Adelochorda--marine wormlike creatures having a notochordin the anterior of the body, and gill slits, both persistent; 2.
Simultaneously, on either side of the notochord appear a series of solid masses of cells, derived mainly by cell division from the cells of the wall of the archenteron, and filling up and obliterating the segmentation cavity.
The cartilage is hard and semi-transparent, the notochord jelly-like, least at the centres of the centra, and at a maximum intervertebrally.
The anterior end of the notochord in the developing rabbit or dog lies where the middle of the basisphenoid bone is destined to be.
Having the notochord reaching anteriorly to the mid-brain, a brain of three primary vesicles and a skull.
This becomes truly cartilaginous, and at regular intervals is alternately thicker and thinner, compressing thenotochord at the thicker parts.
Hence the notochord has a beaded form within this, at first, continuous cartilaginous sheath.
But the notochord in the fowl is not so distinctly connected with the hypoblast, and so distinct from the mesoblast, as it is in the lower type, and no gills, internal or external, are ever developed.
Having the notochord reaching in front of the brain.
The vertebral column is devoid of vertebrae; it is throughout life a rod of gelatinous tissue, the notochord (Figure 1, n.
This sheath is soon cut into a series of vertebral bodies by jointings appearing through the points where the cartilage is thickest and the notochord most constricted.
The hypoblastic notochord is early embraced by a mesoblastic sheath derived from the protovertebrae.
The interesting point is that although the notochord is only a vestige in the adults of these types, it is never absent from the embryo.
The notochord is the supporting axis of the pioneer backboned animals, namely the Lancelets and the Round-mouths (Cyclostomes), such as the Lamprey.
When free fromnotochord and surrounding tissues such a cartilaginous centrum presents a deep conical cavity at each end (amphicoelous).
The outer layer of cells, which are commonly less vacuolated and form a "chordal epithelium," soon secretes a thin cuticle which ensheaths the notochord and is known as the primary sheath.
In ontogeny the notochord is a derivative of the dorsal wall of the archenteron.
The wall of each segment becomes much thickened in the middle so that the notochord becomes constricted within each centrum and the space occupied by it is shaped like the cavity of a dice-box.
This arises in development from a cartilaginous rod (parachordal cartilage) lying on each side of the notochord and possibly representing a fused row of dorsal arcualia.
In the embryonic connective tissue or mesenchyme lying just outside the primary sheath of the notochord there are developed a dorsal and a ventral series of paired nodules of cartilage known as arcualia (fig.
The notochord with its sheaths is in existing fishes essentially the skeleton of early life (embryonic or larval).
These are a pair of chitinous rods which lie ventral to the notochord and in the collar region unite to form a single mass.
The vertebrae are amphicoelous or sometimes nearly flat, and the notochord sometimes persists to some extent.
The notochord is unconstricted, but the neural and haemal arches are well-developed, and the neural spines are long and slender.
In Amphioxus the notochord is immediately surrounded by a structureless cuticular layer, the chordal sheath.
In most reptiles the vertebrae are fully ossified, but in some of the more primitive forms the notochord persists in the centre of the vertebra (i.
They cause the notochord to become constricted and to atrophy to a varying extent, though a remnant of it persists, either permanently or for a long period, within each centrum or between successive centra.
The notochordnever extends further forwards than the mid-brain.
In all Tunicata except Larvacea all trace of the notochord is lost in the adult.
In Anura remains of the notochordare found in the centre of each vertebra, i.
In these forms the notochord persists and retains approximately the same diameter throughout the whole length of the vertebral column.
In some cases the notochord remains unmodified and the skeletogenous layer surrounding it is not segmented to form vertebrae, but in every case the neural arches which protect the spinal cord are segmented.
Portions of the notochord commonly persist in the intervertebral spaces.
The notochord remains continuous throughout the whole vertebral column, but is greatly constricted opposite the middle of each vertebra, and thus rendered moniliform.
