The Role of the Pituitary and the Thyroid in the Development of Teleosts

INTRODUCTION EXTERNAL appearance undergoes a striking transformation at a certain stage in the life cycle of some teleostean fishes. The previous pigmentary pattern of the body is replaced by a brilliant silveriness. Examples are the change from the parr to the smolt condition of Salmo and from the 'yellow eel' to the silver phase of Anguilla. The physiological significance of such changes is not known or is imperfectly understood. It has been assumed that the silvering of the eel is a ' breeding' dress. Of Salmo such an assumption—if less explicitly made—has been shown to be unfounded by recent work of Orton et al. (1938), Jones & Orton (1940) and Jones (1940). These authors have found that a substantial proportion of male salmon parr becomes sexually mature before the change to the silvery smolt dress. The present communication records data resulting from experiments designed to explore the role of endocrine agencies in relation to the change to the silver so-called breeding dress of Anguilla vulgaris L. and Salmo salar L. In the course of these experiments it has become clear that serious misconceptions, to which it is desirable to draw attention, prevail with regard to sexual differentiation in the former species.