In 1912 it was shown that thyroid hormones would change a tadpole into a frog. This metamorphosis is complex at all levels. The tails dissolve away, legs are developed on the side, the lungs are changed over to air breathing, and the liver, without any detectable change in the DNA or cellular morphology, changes over biochemical mechanisms from an ocean water animal to a land animal. Although the effects of thyroid hormone appear to be systemic in the tadpole, in fact, thyroid hormone is affecting each cell individually. But more importantly, if the thyroid gland is removed and Iodine is given in any form--injection, orally or in the bathing solution--metamorphosis will carry along at the same rate as if thyroid hormone was present. This suggests that the ability of tadpoles to synthesize thyroid hormone from iodine alone is retained inside every cell."