fig. 319) take the form of
a cross, while the bases of the arches (_k_ and _k´_) remain as four
cartilaginous radii between the limbs of the osseous cross. In some
instances the bases of the arches also become ossified, and are then
with difficulty distinguishable from the other parts of the secondary
vertebral body. The parts of the arches outside the vertebral bodies
are for the most part ossified (fig. 319). In correlation with the
vertebral constrictions of the notochord the vertebral bodies are
biconcave.
Amphibia. Of the forms of Amphibia so far studied embryologically the
Salamandridæ present the most primitive type of formation of the
vertebral column.
It has already been stated that in Amphibia there is present around
the notochord a cellular sheath, equivalent to the cartilaginous
sheath of Elasmobranchii. In the tissue on the dorsal side of this
sheath a series of cartilaginous processes becomes formed. These
processes are the commencing neural arches; and they rest on the
cellular sheath of the notochord opposite the middle of the vertebral
regions.
A superficial osseous layer becomes very early formed in each
vertebral region of the cellular sheath; while in each of the
intervertebral regions, which are considerably shorter than the
vertebral, there is developed a ring-like cartilaginous thickening of
the sheath, which projects inwards so as to constrict the notochord.
At a period before this thickening has attained considerable
dimensions the notochord becomes sufficiently constricted in the
centre of each vertebral region to give a biconcave form to the
vertebræ for a very short period of foetal life.
[FIG. 319. VERTICAL SECTION THROUGH THE MIDDLE OF A VERTEBRA OF ESOX
LUCIUS (PIKE). (From Gegenbaur.)
_ch._ notochord; _cs._ notochordal sheath; _k._ and _k´._
cartilaginous tissue of the neural and hæmal arches; _h._ osseous
hæmal process; _n._ spinal canal.]
The stage with biconcave vertebræ is retained through life in the
Perennibranchiata and Gymnophiona.
The chief peculiarity which distinguishes the later history of their
vertebral column from that of fishes consists in the immense
development of the _intervertebral_ thickenings just mentioned, which
increase to such an extent as to reduce the notochord, where it passes
through them, to a mere band; while the cartilage of which they are
composed becomes differentiated into two regions, one belonging to the
vertebra in front, the other to that behind, the hinder one being
convex, and the anterior concave. The two parts are not however
absolutely separated from each other.
By these changes each vertebra comes to be composed of (1) a thin
osseous somewhat hourglass-shaped cylinder with a dilated portion of
the notochord in its centre, and (2 and 3) of two halves of two
intervertebral cartilages, viz. an anterior convex half and a
posterior concave half. The vertebræ thus come to be opisthocoelous. A
longitudinal section through the vertebral column at this stage is
diagrammatically shewn in fig. 320 C.
[FIG. 320. DIAGRAM REPRESENTING THE MODE OF DEVELOPMENT OF THE
VERTEBRÆ IN THE DIFFERENT TYPES. (From Gegenbaur.)
A. Ideal type in which distinct vertebræ are not established.
B. Type of Pisces with vertebral constrictions of the notochord.
C. Amphibian type, with intervertebral constrictions of the
notochord by the intervertebral parts of the cellular sheath.
D. Intervertebral constriction of the notochord as effected in
Reptilia and Aves.
E. Vertebral constriction of the notochord as effected in Mammalia,
the intervertebral parts of the cartilaginous sheath being
converted into intervertebral ligaments.
_c._ notochord; _cs._ cuticular sheath of notochord; _s._
cartilaginous sheath; _v._ vertebral regions; _iv._ intervertebral
regions; _g._ intervertebral joints.]
To the centre of each of these vertebræ the neural arches, the origin
of which was described above, become in the meantime firmly attached;
and grow obliquely upwards and backwards, so as to meet and unite
above the spinal cord. The transverse processes of the vertebræ would
seem (Fick) to be developed independently of the arches, though they
very soon fuse with them. According to Götte the transverse processes
are double in the trunk, there being two pairs, one vertically above
the other for each vertebra. The pair on each side eventually fuse
together.
In the tail hæmal arches are formed, which are similar in their mode
of development to the neural arches.
The unconstricted portion of the notochord, which persists in each
vertebra, becomes in part converted into cartilage.
Anura. In the Anura the process of formation of the vertebral column
is essentially the same as that in the Salamandridæ. Two types may
however be observed. One of these occurs in the majority of the Anura,
and mainly differs from that in Salamandra in (1) the earlier fusion
of the arches with the cellular sheath of the notochord; (2) the more
rapid growth of the intervertebral thickenings of the cellular sheath,
which results in the early and complete obliteration of the
intervertebral parts of the notochord; (3) the complete division of
these intervertebral thickenings into anterior and posterior portions,
which unite with and form the articular surfaces of two contiguous
vertebræ. The vertebræ are moreover procoelous instead of being
opisthocoelous.
