No other organ of the body has so weird an influence upon the well-being of the whole. No other organ has an equally mysterious ancestral history. Assuredly the thyroid gland was not always such as we see it now. In prevertebrate animals it must have been quite different, both in structure and in function. From fishes upwards, however, its structure is always the same. It is composed of spherical vesicles or globes. Every globe is lined by a single layer of cubical epithelial cells. Its cavity is filled with a homogeneous semi-solid substance known as “colloid.” The globes are associated into groups or lobules. They are in contact with large wide lymphatic vessels. The organ has a lavish supply of blood. It is also well supplied with nerves. Colloid is the secretion of the epithelial cells which line the globes. As these globes have no openings, the secretion must be passed by osmosis into the lymphatic vessels. There is abundant reason for believing that by this route the products of the gland reach the blood, and are distributed by the blood to all the tissues of the body. And here it is important to notice that associated with the thyroid gland are certain very small masses of tissue termed “parathyroids.” There may be four of these—two on the course of the large arteries which supply the thyroid gland from above, two related with the almost equally large arteries which supply it from below; but the number varies. The parathyroids do not contain vesicles. They are solid masses of epithelial cells, traversed by bloodvessels and lymphatics. Yet, like the epithelial cells of the vesicles, they secrete colloid. Granules of this substance are to be seen within their cells. We cannot pass over the parathyroids without this reference, since, small though they are, they seem to be quite as important as the thyroid gland itself, judging from the effects which follow their removal. In all vertebrate animals the thyroid gland has the characters which we have described. What was it like in the ancestors of the vertebrate races? Its microscopic appearance in vertebrates, the only animals in which we know it, is so anomalous as to convince an histologist that it is a makeshift; it looks like an organ which, at a period no longer visible through the mists of time, had a quite different function to perform. This function it has lost—some other organ has taken it on—yet it must do something which is useful to the organism. Otherwise it would not have been preserved. It has been retained for the sake of its by-function, for the sake of the internal secretion which it produces. This is now the only work it has to do.