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Updated: June 14, 2025
We meet the same simple medullary tube in the first structure of the ascidia larva, in the same characteristic position, above the chorda. On closer examination we find here also a small vesicular swelling at the fore end of the tube, the first trace of a differentiation of it into brain and spinal cord.
At the base is the 'cricoid cartilage, the support and bond of union of the rest. Above are the 'arytenoid cartilages, resting on the 'chorda vocales' and influencing their action.
The base or ventral side of the vesicular cranial capsule corresponds originally to a number of developed vertebral bodies; its vault or dorsal side to their combined upper vertebral arches. Intervertebral disk of a new-born infant, transverse section. a rest of the chorda.
Undoubtedly both the Tunicates and Acrania have inherited the chorda from a common unsegmented stem-form; and these ancient, long-extinct ancestors of all the chordonia are our hypothetical Prochordonia.
Though less sharply defined, this gelatinous nucleus of the elastic cartilaginous disks persists throughout life in the mammals, but in the birds and most reptiles the last trace of the chorda disappears. Then there is a special osseous nucleus formed in each half of the vertebral arch. The ossification does not reach the point at which the three nuclei are joined until after birth.
In consequence of these important folding-processes in the primitive gut, the simple entodermic tube divides into four different sections: 1. underneath, at the ventral side, the permanent alimentary canal or permanent gut; 2. above, at the dorsal side, the axial rod or chorda; and 3. the two coelom-sacs, which immediately sub-divide into two structures:
The primary axial skeleton remains a simple chorda throughout life in the Acrania, the Cyclostomes, and the lowest fishes. In most of the other Vertebrates the chorda is more or less replaced by the cartilaginous tissue of the secondary perichorda that grows round it. In the mammals it disappears for the most part.
The letters have the same meaning throughout: N medullary tube, Ch chorda, M lateral muscles, Lh body-cavity, G part of the body-cavity in which the sexual organs are subsequently formed. But the body afterwards undergoes various modifications, especially in the fore-part.
Bateson believes he has detected a rudimentary chorda between the two. Of all extant invertebrate animals the Enteropneusts come nearest to the Chordonia in virtue of these peculiar characters; hence we may regard them as the survivors of the ancient gut-breathing Vermalia from which the Chordonia also have descended.
On the other hand, the long straight vessel that runs along the dorsal line of the gut above, between it and the chorda, and contains arterial blood, is clearly identical with the aorta or principal artery of the other vertebrates; and on the other side it may be compared to the dorsal vessel in the worms.
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