The master orchestrator of these changes is the pituitary gland in the base of the brain. It sends hormonal signals to the thyroid gland which then begins to release thyroid hormones into the blood stream. At low levels of thyroid hormone, the rear legs begin to form, and the large intestine begins to shorten. (Herbivores need longer intestines to digest food than do carnivores.) As the hormone levels increase, the front legs form and the tail begins a sequence of self-destruction. Tail cell contents are dissolved, moved into the blood stream and reused elsewhere.
The eyes of the tadpole are faced to the side, typical of a prey animal which needs to be alert for predators. The eyes move forward during metamorphosis and become very large. New neurons (nerve cells) form, connecting the right eye to the right side of the brain and the left eye to the left brain, supplementing the tadpoles existing right eye to left brain and left eye to right brain development. With these new connections, the frog has binocular vision, very useful for a predator.
Eye pigments also change. The porphyropsin of the tadpole allows it to see better underwater; this pigment is sensitive to reddish light. The frog eye has rhodopsin, which is sensitive to green-blue light and useful for seeing at night on land.
The middle ear and tympanic membrane (ear drum) form. (Of interest: mammalian embryos have gill pouches which later become the bones of the inner ear.)
A giant new muscle, the tongue, is created, and with it are the restructured neuronal connections to control it. The suction disk and horny teeth of the tadpole are lost and the jaw shape changed. Bone replaces cartilage in the skull and jaw.
The lateral line system, also found in fish, is composed of skin cells sensitive to changes in movement or vibration in water. The fishlike lateral line disappears during metamorphosis.
The tadpole breathes underwater with gills. These gills disappear as the lung enlarges, along with the muscles, cartilage and nerves necessary to pump air. The oxygen-carrying molecule in the red blood cells of the tadpole, hemoglobin, is revised into an adult version which releases oxygen more rapidly in the frog’s tissues.
The tadpole excreted ammonia. Frogs, being terrestrial, need to conserve water, so their liver now makes enzymes which add carbon dioxide to ammonia to form urea, which is excreted instead of ammonia.
A whole lot of changes, don’t you think?
Written by Eve Broughton. Eve lives in Whitethorn and was educated at UC Berkeley.
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