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 Ammocoetes. Dohrn concludes, in my opinion rightly, that the endostyle in the Tunicata and in Amphioxus represents the remnants of the more elaborate organ in Ammocoetes, and that, therefore, in order to explain the meaning of these organs in the former animals, we must first find out their meaning in Ammocoetes. Dohrn, however, goes further than this; for just as he considers Amphioxus and the Tunicata to have arisen by degeneration from an Ammocoetes-like form, so he considers Ammocoetes to have arisen {199}from a degenerated Selachian; therefore, in order to be logical, he ought to show that the thyroid of Ammocoetes 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 Ammocoetes thyroid is so immeasurably more complicated and elaborate a structure than is that of Petromyzon, as to make it impossible to believe that the Ammocoetes 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 Ammocoetes, 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.