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. The fact that the substance of the thyroid may induce general growth in the human is too well known to require more than an allusion in this connection. When growth stops in children as a consequence of a degeneration of the thyroid, feeding of the patient with thyroid again induces growth. It may also suffice merely to call attention to the connection between acromegaly and the hypophysis. It was formerly believed that the nervous system acted as a regulator of the phenomena of metamorphosis in animals, but it was possible to show by simple experiments that the central nervous system does not play this rôle and that the regulator must be the blood or substances contained therein. In the metamorphosis of the _Amblystoma_ larva the gills at the head and tail undergo changes simultaneously, the gills being absorbed completely. The writer showed that in larvæ in which the spinal cord was cut in two, no matter at which level,--the sympathetic nerves were in all probability also cut--the two organs continued to undergo metamorphosis simultaneously.[147] Uhlenhuth found that if the eye of a salamander larva is transplanted into another larva the transplanted eye undergoes its metamorphosis into the typical eye of the adult form, simultaneously with the normal eyes of the individual into which it was transplanted.[148] These and other observations of a similar character leave no doubt that substances circulating in the blood and not the central nervous system are responsible for the phenomena of growth and metamorphosis. [147] Loeb, J., _Arch. f. Entwcklngsmech._, 1897, iv., 502. [148] Uhlenhuth, E., _ibid._, 1913, xxxvi., 211.