The Origin of Vertebrates

by Walter Holbrook Gaskell

It is very suggestive that the direct following out of the original working hypothesis should lead to this conclusion, for it is universally agreed by all morphologists that the present mouth is a new formation, and Dohrn has argued strongly in favour of the mouth being formed by the coalescence of a pair of gill-slits. Interpret this in the language of my theory, and immediately we see, as already explained, gill-slits must mean in this region the spaces between appendages which did not carry gills; the mouth, therefore, was formed by the coalescence of a pair of appendages to form a lower lip just as I have pointed out.

Where, then, must we look for the palæostoma, or original mouth? Clearly, as already suggested, it was situated at the base of the olfactory passage, and the olfactory passage or nasal tube of Ammocœtes was originally the tube of the hypophysis, so that the following out of the theory points directly to the tube of the hypophysis as the place where the palæostoma must be looked for.

This conclusion is not only not at variance with the opinions of morphologists, but gives a straightforward, simple explanation why the palæostoma was situated in the very place where they are most inclined to locate it. Thus, if we trace the history of the question, we see that Dohrn's original view of the comparison of the vertebrate and the annelid led him to the conception that the vertebrate mouth was formed by the coalescence of a pair of gill-slits, and that the original mouth was situated somewhere on the dorsal surface and opened into the gut by way of the infundibulum and the tube of the hypophysis. This, also, was Cunningham's view as far as the tube of the hypophysis was concerned. Beard, in 1888, holding the view that the vertebrates were derived from annelids which had lost their supra-œsophageal ganglia, and that, therefore, there was no question of an œsophageal tube piercing the central nervous system of the vertebrate, explained the close connection of the infundibulum with the hypophysis by the comparison of the tube of the hypophysis with the annelidan mouth, so that the infundibular or so-called nervous portion was a special nervous innervation for the original throat, just as Kleinenberg had shown to be the case in many annelids. Beard therefore called this opening of the hypophysial tube the old mouth, or palæostoma. Recently, in 1893, Kupffer has also put forward the view that the hypophysial opening is the palæostoma. basing this view largely upon his observations on Ammocœtes and Acipenser.

Fig. 125.—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.

As is seen in Fig. [125], the position of this palæostoma is a very suggestive one. At this single point in Ammocœtes, four separate tubes terminate; here is the end of the notochordal tube, the termination of the infundibulum, the blind end of the nasal tube or tube of the hypophysis, and the pre-oral elongation of the alimentary canal.

It is perfectly simple and easy for the olfactory tube to open into any one of the other three. By opening into the infundibulum it reproduces the condition of affairs seen in the scorpion; by opening into the gut it produces the actual condition of things seen in Myxine and other vertebrates; by opening into the notochordal tube it would produce a transitional condition between the other two.

The view held by Kupffer is that this nasal tube (tube of the hypophysis) opened into the anterior diverticulum of the vertebrate gut, and was for this reason the original mouth-tube; then a new mouth was formed, and this connection was closed, being subsequently reopened as in Myxine. My view is that this tube originally opened into the infundibulum, in other words, into the original gut of the palæostracan ancestor, and was for this reason the original mouth-tube, in the same sense as the olfactory passage of the scorpion may be, and often is, called the mouth-tube. When, with the breaking through of the septum between the oral and respiratory chambers, the external opening of the oral chamber became a new mouth, the old mouth was closed but the olfactory tube still remained, owing to the importance of the sense of smell. Subsequently, as in Myxine and the higher vertebrates, it opened into the pharynx, and so formed the nose of the higher vertebrates.

It is not, to my mind, at all improbable that during the transition stage, between its connection with the old alimentary canal, as in Eurypterus or the scorpions, and its blind ending, as in Ammocœtes, the nasal tube opened into the tube of the notochord. This question will be discussed later on when the probable significance of the notochord is considered.