The Origin of Vertebrates

by Walter Holbrook Gaskell

Yet another circumstance in favour of this suggestion is the very striking position of the anterior termination of the notochord. It terminates at the point of convergence of three structures:—

(1) The tube of the hypophysis or nasal tube.

(2) The infundibulum or old mouth-termination.

(3) The notochordal tube.

To these may be added, according to Kupffer, in the embryonic stage, the anterior diverticulum of the gut (Fig. [166]).

This is a very significant point. Here originally, in the invertebrate stage, the olfactory passage opened into the old mouth and œsophagus. Here, finally, in the completed vertebrate the same olfactory passage opens into the new pharynx. In the stage between the two it may well have opened into an intermediate gut, the notochordal tube, its separation from which would leave the end of the notochord blind, just as it had already left the end of the infundibulum blind.

The whole evidence points to the derivation of the notochord from a ventral groove on the surface of the animal, which closed to form a tube capable of acting as an accessory gut at the critical period before the new gut was fully formed. The essentials of a gut tube are absorption and digestion of food; is it likely that a tube formed as I have suggested would be efficient for such purposes?

As far as absorption is concerned, no difficulty would arise. The gut of the arthropod is lined with a thin layer of chitin, which is traversed, like all other chitinous surfaces, by fine canaliculi. Through these canaliculi, absorption of fluid material takes place, from the gut to the body. Similar canaliculi occur in the chitin covering the animal externally, so that, if such external surface formed a tube, and food in the right condition for absorption passed along it, absorption could easily take place through the chitinous surface. The evidence of Apus proves that food does pass along such a tube in the open condition, and in the trilobites the chitinous surface lining a similar groove was apparently very thin, a condition still more favourable to such an absorption process.

At first sight the second essential of a gut-tube—the power of digestion—appears to present an insuperable difficulty to this method of forming an accessory gut-tube, for it necessitates the formation of a secretion capable of digesting proteid material by the external cells of the body, whereas until recently it was supposed that such a function was confined to cells belonging to the so-called hypoblastic layer. Experiments were made now years ago of turning a Hydra inside out so that its internal layer should become external, and vice versâ, and they were said to have been successful. Such an animal could go on living and absorbing and digesting food, although its epiblastic surface was now its digestive internal surface. More recent observations have shown that these experiments were fallacious. At night-time, when the observer was not looking, the hydra reinverted itself, so that again its original digestive surface was inside and it lived and prospered as before.

Another piece of evidence of somewhat similar kind, which has not as yet been discredited, is seen in the Tunicata. In many of these, new individuals are formed from the parent by a process of budding, and it has been proved that frequently the gut of the new individual thus budded off arises not from the gut or hypoblastic layer of the parent, but from the surface or epiblastic layer. Such gut so formed possesses as efficient digestive powers as the gut of the parent.