TEN FIGURES
The administration of thyroxin and dried mammalian thyroid powder t o fish has been used as an experimental method in an effort to determine the normal function of the thyroid gland in the teleosts. The results have been negative or contradictory. Grobstein and Bellamy ('39) fed a "pinch" of desiccated mammalian thyroid to immature fish (Platypoecilns) and reporte'd that growth was inhibited with controls showing a 25% greater body length. Gonopod differentiation in the treated males began considerably earlier but was atypical. All treated fish developed bilateral exophthalmos. Smith and Everett ('43) fed both mammalian thyroid powder and synthetic thyroxin to Lebistes reticulntus in esperiments which lmasted from 60 t o 90 days an'd found no effect on growth nor on the time of beginning of gonopod differentiation. These authors reported that exophthalmos did not develop in the treated fish. Nigrelli, Goldsmith and Charipper ('46) stated that Lebistes previously treated with 0.03% thiourea and retarded in development, showed an increase in growth rate and normal differentiation of the gonopodinm when fed 0.2 gm of mammalian thyroid powder twice a week. This increase in growth rate in thiourea-treated fish is the only report of a positive effect of any thyroid substance on growth processes in the teleosts.