The unconstricted vertebral sections of the notochord always persist
till the ossification of the vertebræ has taken place. In some forms
they remain through life (Rana), while in other cases they eventually
either wholly or partially disappear.
The second type of vertebral development is found in Bombinator,
Pseudis, Pipa, and Pelobates. In these genera the formation of the
vertebra takes place almost entirely on the dorsal side of the
notochord; so that the latter forms a band on the ventral side of the
vertebral column. In other respects the history of the vertebral
column is the same in the two cases; the vertebral unconstricted parts
of the notochord appear however to become in part converted into
cartilage. The type of formation of the vertebral column in these
genera has been distinguished as epichordal in contradistinction to
the more normal or perichordal type.
Amniota. In the Amniota all trace of a distinction between a cellular
notochord sheath and an arch tissue is lost, and the two are developed
together as a continuous whole forming an unsegmented tube round the
notochord, with a neural ridge which does not at first nearly invest
the neural cord. This tube becomes differentiated, in the manner
already described for other types, into (1) vertebral regions with
true arches, and (2) intervertebral regions.
Reptilia. In Reptilia (Gegenbaur, No. 416) a cartilaginous tube is
formed round the notochord, which is continuous with the cartilaginous
neural arches. The latter are placed in the vertebral regions, and in
these regions ossification very early sets in, while the notochord
remains relatively unconstricted. In the intervertebral regions the
cartilage becomes thickened, as in Amphibia, and gradually constricts
the notochord. The cartilage in each of the intervertebral regions
soon becomes divided into two parts which form the articular faces of
two contiguous vertebræ.
The general character of the vertebral column on the completion of
these changes is shewn in fig. 320 D. The later changes are relatively
unimportant. The constricted intervertebral sections of the notochord
rapidly disappear, while the vertebral sections become partially
converted into cartilage, and only cease to be distinguishable at a
considerably later period.
The ossification extends from the bodies of the vertebræ into the
arches and into the articular surfaces, so that the whole vertebræ
eventually become ossified.
The Ascalabotæ (Geckos) present an exceptional type of vertebral
column which has many of the characters of a developmental stage in
other Lizards. The body of the vertebra is formed of a slightly
hourglass-shaped osseous tube, united with adjoining vertebræ by a
short intervertebral cartilage. There is a persistent and continuous
notochord which, owing to the small development of the intervertebral
cartilages, is narrower in the vertebral than in the intervertebral
regions.
Aves. In Birds the cellular tube formed round the notochord is far
thicker than in the Reptilia. It is continuous in the regions of the
future vertebræ with neural arches, which do not at first nearly
enclose the spinal cord.
On about the fifth day, in the case of the chick, it becomes
differentiated into vertebral regions opposite the attachments of the
neural arches, and intervertebral regions between them; the two sets
of regions being only distinguished by their histological characters.
Very shortly afterwards each intervertebral region becomes segmented
into two parts, which respectively attach themselves to the contiguous
vertebral regions. A part of each intervertebral region, immediately
adjoining the notochord, does not however undergo this division, and
afterwards gives rise to the ligamentum suspensorium.
The notochord during these changes at first remains indifferent, but
subsequently, on about the seventh day in the chick, a slight
constriction of each vertebral region takes place; so that the
vertebræ have temporarily, as they have also in Amphibia, a biconcave
form which repeats the permanent condition of most fishes. By the
ninth and tenth days, however, this condition has completely
disappeared, and in all the intervertebral portions the notochord has
become distinctly constricted, and at the same time in each vertebral
portion there have also appeared two constrictions of the notochord
giving rise to a central and to two terminal enlargements.
On the twelfth day the ossification of the cartilaginous centra
commences.
The first vertebra to ossify is the second or third cervical, and the
ossification gradually extends to those behind. It does not commence
in the arches till somewhat later than in the bodies. For each arch
there are two centres of ossification, one on each side.
The notochord persists for the greater part of foetal life and even
into post-foetal life. The larger vertebral portions are often the
first completely to vanish. They would seem in many cases at any rate
(Gegenbaur) to be converted into cartilage, and so form an integral
part of the permanent vertebræ. Rudiments of the intervertebral
portions of the notochord may long be detected in the ligamenta
suspensoria.
