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The investigations of ultimobranchial tissue still remain to be some contradictory questions on its fate and function. Some investigators insist the contribution of ultimobranchial tissue to formation of thyroid tissue and some others deny it. Autoradiographic studies using radioiodine (Gorbman, '47) elucidated a fact in mice that the follicles of ultimobranchial origin are not able to collect P 31 and are physiologically different from the thyroid follicles. On the other hand, the concept of transformation of ultimobranchial tissue into thyroid tissue has been supported by studies of alkaline glycerophosphatase activity of branchiogeneous endocrine organs in rats (McAlpine, '55). It has been observed that, when incorporated into the developing thyroid gland, the ultimobranchial tissue is indistinguishably transformed into thyroid-like follicies which appear to function as such during periods of thyroid activity (van Dyke, '58). Furthermore, parafollicular cells (interfollicular cells) found in the mammalian thyroid glands have been recently described to be derived from residual ultimobranchial tissue (Godwin, '36/37, in dogs; van Dyke, '45, in sheep; Sato, '59, in hamsters). In the electron microscopic study of this article, special attention is paid to (1) the nature of the cytoplasmic components in ultimobranchial tissue, (2) the cytological evidence of glandular activity and (3) the possibility or nonpossibility of the same function as that found in thyroid tissue.

ELECTRON MICROSCOPIC OBSERVATIONS OF THE CYST OF ULTIMOBRANCHIAL ORIGIN FOUND IN THE THYROID GLAND OF A DOG.