There are, too, individuals in whom hyperthyroidism and hypothyroidism are mixed, or rather alternate. At one time they present the phenomena of the one, at another of the other. They are the people who complain of the cyclic quality of their moods and purposes. Their mood will be a heaven of exaltation and exhilaration, and then descend into a slough of despond from which they feel themselves inextricable. They are always talking about the ups and downs of their mental states. Headache and languor and fatigability, dry skin and lack of appetite for food or exertion on one day or for one week, give way on the next day, or for the next week, to an energetic gayety, and sweaty, flushed skin, a prominent appetite for food and every sort of activity. Driven to be forever on the go, for one period, in the next they feel like lying down most of the day, with no inclination for any life whatever. The stage of depression may go as far as a melancholia, the stage of stimulation as far as mania. They may simulate manic-depressive or cyclic insanity. Something restrains them, and holds them bound as in a vise in the one cycle. And then they are driven on beyond themselves by some invisible whip in the next. THYROID AS DIFFERENTIATOR Besides the action of the thyroid as energizer, lubricator, and growth catalyzer, it has a remarkable power as a differentiator of tissues. It determines the embryonic etchings of the different organs which in their totality comprise the unique individual. Every multicellular animal must first have existed as a single cell, the impregnated ovum. With the body and personality of the ovum, the creature is one and continuous, literally something the single cell has made of itself by sub-dividing and differentiating. In the process, the cell mass often goes through stages which stand out as individualities in themselves, that appear on the surface absolutely unrelated. So the caterpillar and the butterfly, to the naïve child, seem as far apart as worm and bird. In the case of the frog, the tadpole as a first sketch seems completely an impossible and wild absurdity. Yet we know that there is an orderly progression of events, a propagation of cells, a forward going arrangement of chemical reactions, that results in expansion and intricate complication of the organism. Just what the forces at work in this most mysterious of all natural processes are, has been an intellectual mystery that the best minds of the race have attempted to get rid of with words like pangenesis (Darwin). Words of Black (Mediterranean or Greek and Latin) origin, as Allen Upward has named them, always cover a multitude of ignorances. The glands of internal secretion, here, as in so many other dark places, provide the open sesame to certain long closed doors of biology. They offer themselves to us as the first definitely tangible agents which are known to keep the process of growth going, and undoubtedly initiate the marvelous