Schwarck (No. 420) states that in both the intervertebral and the
vertebral regions, though less conspicuously in the former, the
cartilage is divided into two layers, an _inner_ and an _outer_. He
holds that the inner layer corresponds to the cartilaginous
notochordal sheath of the lower types, and the outer to the arch
tissue. Ossification (Gegenbaur) of the centra appears in a special
inner layer of cartilage, which is probably the same as the inner
layer of the earlier stage, though this point has not been definitely
established.
[FIG. 321. LONGITUDINAL SECTION THROUGH THE VERTEBRAL COLUMN OF AN
EIGHT WEEKS' HUMAN EMBRYO IN THE THORACIC REGION. (From Kölliker.)
_v._ cartilaginous vertebral body; _li._ intervertebral ligament;
_ch._ notochord.]
Mammalia. The early development of the perichordal cartilaginous tube
and rudimentary neural arches is almost the same in Mammals as in
Birds. The differentiation into vertebral and intervertebral regions
is the same in both groups; but instead of becoming divided as in
Reptilia and Birds into two segments attached to two adjoining
vertebræ, the intervertebral regions _become in Mammals wholly
converted into the intervertebral ligaments_ (fig. 322 _li_). There
are three centres of ossifications for each vertebra, two in the arch
and one in the centrum.
[FIG. 322. LONGITUDINAL SECTION THROUGH THE INTERVERTEBRAL LIGAMENT
AND ADJACENT PARTS OF TWO VERTEBRÆ FROM THE THORACIC REGION OF AN
ADVANCED EMBRYO OF A SHEEP. (From Kölliker.)
_la._ ligamentum longitudinale anterius; _lp._ ligamentum long.
posterius; _li._ ligamentum intervertebrale; _k_, k´. epiphysis of
vertebra; _w._ and _w´._ anterior and posterior vertebræ; _c._
intervertebral dilatation of notochord; _c´._ and _c´´._ vertebral
dilatation of notochord.]
The fate of the notochord is in important respects different from that
in Birds. It is first constricted in the _centre of the vertebræ_
(figs. 320 E and 321) and disappears there shortly after the
ossification; while in the intervertebral regions it remains
relatively unconstricted (figs. 320 E, 321 and 322 _c_) and after
undergoing certain histological changes _remains through life as part
of the nucleus pulposus in the axis of the invertebral ligaments_[201].
There is also a slight swelling of the notochord near the two
extremities of each vertebra (fig. 322 _c´_ and _c´´_). In the
persistent vertebral constriction of the notochord Mammals retain a
more primitive and piscine mode of formation of the vertebral column
than the majority either of the Reptilia or Amphibia.
[201] This view was first put forward by Lushka, and his surmises
have been confirmed by Kölliker and other embryologists. Leboucq
(No. 424) however holds that the cells of the notochord in the
intervertebral regions fuse with those of the adjoining tissue;
and Dursy and others deny that the nucleus pulposus is derived
from the notochord.
BIBLIOGRAPHY _of Notochord and Vertebral column_.
(415) Cartier. "Beiträge zur Entwicklungsgeschichte der Wirbelsäule."
_Zeitschrift für wiss. Zool._, Bd. XXV. Suppl. 1875.
(416) C. Gegenbaur. _Untersuchungen zur vergleichenden Anatomie der
Wirbelsäule der Amphibien und Reptilien._ Leipzig, 1862.
(417) C. Gegenbaur. "Ueber die Entwickelung der Wirbelsäule des
Lepidosteus mit vergleichend anatomischen Bemerkungen." _Jenaische
Zeitschrift_, Bd. III. 1863.
(418) C. Gegenbaur. "Ueb. d. Skeletgewebe d. Cyclostomen." _Jenaische
Zeitschrift_, Vol. V. 1870.
(419) Al. Götte. "Beiträge zur vergleich. Morphol. des Skeletsystems
d. Wirbelthiere." II. "Die Wirbelsäule u. ihre Anhänge." _Archiv f.
mikr. Anat._, Vol. xv. 1878 (Cyclostomen, Ganoiden, Plagiostomen,
Chimaera), and Vol. XVI. 1879 (Teleostier).
(420) Hasse und Schwarck. "Studien zur vergleichenden Anatomie der
Wirbelsäule u. s. w." Hasse, _Anatomische Studien_, 1872.
(421) C. Hasse. _Das natürliche System d. Elasmobranchier auf
Grundlage d. Bau. u. d. Entwick. ihrer Wirbelsäule._ Jena, 1879.
