The Origin of Vertebrates

by Walter Holbrook Gaskell

The series of transverse sections (1-6, Fig. [81]) show that we are dealing here with a central glandular chamber, C (Fig. [81] (6) and Fig. [82]), which opens by the thyroid duct (Th. o.) into the pharyngeal chamber, and is curled upon itself in its more posterior part. This central chamber divides, anteriorly to the thyroid orifice, into two portions, A, A′ (Fig. [82]), giving origin to two tubes, B, B′, which lie close alongside of, and extend further back than, the posterior limit of the curled portion of the central chamber, C. The structure of the central chamber, C, and, therefore, of the separate coils, is given in both Schneider's and Dohrn's pictures, and is represented in Fig. [81] (6), which shows the peculiar arrangement and character of the glandular cells typical of this organ, and also the nature of the central cavity, with the arrangement of the ciliated epithelium. The structure of each of the lateral tubes, B, is different from that of the central chamber, in that only half the central chamber is present in them, as is seen by the comparison of the tube B with the tube C in Fig. [81] (5 and 6), so that we may look upon the central chamber, C, as formed of two tubes, similar in structure to the tubes B, which have come together to form a single chamber by the partial absorption of their walls, the remains of the wall being still visible as the septum, which partially divides the chamber, C, into halves.

In the walls of each of these tubes is situated a continuous glandular line, the structure of the glandular elements being specially characterized by the length of the cells, by the large spherical nucleus situated at the very base of each cell, and by the way in which the cells form a wedge-shaped group, the thin points of all the wedge-shaped cells coming together so as to form a continuous line along the chamber wall. This free termination of the cells of the gland in the lumen of the chamber constitutes the whole method for the secretion of the gland; there is no duct, no alveolus, nothing but this free termination of the cells.

Moreover, sections through the portion A, A′ (Fig. [82]) show that here, as in the central chamber, C, four of these glandular lines open into a common chamber, but they are not the same four as in the case of the central chamber, for if we name these glandular lines on the left side a b, a′ b′ (Fig. [81]), and on the right side c d, c′ d′, then the central chamber has opening into it the glands a b, c d, while the chambers of A and A′ have opening into them respectively a b, a′ b′, and c d, c′ d′. Further, the same series of sections shows that the glands a and b are continuous with the glands a′ and b′ respectively across the apex of A, and similarly on the other side, so that the two glandular rows a b are continuous with the two glandular rows a′ b′, and we see that the cavity of the portion A or A′ is formed by the bending over of the tube or horn, B or B′, with the partial absorption of the septum so formed between the tube and its bent-over part. If, then, we uncoil the curled-up part of C, and separate the portion, B, on each side from the chamber, C, we see that the so-called thyroid of Ammocœtes may be represented as in Fig. [83], i.e. it consists of a long, common chamber, C, which, for reasons apparent afterwards, I will call the palæo-hysteron, which opens, by means of a large orifice, into the respiratory or pharyngeal chamber. The anterior end of this chamber terminates in two tubes, or horns, B, B′, the structure of which shows that the median chamber, C, is the result of the amalgamation of two such tubes, and consequently in this chamber, or palæo-hysteron, the glandular lines are symmetrically situated on each side.

Fig. 82.—Diagrammatic Representation of the so-called Thyroid Gland of Ammocœtes.

C, central chamber; A, A′, anterior extremity; B, B′, posterior extremity; Th. o., thyroid opening into respiratory chamber; Ps. br., Ps. br′., ciliated grooves, Dohrn's pseudo-branchial grooves.

Fig. 83.—Thyroid Gland as it would appear if the Central Chamber were Uncurled and the Two Horns, B, B′, separated from the Central Chamber.

Any explanation, then, of the thyroid gland of Ammocœtes, must take into account the clear evidence that it is composed of two tubes, which have in part fused together to form an elongated central chamber, in part remain as horns to that chamber, and that in its walls there exist lines of gland-cells of a striking and characteristic nature.

Further, this central chamber, with its horns, is not a closed chamber, but is in communication with the pharyngeal or respiratory chamber by three ways. In the first place, the central chamber, as is well known, opens into the respiratory chamber by a funnel-shaped opening—the so-called thyroid duct (Th. o.). In the second place, there exist two ciliated grooves (Ps. br., Ps. br′.), the pseudo-branchial grooves of Dohrn, which have direct communication with the thyroid chamber. The manner in which these grooves communicate with the thyroid chamber has never, to my knowledge, been described previously to my description in the Journal of Physiology and Anatomy; it is very instructive, for, as I have there shown, each groove enters into the corresponding lateral horn, so that, in reality, there are three openings into the thyroid chamber or palæo-hysteron—a median opening into the central chamber, and a separate opening into each lateral horn.