The Origin of Vertebrates

by Walter Holbrook Gaskell

The latter theory derives the alimentary canal of the vertebrate from that of the invertebrate, and finds in the latter the commencement of the notochord. In the comparison which I have made the alimentary canal of the invertebrate ancestor has become the tube of the central nervous system of the vertebrate, and there is no sign of a notochord whatever. All the organs of the arthropod have already been allocated; where the notochord is situated in the vertebrate there is nothing but a gap in the invertebrate, but the position of that gap can be settled with great accuracy from the previous comparison of organs in the two groups. So, also, the alimentary canal of the vertebrate is from the very nature of the case a new organ, yet, as has been shown in Chapter V., the comparison of the respiratory organs in the two groups gives a strong suggestion of the manner in which such a canal was formed.

The Origin of the Notochord.

The time has now come to endeavour to frame a plausible theory of the method of formation of the notochord and the new alimentary canal, and thus to complete the diagram on p. [413]. The comparative method is no longer available, for these structures are both unrepresented as such in the arthropod; any suggested explanation, therefore, must be more tentative, and cannot give the same feeling of certainty as is the case with all the organs already considered. Our only chance of finding out the past history of the notochord lies in the embryological method, in the hope that, according to the 'law of recapitulation,' the ancestral history may be repeated in the ontogeny with sufficient clearness to enable some conclusion to be drawn.

At the outset, one point comes out clearly—the close relationship between the notochord and the vertebrate gut; they are both derived from the same layer, both parts of the same structure. On this point all embryologists are agreed; it is expressed in such statements as, "the notochord, as well as the alimentary canal, is formed from hypoblast"; "the notochord arises as a thickening in the dorsal wall of the alimentary canal." The two structures are so closely connected together that they must be considered together. If we can conjecture the origin of the one, we may be sure that we have the clue to the origin of the other. The two together form the one new organ which distinguishes the vertebrate from the arthropod, the only thing left which requires explanation for the completion of this strange history.

What, then, is the notochord? What are its characteristics? In the highest vertebrates it is conspicuous only in the embryo; with the development of the axial skeleton it is more and more squeezed out of existence, until in the adult it is no longer visible. By the 'law of recapitulation' this developmental history implies that, as we descend the vertebrate phylum, the notochord ought to be more and more conspicuous, more and more permanent during the life of the animal. Such is, indeed, found to be the case, until at last, in the lowest vertebrates, such as the lamprey, and in forms like Amphioxus, the notochord persists throughout the life of the animal as a large important axial supporting rod.

This rod has a number of striking characteristics which distinguish it from all other structures, and are the only means of guessing its probable origin. Its position in the body is always the same in all vertebrates and is very significant, for it lies just ventrally to the central nervous system, along nearly the whole length of the animal, not quite the whole length, for it invariably terminates close to the place where the infundibulum comes to the surface of the brain; it is, in fact, always confined to the infra-infundibular and spinal cord part of the central nervous system. Interpreting this into the language of the arthropod, it means that a rod was formed just ventrally to the nervous system, which extended the whole length of the infraœsophageal and ventral chain of ganglia, and terminated at the orifice of the mouth. Moreover, this rod was unsegmented, for the notochord is devoid of segmentation.

At the anterior end the rod tapers to a point, as in Fig. [166]. In its middle part it is very large and conspicuous, cylindrical in shape; its interior is filled with a peculiar vacuolated tissue, different to any other known vertebrate tissue, which has therefore received the name of notochordal tissue. Outside this is a thick sheath formed of many layers, of which the external one gives the staining reactions of elastin, and is called the external elastic layer. Between this sheath and the notochordal tissue a thin layer of lining cells, of normal appearance, is conspicuous in Ammocœtes. These cells secrete the layers of the sheath, and have originally, by proliferation, given rise to the notochordal tissue. In the notochord of Ammocœtes there is no sign of either nerves, blood-vessels, or muscles.

The centre of the notochord presents the appearance of a slight slit, as though it had originated from a tube, and that is the opinion now generally held, for its mode of formation in the embryo is as that of a tube formed from an open groove, as will be explained immediately.

We may, then, conceive of the notochord as originally a tube lying in the mid-line just ventrally to the central nervous system, and extending from the original mouth to the end of the body. Translate this into the language of the arthropod and it denotes a tube on the mid-ventral surface of the body, which extended from mouth to anus. Such a tube might be formed from the mid-ventral surface as follows:—

In Fig. [163], A, the lining of the ventral surface between two appendages is represented flat, in B is shown how the formation of a solid rod may arise from the bulging of that ventral surface, and in C how a groove on that surface may lead to the formation of a tube between the two appendages. The difference between a notochordal rod formed as in B from that in C would be shown in the sheath, for in B the sheath would be formed from the cuticle of the lining cells, and in C from the basement membrane. The structure of the sheath is in accordance with the embryological evidence that the notochord is formed as a tube from a groove, as in C, and not as a solid rod as in B, for it possesses a well-marked elastin layer, and elastin has never yet been found as a constituent of any cuticular secretion, but invariably in connection with basement-membranes.