On each side of the mouth there is a curved bar resembling the notochord in structure.
The spinal cord and the notochord each begin to be surrounded by rings of cartilage or gristle, which by degrees is changed into bone.
The primitive nature of thenotochord is, however, perhaps the most striking feature of Amphioxus.
When the centrum, instead of being biconcave, is solid, as in the higher Vertebrata, the notochord is obliterated by its encroachment.
The notochord sheath is supplemented, however, by cartilage bars which are equivalent to the beginnings of the vertebrae of the back-bone.
Let us now return to the lowest vertebrate of all, which has a large notochordand no bones.
The presence of this feature is important, in view of the fact that some authorities have sought to throw doubt on the genuineness of the notochord of Balanoglossus.
Nevertheless, traces of the notochordpersist in the back-bone of these fishes.
B, Diagram indicating the position of the Notochord in the vertebra of an adult Common Dog-fish (Scyllium canicula).
Both produce buds and form a colony, and in both a notochord has been distinguished.
This structure is called the Notochord (a string down the back).
The credit of this great discovery belongs to Russia; for the presence of the Notochord in the Ascidian larva was discovered by A.
This notochord consists of a series or cord of cells extending longitudinally through the body from head to tail, above the alimentary canal and below the spinal nerve-cord.
It loses its tail and with it the short notochord possessed by the larva; the eye and the auditory organ are lost, and the nervous system and alimentary canal become much reduced and simplified.
In all the vertebrates excepting a few low forms, the notochord while present in the young, is replaced in the adult by a segmented bony or cartilaginous axis, the spinal or vertebral column.
Labyrinthodonta, with simple biconcave vertebral disks, very slightly pierced by a remnant of the notochord and supporting the loosely articulated neural arch.
Microsauria, nearest the reptiles, with persistentnotochord completely surrounded by constricted cylinders on which the neural arch rests.
The U-shaped intercentrum encloses the notochord and occupies an anteroventral position in the vertebra.
The notochord seems to have been slightly constricted by the intercentra, but not interrupted.
The Liodesmidæ (genus Liodesmus) are much like Amia, but thenotochord is persistent, its sheath without ossification.
In this group the notochord is persistent, and the vertebræ are in various degrees of ossification and of different forms.
The symplectic bone is wanting, the notochord is more or less persistent, the vertebral centra always complete constricted cylinders, none coalesced.
The notochord is persistent, the vertebræ variously ossified.
The chordate animals are those which at some stage of life possess a notochord or primitive dorsal cartilage which divides the interior of the body into two cavities.
Naturalists believe that all animals having a notochord at any stage in their existence are related to each other through being descended from a common ancestor, the first or oldest chordate or back-boned animal.
The notochord is persistent, without intercalary cartilage, such as appear in the higher sharks.
In the lancelets (Leptocardii), the vertebral column consists simply of a gelatinous notochord extending from one end of the fish to the other, and pointed at both ends, no skull being developed.
In the Haplistia the notochord is persistent, and the basal bones of dorsal and anal fins are in regular series, much fewer in number than the fin-rays.
There is no trace of segmentation in the notochord itself in these or any other fishes, but neutral arches are foreshadowed in a series of cartilages on each side of the spinal chord.
The notochord is very simple, scarcely or never constricted, the calcifications of its sheath "arrested at the most primitive or rhachitomous stage, except in the tail.
The notochord and the nervous chord persist through life, the latter with ganglionic segmentations at regular intervals.
The notochord never shows traces of segmentation, although cartilaginous rods above it are thought to forecast apophyses.
The primitive function of the notochord is still far from clear, but its ultimate use of its structures in affording protection and in furnishing a fulcrum for the muscles and limbs is of the greatest importance in the processes of life.
In the chimaeras (Holocephali) the notochord persists and is surrounded by a series of calcified rings.
