Studies on internal secretion.—I. The effect of pituitary (anterior lobe) injection upon normal and thyroidectomised axolotls

The past eight years have witnessed signal advances in the study of internal secretion in relation to Amphibian metamorphosis, a line of enquiry which was opened up by the researches of Adler, Gudernatsch, and others in Germany, immediately before the outbreak of the late war, and later developed more extensively by the American school of experimental zoologists. From the standpoint of developmental mechanics the discoveries made in this connection have illustrated forcefully, to employ Prof. MacBride's felicitous phrase, the influence of theinternal environment ,i. e ., the processes by which the development of one structure conditions the differentiation of another. Of more general interest is the fact that the study of ductless glands in connection with developmental phenomena in frogs, salamanders, and their allies, has vindicated the existence of a specific iodine metabolism in Vertebrates and its relation to the thyroid. While a significant result, from the medical standpoint, is that a way has been opened up for a more precise knowledge of the activity of the anterior lobe of the pituitary gland. For a brief outline of recent advances in this field the work of Gudernatsch (1912-14), who showed that frog tadpoles transform into adults precociously if fed on thyroid tissue, will serve as a convenient starting point. Morse, West, Barthelemez, Lim and Swingle are among those who confirmed this observation. Bennet Allen (1918) succeeded in removing the thyroidanlage in Anuran tadpoles, as did also E. R. and. M. M. Hoskins (1919); and it was shown that the extirpation of this gland prevents metamorphosis in the ordinary course of events. Swingle, however, induced thyroidless tadpoles to undergo metamorphosis readily by feeding them on fresh thyroid gland. The work of Swingle also shed light on the iodine question. The existence of a high percentage of iodine in the thyroid has long been known; and has prompted considerable discussion which the work of Kendall (1917), in isolating a physiologically active iodine compound from thyroid extracts, has greatly illuminated. If any doubt as to the significance of the presence of iodine in the thyroid remained, it should be dissipated by the study of Amphibian metamorphosis. Morse (1915) found that iodised blood albumen possesses the power of stimulating the destruction of the larval characteristics in Anura, and Lenhart (1915) further showed that the rate of metamorphosis in thyroid-fed tadpoles is more rapid when the iodine-content of the glands used is high. Finally Swingle (1918-19) demonstrated the efficacy ofinorganic iodine to induce transformation in thyroidless tadpoles which are normally incapable of further development; and proved the necessity of a certain minimum of iodine in the food or water in order that either normal or thyroidless tadpoles may assume the adult form.