Abstract
IT is WELL ESTABLISHED that the guinea pig is the best mammal to use in tests of the thyrotropic potency of the anterior lobe of the pituitary gland because its thyroid responds quickly and markedly to administration of anterior pituitary substanceof many animals (1, 2). However, there is considerable evidence that the thyroid of a bird, the very young chick, is an even more sensitive indicator of the presence of thyrotropic hormone (3-8) both because the histological picture in the untreated animal is amore uniform one and therefore better adapted to reveal stimu lation in large or small degree and because it responds to a smaller quantity of sub stance. It is also a less expensive and more easily cared for host than the guinea pig. In some experiments in which the chick has been used the degree of the reaction has been judged mainly on the basis of changes in absolute weight in the thyroids of anterior pituitary treated birdscompared with their controls (3-7, 9).
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