In the embryo of the bony fish a similar notochord precedes the segmentation and ossification of the vertebral column.
The back-bone, as usual in primitive fishes, is developed as a persistent notochord imperfectly segmented.
The first appearance of thisnotochord in lower animals is the earliest indication of the vertebrate type, because it is found that in the higher vertebrates it is the forerunner of the bony spinal column and the skull.
Later on these surrounding portions become cartilage, and, still later, bone; the notochordmeanwhile gradually disappearing where the bony spinal column appears.
The nervous system on the dorsal surface of the notochord consists of the brain in the head and the spinal marrow running down the back.
The nerve-centres are therefore here all dorsal, and the notochord or skeleton lies between these and the digestive or alimentary canal.
Only gradually is the notochord crowded out so that the vertebræ join by their whole adjacent surfaces.
Notochord and backbone may be the most striking and apparent characteristic of vertebrates, but their predominant characteristic is brain.
Around this primitive notochord was a layer of connectile tissue which later gave rise to the vertebræ forming our backbone.
Now this is a very peculiar origin for cartilage, and the notochord is a very strange cartilage even if we have not made a mistake in calling it cartilage at all.
These unite with the masses of cartilage surrounding the notochord to form cartilaginous vertebræ, which may be stiffened by an infiltration of carbonate of lime.
The notochord arises from a set of cells which are in close relation with the fused tips of the blastopore.
When the notochord and the nerve-plate appeared, as a result of this abnormal condition, they grew from a cell-material that was quite different to that which gives them origin in normal cases.
As a result of this the notochord and the nerve plate came to arise, not from the usual set of cells, but from those cells that, by the abnormal condition, had come to lie in the place for the notochord.
The protruding cells, which normally would have developed into notochord and nerve plate, grew into a simple fold of the external skin.
He reared gastrulae and older embryos with notochord and nerve-tube, which were perfect and normal, except in size.
The notochordis hardly differentiated, but the complete separation of mesoblast and hypoblast under the primitive groove is clearly shewn.
The front end of thenotochord became indistinct, and I could not see its exact termination.
Peculiar body underlying the notochordderived from the hypoblast.
Sections through a blastoderm in which the first traces ofnotochord and medullary groove have made their appearance.
The epithelium of the alimentary canal (al) is seen closely underlying the notochord and becoming continuous with the epiblast at the hind end of the notochord.
A, A) are seen on each side with the thickened mesoblast, and the mesoblast cells are beginning to form the notochord (nc) under the medullary groove.
Section nearer the tail; it shews the thickening of the hypoblast to form the notochord (ch').
The notochord cannot be traced in the living embryo so far forward as it is represented.
Granular outer portion of the sheath of the notochord in the vertebral regions.
Longitudinal section through the notochord and adjoining parts to shew the first appearance of the cartilaginous notochordal sheath which forms the vertebral centra.
Footnote 1: The apparent structure in the sheath of the notochord in this and the succeeding figure is merely the result of an attempt on the part of the engraver to represent the dark colour of the sheath in the original figure.
The medullary folds and notochord are evident at this stage, but no mesoblastic somites are to be seen.
The entoderm, which has the appearance of being thickened because of the fact that the notochord has not yet completely separated from it, is continuous, through the blastopore, with the ectoderm.
In the comparison which I have made the alimentary canal of the invertebrate ancestor has become the tube of the central nervous system of the vertebrate, and there is no sign of a notochord whatever.
In the notochord of Ammocoetes there is no sign of either nerves, blood-vessels, or muscles.
Embryologically, then, the notochord is looked upon as a tube formed directly from the alimentary canal.
Lwoff and Rolph both describe a direct communication between the spinal cord and the notochord by means of fibres passing through these holes, without however looking upon this connection as a nervous one.
The notochord is a continuous rod of cartilage, or gristle, which in the embryological growth of vertebrate animals supports the spinal nerve cord before the formation of the vertebrae.
