THE HISTOLOGY OF TADPOLES FED WITH THYROID

1. The effect of thyroid is to “accelerate” the ageing of tadpoles, whereby the number of mitoses and development is increased. All organs thus appear more “adult” in character. Metamorphosis is induced and hastened. 2. The alimentary tract is the first to be affected by thyroid treatment, the large and thin‐walled larval type of intestine being replaced by a small thick‐walled type. Masses of pigment are accumulated in the mucosa, and may be seen breaking through the epithelium. 3. Thyroid‐induced metamorphosis results in rapid absorption of the tail, the caudal muscles being disintegrated partly by phagocytes and partly by autolysis. The phagocytes loaded with muscle‐débris simulate eosinophil leucocytes. They wander into the circulation and are, eventually extruded through the intestinal mucosa. 4. The effect of thyroid on blood corpuscles is chiefly that of increasing the numbers undergoing mitosis; the red cells frequently exhibit irregularly shaped nuclei, perhaps a further indication of rapid division. 5. The “fine‐pigment” granules throughout the body are regarded as excretion products. They are well developed in thyroid‐fed tadpoles, and appear to be peculiar to the metabolism of young tadpoles. The effects of thyroid feeding may be summarised in general terms as normal processes which have been hastened and exaggerated by excess of thyroid hormone. This study was commenced in 1917 by Sir Edward Sharpey Schafer, to whom the author is grateful for being permitted to continue it, and for help and advice during its progress. Acknowledgment is due to Messrs C. Siung and A. Dower for their assistance in the preparation of some of the microscopic specimens. A short note has already been published in the Proceedings of the Physiological Society (12, b). The expenses of the research have been assisted by grants from the Earl of Moray Fund of the University of Edinburgh and from the Carnegie Trust.