formulated the idea that the formation of new organs in the plant is determined by the existence of specific substances which are carried by the ascending or descending sap. Specific shoot-producing substances are carried to the apex, while specific root-producing substances are carried to the base of a plant. When a piece is cut from a branch of willow the root-forming substances must continue to flow to the basal end of the piece, and since their further progress is blocked there they induce the formation of roots at the basal end. Goebel[143] and de Vries have accepted this view and the writer made use of it in his first experiments on regeneration and heteromorphosis in animals.[144] At that time the idea of the existence of such specific organ-forming substances was received with some scepticism, but since then so many proofs for their existence have been obtained that the idea is no longer questioned. Such substances are known now under the name of “internal secretions” or “hormones”; their connection with the theory of Sachs was forgotten with the introduction of the new nomenclature. [142] v. Sachs, J., “Stoff und Form der Pflanzenorgane,” _Gesammelte Abhandlungen_, 1892, ii., 1160. Arbeiten a. d. bot. Inst. Würzburg, 1880-82. [143] Goebel, K., _Einleitung in die experimentelle Morphologie der Pflanzen_, 1908. [144] Loeb, J., Untersuchungen zur physiologischen Morphologie der Tiere. I. Heteromorphose. Würzburg, 1891. II. Organbildung und Wachsthum. 1892. Reprinted in _Studies in General Physiology_. Chicago, 1906. It may be well to enumerate some of the cases in which the influence of specific substances circulating in the blood upon phenomena of growth has been proven. One of the most striking observations in this direction is the one made by Gudernatsch on the growth of the legs of tadpoles of frogs and toads.[145] The young tadpoles have no legs, but the mesenchyme cells from which the legs are to grow out later are present at an early stage. From four months to a year or more may elapse before the legs begin to grow. Gudernatsch found that legs can be induced to grow in tadpoles at any time, even in very young specimens, by feeding them with the thyroid gland (no matter from what animal). No other material seems to have such an effect. The thyroid contains iodine, and Morse[146] states that if instead of the gland, iodized amino acids are fed to the tadpole the same result can be produced. We must, therefore, draw the conclusion that the normal outgrowth of legs in a tadpole is due to the presence in the body of substances similar to the thyroid in their action (it may possibly be thyroid substance) which are either formed in the body or taken up in the food. [145] Gudernatsch, J. F., _Zentralbl. f. Physiol._, 1912, xxvi., 323; _Arch. f. Entwcklngsmech._, 1912, xxxv., 457; _Am. Jour. Anat._, 1914, xv., 431. [146] Morse, M., _Jour. Biol. Chem._, 1914, xix., 421.