Moreover, sections through the portion A, A[prime] (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[prime] b[prime]_ (Fig. 81), and on the right side _c d, c[prime] d[prime]_, then the central chamber has opening into it the glands _a b, c d_, while the chambers of A and A[prime] have opening into them respectively _a b, a[prime] b[prime]_, and _c d, c[prime] d[prime]_. Further, the same series of sections shows that the glands _a_ and _b_ are continuous with the glands _a[prime]_ and _b[prime]_ 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[prime] b[prime]_, and we see that the {196}cavity of the portion A or A[prime] is formed by the bending over of the tube or horn, B or B[prime], 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 Ammocoetes 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[prime], 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. [Illustration: FIG. 82.--DIAGRAMMATIC REPRESENTATION OF THE SO-CALLED THYROID GLAND OF AMMOCOETES. _C_, central chamber; _A, A[prime]_, anterior extremity; _B, B[prime]_, posterior extremity; _Th. o._, thyroid opening into respiratory chamber; _Ps. br., Ps. br[prime]._, ciliated grooves, Dohrn's pseudo-branchial grooves.] [Illustration: FIG. 83.--THYROID GLAND AS IT WOULD APPEAR IF THE CENTRAL CHAMBER WERE UNCURLED AND THE TWO HORNS, _B_, _B[prime]_, SEPARATED FROM THE CENTRAL CHAMBER.] Any explanation, then, of the thyroid gland of Ammocoetes, must {197}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.