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Updated: June 15, 2025
The new and characteristic organ was not the vertebral column, or series of vertebræ, or backbone, from which the kingdom has derived its name. This was a later production. The primitive skeleton was the notochord, still appearing in the embryos of all vertebrates and persisting throughout life in fish.
The young of Boltenia promises everything in its tiny backbone or notochord, but it all ends in promise, for that shadow of a great ambition withers away, and the creature is doomed to a lowly and vegetative life.
The late able ichthyologist Heckel pointed out the fact, that, while the Pycnodonts never possess true vertebral centra, they differ in the degree of expansion and extension of the ends of the bony arches of the vertebrae upon the sheath of the notochord; the Carboniferous forms exhibiting hardly any such expansion, while the Mesozoic genera present a greater and greater development, until, in the Tertiary forms, the expanded ends become suturally united so as to form a sort of false vertebra.
Thus these vertebræ join one another by their edges, like two shallow wine-glasses placed rim to rim. Only gradually is the notochord crowded out so that the vertebræ join by their whole adjacent surfaces. Even in highest forms, for the sake of mobility, they are united by washer-like disks of cartilage. Biconcave vertebræ persisted through the oldest amphibia, reptiles, and birds.
The late able ichthyologist Heckel pointed out the fact, that, while the Pycnodonts never possess true vertebral centra, they differ in the degree of expansion and extension of the ends of the bony arches of the vertebrae upon the sheath of the notochord; the Carboniferous forms exhibiting hardly any such expansion, while the Mesozoic genera present a greater and greater development, until, in the Tertiary forms, the expanded ends become suturally united so as to form a sort of false vertebra.
But lying in the central axis of the body it furnishes the very best possible attachment for muscles. Around this primitive notochord was a layer of connectile tissue which later gave rise to the vertebræ forming our backbone. 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.
Constituting a transition type between the amphibians on the one hand, and both the higher reptiles and the mammals on the other, they retained the archaic biconcave vertebra of the fish and in some cases the persistent notochord, while some of them, the theromorphs, possessed characters allying them with mammals.
Goodsir long ago insisted upon the resemblance between Amphioxus and the Ascidians; but the notion of a genetic connection between the two, and especially the identification of the notochord of the Vertebrate with the axis of the caudal appendage of the larva of the Ascidian, is a novelty which, at first, takes one's breath away.
"No thoroughfare" is often written across paths previously followed by blood or nervous impulses, when other paths have been found more economical or effective. But where did the notochord come from? I do not know. It always forms in the embryo out of the entoderm or layer which becomes the lining of the intestine.
He must have been at least ten or twelve years ahead of Yarrell; and he saw the whole significance of it, too he saw where it led to. As I understand it, he actually anticipated in his pamphlet Saint Hilaire's theory of the universal type, and supported the hypothesis by describing the notochord of the amphioxus as a cartilaginous vertebral column.
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