The Origin of Vertebrates

by Walter Holbrook Gaskell

As regards its structure, its tissue is, as already stated, something sui generis. Notochordal tissue has no resemblance to bone or cartilage, or any of the usual supporting tissues. Such a tissue is not, however, entirely confined to the notochord of the vertebrates, but tissue closely resembling it has been found not only in Amphioxus and the Tunicata, but in certain other invertebrates, in the Enteropneusta (Balanoglossus, etc.), in Cephalodiscus, and in Actinotrocha. In all these latter cases, such a tissue is invariably found in disused portions of the alimentary canal; a diverticulum of the alimentary canal becomes closed, vacuolation of its lining cells takes place, and a tissue resembling notochordal tissue is formed.

Owing to the notochord being invariably so striking and mysterious a feature of the lowest vertebrates, the term vertebrate, which is inappropriate in the members of the group which do not yet possess vertebræ, has been largely superseded by the term chordate, with the result of attributing an undue preponderance to this tissue in any system of classification. Hence, wherever any animal has been found with a tissue resembling that of the notochord, enthusiasts have immediately jumped to the conclusion that a relationship must exist between it and the chordate animals; and, accordingly, they have classified such animals as follows: Amphioxus belongs to the group Cephalochorda because the notochord projects beyond the central nervous system; the Tunicata are called Urochorda because it is confined to the tail; the Enteropneusta, Hemichorda, because this tissue is confined to a small diverticulum of the gut, and, finally, Diplochorda has been suggested for Actinotrocha and Phoronis because two separate portions of the gut are transformed in this way.

This exaggerated importance given to any tissue resembling in structure that of the notochord is believed in by many of those who profess to be our teachers on this subject, the very men who can deliberately shut their eyes to the plain reading of the story of the pineal eyes, and say, "In our opinion this pineal organ was not an eye at all."

The only legitimate inference to be drawn from the similarity of structure between the notochord and these degenerated gut-diverticula, is that the structure of the notochord may have arisen in the same way, and that therefore the notochord may once have functioned as a gut. With cessation of its function its cells became vacuolated, as in these other cases, and its lumen became filled with notochordal tissue. This evidence strongly confirms the suggestion that the notochord was once a digestive tube, but by no means signifies that such tissue, wherever found, indicates the presence of a notochord.

In order to resemble a notochord, this tissue must possess not only a definite structure but a definite position, and this position is a remarkably striking and suggestive one. The notochordal tube is unsegmented, although the vertebrate is markedly segmented. But in all segmented animals the only unsegmented tube which extends the whole length of the body, from mouth to anus, is invariably the gut. In the vertebrate there are three such tubes: (1) the gut itself, (2) the central canal of the nervous system, and (3) the notochordal tube.

The first is the present gut, the second the gut of the invertebrate ancestor, and the third the tube in question.

These three unsegmented tubes, extending along the whole length of the segmented animal, constitute the great peculiarity of the vertebrate group; it is not the unsegmented notochord alone which requires explanation, but the presence of three such tubes in the same animal. Any one of them might be the unsegmented gut of the segmented animal. The most ventral tube is the actual gut of the present vertebrate; the most dorsal—the neural canal—was, according to my view, the original gut of the invertebrate ancestor; the middle one—the notochordal tube—was, in all probability, also once a gut, formed at the time when the exigencies of the situation made it difficult for food to pass along the original gut.

Fig. 166.—Diagram to show the Meeting of the Four Tubes in such a Vertebrate as the Lamprey.

Nc., neural canal with its infundibular termination; Nch., notochord; Al., alimentary canal with its anterior diverticulum; Hy., hypophysial or nasal tube; Or., oral chamber closed by septum.