Comparative Histological Studies of the Thyroid Gland of Lower Vertebrates

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Thyroid glands of lower v e rtebrates including several species of birds, reptiles, amphibians and fishes were histologically investigated and the data obtained were discussed by comparing with the data known of mammalian thyroid glands.<BR>1. The thyroid glands of lower vertebrates are composed of independent, separated closed epithelial sacsfollicleswhich are generally round to oval and larger in the periphery than in the center.<BR>2. Follicles of the embryonic or maturing thyroid glands are often associated with each other by direct connection of their walls without open communication of their cavities, and form follicle groups. This may suggest the proliferation, present and past, of follicles through budding process.<BR>3. It was difficult to f i nd any significant difference in follicles in relation to sex and season. The follicles are generally small in birds such as crows, pigeons, domestic fowls, swallows and sparrows. On the other hand, the follicles are considerably large in Chelonia and snakes. The follicles of lizards are small. The follicles of amphibians and small fishes are also small. It is supposed that the size of follicles may depend partly upon the body weight and length of animals used.
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