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Updated: June 14, 2025
We have previously seen how this very rudimentary beginning of the skull in man is formed ontogenetically from the "head-plates," and thus the fore end of the chorda is enclosed in the base of the skull. The phylogeny of the skull has made great progress during the last three decades through the joint attainments of comparative anatomy, ontogeny, and paleontology.
Gegenbaur referred on many other occasions to the close blood-relationship of the Tunicates and Vertebrates, and luminously explained the reasons that justify us in framing the hypothesis of the descent of the two stems from a common ancestor, an unsegmented worm-like animal with an axial chorda between the dorsal nerve-tube and the ventral gut-tube.
I shall be met with the objection that a uniform system will be free from irregularities. I answer that it would be an irregularity to be too uniform, that would offend against the rules of harmony. Et citharoedus Ridetur chorda qui semper oberrat eadem.
The fin-fringe is supported by a number of square elastic fin-plates. The chorda develops no further, but retains its original simplicity throughout life. It is enclosed by a firm membrane, the chorda-sheath or perichorda. The perichorda forms a cylindrical tube immediately over the chorda, and the central nervous system, the medullary tube, is enclosed in it.
A right conception of the genesis of a vertebral column, includes not only the differentiations from which result the chorda dorsalis and the vertebral segments imbedded in it; but quite as much it includes the coalescence of numerous vertebral processes with their respective vertebral bodies.
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