20 GLANDS IN HEALTH AND DISEASE 



of a tadpole. When, however, such superannuated 

 tadpoles are fed with thyroid they promptly under- 

 go metamorphosis. 



"These observations cleared up an old biological 

 puzzle. Salamanders also undergo a metamorpho- 

 sis which is, however, less striking than that of the 

 tadpole of a frog. In the salamander the meta- 

 morphosis consists chiefly in the throwing off of 

 the gills, and in changes in skin and tail. In 

 Mexico a salamander occurs which through its 

 whole life maintains its tadpole form, namely, the 

 axolotl. Attempts to induce the axolotl to meta- 

 morphose failed until after Gudernatsch's discov- 

 ery an investigator fed the axolotl thyroid gland, 

 and this brought about metamorphosis. ... It 

 seemed possible that the iodine contained in the 

 thyroid was the active principle causing metamor- 

 phosis in tadpoles. This was confirmed by Swingle 

 who succeeded in inducing metamorphosis in tad- 

 poles by feeding them with traces of inorganic 

 iodine." 



Administration of thyroid extract. Thyroid ex- 

 tract may be administered in one of three ways: 

 by mouth, by injection beneath the skin (subcu- 

 taneous), or by injection into a vein (intravenous). 

 The last method yields the quickest response. This 

 might be expected if we remember that the thyroid 

 hormone, and all other hormones, reach the various 

 parts of the body by means of the blood stream. 