(422) A. Kölliker. "Ueber die Beziehungen der Chorda dorsalis zur
Bildung der Wirbel der Selachier und einiger anderen Fische."
_Verhandlungen der physical. medicin. Gesellschaft in Würzburg_, Bd.
X.
(423) A. Kölliker. "Weitere Beobachtungen über die Wirbel der
Selachier insbesondere über die Wirbel der Lamnoidei." _Abhandlungen
der senkenbergischen naturforschenden Gesellschaft in Frankfurt_, Bd.
V.
(424) H. Leboucq. "Recherches s. l. mode de disparition de la corde
dorsale chez les vertébrés supérieurs." _Archives de Biologie_, Vol.
I. 1880.
(425) Fr. Leydig. _Anatomisch-histologische Untersuchungen über Fische
und Reptilien._ Berlin, 1853.
(426) Aug. Müller. "Beobachtungen zur vergleichenden Anatomie der
Wirbelsäule." Müller's _Archiv._ 1853.
(427) J. Müller. "Vergleichende Anatomie der Myxinoiden u. der
Cyklostomen mit durchbohrtem Gaumen, I. Osteologie und Myologie."
_Abhandlungen der königlichen Akademie der Wissenschaften zu Berlin._
1834.
(428) W. Müller. "Beobachtungen des pathologischen Instituts zu Jena,
I. Ueber den Bau der Chorda dorsalis." _Jenaische Zeitschrift_, Bd.
VI. 1871.
(429) A. Schneider. _Beiträge z. vergleich. Anat. u. Entwick. d.
Wirbelthiere._ Berlin, 1879.
_Ribs and Sternum._
Ribs. Embryological evidence on the development of the ribs, though
somewhat inadequate, indicates that they arise as cartilaginous bars
in the connective tissue of the intermuscular septa, and that they are
placed, in Elasmobranchii and Amphibia, on the level of division
between the dorso-lateral and ventro-lateral divisions of the
muscle-plates. This does not appear to hold true for either Ganoidei
or Teleostei. In Teleostei they are entirely below the muscles along
the lines of the intermuscular septa, and this is partially true for
Ganoidei, though not wholly so in Lepidosteus. They may be attached
either to the hæmal (Pisces) or neural (Amphibia and Amniota) arches.
The connective tissue from which they are formed is continuous with
the processes of the vertebræ to which they are attached; but the
conversion of the tissue into cartilage takes place more or less
independently of that of the arches, although in many cases the
cartilage of the two becomes continuous, the separation of the ribs
being then effected by a subsequent process of segmentation (Fick, No.
431). It is possible that the ribs of Pisces may not be homologous
with those of Amphibia and the Amniota, but till the reverse can be
proved it is more convenient to assume that the ribs are homologous
structures throughout the vertebrate series.
In Elasmobranchii the ribs are relatively of less importance in the
adult than in the embryo. By a careful examination of their early
development, I have satisfied myself that the differentiation of the
ribs is _independent_ of that of the hæmal processes to which they are
attached, although the differentiation proceeds in such a manner that,
when both are converted into cartilage, they are quite continuous.
Subsequently the ribs become segmented off from the hæmal processes.
At the junction of the tail and trunk, where the hæmal processes
commence to be ventrally prolonged, eventually to unite in the region
of the tail below the caudal vein, the ribs are attached to short
processes which spring from the sides of the hæmal arches (fig. 317).
The ventral hæmal arches of these fishes are therefore clearly in no
part formed by the ribs.
In Ganoidei and Teleostei there is very great difficulty in
determining the homologies of the ribs.
In the cartilaginous Ganoidei there are well developed rib-like
structures, which might be regarded as homologous with Elasmobranch
ribs, and indeed probably are so; but at the same time their relations
are in some respects very different from those of Elasmobranch ribs in
the caudal region. In Ganoids the ribs, in approaching the tail,
become shorter and then fuse with the ends of the hæmal processes, and
finally in the caudal region form together with the hæmal arches a
closed hæmal canal which superficially resembles that in
Elasmobranchii.
In Lepidosteus and Amia, especially the former, the same phenomenon is
still more marked; and in Lepidosteus it is easy, in passing
backwards, to trace the ribs bending ventralwards, and uniting
ventrally in the caudal region to form, with the hæmal processes, a
complete hæmal canal.
