The Origin of Vertebrates

by Walter Holbrook Gaskell

The system of ciliated grooves on the inner ventral surface of the respiratory chamber of Ammocœtes was originally described by Schneider as consisting of a single median groove, which extends from the opening of the thyroid to the posterior extremity of the branchial chamber, and a pair of grooves, or semi-canals, which, starting from the region of the thyroid orifice, run headwards and diverge from each other, becoming more and more lateral, and more and more dorsal, till they come together in the mid-dorsal pharyngeal line below the auditory capsules. The latter are the pseudo-branchial grooves of Dohrn, of which I have already spoken. Schneider looked upon the whole of this system as a single system, for he speaks of "a ciliated groove, which extends from the orifice of the stomach (i.e. anterior intestine) to the orifice of the thyroid, then divides into two, and runs forward right and left of the median ridge, etc." Dohrn rightly separates the median ciliated groove posterior to the thyroid orifice (seen in Fig. [81] (6)) from the paired pseudo-branchial grooves; the former is a shallow depression which opens into the rim of the thyroid orifice, while the latter has a much more intimate connection with the thyroid gland itself.

A series of sections, such as is given in Fig. [81], shows the relation of this pair of ciliated grooves to the thyroid better than any elaborate description. In the first place, it is clear that they remain separate up to their termination—they do not join in the middle line to open into the thyroid duct; in the second place, they are separate from the thyroid orifice—they do not terminate at the rim of the orifice, as is the case with the median groove just mentioned, but continue on each side on the wall of the thyroid duct (Fig. [81] (2)), gradually moving further and further away from the actual opening of the duct into the pharyngeal chamber. During the whole of their course on the wall of the funnel-shaped duct they retain the character of grooves, and are therefore open to the lumen of the duct. The direction of the groove (Ps. br.) shifts as it passes deeper and deeper towards the thyroid, until at last, as seen in Fig. [81] (3 and 4), it is continuous with the narrow diverticulum of the turned-down single part of the thyroid (B), or turned-down horn, as I have called it. In other words, the median chamber opens into the pharyngeal or respiratory chamber by a single large, funnel-shaped opening, and, in addition, the two ciliated grooves terminate in the lateral horns on each side, and only indirectly into the central chamber, owing to their being semi-canals, and not complete canals. If they were originally canals, and not grooves, then the thyroid of Ammocœtes would be derived from an organ composed of a large, common glandular chamber, which opened into the respiratory chamber by means of an extensive median orifice, and possessed anteriorly two horns, from each of which a canal or duct passed headwards to terminate somewhere in the region of the auditory capsule.

Dohrn has pointed out that a somewhat similar structure and topographical arrangement is found in Amphioxus and the Tunicata, the gland-cells being here arranged along the hypobranchial groove to form the endostyle and not shut off to form a closed organ, as in the thyroid of Ammocœtes. Dohrn concludes, in my opinion rightly, that the endostyle in the Tunicata and in Amphioxus represents the remnants of the more elaborate organ in Ammocœtes, and that, therefore, in order to explain the meaning of these organs in the former animals, we must first find out their meaning in Ammocœtes. Dohrn, however, goes further than this; for just as he considers Amphioxus and the Tunicata to have arisen by degeneration from an Ammocœtes-like form, so he considers Ammocœtes to have arisen from a degenerated Selachian; therefore, in order to be logical, he ought to show that the thyroid of Ammocœtes is an intermediate downward step between the thyroid of Selachians and that of Amphioxus and the Tunicates. Here, it seems to me, his argument utterly breaks down; it is so clear that the thyroid of Petromyzon links on to that of the higher fishes, and that the Ammocœtes thyroid is so immeasurably more complicated and elaborate a structure than is that of Petromyzon, as to make it impossible to believe that the Ammocœtes thyroid has been derived by a process of degeneration from that of the Selachian. On the contrary, the manner in which it is eaten up at transformation and absolutely disappears in its original form is, like the other instances mentioned, strong evidence that we are dealing here with an ancestral organ, which is confined to the larval form, and disappears when the change to the higher adult condition takes place. Dohrn's evidence, then, points strongly to the conclusion that the starting-point of the thyroid gland in the vertebrate series is to be found in the thyroid of Ammocœtes, which has given rise, on the one hand, to the endostyle of Amphioxus and the Tunicata, and on the other, to the thyroid gland of Petromyzon and the rest of the Vertebrata.

The evidence which I have just given of the intimate connection of the two pseudo-branchial grooves with the thyroid chamber shows, to my mind, clearly that Dohrn is right in supposing that morphologically these two grooves and the thyroid must be considered together. His explanation is that the whole system represents a modified pair of branchial segments distinct from those belonging to the VIIth and IXth nerves. The cavity of the thyroid and the pseudo-branchial grooves are, therefore, according to him, the remains of the gill-pouches of this fused pair of branchial segments, which no longer open to the surface, and the glandular tissue of the thyroid is derived from the modified gill-epithelium. This view of Dohrn's, which he has urged most strongly in various papers, is, I think, right in so far as the separateness of the thyroid segment is concerned, but is not right, and is not proven, in so far as concerns the view that the thyroid gland is a modified pair of gills.

We may distinctly, on my view, look upon the thyroid segment, with its ciliated grooves and its covering plate of muco-cartilage, as a distinct paired segment, homologous with the branchial segments, without any necessity of deriving the thyroid gland from a pair of gills.

The evidence that such a median segment has been interpolated ventrally between the foremost pairs of branchial segments is remarkably clear, for the limits ventrally of the branchial segments are marked out on each side by the ventral border of the cartilaginous basket-work; and it is well known, as seen in Fig. [80], that whereas this cartilaginous framework on the two sides meets together in the middle ventral line in the posterior branchial region, it diverges in the anterior region so as to form a tongue-shaped space between the branchial segments on the two sides. This space is covered over with a plate of muco-cartilage which bears on its inner surface the thyroid gland.

Fig. 84.—Diagram of (A) Ventral Surface and (B) Lateral Surface of Ammocœtes, showing the arrangement of the Epithelial Pits on the Branchial Region, and their innervation by VII., the Facial, IX., the Glossopharyngeal, and X¹-X⁶, the Vagus Nerves.

In addition to this evidence that we are dealing here with a ventral tongue-like segment belonging to the facial nerve which is interpolated between the foremost branchial segments, we find the most striking fact that at transformation the whole of this muco-cartilaginous plate disappears, the remarkable thyroid gland of the Ammocœtes is eaten up, and nothing is left except a small, totally different glandular mass; and now the cartilaginous basket-work meets together in the middle line in this region as well as in the more posterior region. In other words, the striking characteristic of transformation here is the destruction of this interpolated segment, and the resulting necessary drawing together ventrally of the branchial segments on each side.