“The axolotl is remarkable because it becomes sexually mature while still retaining the external gills of its larval form. It seems, however, that this is due to nutritional problems. The change from larva to adult is triggered by hormones, including thyroxine produced by the thyroid gland. Conditions in this one lake, both chemical and physical, are such that this gland does not develop properly. But that can be corrected. If an axolotl (Ambystoma mexicanum) is kept in a tank and a little thyroxine added to its water, the animal loses its external gills, climbs out of water and assumes a terrestrial life.”
Book:Life in Cold Blood
