The localization of activities among the limited kinds of cells in the anterior lobe of the hypophysis has been an intriguing problem for many years. While there have been certain indications of a functional nature, reliance has been placed largely on cytological evidence which has been summarized by Severinghaus [1937]. The inconclusive nature of the findings was amply illustrated by the numerous points for and against associating the secretion of gonadotrophic hormone with eosinophil cells and with basophils. For example, although there was evidence that during oestrus the basophils were smaller and less granular than during dioestrus, there were also observations of alterations in the eosinophils. Even after castration, when the pituitary gland accumulates more gonadotrophin than at any other time, the characteristic changes in basophils are apparently accompanied by others in eosinophils. In the former, the colloid vacuole has been the subject of intensive study, and it was suggested