It might have been anticipated that the Teleostean Ganoids would
resemble the Teleostei, but, from an examination of adult Teleostei,
it would seem to be clear that the relations of the parts are the same
as in Elasmobranchii, _i.e._ that the ribs have no share in forming
the hæmal canal in the tail. Aug. Müller and Götte have however
brought embryological evidence (though not of a conclusive character),
to shew that in the embryo the ribs really fuse with the hæmal
processes in the tail, and so assist, as in the Ganoids, in forming
the hæmal canal. Götte moreover holds that the ribs in Elasmobranchii
are not homologous with those of Teleostei and Ganoids; but that the
hæmal arches in the tail are homologous in the three groups.
Without necessarily following Götte in these views it is worth
pointing out that the undoubtedly close affinity between the bony
Ganoids and the Teleostei is in favour of the view on the hæmal arches
of Teleostei at which he has arrived on embryological grounds.
In Amphibia the formation of the ribs from the connective tissue of
the intermuscular septa, their secondary attachment to the transverse
processes of the neural arches, and their subsequent separation was
first clearly established by Fick (No. 431), whose statements have
since been confirmed by Hasse, Born, &c., and in part by Götte, who
holds however that, though converted into cartilage independently of
the transverse processes, they are formed in membrane as outgrowths of
these processes.
In the Amniota the ribs are also independently established (Hasse and
Born), though they subsequently become united to the transverse
processes and to the bodies of the vertebræ, or to the transverse
processes only. This junction is however stated by the majority of
authorities, never to be effected by the fusion of the cartilage of
the two parts, but always by fibrous tissue; though Hoffmann (No. 435)
takes a different view on this subject, holding that the ribs are at
first continuous with the intervertebral regions of the primitive
cartilaginous tube surrounding the notochord.
Sternum. In dealing with the development of the sternum it will be
convenient to leave out of consideration the interclavicle or
episternum which is, properly speaking, only part of the
shoulder-girdle and to confine my statements to the sternum proper.
This structure is found in all the Amniota except the Ophidia,
Chelonia, and some of the Amphisbænæ.
From the older researches of Rathke, and from the newer ones of Götte,
etc., it appears that the sternum is always formed from the fusion of
the ventral extremities of a certain number of ribs. The extremities
of the ribs unite with each other from before backwards, and thus give
rise to two cartilaginous bands. These bands become segmented off from
the ribs with which they are at first continuous, and subsequently
fuse in the median ventral line to form an unpaired sternum. The
Mammalian presternum (manubrium sterni) and xiphosternum have the same
origin as the main body of the sternum (Ruge, No. 438).
In the Amphibia there is no structure which admits from its mode of
development of a complete comparison with the sternum of the Amniota;
and it must for this reason be considered doubtful whether the median
structure placed behind the coracoids in the Anura, which is usually
known as the sternum, is really homologous with the sternum of the
Amniota[202].
The remaining Ichthyopsida are undoubtedly not provided with a sternum.
BIBLIOGRAPHY _of Ribs and Sternum_.
(430) C. Claus. "Beiträge z. vergleich. Osteol. d. Vertebraten. I.
Rippen u. unteres Bogensystem." _Sitz. d. kaiserl. Akad. Wiss. Wien_,
Vol. LXXIV. 1876.
(431) A. E. Fick. "Zur Entwicklungsgeschichte d. Rippen und
Querfortsätze." _Archiv f. Anat. und Physiol._ 1879.
(432) C. Gegenbaur. "Zur Entwick. d. Wirbelsäule des Lepidosteus mit
vergleich. anat. Bemerk." _Jenaische Zeit._, Vol. III. 1867.
(433) A. Götte. "Beiträge z. vergleich. Morphol. d. Skeletsystems d.
Wirbelthiere Brustbein u. Schultergürtel." _Archiv f. mikr. Anat._,
Vol. XIV. 1877.
(434) C. Hasse u. G. Born. "Bemerkungen üb. d. Morphologie d. Rippen."
_Zoologischer Anzeiger_, 1879.
(435) C. K. Hoffmann. "Beiträge z. vergl. Anat. d. Wirbelthiere."
_Niederländ. Archiv Zool._, Vol. IV. 1878.
(436) W. K. Parker. "A monograph on the structure and development of
the shoulder-girdle and sternum." _Ray Soc._ 1867.
(437) H. Rathke. _Ueb. d. Bau u. d. Entwicklung d. Brustbeins d.
Saurier._ 1853.
(438) G. Ruge. "Untersuch. üb. Entwick. am Brustbeine d. Menschen."
_Morphol. Jahrbuch._, Vol. VI. 1880.
[202] The so-called sternum of the Amphibia develops in proximity
with certain rudimentary abdominal ribs, and Ruge has with some
force urged (against Götte) that it may be for this reason a
rudimentary structure of the same nature as the sternum of the
higher types.