A study on the histomorphology and histochemistry of the thyroid and parathyroid glands in sheep was conducted in the prenatal and post natal age groups of sheep. The capsule of the thyroid gland consisted of two layers of connective tissue separated by a layer of adipose tissue. The stroma showed the presence of solitary ganglion cells. The parenchyma of the thyroid gland consisted of solid epithelial cords separated by mesenchymal tissue. In sheep embroys at 25 days of gestation, precursors of C cells called dark cells were found in the ultimobranchial bodies. Multiple ciliated follicular cells were observed in the developing thyroid of dogs and were prominent at three weeks prior to the expected date of birth. The thyroid follicles were irregular in outline and the diameter increased in the older subjects. In buffaloes, the larger follicles were present in the deeper zone of the thyroid gland. The follicles were lined by follicular cells and light cells. The solid form of colloid was observed in oldest goats. In the sheep thyroid glands, the parafollicular cells (C cells) were oval to polyhedral in shape, frequently possessed long cytoplasmic protrusions and were located mainly in the intrafollicular and often in the parafollicular area. The ultimobranchial follicles of the sheep thyroid were of various shapes, sizes and forms. The ultimobranchial follicles of the sheep thyroid glands were positive for neutral and acid mucopolysaccharides, lipids and acid phosphatase activity. In the golden hamster, the parathyroid was derived from the third pharyngeal pouch, situated on day 13 of gestation on the dorsolateral side of the thyroid, being surrounded by a common capsule with the thyroid. In the parathyroids of the Wister rats, the chief cell clusters were clearly demarcated from the thyroid by a prominent connective tissue capsule with poor vascularity and the cell clusters were separated by an indistinct stroma of connective tissue with occasional fibrocytes. In the sheep parathyroids, the follicles were irregularly distributed in the gland parenchyma and consisted of chief cells, but not of water-clear cells and oxyphil cells. Two types of follicles namely dark and light follicles were observed. In goat, the secretory granules of the chief cells of the internal parathyroids were PAS positive. Chief cells in the parathyroids of duck showed acid and alkaline phosphatase activities.

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