Abstract
A relationship between the thyroid gland and metamorphosis has been fully established in amphibians. 1 and has been suggested for at least one species of fish. 2 Two factors appear to be necessary for that relationship, an actively secreting gland, and sensitivity of the body cells to its hormone.
The adult cyclostome possesses a single thyroid gland which develops at metamorphosis from the endostyle, a mucus-secreting structure ventral to the pharynx. According to Horton, 3 the thyroid of the adult lamprey will induce metamorphosis in tadpoles. Subjection of larval lampreys to thyroid treatment has produced no effect on external metamorphosis; 4 and potassium iodide solutions, effective when administered to amphibian larvae, do not alter the histology of the endostyle. 5
The present study was undertaken to test the results of earlier investigators with a larger number of animals. A careful histological study of the endostyle was also made, on the possibility that slight metamorphic changes not reflected in external appearance might have been produced by thyroid treatment.
Ammocoetes larvae 50-80 mm in length, of the genus Ento-sphenus, were collected in midsummer and divided into lots of 20. Larger animals were discarded to eliminate the complication of normal metamorphosis, and the larvae were placed in finger bowls in groups of 5. The solutions used were as follows :
1. Control—pond water.
2. Crystalline thyroxin∗ in pond water, 1:500,000.
3. Desiccated thyroid (crushed tablets), 1:120,000.
4. Desiccated thyroid (powder), 1:500,000.
(A similar concentration of this preparation produced distinct metamorphic changes in Hyla tadpoles in 7 days. Stronger doses were fatal.)
In contrast to most of the earlier work, where animals were exposed only at intervals, the Ammocoetes were kept in the solutions, which were renewed every 3 days.
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