unfolding of tissues and functions, organs and faculties summed up as development or differentiation. Thus by the direct feeding of thyroid at particular points in the differentiating history most curious effects have been elicited. If the gland is made part of the nutriment, the bathing environment, of the tadpole, a hastening of its metamorphosis is attained. The tadpole lives not out its day as a tadpole, but precociously turns into a frog. But such a frog! It is a miniature frog, a dwarf frog, a frog seen by looking through the wrong end of the telescope, a frog not magnified, but micrified. Frogs have been so created the size of flies. There has occurred a splitting of the two reactions which ordinarily go hand in hand: the reaction of growth which is just brute increase of total mass or weight and volume, and the reaction of differentiation which is the finer process. The picture is a frog, but a frog the size of a tadpole, a frog which has missed its childhood, adolescence and youth, skipping over these transition stages into the adult age, as a pigmy. It is all as if a baby were suddenly to grow a beard and moustache, evolve and shed teeth, and acquire the manner of an earnest citizen, and yet retain the height and weight of a baby. That the spectacle of such a superbaby is not quite the most fantastic of all improbabilities is shown by the condition of progeria, first recorded by the Briton, Hastings Guilford. A queer spectacle in which a child incontinently grows old without having lived--in the course of a few weeks or months. You look upon him and see senility on a small scale, but with all its peculiarities: wrinkled skin, apathy, gray hair and all the rest of it. All we can say about it is that it is probably due to a paralysis of all the glands of internal secretion, a removal of their influence upon the cells. Contrariwise to the feeding of thyroid, removal of the thyroid of tadpoles will prevent their development into frogs. If iodine is then fed to them, say mixed with flour, normal metamorphosis will occur. If Body is the tool chest which we carry about with us, as Samuel Butler said, then to the thyroid belongs the name of tool-maker.