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THE NOTOCHORD
     In most vertebrates today, a bony vertebral column provides the primary support along the long axis of the body, as in the model of the human vertebral column below.
SPINE
     A vertebral column composed of a series of solid bony vertebrae did not exist for the first several hundred years of vertebrate evolution.  The first “vertebrae” were small bony elements which supported a structure which was the primary longitudinal support in ancestral vertebrates, the notochord.  The notochord is a flexible, elastic rod that runs down the longitudinal axis of the body made of a core of cells in a tough fibrous coat.   It is important for movement since without it, muscle contraction would cause the entire body to shorten instead of the side-to side body flexion which the chordates depend on. Leeches, for example, lack this longitudinal support rod and extend and retract their bodies rather than swim by laterally moving their backs.
LEECH LEECH LEECH

     Although the notochord no longer serves this support function in most modern vertebrates (because it has been replaced by a vertebral column), the notochord is still present in all vertebrate embryos where it promotes the formation of other structures (like the central nervous system).  In adult humans, the remnants of the notochord form the central region of the intervertebral disks between our vertebrae (called the nucleus pulposus).

     Nemertine worms possess a rod-like structure which has been compared to the notochord, although its identity as a homologous structure has not been established (Sarnat, 1985).  Some hemichordates (the acorn worms) possess a rod-like structure whose homology to the chordate notochord has long been debated, although most experts today feel that the two are not homologous.  The most primitive group of animals which possess a notochord are the urochordates (such as the tunicate larva below), although the notochord only exists in the larval stage. 

TUNICATE
TUNICATE

The notochord is one of four major features which define chordates as a group (in addition to a postanal tail, pharyngeal arches, and a hollow, dorsal nerve cord).  It is well developed in cephalochordates, the lancelets.

Amphioxus

lancelet lancelet
lancelet

In lancelets, the notochord extends to the very tip of the head, which is the source of their name, the cephalochordates.  In jawless fish, the formation of hypophysis limits the cranial extension of the notochord. The most primitive fish today, the jawless fish, do not have full-fledged vertebrae, and still depend on the notochord as their primary axial support (Barteczko, 1999; Stach, 2000). 

Lamprey larva

lamprey
hagfish lamprey
Even once vertebrates began to develop the bony elements which sould contribute to vertebrae, these structures arose in pieces around a notochord which still ran uninterrupted from head to tail.  The following vertebrae from a shark have formed around a notochord which runs interrupted from head to tail.
shark
Human embryos possess a notochord running from head to tail as their initial longitudinal support.
human embryo
human embryo
human embryo

    The notochord develops cranially until it reaches the site of the mouth (oropharyngeal membrane).  It establishes the long axis of the embryo, provides support, and sends signals to surrounding tissues.  The nerve chord develops dorsal to it and vertebrae develop around it.  In adults, remnants of the notochord form the nucleus pulposus of the intervertebral disks and additional remnants in some individuals may cause benign or malignant chordomas. (Moore, p. 69-70)

Even in animals which rely on a vertebral column for longitudinal support in adults, an uninterrupted notochord forms in the embryo.  In developing chick embryos, the cells lateral to the primitive streak are capable of forming a new notochord if the notochord is removed (Yuan, 1995).

 

Frog

frog embryo frog embryo
frog embryo frog embryo
frog embryo
chick embryo
chick embryo chick embryo
chick embryo chick embryo
chick embryo chick embryo
pig embryo
pig embryo pig embryo