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.
An interesting observation on the rôle of internal secretion in growth was made by Leo Loeb.[149] When the fertilized ovum comes in contact with the wall of the uterus it calls forth a growth there, namely the formation of the maternal placenta (decidua). This author showed that the corpus luteum of the ovary gives off a substance to the blood which alters the tissues in the uterus in such a way that contact with any foreign body induces this deciduoma formation. The case is of interest since it indicates that the substance given off by the corpus luteum does not induce growth directly, but that it allows mechanical contact with a foreign body to do so while without the intervention of the corpus luteum substance no such effect of the mechanical stimulus would be observable. The action of the substance of the corpus luteum is independent of the nervous system, since in a uterus which has been cut out and retransplanted the same phenomenon can be observed.