1. The histogenesis of the pituitary of Frog and Toad tadpoles has been followed from the time of emergence from the egg-jelly until they leave the water, and that of the young Frog to four months. The development of the thyroids, particularly in relation to the pituitary and to metamorphosis, and also that of the gonads has been followed.
2. Histological differentiation of the two glands in both species commences at the same time and at the stage when the larvae become completely free-swimming. Eosinophile granules appear in cells towards the posterior end of the anterior lobe of the pituitary, and minute droplets of colloid appear in the centre of groups of thyroid cells.
3. There is a steady increase in eosinophiles and in the amount of colloid in the early stages up to the beginning of rapid hind-leg growth. Colloid is not only stored but discharged during this period, when only eosionophiles are present in the pituitary. The early organization of the gonad proceeds meanwhile, seemingly independently of the pituitary.
4. In the next period, which can be regarded as the most active period of metamorphosis since the hind legs grow rapidly, the fore legs appear and the tail is absorbed, the thyroid is extremely active, both in the formation and release of colloid. This activity is much more pronounced in the Frog than in the Toad, which develops later. A marked increase in the amount of thyroid tissue also occurs. The eosinophiles continue to increase in number but lose their staining power somewhat, possibly indicating an active discharge of secretion. These observations indicate that general body-growth and thyrotropic function are associated with the eosinophile cells. A few faintly staining basophiles appear at this time in the anterior pituitary lobe in the Frog, and later in the Toad. They increase regularly but slowly and show no close relationship with thyroid activity. Basophiles can be seen also in the intermediate lobe, but remain few and primitive throughout early development.
5. As the absorption of the tail is completed and the animals leave the water the eosinophiles continue to increase slowly and the basophiles to increase more rapidly, whilst the pars tuberalis becomes separated from the anterior lobe. The largely emptied vesicles of the thyroid become refilled with colloid and greatly distended, while the epithelial cells begin to lose their active cubical form and to take on the flattened form of the adult. The gonad is now becoming organized into a definite ovary or testis.
6. The further development of the pituitary in the Frog up to the first hibernation period produces a gland in which the intermediate lobe still remains rather undeveloped but in which the anterior lobe is beginning to resemble that of the adult. Eosinophiles are deeply staining and adult in appearance, basophiles are much more numerous and are approaching the adult state both in proportion to eosinophiles and depth of staining, whilst neutrophils can be distinguished. The increase in number and maturity of the basophiles as the definitive gonads form supports the view that they are primarily sex-stimulating in function.