notochord

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notochord

(nō`təkôrd'), in biology, supporting rod running most of the length of animals of the phylum ChordataChordata
, phylum of animals having a notochord, or dorsal stiffening rod, as the chief internal skeletal support at some stage of their development. Most chordates are vertebrates (animals with backbones), but the phylum also includes some small marine invertebrate animals.
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 and present at varying times in the life cycle. Composed of large cells packed within a firm connective tissue sheath, the notochord lies between the neural tube (spinal cord) and the gut. The division of the phylum Chordata into subphyla is based on the structure of the notochord and the time of life in which it is present: In the subphylum Urochordata (tunicates) the notochord characterizes the larval, swimming, stage of the animals and does not extend into the head; in the subphylum Cephalochordata (lancelets) the notochord extends to the extreme tip of the head in both young and adults; and in the subphylum Vertebrata the notochord becomes surrounded by skeletal vertebrae during embryonic development—in higher vertebrates it is present in the early embryo only and is later completely replaced by the vertebrae.

Notochord

 

the elastic, unsegmented skeletal axis in chordate animals and in man. In some chordate animals, the notochord is retained throughout life. These include such tunicates as the Appendicularia, such acraniates as the lance lets, and such vertebrates as the cyclostomates. Fishes in which the notochord is retained throughout life include the holocephalans, chondrosteans, and dipnoans. In other vertebrates and tunicates, the notochord is present only during embryonic development; in adult vertebrates it is replaced by the vertebrae.

The notochord’s position in the body differs among various animals. In protochordates, a rudimentary notochord is located on the dorsal side of the intestine in the proboscis. In the larvae of tunicates, it is located in the caudal section of the body. In acraniates and vertebrates, the notochord is located on the dorsal side of the body, under the neural tube and between the bands of the segmented musculature of the trunk. Among lancelets, the notochord extends from the end of the tail to the anterior end of the body. This is apparently a secondary adaptation caused by a need to strengthen the anterior end of the body owing to the lancelets’ burrowing mode of life. In vertebrates, the cephalic section of the notochord ends behind the hypophysis.

The notochord develops from the middle part of the operculum of the chordamesoderm in the form of a protrusion that narrows into an elongated cylindrical cord. The cells of the notochord’s rudiment flatten and then vacuolate. The periphery retains a layer of nonvacuolate cells that are rich in cytoplasm. This layer is the notochord’s epithelium, which plays a role in forming the notochord’s membrane. At first a thin elastic outer membrane is formed, well supplied with elastic fibers. Within the outer membrane a fibrous membrane forms, consisting of collagen fibers. In elasmobranchians and teleosts the inner part of the fibrous membrane forms a separate entity within the inner elastic membrane, and the cartilaginous elements of the vertebrae develop within the fibrous membrane. In holocephalans, numerous

Figure 1. Numeral systems of various people

calcic rings develop within the fibrous membrane. As the vertebrae develop, the spinal column performs the function of the notochord as the skeletal axis of the body.

In vertebrates the notochord is the central structure around which the cartilaginous or bony axial skeleton develops. The vertebrae developing from certain parts of the notochord at first supplement and then to a greater or lesser degree displace it. Vestiges of the notochord remain between the vertebrae in fishes and within the vertebrae in amphibians. The notochord disappears in birds, but in mammals it is retained in the intervertebral cartilage in the form of a jelly-like inner substance.

The supportive properties of the notochord result from its elasticity and the strength of its outer membranes. When lower vertebrates propel themselves with wavelike movements, the notochord flexes owing to the action of the segmented lateral muscle of the trunk, and extends owing to its own elasticity. The notochord is retained during the embryogenesis of higher vertebrates as a result of the supportive function of the skeletal axis in embryos and also owing to the inductive action of the notochord’s rudiment on the formation of the neural tube.

The dried notochord of fishes of the genus Acipenser (sturgeons) is known as viaziga and is used in cooking.

N. S. LEBEDKINA

notochord

[′nōd·ə‚kȯrd]
(vertebrate zoology)
An elongated dorsal cord of cells which is the primitive axial skeleton in all chordates; persists in adults in the lowest forms (Branchiostoma and lampreys) and as the nuclei pulposi of the intervertebral disks in adult vertebrates.