The Effect of Mammalian Thyroid Extract on the Growth Rate and Sexual Differentiation in the Fish, Lebistes reticulatus, Treated with Thiourea

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Abstract
The effect of thiourea growth and differentiation in fishes was studied by adding the drug to the water in which young growing guppies (Lebistes reticulatus) were kept. Growth rate was retarded and sexual differentiation delayed. On cessation of thiourea the usual growth rate was resumed within 2 weeks. Addition of mammalian thyroid fed to thiourea-treated fish did not prevent the retardation of growth and delay in time of sexual differentiation. Fish fed thyroid prior to treatment ceased to grow and differentiate when thiourea was added to the water even though the thyroid was continued. Fish treated with both thiourea and thyroid simultaneously showed a marked increase in growth when thiourea was discontinued.