The skeleton is completely ossified and thenotochord removed.
In most modern fishes and in all higher vertebrates the notochord is gradually removed as the bodies of the vertebrae are formed about it; but in the Devonian fishes it persists through maturity and the vertebrae remain incomplete.
As we approach the front end of the primitive streak the notochord becomes (Ser.
Around the cuticular sheath of the notochord there is formed an elastic membrane--the membrana elastica externa.
Thus, curiously enough, the membrana elastica externa and the sheath of the notochord in the intervertebral regions are folded in different directions, the folds of the one being only visible in transverse sections (Plate 42, fig.
Between it and the gelatinous tissue of the notochord within there is a very thin unmodified portion of the sheath, which is continuous with the thinner intervertebral parts of the sheath.
The last of these rings lay just where the notochord began to bend up.
The protoplasm of the notochord has now again retreated towards the centre, shewing a clear space all round.
As already stated, thenotochord extends beyond the mouth to the tip of the rostrum.
The ordinal name for the genera and species of Amphioxus is Cephalochorda, the term referring to the extension of the primary backbone or notochord to the anterior extremity of the body; the family name is Branchiostomidae.
This notochord represents the persistent primordial skeletal axis which, in the higher Craniata (though not so in the lower), gives way by substitution to the segmented vertebral column.
Immediately above the notochord there lies another subcylindrical cord, also surrounded by a sheath of connective tissue.
Vertebrae of Sturgeon in side view A; and vertical transverse section B, showing Notochord ch, and Neural Canal m] =The tail fin is vertebrated.
The investing mass of the head and of the notochord is the skeleton of the occipital bone.
Around the notochord is later formed a species of membrane which protects it, called the perichord.
The spinal cord is at first essentially a notochord as in the lowest types of vertebrates.
This primary skull extended in front of the notochord (the spinal cord of the human embryo, and the permanent spinal cord of the lancelet), where it gave off two trabeculae cranii or front skull plates.
When the medullary tube of the notochord enlarges to form the brain the end of the head bends over to make room for that enlargement.
The notochord possesses an inner and an outer sheath and the outer sheath is continuous with the basis cranii (p.
With the notochord he homologised the supporting rod in the proboscis of Balanoglossus, which like the notochord arises from the dorsal wall of the archenteron, and has a vacuolated structure.
He recognises as the basis of his comparisons the homology of the notochord in all vertebrate embryos with the persistent notochord which forms the chief part or the whole of the vertebral column in the Cyclostomes.
He admits that there exists a real homology between, for instance, the notochordof Vertebrates and that of Ascidians.
In no Vertebrate does thenotochord extend farther forward (p.
The notochordends in this plate, which is the basis cranii, just at the level of the ear-labyrinth.
It is interesting to notice the remarkable relations of the notochord to the walls of the neurenteric passage.
Bibliography of the notochordand vertebral column, p.
The notochord appears also to enter into the posterior part of the region which ossifies as the basisphenoid.
The notochord (ch) is seen as an independent cord for the greater part of the length of the embryo, but falls into the hypoblast shortly in front of the neurenteric passage.
This interpretation receives support from the fact that at a later stage, when the neural and alimentary canals become separated, the neural canal extends round the posterior end of the notochord to the ventral side.
Vertebral constriction of the notochord as effected in Mammalia, the intervertebral parts of the cartilaginous sheath being converted into intervertebral ligaments.
These plates together with the notochord (nc) enclosed between them form a floor for the hind- and mid-brain.
Gadus, Salmo, owing to the simultaneous appearance of a number of fin rays on the dorsal and ventral side of the notochord the external symmetry of the tail is not interfered with in the above processes.
The brain is at first enveloped in a continuous layer of mesoblast known as the membranous cranium, into the base of which the anterior part of the notochord is prolonged for some distance.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "notochord" in a variety of sentences. We hope that you will now be able to make sentences using this word.
