. The biology of the amphibia. Amphibians. 294 THE BIOLOGY OF THE AMPHIBIA sis here are not sensitive to the thyroid hormone. Jensen, Swin- gle, and others found that thyroid substances induced no change in Proteus or Necturus, although the thyroid of the latter is capable of hastening metamorphosis when transplanted to Rana tadpoles (Swingle, 1922). When Cryptobranchus and Siren are subjected soon after hatching to thyroid extracts or thyroxin, they shed their larval skin and assume the characteristic integu- ment of metamorphosed salamanders. This change normally occurs much later in the lif

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. The biology of the amphibia. Amphibians. 294 THE BIOLOGY OF THE AMPHIBIA sis here are not sensitive to the thyroid hormone. Jensen, Swin- gle, and others found that thyroid substances induced no change in Proteus or Necturus, although the thyroid of the latter is capable of hastening metamorphosis when transplanted to Rana tadpoles (Swingle, 1922). When Cryptobranchus and Siren are subjected soon after hatching to thyroid extracts or thyroxin, they shed their larval skin and assume the characteristic integu- ment of metamorphosed salamanders. This change normally occurs much later in the lif Stock Photo
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. The biology of the amphibia. Amphibians. 294 THE BIOLOGY OF THE AMPHIBIA sis here are not sensitive to the thyroid hormone. Jensen, Swin- gle, and others found that thyroid substances induced no change in Proteus or Necturus, although the thyroid of the latter is capable of hastening metamorphosis when transplanted to Rana tadpoles (Swingle, 1922). When Cryptobranchus and Siren are subjected soon after hatching to thyroid extracts or thyroxin, they shed their larval skin and assume the characteristic integu- ment of metamorphosed salamanders. This change normally occurs much later in the lif
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. The biology of the amphibia. Amphibians. 294 THE BIOLOGY OF THE AMPHIBIA sis here are not sensitive to the thyroid hormone. Jensen, Swin- gle, and others found that thyroid substances induced no change in Proteus or Necturus, although the thyroid of the latter is capable of hastening metamorphosis when transplanted to Rana tadpoles (Swingle, 1922). When Cryptobranchus and Siren are subjected soon after hatching to thyroid extracts or thyroxin, they shed their larval skin and assume the characteristic integu- ment of metamorphosed salamanders. This change normally occurs much later in the life of Cryptobranchus and Siren, and presumably takes place under the influence of the salamanders' own thyroids. It is evidence for the fact that the skin, alone of all the tissues, which normally metamorphose in related sal- amanders, is in these forms sen- sitive to the thyroid hormone. Neoteny.—Urodele larvae are frequently found sexually ma- ture in nature, showing that the development of the gonads is f^not dependent on the thyroid hormone. The removal of the thyroid in mammals prevents growth and leads to cretinism, but the growth of the larvae of both frogs and urodeles is un- affected by this operation. In the perennibranch Typhlomolge the thyroid may be absent, but in view of the fact that none of the other perennibranchs fully metamorphoses after thyroid treatment it appears doubtful if neoteny in this genus can be attributed to the loss or reduction of this organ. Blacher (1928) found that the intestine of tadpoles was stimulated to metamorphic change by weaker solutions of thyroid extracts than those necessary to produce changes in the tail, while the latter responded to weaker solutions than those required by jaws or trunk. Fontes and Aron (1929) by using minimum doses of thyroxin showed that the skin of tadpoles was more sensitive than the tail in the species they were considering. The quantity of thyroid hormone necessary to produce a metamorphic change varies w