[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.
Thus we see that the mesenchyme cells giving rise to legs may lie dormant for months or a year but will grow out when a certain type of substances, _e. g._, thyroid, circulates in the blood. There may exist an analogy between the activating effect of the thyroid substance and the activating effect of the spermatozoön or butyric acid (or other parthenogenetic agencies) upon the egg, but we cannot state that the thyroid substance activates the mesenchyme cells by altering their cortical layer.